© 2000 American Heart Association, Inc.
Editorial |
Knockout Blow for Channel Identity Crisis
Vasodilation to Potassium Is Mediated via Kir2.1
From the Department of Pharmacology, The University of Melbourne, Parkville, Victoria, Australia; Departments of Internal Medicine and Pharmacology, Cardiovascular Center, University of Iowa College of Medicine, Iowa City, Iowa.
Correspondence to Frank M. Faraci, PhD, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA 52242-1081. E-mail frank-faraci{at}uiowa.edu
Key Words: potassium channels • genetics • mice • cerebral arteries
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Introduction |
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 Top Introduction References |
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The normal concentration of potassium ion (K+) in extracellular fluid is
3 to 5
mmol/L.1 In contrast to the depolarization and contraction
of vascular muscle that are commonly produced by high concentrations of
K+, small to moderate increases in the
concentration of extracellular K+ produce
membrane hyperpolarization and relaxation in a
variety of blood vessels in vitro.2 3 4 5 6 7 8 9 This vasodilator
response is particularly prominent in cerebral
arteries.2 3 4 5 Because K+ is
released during normal neuronal and muscle activity, this mechanism may
play a role in the coupling of cellular metabolism and
local blood flow.10 11 12 Several mechanisms have been proposed to contribute to K+-induced hyperpolarization of vascular muscle and vasodilatation. The two most studied have been (1) increased activation of Na+/K+-ATPase and (2) increased activity of inwardly rectifying K+ channels (Kir).2 3 4 8 These studies have relied almost exclusively on the use of pharmacological inhibition with ouabain and extracellular barium ion (Ba2+), respectively. Although a ouabain-sensitive vascular response may be present for very modest increases in extracellular K+ (<5 mmol/L), the major sustained component elicited by higher concentrations of K+ (7 to 20 mmol/L) is Ba2+-sensitive.2 3 4 5 8 Because of these findings, recent interest has focused on Kir channels as the key signaling pathway that produces vasodilatation through physiological elevations in extracellular K+.
K+ channels are thought to play a major role in regulation of vascular tone by producing hyperpolarization of vascular muscle in response to diverse stimuli including receptor-mediated agonists, second messengers, and Ca2+ sparks.13 14 15 Hyperpolarization of vascular muscle leads to vasorelaxation by a mechanism that is thought to involve closure of voltage-operated Ca2+ channels and a lowering in the levels of intracellular Ca2+. As the name indicates, Kir channels display inward rectification, ie, they conduct inward current much more readily than outward current. Importantly, though, a small increase above the physiological extracellular K+ concentration leads to a shift in the channel gating properties and an increase in the resting outward K+ current through Kir channels. Hence, a modest increase in extracellular K+ can paradoxically lead to vascular hyperpolarization due to K+ efflux through Kir channels.
The biology of K+ channels is extremely complex, given that there is enormous diversity at the molecular level in mammalian cells.16 Even if one focuses on vascular muscle, the array of K+ channels that have been reported to be expressed is intimidating for those interested in precisely defining mechanisms that regulate vascular tone. For example, at least five voltage-dependent K+ channels are expressed in pulmonary arteries.17 Overall, there are at least seven subfamilies of Kir channels.16 Within the brain, immunocytochemistry has suggested that three isoforms of Kir are present in cerebral arteries (Kir2.1, Kir2.2, and Kir2.3).18 In contrast, another study suggested that only the Kir2.1 channel was functionally expressed in cerebral vascular muscle.19 This conclusion was formed on the basis of several lines of evidence including the finding that the Kir currents in normal vascular muscle most closely resemble those observed in Xenopus oocytes expressing cloned Kir2.1 channels.19 Unfortunately, although pharmacological inhibitors such as Ba2+ can be very useful in implicating the involvement of a particular family of K+ channels in a functional response, these inhibitors cannot distinguish which K+ channel protein is the key for carrying a specific K+ current.
Gene targeting in mice is increasingly being used to establish the functional importance of a particular gene product and is now used widely in many disciplines. A major strength of the gene targeting approach is that it allows the use of a precise genetic alteration to study complex responses in cells or tissue or in intact animals. Gene targeting offers a level of specificity that traditional K+ channel pharmacology cannot achieve.
