CALCIUM-CHANNEL CURRENTS IN ISOLATED SMOOTH-MUSCLE CELLS FROM THE BASILAR ARTERY OF THE GUINEA-PIG

Ca2+ channel currents were studied in smooth muscle cells from the basilar artery of the guinea pig using the whole-cell patch-clamp technique, 10 mM Ba2+ as the charge carrier and strong buffering of intracellular Ca2+ with EGTA (pCa(i) = 8). Cell capacitance was 18.8 +/- 6.6 pF (n = 96) and maximum current density at +10 to +20 mV (holding potential < -55 mV), measured early in dialysis, was -14.8 +/- 4.9 pA/pF (n = 83). Currents reversed at approximately +95 mV and, at more positive potentials, outward Cs+ currents were recorded that were blocked by either external Cd2+ or Ca2+. One component of current was identified that had properties consistent with L-type channels. On the basis of measurements of tail currents, its threshold for activation was -15 mV, its voltage dependence of activation was steep and it was half-activated at +8.5 mV. It inactivated very slowly at +15 mV (2787 +/- 511 ms) and it deactivated rapidly (251 +/- 55-mu-s) at -55 mV. It was quickly lost during dialysis and was largely blocked by 1 nM nifedipine (1-s pulses, holding potential = -55 mV). A second component, termed B-type current, was identified that had properties inconsistent with those of T-type channels. On the basis of tail currents, its threshold for activation was -30 mV, its voltage dependence of activation was less steep and it was half-activated at +33.7 mV. It was half-inactivated at -32.1 mV, it inactivated with a time constant of 162 +/- 13 ms at +15 mV and it deactivated relatively slowly (3.8 +/- 0.8 ms) at -55 mV. It was less sensitive than L-type current to dialysis and to block by nifedipine.