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(Circulation Research. 2006;99:462.)
© 2006 American Heart Association, Inc.

Editorials

Realizing Its Potential

The Intermediate Conductance Ca²⁺-Activated K⁺ Channel (K_Ca3.1) and the Regulation of Blood Pressure

Ingrid Fleming

From the Vascular Signalling Group, Institut für Kardiovaskuläre Physiologie, Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany.

Correspondence to Ingrid Fleming, PhD, Vascular Signalling Group, Institut für Kardiovaskuläre Physiologie, Johann Wolfgang Goethe-Universität, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany. E-mail fleming{at}em.uni-frankfurt.de

See related article, pages 537–544

Key Words: endothelium-derived hyperpolarizing factor • hypertension • membrane potential • microcirculation • vasodilatation

Local vascular tone is generally determined by a variety of factors such as neurotransmitters released from autonomic nerves, circulating vasoactive compounds, tissue metabolites, and endothelium-derived autacoids. The best characterized autacoids are the potent vasodilators nitric oxide (NO) and prostacyclin (PGI₂) and the vasoconstrictor peptide endothelin-1. Several studies have, however, convincingly demonstrated the existence of an NO/PGI₂-independent component of endothelium-dependent relaxation in various arterial beds, most notably in mesenteric, carotid, cerebral, coronary, and renal arteries. Because the NO/PGI₂-independent vasodilatation originally described was coincident with vascular smooth muscle hyperpolarization and was abolished by depolarizing concentrations of potassium, it was proposed to be mediated by an endothelium-derived hyperpolarizing factor or "EDHF."¹

When the term EDHF was initially coined, researchers expected to be able to identify a specific chemical entity synthesized in, and released from, the endothelium which hyperpolarizes vascular smooth muscle cells and elicits relaxation. However, there does not seem to be a specific EDHF, as at least 3 principal mechanisms have been linked to the EDHF phenomenon: (1) an increase in endothelial [Ca²⁺]_i after cell stimulation triggers the synthesis of a cytochrome P450 metabolite which is essential for the subsequent EDHF-mediated responses; (2) K⁺, released from endothelial cells via Ca²⁺-dependent K⁺ (K⁺_Ca) channels induces smooth muscle hyperpolarization by activating inwardly rectifying K⁺ channels or the Na⁺-K⁺-ATPase on vascular smooth muscle cells; and (3) endothelial cell hyperpolarization is transmitted to the vascular smooth muscle via gap junctions. The strengths and weaknesses of the arguments for each of these specific types of EDHF have been reviewed recently.^2,3

The resting membrane potential of smooth muscle cells in normal pressurized arteries and arterioles is mainly determined by the open probability of K⁺ and, to a lesser extent, Cl^– channels and ranges in vivo between –55 and –35 mV, reflecting the fact that arteries are usually partially contracted, a state from which they can constrict or dilate. The mechanism by which hyperpolarization elicits relaxation is controversial, but the most direct and obvious explanation is that the opening of K⁺ channels hyperpolarizes the smooth muscle cell membrane, reducing the open probability of voltage-dependent Ca²⁺ channels and activating Ca²⁺ sequestration and extrusion mechanisms, so that [Ca²⁺]_i is lowered and relaxation can take place. However although the literature relating to EDHF seems confusing, there is widespread agreement that the initial step in all of the EDHF-mediated responses studied to date is activation of small (SK_Ca) and intermediate conductance K⁺_Ca (IK_Ca) channels in the endothelium.² This explains why NO/PGI₂-independent hyperpolarization and relaxation are exquisitely sensitive to the combination of charybdotoxin and apamin,^4–7 or the more specific inhibitors TRAM-34⁸ and UCL1684,⁹ and insensitive to iberiotoxin, an inhibitor of large conductance (BK_Ca) K⁺_Ca channels. However, the lack of selectivity of the available tools has made it difficult to determine whether the activation of the endothelial IK_Ca or SK_Ca or the release of a hyperpolarizing factor such as the cytochrome P450 epoxygenase-derived epoxyeicosatrienoic acids underlies the NO/PGI₂-independent responses.^10,11 For example, clotrimazole, an inhibitor of cytochrome P450 epoxygenases, also blocks IK_Ca, whereas charybdotoxin blocks IK_Ca as well as BK_Ca.¹² Thus it seems that a genetic approach is the only way to elucidate the relative importance of the two channels in EDHF- as well as NO- and PGI₂-dependent responses.

In this issue of Circulation Research, Si et al¹³ report that the targeted deletion of K_Ca3.1 (to give the IK_Ca its proper name¹⁴) attenuates the acetylcholine (ACh)-induced hyperpolarization of endothelial and vascular smooth muscle cells. These effects were associated with an attenuated EDHF-dependent relaxation of the isolated carotid artery as well as resistance-sized vessels in the cremaster microcirculation indicating that a significant portion of the NO and PGI₂-independent relaxation in the arteries studied was related to the opening of K_Ca3.1 (Figure).

