Increased Ca2+ Influx in the Resting State Maintains the Myogenic Tone and Activates Charybdotoxin-Sensitive K+ Channels in Dog Basilar Artery

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Abstract
We examined whether Ca2+ channel function in the resting state alters the resting tone and Ca2+ -activated K+ (KCa) channel function in dog basilar artery: data were compared with findings in the mesenteric artery. Isolated dog basilar artery maintained a myogenic tone; that is, the resting tone decreased when either the Krebs solution was replaced with a Ca2+ -free solution or nifedipine was added. The basal 45Ca influx in the resting state of the basilar artery was significantly increased compared with that in the mesenteric artery, and this increase in the basilar artery was reduced by nifedipine. The addition of charybdotoxin (ChTX), a blocker of large-conductance KCa channels, to the resting strips caused a concentration-dependent contraction in the basilar artery but not in the mesenteric artery. The ChTX-induced contraction in the basilar artery was abolished by nifedipine. In resting strips preloaded with 86Rb, the basal 86Rb efflux rate constant was significantly greater in the basilar artery than in the mesenteric artery. The addition of nifedipine to the resting strips decreased the basal 86Rb efflux rate constant only in the basilar artery. These results suggest that the transmembrane Ca2+ influx via L-type voltage-dependent Ca2+ channels was significantly increased in the resting state of the basilar artery and that the myogenic tone was therefore maintained and the ChTX-sensitive KCa channels were highly activated.