Analysis of relaxation and repolarization mechanisms of nicorandil in rat mesenteric artery

Abstract

1 The mechanism by which nicorandil causes relaxation of rat isolated small mesenteric arteries mounted on a Mulvany myograph was investigated by use of a combination of putatively mechanismspecific antagonists. 2 In arteries precontracted by the thromboxane-mimetic, U46619, the EC50 for cromakalim and levcromakalim-induced relaxation curves were raised by 36 and 17 fold by glibenclamide (3 giM) while the EC50 for nicorandil-induced relaxation was unaffected by either glibenclamide or methylene blue (10 gLM). A combination of these antagonists raised the EC50 for nicorandil by 8 fold.3 In U46619-contracted arteries, nifedipine (100 nM) did not affect the cromakalim relaxation curve but it raised the EC50 for nicorandil by 5 fold. The combination of methylene blue, glibenclamide and nifedipine further inhibited the maximum relaxation to nicorandil. 4 In separate experiments, membrane potential (Em) and force responses were measured simultaneously. In arteries depolarized and contracted by U46619 both nicorandil and cromakalim repolarized (AEm, 35 mV) and relaxed (100%) the vessels. Glibenclamide (3 gIM) did not alter the relaxation-concentration curve to nicorandil but reduced the maximum repolarization to A10.8 mV. In contrast the Em and relaxation-response curves to cromakalim were shifted to the right by glibenclamide by 30-100 fold. 5 In unstimulated arteries, nicorandil (but not cromakalim) -induced hyperpolarization was significantly antagonized by methylene blue (10 giM) (6.6 fold rightward shift) or nifedipine (100 nM) (2.6 fold). In depolarized arteries (U46619), nifedipine but not methylene blue inhibited the nicorandilinduced hyperpolarization. 6 In arteries precontracted to 50% tissue maximum by either KCl or U46619, nifedipine (100 nM) relaxed the artery but failed to repolarize the Em. Presumably voltage-operated calcium channels (VOCC) were blocked preventing contraction but the artery remained depolarized, presumably through non VOCC mechanisms. 7 These data suggest that nicorandil may relax small arteries through 3 parallel pathways, (i) NOdonor mediated stimulation of guanylate cyclase and increase in cyclic GMP, (ii) K+ATP channel opening, and (iii) nifedipine-sensitive VOCC inhibition. Em data suggest that nicorandil-induced repolarization is caused principally through opening K+ATP channels. Blockade of this hyperpolarization by glibenclamide is not sufficient to alter the relaxation, indicating dissociation of nicorandil-induced changes in membrane potential and relaxation. 8 These results highlight the 'chameleon' actions of nicorandil where there is no apparent association of Em repolarization with relaxation, in contrast to the parallel responses for cromakalim.

Potassium channel activation and relaxation by nicorandil in rat small mesenteric arteries

“…It appears that the relative contribution of the two pathways varies between tissues, and according to the size of the vessel studied (Yoneyama et al, 1990;Holzmann et al, 1992;Akai et al, 1995). Our results di er in some respects from those obtained recently by Fujiwara & Angus (1996), using the same preparation, rings of rat small mesenteric arteries. They found that neither glibenclamide nor methylene blue alone shifted the concentration-response curve for relaxation by nicorandil, though both antagonists in combination were e ective.…”

“…Under these conditions it may be that either K + channel opening or guanylyl cyclase activation is su cient to cause relaxation, so that inhibition of both pathways is necessary to shift the concentration-response curve. Fujiwara & Angus (1996) also found that the nicorandil relaxations of contractions to U46619 were inhibited by nifedipine, and suggested that direct inhibition of Ca 2+ channels might provide a third mechanism for the action of nicorandil. We addressed this possibility directly, by investigating the e ect of nicorandil on Ca 2+ channel currents measured under voltage clamp.…”

“…Since it has been suggested that direct inhibition of L-type Ca 2+ channels may contribute to the relaxant action of nicorandil in rat mesenteric artery (Fujiwara & Angus, 1996) we investigated this possibility directly by measuring the e ect of nicorandil on currents through Ca 2+ channels in isolated cells. Currents were measured in response to 100 ms voltage steps from a holding potential of 760 mV, using 10 mM Ba 2+ as the charge carrier.…”

“…It is also possible, however, that channel activation occurs through activation of GC as a result of the nitrovasodilator action of nicorandil, since nitric oxide has been reported to cause glibenclamide-sensitive hyperpolarization of rat and rabbit mesenteric arteries (Garland & McPherson, 1992;Murphy & Brayden, 1995) and isosorbide dinitrate, atrial natriuretic peptide, and 8-bromo cyclic GMP can activate K ATP channels in cultured rat aortic cells (Kubo et al, 1994). Finally, on the basis of experiments using myography and membrane potential recording in rat mesenteric artery, Fujiwara & Angus (1996) have recently suggested that nicorandil might have a third mechanism of action, causing inhibition of nifedipinesensitive Ca 2+ channels directly.…”

Human vascular to cardiac tissue selectivity of L‐ and T‐type calcium channel antagonists

“…The atrial muscle would be depolarized/repolarized continuously at 1 Hz while the blood vessel would be at its resting Em for 30 min in the presence of the calcium antagonist before the 2 min exposure to K + 62 mM and depolarization. In rat mesenteric small arteries (326 mm diameter) K + 60 mM depolarized the arteries mounted in a similar myograph as used in the present study, from 7 60 to 725 mV (Fujiwara & Angus, 1996). In a detailed study in 200 mm rat mesenteric resistance arteries, noradrenaline 10 mM depolarized the membrane from 759+0.4 to 733.7+1.0 mV while K + 125 mM caused the membrane to depolarize to 76.9+1.7 mV although the contraction was only 70% of that to noradrenaline (Mulvany et al, 1982).…”