Published online before print February 9, 2007, doi: 10.1161/01.RES.0000260182.36481.c9
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© 2007 American Heart Association, Inc.
Cellular Biology |
Diabetes Downregulates Large-Conductance Ca2+-Activated Potassium ß1 Channel Subunit in Retinal Arteriolar Smooth Muscle
From the Centre for Vision Sciences (M.K.M., D.P.D., A.A., N.W., J.D., D.A.S., T.M.C.), The Queen’s University of Belfast, Institute of Clinical Sciences, Royal Victoria Hospital, Belfast, Northern Ireland; Cell and Metabolic Signalling Group (C.N.S., J.G.M.), School of Medicine and Dentistry, Queen’s University of Belfast, Medical Biology Centre, Belfast, Northern Ireland.
Correspondence to Dr Tim M. Curtis, Centre for Vision Sciences, School of Biomedical Sciences, The Queen’s University of Belfast, Institute of Clinical Sciences, The Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, Northern Ireland. E-mail t.curtis{at}qub.ac.uk
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Abstract |
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Retinal vasoconstriction and reduced retinal blood flow precede the onset of diabetic retinopathy. The pathophysiological mechanisms that underlie increased retinal arteriolar tone during diabetes remain unclear. Normally, local Ca2+ release events (Ca2+-sparks), trigger the activation of large-conductance Ca2+-activated K+(BK)-channels which hyperpolarize and relax vascular smooth muscle cells, thereby causing vasodilatation. In the present study, we examined BK channel function in retinal vascular smooth muscle cells from streptozotocin-induced diabetic rats. The BK channel inhibitor, Penitrem A, constricted nondiabetic retinal arterioles (pressurized to 70mmHg) by 28%. The BK current evoked by caffeine was dramatically reduced in retinal arterioles from diabetic animals even though caffeine-evoked [Ca2+]i release was unaffected. Spontaneous BK currents were smaller in diabetic cells, but the amplitude of Ca2+-sparks was larger. The amplitudes of BK currents elicited by depolarizing voltage steps were similar in control and diabetic arterioles and mRNA expression of the pore-forming BK
subunit was unchanged. The Ca2+-sensitivity of single BK channels from diabetic retinal vascular smooth muscle cells was markedly reduced. The BKß1 subunit confers Ca2+-sensitivity to BK channel complexes and both transcript and protein levels for BKß1 were appreciably lower in diabetic retinal arterioles. The mean open times and the sensitivity of BK channels to tamoxifen were decreased in diabetic cells, consistent with a downregulation of BKß1 subunits. The potency of blockade by Pen A was lower for BK channels from diabetic animals. Thus, changes in the molecular composition of BK channels could account for retinal hypoperfusion in early diabetes, an idea having wider implications for the pathogenesis of diabetic hypertension.
Key Words: Ca2+ sparks • diabetes mellitus • microcirculation • potassium channels • vascular smooth muscle cells
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Introduction |
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Diabetes causes changes to the structure and function of blood vessels in the retina leading to visual impairment and blindness.1 The cellular and molecular basis of diabetic retinopathy is not wholly understood although large prospective clinical trials have established the importance of hyperglycaemia in precipitating this disease in both type 1 and type 2 diabetic patients.2,3 A major pathway through which hyperglycaemia is believed to contribute to retinal microangiopathy is the disruption of retinal blood flow. Patient-based studies have shown that retinal hemodynamic abnormalities occur before the onset of clinical diabetic retinopathy4 and that the development and progression of diabetic retinopathy correlates with the extent of the blood flow changes observed.5–8 In diabetic patients without retinopathy, retinal arteriolar vasoconstriction9,10 and decreased total retinal blood flow has been reported.4,11 However, as the disease progresses, the arterioles begin to dilate12 and bulk retinal blood flow increases in proportion to the severity of retinopathy, and thus the degree of retinal ischemia.11 Presently, the molecular mechanisms underlying retinal vasoconstriction during early diabetes are unknown, yet an improved understanding of the pathophysiology involved could be crucial to the development of better therapies for the treatment of diabetic retinopathy.
