Analysis of calcium channel properties in cultured leech retzius cells by internal perfusion, voltage-clamp and single-channel recording.

1. The properties of voltage-dependent calcium channels have been measured in Retzius cells isolated from the central nervous system of the leech and maintained in tissue culture. 2. Macroscopic divalent cation currents were isolated after blocking Na and K currents by bathing the cells with Na-free solutions containing TEA and 4-AP, and internally perfusing them with Cs and TEA. Depolarizing voltageclamp pulses activated inward currents that were larger for Sr" than for Ba or Ca. The peak currents were observed at +15mV for Ca and Ba and at +7 mV for Sr"". Divalent cation currents were blocked by Cd and Mn but not by dihydropyridine blockers. 3. The activation kinetics of Ba currents was sigmoid. The inactivation was approximately 10% at the end of a 50ms depolarizing pulse. Decay of Sr"" and Ca currents was larger and showed two kinetic phases. Activation and inactivation of the calcium channels were not significantly influenced by the holding potential. 4. Deactivation kinetics observed during tail currents consisted of two exponential components. At a closing voltage of — 60mV, the time constant was ==200 /JS for the fast component and 1.9 ms for the slow component. Both time constants of deactivation were voltage-dependent over the range from —80 to —20mV, and increased at more depolarized closing voltages. 5. Single-channel activity was recorded with cell-attached patches in solution containing TSmmoll" Ba. 6. Taken together, the results define the characteristics of a distinctive type of calcium channel in isolated Retzius cells.