Kinetic properties of skeletal-muscle-like high-threshold calcium currents in a non-fusing muscle cell line

Abstract

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 (τ ₀<1 ms), two ascending exponential terms with voltage-dependent time constants (2<τ ₁<20 ms and 10<τ ₂<200 ms), and a single exponential decay term (0.5<τ ₃⩽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 4-fold respectively. These two distributions were correlated with the steady-state voltage dependence of the two ascending kinetic terms described by τ ₁ and τ ₂ respectively. Rundown of the DHP-sensitive slow current was correlated with a negative shift in the voltage dependence of current decay (τ 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).