Rabbit intestinal smooth muscle calcium current in solutions with physiological calcium concentration

The calcium channel current in enzymatically isolated cells of the longitudinal muscle of rabbit jejunum was studied using the whole-cell voltage clamp technique. The current-voltage relationship was measured in cells held at potentials ranging from -90 to -30 mV in the presence of 1.5 mM-calcium or barium in order to test for the presence of multiple current components. The kinetics and current-voltage relationship of the current showed no evidence of a discrete low-threshold or 'transient' current. Measurable current was observed at -55 mV. Current availability was dependent on the holding potential but not on the test potential, suggesting the presence of only one type of calcium channel. A component of current persisted for at least 25 s following depolarization. Nifedipine reduced the Ca2+ current amplitude without shifting the current-voltage relationship; the degree of inhibition was enhanced by depolarization of the holding potential. The peak and sustained currents were similarly inhibited by nifedipine. The results indicate the presence of a single type of dihydropyridine-sensitive calcium channel current which is partially activated at or near the physiological resting membrane potential of these cells.