A major factor controlling the contractile state of arterioles is the activity of ion channels on the plasma membranes of the vascular smooth muscle cells (VSMCs).13 Large-conductance Ca2+-activated K+ (BK) channels are known to play a crucial role in the regulation of arterial smooth muscle tone because blockade of these channels using the specific inhibitor iberiotoxin, causes membrane depolarisation and vasoconstriction in pressurized isolated vessels.14 BK channels are activated by local Ca2+ release events, termed Ca2+ sparks, resulting from opening of ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR15). The activation of BK channels results in outward K+ current that opposes VSMC contraction by causing membrane hyperpolarisation. This reduces Ca2+ influx by reducing activation of voltage-dependent Ca2+ channels.15 BK channels are composed of -subunits and accessory ß-subunits.16 The -subunit forms the K+ selective pore, whereas the ß subunits influence the kinetics, pharmacology and Ca2+ sensitivity of BK currents.17,18 Four members of the BK ß-subunit family have been identified to date (ß1-ß4) and the ß1-subunit is expressed predominantly in VSMCs.19 Targeted deletion of the ß1-subunit gene reduces the Ca2+ sensitivity of BK channels and the coupling of Ca2+ sparks to BK channel activity in VSMCs from cerebral arteries.19,20 The functional significance of the ß1-subunit of VSMC BK channels is underlined by the observation that knockout mice are hypertensive and display enhanced vascular reactivity to application of vasoconstrictors.19,20
In the present study we have examined the effects of streptozotocin (STZ)-induced diabetes on the properties of BK channels in VSMCs of the rat retinal microcirculation. A clear advantage of using STZ-diabetic rats is that it is well documented that retinal arteriolar vasoconstriction and decreased retinal blood flow occur in this animal model following several weeks of diabetes.21 These animals subsequently exhibit many of the vasodegenerative changes associated with human diabetic retinopathy.22 We hypothesized that diabetes causes a downregulation of the - and/or ß1-subunit, thereby reducing the capacity of the BK channels to hyperpolarise retinal VSMCs and resist vasoconstriction. We show that diabetes reduces coupling between Ca2+ release from RyR sensitive Ca2+ stores and BK channel activation. No alteration in the expression of the pore-forming -subunit was evident, but ß1-subunit expression was reduced at both the mRNA and protein level. Consistent with this, we found that BK channels in retinal VSMCs from STZ-diabetic animals exhibit a diminished sensitivity to Ca2+ and we provide pharmacological evidence supporting the idea that the expression of functional +ß1 subunit complexes is reduced in diabetes. These results suggest that changes in the molecular composition of BK channels in retinal VSMCs during diabetes might contribute to the onset and early progression of diabetic retinopathy.
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Materials and Methods |
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All procedures with animals were performed in accordance with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85 to 23, revised 1996) and the United Kingdom Animals (Scientific Procedures) Act, 1986. Full details of the methods and materials used are in the online data supplement available at http://circres.ahajournals.org.
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Results |
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Characteristics of Experimental Animals
Diabetic animals had higher mean plasma glucose levels (27.8±1.3mmol/L; n=41) than nondiabetic animals (7.6±0.3mmol/L; n=37 P<0.001). Mean glycosylated hemoglobin values were 5.9±0.2% and 17.6±0.8% in the nondiabetic and diabetic groups, respectively (P<0.001). Both sets of animals gained weight during the 3-month experimental period, but the increase was greater in nondiabetic than diabetic rats (304±17g versus 141±9.7g, respectively; P<0.001).
Functional Significance of BK Channels in Retinal VSMCs
We examined the contribution of BK channels to the regulation of retinal arteriolar tone by pressurizing freshly isolated retinal arterioles from nondiabetic animals to 70mmHg and measuring the change in internal diameter induced by 100nmol/L Penitrem A (Pen A), a potent inhibitor of BK channels.23 Pen A caused a 28% decrease in the diameter of pressurized retinal vessels (Figure 1). These results suggest that BK activity plays an important vasodilatory role in retinal blood vessels.
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Caffeine-Induced BK Currents, but Not Ca2+ Activated Cl– Currents, Are Smaller in Retinal VSMCs From Diabetic Animals
To investigate BK channel activity during diabetes we tested activation following Ca2+ release from caffeine-sensitive Ca2+ stores. Retinal arterioles were bathed in low Cl– Hanks’ solution containing 1mmol/L 9-anthracene carboxylic acid (9AC) to block Ca2+ activated Cl– channels (ClCa channels). Application of 10mmol/L caffeine for 5 seconds evoked large, noisy transient outward currents at positive membrane potentials (Figure 2A and 2C). These currents were fully abolished by addition of 100nmol/L Pen A (Figure 2A; n=6). Figure 2B shows typical caffeine-induced BK currents in retinal VSMCs from nondiabetic and diabetic animals. Figure 2C shows the average peak current density plotted as a function of voltage for the caffeine-induced BK currents. BK currents evoked by Ca2+ release from caffeine-sensitive Ca2+ stores were dramatically reduced in VSMCs from diabetic animals.
