Abstract
N-type and P/Q-type Ca(2+) channels are inhibited by neurotransmitters acting through G protein-coupled receptors in a membrane-delimited pathway involving Gbetagamma subunits. Inhibition is caused by a shift from an easily activated "willing" (W) state to a more-difficult-to-activate "reluctant" (R) state. This inhibition can be reversed by strong depolarization, resulting in prepulse facilitation, or by protein kinase C (PKC) phosphorylation. Comparison of regulation of N-type Ca(2+) channels containing Cav2.2a alpha(1) subunits and P/Q-type Ca(2+) channels containing Ca(v)2.1 alpha(1) subunits revealed substantial differences. In the absence of G protein modulation, Ca(v)2.1 channels containing Ca(v)beta subunits were tonically in the W state, whereas Ca(v)2.1 channels without beta subunits and Ca(v)2.2a channels with beta subunits were tonically in the R state. Both Ca(v)2.1 and Ca(v)2.2a channels could be shifted back toward the W state by strong depolarization or PKC phosphorylation. Our results show that the R state and its modulation by prepulse facilitation, PKC phosphorylation, and Ca(v)beta subunits are intrinsic properties of the Ca(2+) channel itself in the absence of G protein modulation. A common allosteric model of G protein modulation of Ca(2+)-channel activity incorporating an intrinsic equilibrium between the W and R states of the alpha(1) subunits and modulation of that equilibrium by G proteins, Ca(v)beta subunits, membrane depolarization, and phosphorylation by PKC accommodates our findings. Such regulation will modulate transmission at synapses that use N-type and P/Q-type Ca(2+) channels to initiate neurotransmitter release.