KINETIC-PROPERTIES OF SKELETAL-MUSCLE-LIKE HIGH-THRESHOLD CALCIUM CURRENTS IN A NON-FUSING MUSCLE-CELL LINE

Macroscopic kinetics and single-channel properties of skeletal-muscle-type calcium currents were studied in the non-fusing, clonal muscle cell line, BC3H1. Slowly activating, dihydropyridine(DHP)-sensitive currents, associated with T-tubular DHP receptors and ion channels, could be isolated from rapidly activating, DHP-resistant currents. Description of macroscopic current activation kinetics required only a brief delay term (tau(0) < 1 ms), two ascending exponential terms with voltage-dependent time constants (2 < tau(1) < 20 ms and 10 < tau(2) < 200 ms), and a single exponential decay term (0.5 < tau(3) less than or equal to 5 s). Steady-state activation voltage dependence required description by two Boltzmann distribution terms with V-1/2 and slope factors differing by 20 mV and 3.5- to LC-fold respectively. These two distributions were correlated with the steady-state voltage dependence of the two ascending kinetic terms described by tau(1) and tau(2) respectively. Rundown of the DHP-sensitive slow current was correlated with a negative shift in the voltage dependence of current decay (tau(3)). Three conductance levels (4.5 pS, 8 pS and 12 pS) were detected in single-channel records, two of which (the 8-pS and 12-pS events) were prolonged by BayK8644 and thus associated with DHP-sensitive single-channel events. Description of single-channel open time distributions required a minimum of two exponential terms (2.5 +/- 0.9 ms and 10.3 +/- 5.4 ms at -10 mV). Slow transitions among closed states result in biexponential latency-to-first-event distributions (47 +/- 12 ms and 470 +/- 123 ms at -10 mV).