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The attenuated BK currents in arterioles from diabetic rats could be explained by decreased Ca2+ release from the caffeine-sensitive stores. To test this, global [Ca2+]i responses were measured in retinal VSMCs from nondiabetic and diabetic animals using fura 2 based Ca2+ microfluorimetry (Figure 3A). No differences in the peak amplitude of [Ca2+]i transients evoked by 10mmol/L caffeine were observed. We also compared the size of caffeine-evoked ClCa currents at a range of voltages in myocytes from nondiabetic and diabetic arterioles bathed in normal Hanks’ solution containing 100nmol/L Pen A to block BK channels. Application of 10mmol/L caffeine evoked currents that reversed close to ECl (0mV; Figure 3B and 3D). These were completely abolished in low Cl– Hanks’ solution containing the Cl– channel inhibitor, 9AC (Figure 3B; n=6). Isolation of the caffeine-evoked Cl– currents in diabetic arterioles required longer preincubation times with Pen A (
20 minutes as compared with 5-minute for nondiabetic vessels). Figure 3C and 3D show representative traces and summary data for the caffeine-evoked Cl– currents in nondiabetic and diabetic retinal VSMCs. The mean peak current densities were similar for normal and diabetic cells at all voltages tested. Thus, it appears that the smaller caffeine-induced BK currents in diabetic retinal VSMCs reflect reduced coupling efficiency between Ca2+ release and BK channel activation rather than defective Ca2+ release.
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Ca2+ Sparks Are Larger, but STOCs Are Smaller in Diabetic Retinal VSMCs
From a physiological perspective it is well established that Ca2+ sparks activate BK channels generating spontaneous transient outward K+ currents (STOCs), modulating vascular tone.15,24 STOCs were recorded from retinal VSMCs of nondiabetic and diabetic animals at test potentials between –80mV to +80mV (increased in 40mV increments) using the perforated patch clamp technique. STOCs were evident at membrane potentials positive to –40mV and were completely abolished in the presence of 100nmol/L Pen A (n=6, nondiabetic VSMCs). Individual STOC events were superimposed (Figure 4A) precluding an accurate assessment of STOC amplitudes and frequencies, so we calculated the integral of the STOC densities for recordings lasting 5 to 10 minutes. Visual inspection of the original traces suggested that STOC amplitudes and frequencies were greatly reduced in diabetic retinal VSMCs, and integrated current densities were considerably smaller (Figure 4A). These data confirm that retinal VSMCs from diabetic animals demonstrate less spontaneous BK current activity than cells from nondiabetic animals.
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Using confocal imaging techniques, we have recently described the presence of 2 distinct populations of spontaneous Ca2+ sparks in retinal VSMCs: "basal" sparks that arise from resting fluorescence levels (ie, from F/Fo=0.95 to 1.05) and "sparks on oscillations" that overlay global Ca2+ transients.25 Figure 4B shows representative images of basal Ca2+ sparks, Ca2+ sparks on oscillations and global Ca2+ oscillations in nondiabetic arteriolar myocytes. We considered the possibility that the lower STOC activity in diabetic retinal VSMCs may reflect reduced spontaneous subcellular [Ca2+]i signaling in these cells. Figure 4C shows typical images of basal Ca2+ sparks in nondiabetic and diabetic arteriolar myocytes. Below the image panels, the time course of the normalized fluorescence for each event has been plotted and the traces superimposed. It is evident that the peak amplitude of the Ca2+ spark in the diabetic cell is around twice that of the nondiabetic. Quantitative data for basal Ca2+ sparks, Ca2+ sparks on oscillations and global oscillations in nondiabetic and diabetic VSMCs are summarized in supplemental Table I of the online data supplement. The peak amplitude of both populations of Ca2+ sparks was substantially larger in diabetic than in nondiabetic VSMCs (
F/Fo basal sparks, 0.92±0.06 and 0.42±0.03, respectively; F/Fo sparks on oscillations, 1.56±0.21 and 0.36±0.04, respectively; P<0.001 in both cases), whereas the frequency and duration (FDHM) of these events remained unchanged. No differences were observed in amplitude, frequency or duration of global Ca2+ oscillations between nondiabetic and diabetic VSMCs. These results show that decreased subcellular Ca2+ signaling activity cannot explain the decreased STOC activity observed in diabetic VSMCs.
