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 ([Ca2+]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 [Ca2+]i followed by a small sustained rise. In individual cells, the peak rise in [Ca2+]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 [Ca2+]i, but not the sustained rise, was due to the release of stored Ca2+, because it was unchanged after removal of external Ca2+ and the addition of nifedipine (1 microM) or La3+ (1 mM). 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 Ca2+. 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 Ca2+ takes place in an all-or-nothing fashion in individual cells of the rat intestinal smooth muscle.