Whole-cell and unitary Ca channel currents in mammalian intestinal smooth muscle cells: evidence for the existence of two types of Ca channels.

Whole-cell and single Ca channel currents were recorded in smooth muscle cells isolated from guinea-pig taenia coli to examine whether multiple types of Ca channels exist. Two different types of voltage-dependent Ca channels with different conductances and inactivation kinetics were identified from cell-attached patch clamp recordings using 50 mM Ba in the patch pipettes. One type of channel, with a large conductance of 25 pS, had a threshold of activation near -40 mV and the mean current reconstructed by averaging individual current responses inactivated slowly. A second type of channel, with a small conductance of 12 pS, had a similar threshold value to that of the 25 pS channel, but the averaged current inactivated rapidly. The steady state inactivation of the 12 pS channel was complete at a holding potential of -40 mV. We concluded that both channel types represent fast and slow inactivating voltage-dependent Ca channels which have been found in many excitable cells.