Effects of external pH on ionic currents in smooth muscle cells from the basilar artery of the guinea pig

Abstract

pH0 is an important determinant of vascular tone in cerebral blood vessels. We investigated the effects of changes in pH0 on isolated smooth muscle cells from the basilar artery of the guinea pig. Single cells contracted rapidly in response to an elevation in pH0 (constant CO2), and contraction was blocked by nifedipine, suggesting a role for dihydropyridine-sensitive Ca2+ channels. In whole-cell patch-clamp experiments, changes in pH0 (pH0 5.7-8.1, pHi 7.2 with 10 mM HEPES) strongly affected the amplitude of the peak Ca2+ channel current (10 mM Ba2+, +15 mV, holding potential of -55 mV), with an apparent pK of 6.9. The current-voltage curves were minimally shifted, indicating no important effect of surface charge. To separate the slowly inactivating L-type Ca2+ channel current from the more rapidly inactivating B-type current, the decaying portions of inward currents from cells studied with repetitive 1-second pulses (+15 mV, holding potential of -55 mV) were fit to a two-component model. Titration curves for the L-type and B-type currents indicated maximum increases by factors of 3.65 and 1.28 at alkaline pH0 and gave apparent pK values of 7.71 and 6.47 (Hill coefficient unity). The time constant of inactivation for the B-type current at +15 mV was little affected by pH0, whereas that for the L-type current increased somewhat with increasing pH0. Additional experiments showed no significant effect of pH0 on holding current or on voltage-activated outward currents (PCai 7 with 11 mM EGTA). Our results provide additional evidence for participation of Ca2+ channels in regulating basal tone in cerebral smooth muscle and indicate that pH0 regulates current through slowly inactivating, dihydropyridine-sensitive L-type Ca2+ channels.