All‐or‐nothing responses to carbachol in single intestinal smooth muscle cells of rat

1 Concentration‐response relationships for carbachol (CCh)‐induced increases in the cytosolic calcium concentration ([Ca 2+ ] i ) and membrane currents were studied by use of fura‐2 microfluorimetry and nystatin‐perforated whole‐cell recording in single smooth muscle cells isolated from rat intestine. 2 CCh produced an initial peak rise in [Ca 2+ ] i followed by a small sustained rise. In individual cells, the peak rise in [Ca 2+ ] i did not increase in amplitude even with increasing concentrations of CCh, though the threshold concentration varied in different cells. The initial peak rise in [Ca 2+ ] i , but not the sustained rise, was due to the release of stored Ca 2+ , because it was unchanged after removal of external Ca 2+ and the addition of nifedipine (1 μ m ) or La 3+ (1 m m ). 3 CCh elicited an outward and inward current in a cell dialyzed with a pipette solution containing KCl at a holding potential of – 30 mV and with one containing NaCl at – 60 mV, respectively. In individual cells, the amplitude of each current was similar in cells stimulated at over the threshold concentration of CCh, but the threshold was different among cells. 4 The percentage of cells showing Ca 2+ ‐transient responses to CCh at given concentrations was similar to those showing current responses and contractile responses. 5 In thin muscle bundles, a concentration‐dependent contraction was evoked by CCh in the absence of external Ca 2+ . Its threshold was similar to those of Ca 2+ ‐transient and current responses in single cells. 6 These results suggest that CCh‐induced release of stored Ca 2+ takes place in an all‐or‐nothing fashion in individual cells of the rat intestinal smooth muscle.