Unitary currents through single calcium channels were measured from cell-attached patches on smooth muscle cells isolated from resistance-sized branches of posterior cerebral arteries from Wistar-Kyoto normotensive rats. Barium (80 and 10 mM) was used as the charge carrier, with and without the dihydropyridine calcium channel agonist BAY R 5417. Unitary currents decreased on membrane depolarization, with a slope conductance of 19.4 pS (80 mM barium). Channel open-state probability (Po) was steeply voltage dependent. Peak Po during test pulses from -70 mV increased e-fold per 4.5-mV depolarization. Mean peak Po at potentials positive to +10 mV was 0.44. Po at steady membrane potentials was also steeply voltage dependent, changing e-fold per 4.5 mV in the absence of inactivation. Steady-state Po at positive potentials was substantially lower than peak Po elicited by test pulses, suggesting that steady-state inactivation can reduce Po by as much as 10-fold. Membrane depolarization decreased the longest mean closed time but had little effect on the mean open time of single calcium channels measured during steady-state recordings. Lowering the external barium concentration from 80 to 10 mM reduced the single channel conductance to 12.4 pS and shifted the relationship between steady-state Po and membrane potential by about -30 mV. BAY R 5417 also shifted this relationship by about -15 mV.
Theoretical study of the voltage and concentration dependence of the anomalous mole fraction effect in single calcium channels. New insights into the characterization of multi-ion channels