Calcium channel effectors are potent non-competitive blockers of acetylcholine receptors

Nicardipine and other calcium channel effectors (CCEs) were studied for their effects on nicotinic acetylcholine receptor (nAChR) activity. While CCEs had no effect on frog (Rana pipiens) skeletal muscle contractions resulting from nerve stimulation or direct stimulation of the muscle, nicotinic agonist-induced contractures were blocked. Nicardipine did not affect nAChR single-channel open time or amplitude, corroborating data from endplate currents (EPCs); EPC amplitudes and decays were unaffected. All the CCEs tested, however, non-competitively blocked nAChRs. The block of nAChRs resulted in a shortened agonist-induced depolarization and thus a diminished contracture response. An increase in cultured mouse skeletal muscle (C-2) cell single-channel closed times was observed with the intracellular addition of nicardipine, verifying a direct block of nAChRs. Using single-channel analysis, nicardipine's site of action, or at least access to its site of action, was determined to be at the intracellular side of the receptor. A direct action of the CCEs on the nAChR was also shown by their ability to block phencyclidine (PCP) binding to Torpedo nobiliana membranes. All the CCEs blocked specific binding of [3H]-PCP to its binding site on the nAChR-channel complex, with bepridil and nicardipine being the most potent. These data are compatible with a model in which nicardipine and other CCEs, at concentrations which do not alter nAChR channel open time or conductance, block the effects of superfused nicotinic agonist on nAChRs either by stabilizing the formation of the nAChR desensitized state or by effecting a slow channel block.