Diabetes Reduces the Ca2+ Sensitivity of BK Channels in Diabetic Retinal VSMCs
Both the comparisons of caffeine-evoked currents and Ca2+ transients, and of STOC and spark activity, suggest decreased coupling between Ca2+ release and BK channel activation in retinal VSMCs after short-term diabetes. This might be explained by a reduced number of functional BK channels in diabetic myocytes. Quantitative RT-PCR, however, failed to reveal any differences in BK transcript expression in nondiabetic and diabetic retinal arterioles (Figure 5A). To assess BK channel density, we also compared depolarization dependent whole-cell BK currents (Figure 5Bi and 5Bii). Vessels were bathed in low Cl– Hanks’ solution containing 1mmol/L 9AC and 10 mmol/L 4-aminopyridine (4AP) to block ClCa and Kv channels,26 respectively. 10 mmol/L 4AP has no effect on BK channels in retinal arterioles; caffeine-evoked BK currents at +40mV in the absence and presence of 10 mmol/L 4AP were 115±29.8 and 114±26pA/pF, respectively (n=4; P=0.84; paired t-test). Voltage-activated BK currents in control VSMCs were unaffected by the removal of extracellular Ca2+, inhibition of voltage-dependent Ca2+ channels with 10 µmol/L nifedipine or blockade of RyR receptors with 100 µmol/L tetracaine (Figure 5Biii-v). This demonstrates that BK currents evoked by depolarisation are independent of Ca2+ influx and Ca2+ store release in these cells ie, they appear to be entirely dependent on voltage gating of the BK channels. As such, the peak current density should be proportional to the number of functional BK channels, assuming that the single-channel conductance and the voltage dependence of activation remain constant. Single channel conductance’s of BK channels were similar for nondiabetic and diabetic VSMCs (160±2pS and 160±4pS, respectively; n=21 and 15 patches, P>0.05). Figure 5C shows the average peak current density as a function of voltage for the voltage-activated BK currents in retinal VSMCs from nondiabetic and diabetic animals. No differences were observed, suggesting that BK channel density is unaltered in retinal VSMCs during diabetes. Consistent with this, in single channel recordings we found no difference in the number of BK channels per membrane patch in nondiabetic (2.4±0.2 channels) and diabetic (2.1±0.2 channels) retinal VSMCs (P>0.05, n=40 patches).
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Another possible explanation for the reduced coupling between Ca2+ release and BK channel activation is a decrease in the sensitivity of the BK channels to activation by Ca2+. We examined the Ca2+-sensitivity of BK channels using inside-out membrane patches from retinal VSMCs (Figure 6). The open probability (Po) at +80mV was determined for a range of Ca2+ concentrations between 0.01 to 100µmol/L (Figure 6A and 6B). The activity of BK channels from both nondiabetic and diabetic animals increased with increasing Ca2+ concentrations, but the Po versus Ca2+ curve was shifted to the right and the Hill slope was reduced for diabetics (Figure 6B). No differences in Po’s were observed at Ca2+ concentrations of 0.01 and 0.1µmol/L, and this may explain the similarity in voltage-activated, whole cell BK-currents described above. Po versus voltage relations were also determined at a single Ca2+ concentration, 10µmol/L (Figure 6C). There was a strong rightward shift along the voltage axis (>100mV) for BK channels from diabetic animals.
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ß1 Expression and Function is Lower in Diabetic Retinal VSMCs
The results above suggest that BK channels in diabetic VSMCs have a reduced Ca2+ sensitivity. Because the Ca2+ sensitivity of BK channels is known to be dependent on the presence of ß1 accessory subunits,17,18 a downregulation of the ß1 subunit could explain the changes observed in diabetes. Expression of the BKß1 subunit in retinal arterioles from nondiabetic and diabetic animals was investigated at the mRNA level. ß1 transcripts were approximately 60% less abundant in diabetic than in nondiabetic arterioles (Figure 7A). We also estimated changes in expression of the ß1 subunit by immunohistochemistry. A punctuate distribution of BKß1-associated fluorescence was apparent throughout the VSMC layer of retinal arterioles from nondiabetic animals (Figure 7B). Consistent with the RT-PCR analysis, BKß1 immunostaining decreased dramatically in retinal arterioles of diabetic rats (Figure 7B) ie, BKß1 expression is reduced in retinal VSMC in diabetes.
