SINGLE CALCIUM-CHANNEL BEHAVIOR IN NATIVE SKELETAL-MUSCLE

The purpose of this study was to use whole-cell and cell-attached patc hes of cultured skeletal muscle myotubes to study the macroscopic and unitary behavior of voltage-dependent calcium channels under similar c onditions. With 110 mM BaCl2 as the charge carrier, two types of calci um channels with markedly different single-channel and macroscopic pro perties were found. One class was DHP-insensitive, had a single-channe l conductance of similar to 9 pS, yielded ensembles that displayed an activation threshold near -40 mV, and activated and inactivated rapidl y in a voltage-dependent manner (T current). The second class could on ly be well resolved in the presence of the DHP agonist Bay K 8644 (5 m u M) and had a single-channel conductance of similar to 14 pS (T curre nt). The 14-pS channel produced ensembles exhibiting a threshold of si milar to -10 mV that activated slowly (tau(act) similar to 20 ms) and displayed little inactivation. Moreover, the DHP antagonist, (+)-PN 20 0-110 (10 mu M), greatly increased the percentage of null sweeps seen with the 14-pS channel. The open probability versus voltage relationsh ip of the 14-pS channel was fitted by a Boltzmann distribution with a V-P0.5 = 6.2 mV and K-p = 5.3 mV. L current recorded from whole-cell e xperiments in the presence of 110 mM BaCl2 + 5 mu M Bay K 8644 display ed similar time- and voltage-dependent properties as ensembles of the 14-pS channel. Thus, these data are the first comparison under similar conditions of the single-channel and macroscopic properties of T curr ent and L current in native skeletal muscle, and identify the 9- and 1 4-pS channels as the single-channel correlates of T current and L curr ent, respectively.