In the study by Zaritsky et al,20 gene-targeted mice were produced that were deficient in expression of either of two channels, Kir2.1 or Kir2.2. After generation of the mice, electrophysiological studies of isolated cerebral myocytes revealed that Ba2+-sensitive inward K+ currents were present in wild-type mice but not in Kir2.1-deficient mice. Functional studies were also performed using isolated, but pressurized, cerebral arteries. In wild-type mice, raising extracellular K+ from 6 to 15 mmol/L produced marked Ba2+-sensitive vasodilation that is consistent with many previous studies.2 3 4 5 6 The key novel finding was that the dilator response to K+ was completely absent in Kir2.1-deficient mice. The absence of vasodilation in response to K+ in Kir2.1 mice represented a selective change, because both the myogenic contractile response to increased intravascular pressure and the vasodilator response to forskolin (an activator of adenylate cyclase) were not altered in these animals. The latter finding is of interest because K+ channels are thought to be key mediators of vasorelaxation in response to cAMP.13 15 An additional important finding was that in contrast to Kir2.1-deficient mice, mice that were deficient in Kir2.2 exhibited normal vasodilator responses to K+. Thus, the Kir2.1 gene (but not the Kir2.2 gene) is essential for expression of Kir currents and dilation of cerebral vessels in response to an elevation in extracellular K+. These results also suggest that Kir2.2 does not compensate and become functionally important in vascular muscle in the absence of Kir2.1. Many studies of vascular function have now been performed using genetically altered mice, but most have focused on the role of endothelium.21 To our knowledge, the present study is the first in which gene-targeted mice were used to examine the role of any ion channel in blood vessels.
Because the present study was performed using blood vessels studied in vitro, a question that often arises is whether similar mechanisms participate in regulation of vascular tone in vivo. Several studies have shown that modest elevations in extracellular K+ dilate cerebral blood vessels in vivo and increase cerebral blood flow.12 22 23 24 25 We have recently observed that raising extracellular K+ from 3 to 10 mmol/L produces marked dilation of the basilar artery in vivo. The majority of this vasodilator response was inhibited by Ba2+, at concentrations that completely inhibited K+-induced hyperpolarization of the same artery in vitro. RT-PCR experiments confirmed the presence of mRNA for Kir2.1 in the basilar artery (S. Chrissobolis, J. Ziogas, Y. Chu, F.M. Faraci, C.G. Sobey, unpublished observations). Considering these findings and the results of Zaritsky et al,20 it seems likely that a Kir2.1-mediated response to K+ is functionally important in vivo. One approach to address this question more directly would be to study vascular responses in Kir2.1-deficient mice in vivo. Although a worthwhile goal, such experiments may also be very challenging, given that Kir2.1-deficient mice die within hours after birth. The cause of premature death in these mice is apparently respiratory insufficiency due to a complete cleft of the secondary palate.20 This developmental abnormality was not seen in Kir2.2-deficient mice.20 Thus, these studies provide another example in which genetically altered mice may exhibit surprising phenotypes and may potentially represent new models of human-inherited or acquired disease.
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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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TopIntroductionReferences |
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1. Davson H, Welch K, Segal MB. Physiology and Pathophysiology of the Cerebrospinal Fluid. New York, NY: Churchill Livingstone; 1987:189–246.
2.
McCarron JG, Halpern W. Potassium dilates rat cerebral
arteries by two independent mechanisms. Am J Physiol. 1990;259:H902–H908.
3.
Knot HJ, Zimmerman PA, Nelson MT. Extracellular
K+-induced hyperpolarizations
and dilatations of rat coronary and cerebral arteries involve
inward rectifier K+ channels. J
Physiol (Lond). 1996;492:419–430.
4.
Johnson TD, Marrelli SP, Steenberg ML, Childres WF,
Bryan RM. Inward rectifier potassium channels in the rat middle
cerebral artery. Am J Physiol. 1998;274:R541–R547.
5.
Marrelli SP, Johnson TD, Khorovets A, Childres WF,
Bryan RM. Altered function of inward rectifier potassium channels in
cerebrovascular smooth muscle after ischemia/reperfusion.
Stroke. 1998;29:1469–1474.
6. Edwards G, Dora KA, Gardener MJ, Garland CJ, Weston AH. K+ is an endothelium-derived hyperpolarizing factor in rat arteries. Nature. 1998;396:269–272.[Medline] [Order article via Infotrieve]
7. Quayle JM, McCarron JG, Brayden JE, Nelson MT. Inward rectifier K+ currents in smooth muscle cells from rat resistance-sized cerebral arteries. Am J Physiol. 1993;1993:265:C1363–C1370.
