Cytoplasmic calcium and the relaxation of canine coronary arterial smooth muscle produced by cromakalim, pinacidil and nicorandil

Abstract

1 In order to investigate the vasodilator mechanisms of the K⁺ channel openers, cromakalim, pinacidil and nicorandil, we measured changes in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) simultaneously with force by a microfluorimetric method using fura-2, a calcium indicator, in canine coronary arterial smooth muscle cells. 2 The three K⁺ channel openers all produced a concentration-dependent reduction of [Ca²⁺]j in 5 and 30mM KCl physiological salt solution (PSS) but failed to affect [Ca²⁺]_i in 45 and 90 mM KC1-PSS. 3 Cromakalim only partly inhibited (-45%) the 30 mM KCl-induced contractures, whereas pinacidil and nicorandil nearly abolished contractions produced by 45 mM, 90 mM and 30 mM KC1-PSS. 4 Tetrabutylammonium (TBA), a nonselective K⁺ channel blocker, or glibenclamide, a supposed adenosine 5′-triphosphate (ATP)-sensitive K⁺ channel blocker, abolished the reduction of [Ca²⁺]_i caused by the three K⁺ channel openers and the relaxant effect of cromakalim, whereas they only slightly attenuated the relaxant effects of pinacidil and nicorandil. 5 The increase in [Ca²⁺]_i produced by 45 or 90 mM KCl-PSS in the presence of pinacidil or nicorandil was abolished by 10⁻⁵m verapamil, indicating that the increase in [Ca²⁺]_i was caused by the influx of extracellular Ca²⁺ and that pinacidil and nicorandil did not affect the voltage-dependent Ca²⁺ channel directly. 6 The [Ca²⁺]_i-force relationship in the presence of cromakalim was not distinguishable from that of control. 7 The [Ca²⁺]_i-force curve was shifted to the right by pinacidil and nicorandil. 8 These results show that cromakalim is a more specific K⁺ channel opener than pinacidil and nicorandil, and that vasodilatation produced by cromakalim in this study is predominantly a result of a reduction of [Ca²⁺]_i due to the closure of voltage-dependent Ca²⁺ channels by hyperpolarization. In contrast, additional mechanisms are involved in the vasodilator actions of pinacidil and nicorandil. One of these is related to a reduction in the sensitivity of contractile proteins to Ca²⁺. The latter mechanism of nicorandil is akin to that of nitroglycerin. K⁺ channels opened by these K⁺ channel openers may be ATP-sensitive ones which are blocked by glibenclamide.