View larger version (38K): [in this window] [in a new window]   Role of KCa2.3 and KCa3.1 in the regulation of agonist-induced EDHF-dependent relaxation. In endothelial cells expressing KCa3.1 and KCa2.3 channels (A), an agonist-induced increase in intracellular Ca2+ ([Ca2+]i) increases the open probability of both channels. Inhibitor sensitivity indicates that the opening of KCa3.1 plays greater role in mediating endothelial cell hyperpolarization and EDHF-dependent relaxation. B, In the absence of KCa3.1 a residual hyperpolarization and EDHF-dependent relaxation is maintained by KCa2.3 but compensation by this channel and by NO is incomplete, the consequence being an increase in systolic blood pressure (SBP).

With the type of genetic approach used there is a risk that compensation occurs and that the relative importance of remaining K_Ca channels, in this case the SK_Ca (or K_Ca2.3), increases after specific deletion of the K_Ca3.1. Although resting membrane potential was not different in aortic and carotid artery endothelial cells from wild-type and K_Ca3.1^–/– mice, the response to the application of ACh was significantly smaller in cells from animals lacking K_Ca3.1 and completely reversed by the UCL1684, a specific K_Ca2.3 blocker. In wild-type mice, UCL1684 had a minimal effect on the ACh-induced hyperpolarization whereas the K_Ca3.1 blocker TRAM-34 induced complete repolarization. These data indicate that while there does not seem to be a significant role of the endothelial K_Ca3.1 in the regulating of the resting membrane potential, it plays a key role in the hyperpolarization elicited by agonist stimulation.

That the effects described are related to, and have consequences on, EDHF-mediated responses was demonstrated using a number of approaches. Firstly, there was a marked attenuation (by 88%) of the maximal ACh-induced hyperpolarization of carotid artery smooth muscle cells from K_Ca3.1^–/– compared with wild-type mice. Secondly, ACh-induced relaxation in the presence of a NO synthase and cyclooxygenase inhibitor was significantly attenuated in carotid arteries from K_Ca3.1^–/– animals. It should be noted that vasodilator responses were also attenuated in arteries from K_Ca3.1^–/– mice even in the absence of a NO synthase and cyclooxygenase inhibitor. As the effects of NO synthase inhibition were similar in KCa3.1^–/– and wild-type animals Si et al¹³ conclude that NO is unable to compensate for the attenuated EDHF response. This may indeed be the case as while NO has been reported to affect K_Ca channel opening,¹⁵ that of IK_Ca channels is reported to be NO-independent (at least in rat cerebral arteries) and SK_Ca are thought to contribute to hyperpolarization only when the endothelial NO synthase is active.¹⁶ Finally, the vasodilatation measured in resistance-sized arteries in the cremaster microcirculation was also significantly attenuated by the loss of K_Ca3.1. In none of the experiments performed was the lack of K_Ca3.1 associated with any impairment of contractile responses elicited with either an agonist (phenylephrine) or by an increase in transmural pressure (myogenic tone); similarly, endothelium-independent relaxation to sodium nitroprusside was similar in arteries from K_Ca3.1^–/– and wild-type animals.

A number of studies relying on a combination of genetic and pharmacological approaches have indicated a potential role for an EDHF in the regulation of blood pressure.^17,18 In agreement, the loss of K_Ca3.1 led to a significant increase in arterial blood pressure (assessed using tail cuff plethysmography and telemetry), and to mild left ventricular hypertrophy. Although the article by Si et al¹³ indicates the potential importance of K_Ca3.1 in EDHF-mediated responses, some points regarding the potential interaction between K_Ca3.1 and K_Ca2.3 remain to be cleared up, especially as conditional knockout of the latter channel increases myogenic and agonist (phenylephrine)-induced tone as well as systemic blood pressure.¹⁹ Unfortunately, there is currently no information regarding EDHF-dependent responses in K_Ca2.3^–/– mice. Determining the relevance of an EDHF in the acute regulation of vascular tone as well as in the development of certain pathologies such as hypertension or heart failure is hampered by the fact that there is a certain amount of redundancy in the palette of endothelium-derived vasodilator autacoids that are generated by most vascular beds. The availability of K_Ca3.1^–/– animals is likely to prove invaluable in the elucidation of the pathophysiological relevance of the EDHF pathway in different organs.

	Acknowledgments

 
Source of Funding

Studies performed in the author’s own laboratory were supported by the Deutsche Forschungsgemeinschaft (SFB-TR 23: A6).

Disclosures

None.

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

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Related Article:

Impaired Endothelium-Derived Hyperpolarizing Factor-Mediated Dilations and Increased Blood Pressure in Mice Deficient of the Intermediate-Conductance Ca²⁺-Activated K⁺ Channel

Han Si, Willm-Thomas Heyken, Stephanie E. Wölfle, Marcin Tysiac, Rudolf Schubert, Ivica Grgic, Larisa Vilianovich, Günter Giebing, Tanja Maier, Volkmar Gross, Michael Bader, Cor de Wit, Joachim Hoyer, and Ralf Köhler
Circ. Res. 2006 99: 537-544. [Abstract] [Full Text] [PDF]

This Article Extract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Fleming, I. Search for Related Content PubMed PubMed Citation Articles by Fleming, I. Related Collections Related Article