doi: 10.1161/CIRCRESAHA.107.160325
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© 2007 American Heart Association, Inc.
Editorials |
Stoking Up BKCa Channels in Hemorrhagic Shock
Which Channel Subunit Is Really Fueling the Fire?
From the BHF Laboratories, Department of Medicine, UCL, London, UK.
Correspondence to Professor Lucie Clapp, BHF Laboratories, Department of Medicine, UCL, 4th floor Rayne Building, 5 University Street, London WC1E 6JF. E-mail l.clapp{at}ucl.ac.uk
See related article, pages 493–502
Key Words: potassium channels • shock • hypotension • vascular hyporeactivity • Ca2+ sparks
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Introduction |
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 Top Introduction Role of BKCa in...Enhanced STOC Activity in...Concluding RemarksReferences |
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Large conductance calcium-activated potassium channels (BKCa) are abundantly expressed in smooth muscle cells (SMCs) lining the blood vessel wall. They are composed of an
-subunit (Slo) and a modulatory ß1-subunit, which serves to maintain the normal high voltage- and Ca2+-sensitivity of the pore-forming -subunit (reviewed in1,2). In the vasculature, BKCa operate by limiting Ca2+ entry and arterial contraction by repolarizing SMCs and closing voltage-dependent Ca2+ channels previously opened by pressure or vasoconstrictor agents.1 BKCa can also mediate cellular hyperpolarization and vasorelaxation as a result of spontaneous transient outward currents (STOCs) activated by the localized release of micromolar concentrations of Ca2+ (Ca2+ sparks) from ryanodine receptors located in the sarcoplasmic reticulum (SR).1 Moreover, increased frequency of Ca2+ sparks may underlie activation of BKCa by endogenous vasodilators,1 though other mechanisms undoubtedly contribute.2,3
Genetic experiments also highlight BKCa as important regulators of vascular tone and blood pressure. Deletion of the -subunit in mice results in membrane depolarization, a complete lack of STOCs, and attenuates cGMP relaxation in isolated blood vessels.4 On the other hand, deletion of ß1 impairs the coupling of Ca2+ sparks to the activation of hyperpolarizing BKCa currents and enhances agonist-induced vasoconstriction without affecting nitric oxide (NO) mediated vasorelaxation.3 Knockout of both genes leads to systemic hypertension, though in BKCa ß1–null mice this is more pronounced3,4 suggesting physical interactions of the ß1-subunit with other proteins, possibly other ion-conducting pores.5 Moreover, depending on the hypertensive model, ß1-subunit expression can either increase2,6 or decrease.7 The latter might argue that ß1 acts as a compensatory mechanism to limit development of hypertension. Consistent with this, a gain-of-function mutation in the human ß1-subunit gene (KCNMB1), involving an increase in the apparent Ca2+ and voltage sensitivity of the channel, protected patients against diastolic hypertension.8 Thus, while BKCa dynamically regulate vascular tone and blood pressure, the precise role/function of the pore-forming and ß1 subunit requires further evaluation.
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Role of BKCa in Shock |
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TopIntroductionRole of BKCa in...Enhanced STOC Activity in...Concluding RemarksReferences |
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Shock is a condition of profound hemodynamic and metabolic disturbance characterized by failure of the circulatory system to maintain adequate perfusion of vital organs.9 This is largely attributable to the failure of blood vessels to constrict to catecholamines resulting in excessive vasodilatation. Several studies conclude that BKCa mediate, at least in part, SMC membrane hyperpolarization and vascular hyporeactivity in experimental models of hemorrhagic and endotoxic shock.9–12 The mechanism of activation is largely unknown but probably involves NO9,10 which can phosphorylate the
-subunit of BKCa through cGMP-dependent protein kinase (PKG)2 or tyrosine protein kinase.13,14 In this issue, Zhao and colleagues investigate the mechanism by which BKCa are enhanced in acute hemorrhagic shock (HS).15 They tested the hypothesis that increased ß1-subunit expression is responsible for enhanced coupling of Ca2+ sparks to BKCa, and this contributes to vascular hyporeactivity and hypotension in HS. |
Enhanced STOC Activity in HS |
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TopIntroductionRole of BKCa in...Enhanced STOC Activity in...Concluding RemarksReferences |
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A number of interesting and related observations were made. First the authors show that STOC activity at depolarized potentials was enhanced in terms of amplitude, duration, and charge transfer in isolated mesenteric arterial SMCs (ASMCs) from HS rats. Frequency of Ca2+ sparks remained unchanged whereas mean amplitude was increased. In examining the relationship between current and Ca2+ spark amplitude (Figure 2C), the authors convincingly showed that Ca2+ spark-STOC coupling efficiency increased as did the Ca2+ sensitivity of BKCa at micromolar Ca2+ levels in isolated patches. This latter result is expected, given that high Ca2+ promotes a functional interaction between
- and ß1-subunits.16,17 Coupled to Western blotting and immunostaining experiments showing ß1 but not the -subunit protein was elevated in HS, are all observations entirely consistent with their hypothesis. Another prediction is that the activation/deactivation kinetics of BKCa would be slowed if ß1-subunit levels had a functional impact on the number of channels expressing both subunits at the membrane.16–18 This was indeed shown for STOC activation at depolarized potentials (Figure 1B).
