Comparison of ionic currents from interstitial cells and smooth muscle cells of canine colon

1. Voltage-dependent ionic currents of isolated interstitial cells were characterized using the whole-cell voltage clamp technique, and compared with currents recorded from circular muscle cells. Both cell types were isolated from the submucosal pacemaking region in the canine distal colon. 2. Upon depolarization, interstitial cells and smooth muscle cells generated transient inward, followed by slowly inactivating outward, currents. 3. After blocking inward current and much of the Ca(2+)-dependent outward current, interstitial cells displayed voltage-dependent outward current that rapidly activated, reached a peak, and then inactivated. This current was resistant to 4-aminopyridine(4-AP; 1 mM). Smooth muscle cells expressed a similar current but it was reduced by about 40% at a test potential of +20 mV by 4-AP (1 mM). 4. The inactivation characteristics of the voltage-dependent outward currents of interstitial cells and smooth muscle cells were compared. The outward current of interstitial cells inactivated at more negative potentials; half-inactivation occurred at -53 mV, whereas half-inactivation occurred at -20 mV in smooth muscle cells. 5. Inward currents were not strikingly different in the two cell types when dialysing pipettes were used. When the perforated patch technique (using Amphotericin-B) was used, a negatively activating inward current was observed in interstitial cells that had a resolution threshold of -70 to -60 mV. This current peaked at -10 mV. Inward currents in smooth muscle cells were resolved at test potentials positive to -50 mV and peaked at 0 to +10 mV. 6. When interstitial cells were held at -40 mV, inward current could not be resolved with test depolarization negative to -30 mV. From this holding potential, peak amplitude was reduced by 85% with test depolarizations to -10 mV. Holding smooth muscle cells at -40 mV also reduced inward current, but the peak current in these cells was reduced by only 39% at 0 mV. 7. Ni2+ partially inhibited peak inward current in interstitial cells and abolished a 'hump' in the I-V curve that occurred at negative potentials. In dialysed cells where this 'hump' was not apparent, addition of nifedipine unmasked a 'hump'. The presence of both nifedipine and Ni2+ abolished inward current. 8. A portion of the inward current in smooth muscle cells was sustained and persisted for the duration of test pulses. Very little sustained inward current was observed in interstitial cells. 9. The time course of inactivation of inward current in interstitial cells was fitted with two exponentials.(ABSTRACT TRUNCATED AT 400 WORDS)