[Electrical properties and transmembrane ion currents of single smooth-muscle cells]

Current and voltage clamp investigations of freshly isolated smooth muscle cells from guinea-pig ileum and taenia coli were performed using single suction micropipette technique. Specific membrane capacity of smooth muscle cells was calculated and accounted for 1.6 microF/cm2, with specific resistance varying from 50 to 150 k omega X cm2. Transmembrane currents consisted of two inward components, inactivating and noninactivating ones, carried by Ca2+ ions, overlapping with early activated potassium outward current. Time constant of inward current activation was not only voltage-sensitive but also ion-dependent. When Ca2+ ions in Krebs solution were replaced by Ba2+, both the rate of activation and inactivation of inward current were significantly reduced. Estimation of intracellular Ca2+ concentration increase has indicated that inward calcium current transports enough Ca2+ for direct contraction activation.