Whether similar effects occur at more physiological resting potentials (ie, 
–40 mV) and can account for enhanced depolarizing and contractile effects of iberiotoxin in HS remains to be demonstrated. These are important considerations given that Ca2+ spark frequency was actually decreased in resting SMCs in HS and associated with only a marginal increase in amplitude. Without these crucial experiments it remains unclear whether in resting cells there would be significant increases either in the charge transfer of a single STOC or spontaneous transient hyperpolarizations, produced by STOCs and readily recorded in single cells in current clamp mode.4 Also the mechanism underlying enhanced spark amplitude is intriguing. That the Ca2+ load of the SR was not altered, and deletion of or ß1 had no effect on the kinetics of Ca2+ sparks3,4 is indicative of a long-term alteration in ryanodine receptor function induced by HS. Conceivably, reactive oxygen species, which are good activators of Ca2+ release from these channels, may contribute.
How the ß1-subunit enhances the apparent Ca2+ sensitivity of BKCa is not well understood, although most studies conclude that it does not intrinsically alter Ca2+ binding.17–19 Single channel analysis have shown that the main effect of ß1 is to increase the length of time that BKCa spends in bursting states, an effect preserved over a wide range of voltages or Ca2+ concentrations (1 nmol/L to 10 µmol/L).18,19 The other less significant consequence of ß1 expression is to increase the gap between bursts, an effect observed at low Ca2+ but negated at higher levels. Based on these results, it has been proposed that ß1-subunits act to shift the equilibrium for voltage-sensor activation to more negative potentials as well as to reduce the work that Ca2+ binding must do to open the channel. From the initial inspection of their data, both types of kinetic changes appear to occur (Figure 4E), though increased burst duration at low Ca2+ was not evident. Thus a detailed analysis of dwell-time distributions is required to demonstrate that these ß1-related changes in channel kinetics are taking place. Such analysis has already been performed in mesenteric SMCs isolated from rats with HS.11 In these studies, enhanced BKCa activity was characterized by a dramatic decrease in mean close times and the slow close time constant, effects not attributable to ß1-subunit function.18 These changes were best described by -subunit modulation, most likely through NO-mediated sulfhydryl activation of cSrc leading to phosphorylation of the Tyr766 residue in the C terminus.11,14 Interestingly, this form of channel modification leads to enhanced Ca2+ sensitivity of BKCa.14 This is also in keeping with genetic data showing the major downstream target for NO is the -subunit.3,4 However, one cannot rule out that small changes in ß1-subunit expression could facilitate the opening of the pore and either mask or mimic channel effects predicted by ß1 alone. For example, like ß1, methionine oxidation of hSlo1 slows deactivation kinetics, an effect dramatically potentiated by coexpressing ß1.20 Nevertheless, ß1 was reported to confer a novel outcome of oxidation not observed with hSlo1 alone, namely a distinct acceleration of current activation. Together these effects shifted channel activation within the physiological range of membrane potential even in the absence of Ca2+.
Contrary to what increased ß1-subunit expression would do,19 the open channel probability (NPO) was found to be decreased at low nanomolar Ca2+ (Figure 4F). This is surprising, given that voltage-sensitivity of BKCa are not altered by the ß1-subunit below 100 nmol/L.16,17 This argues that channel expression at the membrane might be lower in HS. Recently, the human ß1-subunit was reported to contain an endocytic signal in its C terminus that results in a reduction of surface expression of Hslo but not in total protein.21 Whether the ß1-subunit modulates trafficking in ASMCs is unknown, though no increase in active channel number was reported in cells from hypertensive animals where reduced ß1 expression was demonstrated.3,22
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Concluding Remarks |
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TopIntroductionRole of BKCa in...Enhanced STOC Activity in...Concluding RemarksReferences |
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Although the new findings of Zhao and colleagues elegantly support a role for the ß1 subunit in HS,15 we would modify their scheme to include that Slo probably also enhances Ca2+-senstivity of BKCa (see Figure above). On a cautionary note, whether these mechanisms translate into the in vivo situation remains inconclusive because endothelial-derived factors or circulating hormones could well alter how BKCa open. The ultimate test would be to delete
and ß1-subunits separately to see how each impacts on vascular hyporeactivity and blood pressure responses in HS. Finally, given that ATP-sensitive K+ channels are also important contributors of the cardiovascular collapse in shock9,10 adds to the growing body of evidence that K+ channels are important regulators of vascular reactivity in shock. Which channel, if either, assumes a greater role in the clinical scenario remains a moot point.
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Acknowledgments |
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Sources of Funding
The authors were supported by research grants from the Medical Research Council and British Heart Foundation.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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