Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pHi) changers such as sodium acetate (SA) and NH4Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH4Cl. Intracellular Ca2+ concentration ([Ca2+]i) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R340/380). SA (20 mM) increased and NH4Cl (20 mM) decreased R340/380 by 0.2 ± 0.03 and 0.1 ± 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R340/380 by 0.9 ± 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH4Cl, implying that SA and NH4Cl may affect [Ca2+]i (p < 0.05). Accordingly, we studied the effects of SA and NH4Cl on Ca2+-activated K+ current (IKCa) under patch clamp technique. Caffeine produced transient outward current at holding potential (Vh) of 0 mV, caffeine induced transient outward K+ current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH4Cl. In addition, IKCa was significantly increased by acidotic condition when pHi was lowered by altering the NH4Cl gradient across the cell membrane. Finally, the effects of SA and NH4Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was −28 ± 2.3 mV in a single cell level and was depolarized by 13 ± 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH4Cl depolarized RMP by 10 ± 1.9 and 16 ± 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH4Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca2+]i and Ca2+-activated K channels in rabbit basilar artery.