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Department of Physiology, University of Leicester.

1. Whole-cell patch clamp methods were used to record Ca2+ channel currents from isolated rat basilar arterial myocytes either in response to voltage steps or at steady holding potentials. Inward currents were rapidly and reversibly blocked by 2 mM Co2+, and this solution was routinely used to subtract leakage currents. 2. Peak currents measured in response to voltage steps were -85 +/- 23 pA (n = 7) in physiological Ca2+ (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 component of current flowing through transient Ca2+ channels. 3. Activation curves were constructed by dividing the macroscopic current by the single Ca2+ channel current. The product of the number of functional channels and the open-state probability (NPo) measured in this way rose to around 2000. Its dependence on voltage was fitted by a Boltzmann function with half-activation at -8.6 mV in physiological Ca2+ concentrations and a steepness factor in the range 6-8 mV. 4. In physiological solution, block by external Mg2+ reduced peak current through Ca2+ channels by 17%. 5. Steady-state currents were measured by holding the cell at a fixed voltage and rapidly applying Co2+ to block the current through Ca2+ channels. Steady-state currents could be detected at voltages as negative as -58 mV, and persisted for more than 15 min, suggesting that Ca2+ channels in this artery may provide a steady-state Ca2+ influx that contributes to resting contractile tone.

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