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

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+]i in 5 and 30 mM KCl physiological salt solution (PSS) but failed to affect [Ca2+]i in 45 and 90 mM KCl-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 KCl-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(-5) M 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 Ca21 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.