Thromboxane receptor stimulation associated with loss of SK_Ca activity and reduced EDHF responses in the rat isolated mesenteric artery

Abstract

1 The possibility that thromboxane (TXA 2 ) receptor stimulation causes differential block of the SK Ca and IK Ca channels which underlie EDHF-mediated vascular smooth muscle hyperpolarization and relaxation was investigated in the rat isolated mesenteric artery. 2 Acetylcholine (30 nM-3 mM ACh) or cyclopiazonic acid (10 mM CPA, SERCA inhibitor) were used to stimulate EDHF-evoked smooth muscle hyperpolarization. In each case, this led to maximal hyperpolarization of around 20 mV, which was sensitive to block with 50 nM apamin and abolished by repeated stimulation of mesenteric arteries with the thromboxane mimetic, U46619 (30 nM-0.1 mM), but not the a 1 -adrenoceptor agonist phenylephrine (PE). 3 The ability of U46619 to abolish EDHF-evoked smooth muscle hyperpolarization was prevented by prior exposure of mesenteric arteries to the TXA 2 receptor antagonist 1 mM SQ29548. 4 Similar-sized smooth muscle hyperpolarization evoked with the SK Ca activator 100 mM riluzole was also abolished by prior stimulation with U46619, while direct muscle hyperpolarization in response to either levcromakalim (1 mM, K ATP activator) or NS1619 (40 mM, BK Ca activator) was unaffected. 5 During smooth muscle contraction and depolarization to either PE or U46619, ACh evoked concentration-dependent hyperpolarization (to À67 mV) and complete relaxation. These responses were well maintained during repeated stimulation with PE, but with U46619 there was a progressive decline, so that during a third exposure to U46619 maximum hyperpolarization only reached -52 mV and relaxation was reduced by 20%. This relaxation could now be blocked with charybdotoxin alone. The latter responses could be mimicked with 300 mM 1-EBIO (IK Ca activator), an action not modified by exposure to U46619. 6 An early consequence of TXA 2 receptor stimulation is a reduction in the arterial hyperpolarization and relaxation attributed to EDHF. This effect appears to reflect a loss of SK Ca activity.

Elevated pressure selectively blunts flow‐evoked vasodilatation in rat mesenteric small arteries

“…It is possible that the drug used to increase the tone of the preparation influences the release and/or smooth muscle effects of endothelium-derived factors. Moreover, U46619 has been found to cause marked inhibition of acetylcholine relaxation in mesenteric arteries in the presence of NOS blockade (Plane and Garland 1996), probably by inhibition of the small-conductance calciumactivated K þ channel, which is involved in EDHF-type relaxation (Crane and Garland 2004). However, in the present study only two to three contractions to U46619 were performed, and a comparison of the first and second concentration-response curves showed that flow-and acetylcholine-induced vasodilatations were unaltered.…”

Regulation of Cellular Communication by Signaling Microdomains in the Blood Vessel Wall

“…This was proven to be true with the further development of the IK Ca inhibitors TRAM39 and TRAM34 (Wulff et al, 2000(Wulff et al, , 2001, which clarified a role of these channels in the EDH-mediated relaxation (Crane et al, 2003;Hinton and Langton, 2003). In parallel, IK Ca channel openers such as 1-ethyl-2-benzimidazolinone (1-EBIO) or SK Ca openers (riluzole) were able to reproduce the EDH induced by acetylcholine (Edwards et al, 1999a,b;Walker et al, 2001;Crane and Garland, 2004). These pharmacological studies led to the conclusion that the hyperpolarization observed in the EDH pathway reflects the activation of two potassium channels, IK Ca and SK Ca , which was a key step in our understanding of the EDH-mediated response.…”

Impaired Endothelium-Derived Hyperpolarizing Factor-Mediated Dilations and Increased Blood Pressure in Mice Deficient of the Intermediate-Conductance Ca 2+ -Activated K + Channel

“…Especially the latter was shown to interfere with EDHF signaling in a negative fashion. 25 However, the loss of K Ca 3.1 reduced the overall endothelium-dependent dilation in the carotid artery. This reduction was not caused by an attenuation of the NO-mediated part of the response because inhibition of NO synthase was equally effective in both genotypes.…”