[Biophysical and pharmacological characterization of receptor-operated nonselective cation channels (ROCC) and their regulatory mechanisms in smooth muscle].

Stimulation of excitatory receptors in smooth muscle often leads to the opening of ROCC. These channels exhibit considerable permeability to Ca2+, and they have been regarded as the most probable candidate for the "receptor-operated Ca2+ entry" pathway. The muscarinic receptor ROCC in guinea pig ileum (mROCC) have a unitary conductance of -25pS and are activated through a pertussis toxin-sensitive G protein. mROCC permeate Ca2+ and Ba2+ several fold more preferably than monovalent cations, and they are inhibited by various types of K channel blockers, diphenylamine-2-carboxylate derivatives and even by nicardipine and D-600 at high concentrations. mROCC are efficiently regulated by various physiological factors including the membrane potential, intracellular Ca2+ concentration, external pH and osmolarity. The effective ranges of these factors span their dynamic ranges under physiological conditions. In addition to these properties, mROCC have several sites sensitive to external polyvalent cations. The alpha 1-adrenergic receptor ROCC in rabbit portal vein resemble mROCC in many respects, e.g., the unitary conductance, ionic selectivity, activation kinetics, sensitivity to polyvalent cations and voltage-dependence. These complex characteristics of ROCC suggest that they play other roles in addition to being just a passive cation permeable pore in agonist-mediated Ca2+ mobilization in smooth muscle.