Single Ca channel currents in mammalian visceral smooth muscle cells

Recordings of single Ca channel currents in mammalian visceral smooth muscle cells were obtained using patch clamp techniques. Smooth muscle cells from guinea-pig taenia coli were prepared by enzymatic dispersion using 0.3% collagenase. The recordings were obtained from cell-attached membrane patches of isolated cells with a pipette filled with isotonic 50 mM Ba2+. When the membrane patch was depolarized, brief inward current pulses of unitary size and small amplitude were observed. The amplitude of these single channel currents decreased linearly with increasing depolarization in a voltage range from -20 mV to +50 mV about the resting potential. The slope conductance was estimated to be about 30 pS. The mean current reconstructed by averaging individual current responses showed kinetic behaviour with a rapid activation and a slower inactivation process similar to the macroscopic Ca2+ current observed in strips of guinea-pig taenia coli. The present study suggests that the inward current pulses of unitary size induced by voltage-clamp pulses were due to Ba ions passing through a single voltage dependent Ca channel.