Analysis of the T-type calcium channel in embryonic chick ventricular myocytes.

T-type calcium channels (IT channels) were studied in cell-attached patch electrode recordings from the ventricular cell membrane of 14-day embryonic chick heart. All experiments were performed in the absence of Ca2+ with Na+ (120 mM) as the charge carrier. IT channels were distinguished from L-type calcium channels (IL) by their more negative activation and inactivation potential ranges; their smaller unitary slope conductance (26 pS), and their insensitivity to isoproterenol or D600. Inactivation kinetics were voltage dependent. The time constant of inactivation was 37 msec when the membrane potential was depolarized 40 mV from rest (R + 40 mV), and 20 msec at R + 60 mV. The frequency histogram of channel open times (tau o) was fit by a single-exponential curve while that of closed times (tau c) was bi-exponential. tau o was the same at R + 40 mV and R + 60 mV whereas tau c was shortened at R + 60 mV. The open-state probability (P o) increased with depolarization: 0.35 at R + 40 mV, 0.8 at R + 60 mV and 0.88 at R + 80 mV. This increase in Po at depolarized potentials could be accounted for by the decrease in tau c.