Calcium Currents in the A7r5 Smooth Muscle-derived Cell Line An AUosteric Model for Calcium Channel Activation and Dihydropyrid ne Agonist Action

We have investigated the gating kinetics of calcium channels in the ATr5 cell line at the level of single channels and whole cell currents, in the absence and presence of dihydropyridine (DHP) calcium channel agonists. Although latencies to first opening and macroscopic currents are strongly voltage dependent, analysis of amplitude histograms indicates that the primary open-closed transition is voltage independent. This suggests that the molecular mechanisms for voltage sensing and channel opening are distinct, but coupled. We propose a modified Monod-Wyman-Changeux (MWC) model for channel activation, where movement of a voltage sensor is analogous to ligand binding, and the closed and open channels correspond to inactive (T) and active (R) states. This model can account for the activation kinetics of the calcium channel, and is consistent with the existence of four homologous domains in the main subunit of the calcium channel proteinl DHP agonists slow deactivation kinetics, shift the activation curve to more negative potentials with an increase in slope, induce intermingled fast and slow channel openings, and reduce the latency to first opening. These effects are predicted by the MWC model if we make the simple assumption that DHP agonists act as aUosteric effectors to stabilize the open states of the channel. I N T R O D U C T I O N Calcium channel gating has generally been described with models containing a series of closed states and an open state. Multiple closed states are required to explain delays in activation kinetics and multi-exponential distributions of channel closed times (Fenwick, Marty, and Neher, 1982). Technical difficulties, particularly the rapid gating kinetics and poor signal to noise ratio at the single channel level, have limited the amount of kinetic information available about calcium channels. Dihydropyridine (DHP) agonists, which improve resolution of channel activity by greatly prolonging Address reprint requests to Dr. Stephen W. Jones, Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106. J, GE~. PHYSIOL. © The Rockefeller University Press • 0022-1295/92/03/0367/24 $2.00 Volume 99 March i992 367-390 367 on Jne 5, 2017 D ow nladed fom Published March 1, 1992