L-TYPE CA2-K-8644-DEPENDENT CURRENT IN GH(3) CELLS( CHANNELS ACCESS MULTIPLE OPEN STATES TO PRODUCE 2 COMPONENTS OF BAY)

Tow determine the number of L-channel populations responsible for prod ucing the two components of whole-cell L-type Ca2+ channel current rev ealed by Bay K 8644 (Fass, D.M., and E.S. Levitan. 1996. J. Gen. Physi ol. 108:1-11), L-type Ca2+ channel activity was recorded in cell-attac hed patches. Ensemble tail currents from most (six out of nine) single -channel patches had double-exponential time courses, with time consta nts that were similar to whole-cell tail current decay values. Also, i n single-channel patches subjected to two different levels of depolari zation, ensemble tail currents exactly reproduced the voltage dependen ce of activation of the two whole-cell components: The slow component is activated at more negative potentials than the fast component. In a ddition, deactivation of Bay K 8644-modified whole-cell L-current was slower after long (100-ms) depolarizations than after short (20-ms) de polarizations, and this phenomenon was also evident in ensemble tail c urrents from single L-channels. Thus, a single population of L-channel s can produce the two components of macroscopic L-current deactivation . To determine how individual L-channels produce multiple macroscopic tail current components, we constructed ensemble tail currents from tr aces that contained a single opening upon repolarization and no reopen ings. These ensemble tails were biexponential. This type of analysis a lso revealed that reopenings do not contribute to the slowing of tail current deactivation after long depolarizations. Thus, individual L-ch annels must have access to several open states to produce multiple mac roscopic current components. We also obtained evidence that access to these open states can vary over time. Use of several open states may g ive L-channels the flexibility to participate in many cell functions.