BK channel activation by NS-1619 is partially mediated by intracellular Ca2+ release in smooth muscle cells of porcine coronary artery

Effects of NS-1619. an opener of large conductance Ca2+-activated K+ (BK) c hannel, on intracellular Ca2+ concentration ([Ca2+](i)) and membrane potent ial were examined in single myocytes freshly isolated from porcine coronary artery. 2 Under current clamp mode, the application of 1-30 muM NS-1619 hyperpolari zed the membrane in concentration-dependent manner. The NS-1619-induced hyp erpolarization was abolished by the presence of 100 nM iberiotoxin. 3 Application of 1-10 muM NS-1619 hyperpolarized the membrane by approximat ely 6 mV or less but did not change significantly the [Ca2+](i).. When memb rane hyperpolarization of 12 mV or so was caused by 30 muM NS-1619, [Ca2+]( i) was unexpectedly increased by approximately 200 nM. This increase in [Ca 2+]i and the concomitant outward current activation were also observed unde r voltage-clamp at holding potential of -40 mV. 4 The increase in [Ca2+](i) by 30 muM NS-1619 occurred mainly in peripheral regions than in the centre of the myocytes. The removal of extracellular C a2+ affected neither the membrane hyperpolarization nor the increase in [Ca 2+](i). 5 In the presence of 10 mM caffeine and 10 muM ryanodine, the increase in [ Ca2+](i) by 30 muM NS-1619 was not observed and the membrane hyperpolarizat ion was reduced to approximately 67% of the control. 6 These results indicate that the opening of BK channels by NS-1619 at 30 m uM, which is the most frequently used concentration of this agent, is partl y due to Ca2+ release from caffeine/ryanodine-sensitive intracellular stora ge sites but is mainly due to the direct activation of the channels.