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Besides increasing the Ca2+ sensitivity of the BK-subunit, the BKß1-subunit also modifies the kinetics and pharmacological properties of BK channels.18 The ß1 subunit increases the stability of BK channel open states.27 If there is a decrease in the coupling ratio of /ß1 subunits in diabetes then BK channel open times should be reduced. We compared the open times of BK channels from control and diabetic retinal VSMCs by constructing open time histograms at +80 mV with 10 µmol//L Ca2+. Histograms were fitted with a single exponential function. BK channels from diabetic myocytes (
open=2.6±1.5ms) had shorter open times than those from control VSMCs (open=6.36±0.54ms; P<0.05), supporting the view that ß1 subunit function is decreased during diabetes. Recently it has been shown that the xenoestrogen, tamoxifen, markedly increases the Po of BK channels but only when they are associated with a ß1 subunit.28 Exposure of BK channels from nondiabetic cells to 1µmol/L tamoxifen increased the Po by approximately 1.5-fold but had no significant effect on BK channels from diabetic retinal VSMCs (Figure 7C). From these data, we conclude that the reduced coupling efficiency between Ca2+ release and BK channel activation in diabetic retinal VSMCs results from reduced Ca2+ sensitivity of the BK channels, reflecting a decrease in the functional expression of the BKß1 accessory subunit.
It has also previously been reported that BK+ß channels are markedly less sensitive to blockade by iberiotoxin when compared with BK channels alone.17 To determine whether the BKß1 subunit might affect the sensitivity of BK channels to Pen A, we compared the effects of this inhibitor on the Po of BK channels from nondiabetic and diabetic retinal VSMCs (Figure 7D). Application of 100 nmol/L Pen A to BK channels from nondiabetic cells caused close to 100% block of channel activity. By contrast, the Po of BK channels from diabetic cells was reduced by only 27%. These results are consistent with the possibility that BKß1 increases the sensitivity of BK channels to Pen A, although further studies using heterologous expression systems are required to confirm this.
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Discussion |
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It has been recognized for the past 25 years that abnormal blood flow to the retina occurs during early diabetes and that this may contribute to the pathogenesis of diabetic retinopathy.12 Despite this, surprisingly little is known about the precise mechanisms linking chronic hyperglycaemia to retinal arteriolar vasoconstriction and reduced retinal blood flow before the onset of overt retinopathy. In the present study, we have identified a major pathophysiological mechanism that may play a specific role in the development of retinal perfusion abnormalities in diabetes. We have obtained molecular and functional data to suggest that during diabetes, the BKß1 subunit is downregulated in retinal VSMCs, whereas the expression of the pore-forming
-subunit remains unaltered. Consistent with this, we observed a marked reduction in the Ca2+ sensitivity of BK channels and an uncoupling of BK channel activation from Ca2+ release in diabetic retinal VSMCs. Of particular note, the decreased expression of the ß1 subunit drastically reduced the ability of spontaneous Ca2+ sparks to activate BK channel-mediated STOCs. Because STOCs act to hyperpolarise and relax VSMCs,15,29 loss of STOC activity could well underlie the observed arteriolar vasoconstriction seen in the development of diabetic retinopathy. Information is generally lacking regarding the effects of diabetes on vascular BK channel function. Coronary microvessels from diabetic dyslipidemic swine exhibit an uncoupling in the relationship between Ca2+ sparks and STOC activation30 and whole-cell BK current density is reduced in microvascular smooth muscle cells of mesenteric arteries from Zucker diabetic fatty rats31 and fructose-fed, insulin resistant rats.32 The precise mechanisms underlying the changes in vascular BK channel activity were not resolved in these studies, although it is interesting that an alteration in BKß1 expression was not apparent in Zucker diabetic fatty rats.31 This discrepancy with the present study could possibly be attributed to the different animal models used and the origins of the microvessels studied. In general terms, however, the findings from the current work do strengthen the view that BK channel function is impaired in VSMCs of the microcirculation during diabetes. Intriguingly, impaired BK channel function may be limited to the microvasculature because the Po of BK channels is increased in thoracic aortic VSMCs of STZ-induced diabetic mice.33
Hypertension is approximately twice as frequent in patients with diabetes compared with patients without the disease.34 Furthermore work linked to the United Kingdom Prospective Diabetes Study (UKPDS) showed that the incidence of micro- and macrovascular diabetic complications was strongly associated with elevated blood pressure.35 Presently the etiology of hypertension in diabetic patients is not fully understood, although loss of the normal vascular relaxation to insulin in both type 1 (insulin-deficiency) and type 2 (insulin-resistance) diabetes may contribute.36 The present study is the first to raise the possibility that there is a selective downregulation of BKß1 resulting in an impairment of RyR-BK channel signaling which could be an important component in the pathogenesis of hypertension in diabetic patients. However, detailed profiling of BKß1 expression in a range of vascular tissues during diabetes, particularly in small arteries and arterioles that are central to blood pressure regulation,37 is required before any firm conclusions can be made. It is likely that a widespread downregulation of BKß1 in diabetes could be sufficient to induce hypertension because BKß1 knockout mice are indeed hypertensive.19,20 Furthermore, downregulation of BKß1 has been reported in animal models of both acquired and genetic hypertension.38,39 Also, a recent population-based epidemiological study has reported that a gain-of-function polymorphism (E65K) in the BKß1 subunit, which increased the apparent Ca2+- and voltage-sensitivity of the pore-forming BK subunit, is associated with a low prevalence of diastolic hypertension in this population.40 Future work should be directed toward a clearer understanding of the role BKß1 downregulation in the development of hypertension in diabetic patients as well as elucidating the nature of the signaling mechanisms that regulate BKß1 expression during diabetes.
