Evidence that ionic channels associated with the muscarinic receptor of smooth muscle may admit calcium.

1 The actions of carbachol on the membrane potential and conductance of smooth muscle of the guinea-pig intestine were investigated using microelectrode recording and the double sucrose-gap method in solutions in which calcium was the only cation creating an inwardly-directed electrochemical gradient.2 In a calcium chloride solution containing a small amount of potassium but no sodium and buffered to physiological pH (Ca Locke) the membrane was hyperpolarized to more than -80 mV. Carbachol (2 x 10(-7)-10(-4)M) depolarized the membrane and increased the membrane conductance.3 By passing current the membrane potential of the smooth muscle cells could be varied. In Ca Locke the depolarization produced by carbachol was shown to be reduced if the membrane was depolarized. The relationship between the size of the carbachol depolarization and the membrane depolarization was linear, giving an apparent reversal potential for carbachol depolarization some 20 mV positive to the resting membrane potential, as measured by extracellular electrodes in the sucrose gap.4 Carbachol depolarization was reduced if the calcium concentration was reduced below 2.5 mM by replacing calcium with Tris, but the depolarization in 2.5 mM Ca and in Ca Locke (100 mM Ca) were of similar size. In Ca-free Na-free solution with EGTA, carbachol depolarization was soon abolished.5 In the sucrose-gap when the chloride gradient across the cell membrane was reversed by replacing the chloride of Ca Locke by an impermeant anion the membrane depolarized. Carbachol now contracted the muscle but produced a hyperpolarization.6 These results are consistent with the hypothesis that activation of muscarinic receptors opens ionic channels which at least in the solutions used, can admit sufficient calcium ions to depolarize the cell and cause tension development.