L-type Ca2+ channels access multiple open states to produce two components of Bay K 8644-dependent current in GH3 cells

To determine the number of L-channel populations responsible for producing the two components of whole-cell L-type Ca2+ channel current revealed by Bay K 8644 (Fass, D.M., and E.S. Levitan. 1996. J. Gen. Physiol. 108:1-11), L-type Ca2+ channel activity was recorded in cell- attached patches. Ensemble tail currents from most (six out of nine) single-channel patches had double-exponential time courses, with time constants that were similar to whole-cell tail current decay values. Also, in single-channel patches subjected to two different levels of depolarization, ensemble tail currents exactly reproduced the voltage dependence of activation of the two whole-cell components: The slow component is activated at more negative potentials than the fast component. In addition, deactivation of Bay K 8644-modified whole-cell L-current was slower after long (100-ms) depolarizations than after short (20-ms) depolarizations, and this phenomenon was also evident in ensemble tail currents from single L-channels. Thus, a single population of L-channels 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 traces that contained a single opening upon repolarization and no reopenings. These ensemble tails were biexponential. This type of analysis also revealed that reopenings do not contribute to the slowing of tail current deactivation after long depolarizations. Thus, individual L-channels must have access to several open states to produce multiple macroscopic current components. We also obtained evidence that access to these open states can vary over time. Use of several open states may give L-channels the flexibility to participate in many cell functions.