Another novel observation in the current study was that Ca2+ spark amplitudes were greater in retinal VSMCs from diabetic animals, even though the frequency and duration of these events were unaltered. It seems unlikely that this represents a compensatory response to the downregulation of BKß1 and loss of Ca2+ spark/BK channel coupling because the spatiotemporal properties of Ca2+ sparks were unaltered in VSMCs of BKß1 knockout animals.19,20 Ca2+ spark amplitude should depend on the number and activity (open times) of RyR channels in each spark unit and the driving force for Ca2+ efflux from the SR. It seems unlikely that the larger Ca2+ spark amplitudes can be attributed to changes in the SR Ca2+ load, because caffeine-evoked [Ca2+]i transients and caffeine-activated ClCa currents were similar in retinal VSMCs from nondiabetic and diabetic animals. Diabetic retinal vessels are known to accumulate increased levels of advanced glycation end-products41 and these adducts are known to accumulate heavily on RyR channels during diabetes.42 The possible contribution of AGE crosslinking of RyRs to alterations in Ca2+ spark activity in diabetic retinal VSMCs deserves further investigation.
In summary, we have presented data that strongly supports the hypothesis that diabetes downregulates the expression of the BKß1 subunit and consequently decreases Ca2+ dependent activity of BK channels in retinal VSMCs. Our findings may have important implications with respect to the early pathogenesis of diabetic retinopathy. It is also fascinating to speculate that this mechanism might contribute to the development of hypertension in diabetic patients, although more extensive studies are needed to fully evaluate this possibility.
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Acknowledgments |
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Sources of Funding
This work was supported by grants from The Juvenile Diabetes Research Foundation, USA; Fight for Sight, UK; and The Wellcome Trust.
Disclosures
None.
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Footnotes |
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Original received August 17, 2006; revision received January 25, 2007; accepted January 29, 2007.
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References |
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1. Curtis TM, Scholfield CN. The role of lipids and protein kinase Cs in the pathogenesis of diabetic retinopathy. Diabetes Metab Res Rev. 2004; 20: 28–43.[CrossRef][Medline] [Order article via Infotrieve]
2. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. The Diabetes Control and Complications Trial Research Group. N Engl J Med. 1993; 329: 977–986.
3. Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). UK Prospective Diabetes Study (UKPDS) Group. Lancet. 1998; 352: 837–853.[CrossRef][Medline] [Order article via Infotrieve]
4. Bursell SE, Clermont AC, Kinsley BT, Simonson DC, Aiello LM, Wolpert HA. Retinal blood flow changes in patients with insulin-dependent diabetes mellitus and no diabetic retinopathy. Invest Ophthalmol Vis Sci. 1996; 37: 886–897.
5. Clermont AC, Aiello LP, Mori F, Aiello LM, Bursell SE. Vascular endothelial growth factor and severity of nonproliferative diabetic retinopathy mediate retinal hemodynamics in vivo: a potential role for vascular endothelial growth factor in the progression of nonproliferative diabetic retinopathy. Am J Ophthalmol. 1997; 124: 433–446.[Medline] [Order article via Infotrieve]
6. Cunha-Vaz JG, Fonseca JR, de A, Jr., Lima JJ. Studies on retinal blood flow. II. Diabetic retinopathy. Arch Ophthalmol. 1978; 96: 809–811.
7. Kohner EM, Hamilton AM, Saunders SJ, Sutcliffe BA, Bulpitt CJ. The retinal blood flow in diabetes. Diabetologia. 1975; 11: 27–33.[CrossRef][Medline] [Order article via Infotrieve]
8. Yoshida A, Feke GT, Morales-Stoppello J, Collas GD, Goger DG, McMeel JW. Retinal blood flow alterations during progression of diabetic retinopathy. Arch Ophthalmol. 1983; 101: 225–227.