8.
Edwards FR, Hirst GDS, Silverberg GD. Inward
rectification in rat cerebral arterioles: involvement of potassium ions
in autoregulation. J Physiol (Lond). 1988;404:455–466.
9.
Quayle JM, Nelson MT, Standen NB. ATP-sensitive and
inwardly rectifying potassium channels in smooth muscle. Physiol
Rev. 1997;77:1165–1232.
10. Iadecola C. Regulation of the cerebral microcirculation during neural activity: is nitric oxide the missing link? Trends Neurosci. 1993;16:206–214.[Medline] [Order article via Infotrieve]
11.
Paulson OB, Newman EA. Does the release of potassium
from astrocyte endfeet regulate cerebral blood flow?
Science. 1987;237:896–898.
12.
Caesar K, Akgoren N, Mathiesen C, Lauritzen M.
Modification of activity-dependent increases in cerebellar blood flow
by extracellular potassium in anaesthetized rats. J Physiol
(Lond). 1999;520:281–292.
13. Faraci FM, Sobey CG. Role of potassium channels in regulation of cerebral vascular tone. J Cereb Blood Flow Metab. 1998;18:1047–1063.[Medline] [Order article via Infotrieve]
14.
Nelson MT, Quayle JM. Physiological
roles and properties of potassium channels in arterial
smooth muscle. Am J Physiol. 1995;268:C799–C822.
15.
Jaggar JH, Porter VA, Lederer WJ, Nelson MT. Calcium
sparks in smooth muscle. Am J Physiol. 2000;278:C235–C256.
16. Coetzee WA, Amarillo Y, Chiu J, Chow A, Lau D, McCormack T, Moreno H, Nadal MS, Ozaita A, Pountney D, Saganich M, Vega-Saenz De Miera E, Rudy B. Molecular diversity of K+ channels. Ann N Y Acad Sci. 1999;868:233–285.[Medline] [Order article via Infotrieve]
17.
Yuan X-J, Wang J, Juhaszova M, Golovina VA, Rubin LJ.
Molecular basis and function of voltage-gated K+
channels in pulmonary arterial smooth muscle cells.
Am J Physiol. 1998;274:L621–L635.
18. Stonehouse AH, Pringle JH, Norman RI, Stanfield PR, Conley EC, Brammar WJ. Characterisation of Kir2.0 proteins in the rat cerebellum and hippocampus by polyclonal antibodies. Histochem Cell Biol. 1999;112:457–465.[Medline] [Order article via Infotrieve]
19.
Bradley KK, Jaggar JH, Bonev AD, Heppner TJ, Flynn ERM,
Nelson MT, Horowitz B. Kir2.1 encodes the inward rectifier potassium
channel in rat arterial smooth muscle cells. J
Physiol (Lond). 1999;515:639–651.
20.
Zaritsky JJ, Eckman DM, Wellman GC, Nelson MT, Schwarz
TL. Targeted disruption of Kir2.1 and Kir2.2 genes reveals the
essential role of the inwardly rectifying K+
current in K+-mediated vasodilation. Circ
Res. 2000;87:160–166.
21.
Faraci FM, Sigmund CD. Vascular biology in genetically
altered mice: smaller vessels, bigger insight. Circ Res. 1999;85:1214–1225.
22.
Kuschinsky W, Wahl M, Bosse O, Thurau K. Perivascular
potassium and pH as determinants of local pial arterial
diameter in cats. Circ Res. 1972;31:240–247.
23.
Fujii K, Heistad DD, Faraci FM. Ionic mechanisms in
spontaneous vasomotion of the rat basilar artery in vivo. J
Physiol (Lond).. 1990;430:389–398.
24.
Busija DW, Chen J. Reversal by increased CSF
[H+] and [K+] of
phorbol ester-induced arteriolar constriction in piglets. Am
J Physiol. 1992;263:H1455–H1459.
25. Iadecola C, Kraig RP. Focal elevations in neocortical interstitial K+ produced by stimulation of the fastigial nucleus in rat. Brain Res. 1991;563:273–277.[Medline] [Order article via Infotrieve]
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