REGULATION OF SINGLE CALCIUM CHANNELS IN CEREBRAL-ARTERIES BY VOLTAGE, SEROTONIN, AND DIHYDROPYRIDINES

Unitary currents through Ca channels were measured from cell-attached patches on smooth muscle cells isolated from rabbit cerebral (basilar) arteries. Barium (80 and 10 mM) and calcium (80 and 10 mM) were used as the charge carriers. The dihydropyridine Ca channel agonist BAY R 5417 was used to increase open-state probability (P(open)), with 500 nM BAY R 5417 increasing P(open) 10-fold at 0 mV. Barium currents through single Ca channels were greater than calcium currents at any voltage, with single-channel conductances negative to -20 mV of 24.6 pS (80 mM barium), 15.1 pS (80 mM calcium), 17.2 pS (10 mM barium), and 5.8 pS (10 mM calcium). The single-channel P(open) increased 2.7-fold per 5- to 7-mV membrane depolarization (negative to 0 mV) and was half maximal at +0.4 mV (80 mM calcium) and +13.5 mV (80 mM barium). Ca channels with calcium but not with barium as the charge carrier exhibited pronounced inactivation positive to -20 mV (half time, 112 ms at 0 mV). The dihydropyridine nimodipine (2 nM) inhibited average currents through Ca channels. The cerebral artery constrictor serotonin increased P(open) of single Ca channels by as much as 200-fold without an effect on single-channel conductance. A second distinct amplitude of unitary currents was often observed, corresponding to a channel conductance of about one-half the more commonly observed level. The small-conductance-level channel was voltage dependent, did not inactivate over 0.5-s test pulses (with barium), and could be activated by serotonin.