9. Klein R, Klein BE, Moss SE, Wong TY, Hubbard L, Cruickshanks KJ, Palta M. Retinal vascular abnormalities in persons with type 1 diabetes: the Wisconsin Epidemiologic Study of Diabetic Retinopathy: XVIII. Ophthalmology. 2003; 110: 2118–2125.[CrossRef][Medline] [Order article via Infotrieve]
10. Wong TY, Klein R, Sharrett AR, Schmidt MI, Pankow JS, Couper DJ, Klein BE, Hubbard LD, Duncan BB. Retinal arteriolar narrowing and risk of diabetes mellitus in middle-aged persons. JAMA. 2002; 287: 2528–2533.
11. Ciulla TA, Harris A, Latkany P, Piper HC, Arend O, Garzozi H, Martin B. Ocular perfusion abnormalities in diabetes. Acta Ophthalmol Scand. 2002; 80: 468–477.[CrossRef][Medline] [Order article via Infotrieve]
12. Schmetterer L, Wolzt M. Ocular blood flow and associated functional deviations in diabetic retinopathy. Diabetologia. 1999; 42: 387–405.[CrossRef][Medline] [Order article via Infotrieve]
13. Jackson WF. Ion channels and vascular tone. Hypertension. 2000; 35: 173–178.
14. Brayden JE, Nelson MT. Regulation of arterial tone by activation of calcium-dependent potassium channels. Science. 1992; 256: 532–535.
15. Nelson MT, Cheng H, Rubart M, Santana LF, Bonev AD, Knot HJ, Lederer WJ. Relaxation of arterial smooth muscle by calcium sparks. Science. 1995; 270: 633–637.
16. Knaus HG, Folander K, Garcia-Calvo M, Garcia ML, Kaczorowski GJ, Smith M, Swanson R. Primary sequence and immunological characterization of beta-subunit of high conductance Ca(2+)-activated K+ channel from smooth muscle. J Biol Chem. 1994; 269: 17274–17278.
17. Dworetzky SI, Boissard CG, Lum-Ragan JT, McKay MC, Post-Munson DJ, Trojnacki JT, Chang CP, Gribkoff VK. Phenotypic alteration of a human BK (hSlo) channel by hSlobeta subunit coexpression: changes in blocker sensitivity, activation/relaxation and inactivation kinetics, and protein kinase A modulation. J Neurosci. 1996; 16: 4543–4550.
18. McManus OB, Helms LM, Pallanck L, Ganetzky B, Swanson R, Leonard RJ. Functional role of the beta subunit of high conductance calcium-activated potassium channels. Neuron. 1995; 14: 645–650.[CrossRef][Medline] [Order article via Infotrieve]
19. Brenner R, Perez GJ, Bonev AD, Eckman DM, Kosek JC, Wiler SW, Patterson AJ, Nelson MT, Aldrich RW. Vasoregulation by the beta1 subunit of the calcium-activated potassium channel. Nature. 2000; 407: 870–876.[CrossRef][Medline] [Order article via Infotrieve]
20. Pluger S, Faulhaber J, Furstenau M, Lohn M, Waldschutz R, Gollasch M, Haller H, Luft FC, Ehmke H, Pongs O. Mice with disrupted BK channel beta1 subunit gene feature abnormal Ca(2+) spark/STOC coupling and elevated blood pressure. Circ Res. 2000; 87: E53–E60.[Medline] [Order article via Infotrieve]
21. Takagi C, Bursell SE, Lin YW, Takagi H, Duh E, Jiang Z, Clermont AC, King GL. Regulation of retinal hemodynamics in diabetic rats by increased expression and action of endothelin-1. Invest Ophthalmol Vis Sci. 1996; 37: 2504–2518.[Abstract]
22. Engerman RL, Kern TS. Retinopathy in animal models of diabetes. Diabetes Metab Rev. 1995; 11: 109–120.[Medline] [Order article via Infotrieve]
23. Knaus HG, McManus OB, Lee SH, Schmalhofer WA, Garcia-Calvo M, Helms LM, Sanchez M, Giangiacomo K, Reuben JP, Smith AB, III,. Tremorgenic indole alkaloids potently inhibit smooth muscle high-conductance calcium-activated potassium channels. Biochemistry. 1994; 33: 5819–5828.[CrossRef][Medline] [Order article via Infotrieve]
24. Jaggar JH, Porter VA, Lederer WJ, Nelson MT. Calcium sparks in smooth muscle. Am J Physiol Cell Physiol. 2000; 278: C235–C256.
25. Curtis TM, Tumelty J, Dawicki J, Scholfield CN, McGeown JG. Identification and spatiotemporal characterization of spontaneous Ca2+ sparks and global Ca2+ oscillations in retinal arteriolar smooth muscle cells. Invest Ophthalmol Vis Sci. 2004; 45: 4409–4414.
