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.