Acetylcholine activates single sodium channels in smooth muscle cells

Using the whole-cell clamp technique with a patch electrode, single channel activities were recorded in dispersed single smooth muscle cells of the guinea-pig ileum in response to application of greater than 1 microM acetylcholine (ACh) or carbachol (CCh). Under physiological conditions (bath:Krebs, pipette:High K), these channels have a single channel conductance of 20-25 pS at the membrane potentials ranging between -100 and -40 mV and are activated in the membrane potential dependent manner. The amplitude of the channel current showed a strong dependence on the extracellular Na concentration but the reversal potential obtained by the extrapolation of the I-V relationship was not consistent with the equilibrium potential of Na ion. An approximate estimation of permeability ratio based on the independent principle described by Hodgkin et al. gave a value of Na:K = 1.0:0.3-0.4 to this channel. From features of the macroscopic and single channel currents, it is concluded that these muscarinic ACh(CCh)-activated channels mainly pass Na ion and play a major part in the membrane depolarization produced by ACh or CCh in mammalian intestinal smooth muscles.