26. McGahon MK, Dawicki JM, Scholfield CN, McGeown JG, Curtis TM. A-type potassium current in retinal arteriolar smooth muscle cells. Invest Ophthalmol Vis Sci. 2005; 46: 3281–3287.
27. Nimigean CM, Magleby KL. The beta subunit increases the Ca2+ sensitivity of large conductance Ca2+-activated potassium channels by retaining the gating in the bursting states. J Gen Physiol. 1999; 113: 425–440.
28. Dick GM, Rossow CF, Smirnov S, Horowitz B, Sanders KM Tamoxifen activates smooth muscle BK channels through the regulatory beta 1 subunit. J Biol Chem. 200;276:34594–34599.
29. Jaggar JH, Porter VA, Lederer WJ, Nelson MT. Calcium sparks in smooth muscle. Am J Physiol Cell Physiol. 2000; 278: C235–C256.
30. Mokelke EA, Dietz NJ, Eckman DM, Nelson MT, Sturek M. Diabetic dyslipidemia and exercise affect coronary tone and differential regulation of conduit and microvessel K+ current. Am J Physiol Heart Circ Physiol. 2005; 288: H1233–H1241.
31. Burnham MP, Johnson IT, Weston AH. Reduced Ca2+-dependent activation of large-conductance Ca2+-activated K+ channels from arteries of Type 2 diabetic Zucker diabetic fatty rats. Am J Physiol Heart Circ Physiol. 2006; 290: H1520–H1527.
32. Dimitropoulou C, Han G, Miller AW, Molero M, Fuchs LC, White RE, Carrier GO. Potassium (BK(Ca)) currents are reduced in microvascular smooth muscle cells from insulin-resistant rats. Am J Physiol Heart Circ Physiol. 2002; 282: H908–H917.
33. Ye CL, Shen B, Ren XD, Luo RJ, Ding SY, Yan FM, Jiang JH. An increase in opening of BK(Ca) channels in smooth muscle cells in streptozotocin-induced diabetic mice. Acta Pharmacol Sin. 2004; 25: 744–750.[Medline] [Order article via Infotrieve]
34. Sowers JR, Epstein M, Frohlich ED. Diabetes, hypertension, and cardiovascular disease: an update. Hypertension. 2001; 37: 1053–1059.
35. Adler AI, Stratton IM, Neil HA, Yudkin JS, Matthews DR, Cull CA, Wright AD, Turner RC, Holman RR. Association of systolic blood pressure with macrovascular and microvascular complications of type 2 diabetes (UKPDS 36): prospective observational study. BMJ. 2000; 321: 412–419.
36. Sowers JR, Khoury S, Standley P, Zemel P, Zemel M. Mechanisms of hypertension in diabetes. Am J Hypertens. 1991; 4: 177–182.[Medline] [Order article via Infotrieve]
37. Nelson MT, Quayle JM. Physiological roles and properties of potassium channels in arterial smooth muscle. Am J Physiol. 1995; 268: C799–C822.[Medline] [Order article via Infotrieve]
38. Amberg GC, Bonev AD, Rossow CF, Nelson MT, Santana LF. Modulation of the molecular composition of large conductance, Ca(2+) activated K(+) channels in vascular smooth muscle during hypertension. J Clin Invest. 2003; 112: 717–724.[CrossRef][Medline] [Order article via Infotrieve]
39. Amberg GC, Santana LF Downregulation of the BK channel beta1 subunit in genetic hypertension. Circ Res. 2003;14:93:965–971.
40. Fernandez-Fernandez JM, Tomas M, Vazquez E, Orio P, Latorre R, Senti M, Marrugat J, Valverde MA. Gain-of-function mutation in the KCNMB1 potassium channel subunit is associated with low prevalence of diastolic hypertension. J Clin Invest. 2004; 113: 1032–1039.[CrossRef][Medline] [Order article via Infotrieve]
41. Stitt AW, Li YM, Gardiner TA, Bucala R, Archer DB, Vlassara H. Advanced glycation end products (AGEs) co-localize with AGE receptors in the retinal vasculature of diabetic and of AGE-infused rats. Am J Pathol. 1997; 150: 523–531.[Abstract]
42. Bidasee KR, Nallani K, Yu Y, Cocklin RR, Zhang Y, Wang M, Dincer UD, Besch HR, Jr. Chronic diabetes increases advanced glycation end products on cardiac ryanodine receptors/calcium-release channels. Diabetes. 2003; 52: 1825–1836.
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