CALCIUM CURRENTS ELICITED BY VOLTAGE STEPS AND STEADY VOLTAGES IN MYOCYTES ISOLATED FROM THE RAT BASILAR ARTERY

1. Whole-cell patch clamp methods were used to record Ca2+ channel cur rents from isolated rat basilar arterial myocytes either in response t o voltage steps or at steady holding potentials. Inward currents were rapidly and reversibly blocked by 2 mM Co2+, and this solution was rou tinely used to subtract leakage currents. 2. Peak currents measured in response to voltage steps were -85+/-23 pA (n = 7) in physiological C a2+ (1.8 mM) and -256+/-39 pA (n = 19) in 10 mM Ba2+. The time course of activation and inactivation were unaffected by changing the holding potential from -88 to -48 mV, and thus we found no evidence for a com ponent of current flowing through transient Ca2+ channels. 3. Activati on curves were constructed by dividing the macroscopic current by the single Ca2+ channel current. The product of the number of functional c hannels and the open-state probability (NP(o)) measured in this way ro se to around 2000. Its dependence on voltage was fitted by a Boltzmann function with half-activation at -8.6 mV in physiological Ca2+ concen trations and a steepness factor in the range 6-8 mV. 4. In physiologic al solution, block by external Mg2+ reduced peak current through Ca2channels by 17%. 5. Steady-state currents were measured by holding the cell at a fixed voltage and rapidly applying Co2+ to block the curren t through Ca2+ channels. Steady-state currents could be detected at vo ltages as negative as -58 mV, and persisted for more than 15 min, sugg esting that Ca2+ channels in this artery may provide a steady-state Ca 2+ influx that contributes to resting contractile tone.