Introducing an alternative biophysical method to analyze direct G protein regulation of voltage-dependent calcium channels.

Direct G protein inhibition of voltage-dependent calcium channels is currently indirectly assessed by the gain of current produced by depolarizing prepulse potentials (PP). Indeed, PPs produce a channel opening- and time-dependent dissociation of G proteins from the channel that is responsible for the increase in Ca(2+) permeation. Parameters of G protein dissociation are essential to describe the characteristic landmark modifications in channel activities that underlie G protein regulation. From the kinetics and opening-dependence of this dissociation, crucial biophysical parameters are extracted such as the extent and the rate of G protein unbinding from the channel. Unfortunately, the method used so far assumes that G protein regulated channels undergo the same inactivation kinetics than control channels. Herein, we demonstrate for the first time that G protein-bound channels undergo a much slower inactivation than control channels. We thus introduce a novel simple-to-use method that avoids the use of PPs and that is not affected by potential changes in channel inactivation kinetics conferred by G protein binding. This method extracts G protein unbinding parameters from ionic currents induced by regular depolarizing pulses by separating the ionic currents due to non-regulated channels from the ionic currents that result from a progressive departure of G proteins from regulated channels.