Multiple types of voltage-dependent Ca channels in mammalian intestinal smooth muscle cells

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Abstract
(1) Whole-cell and single channel recording techniques have been applied to smooth muscle cells isolated from guinea-pig taenia coli to examine whether multiple types of Ca channels exist. (2) Whole-cell recordings under physiological Ca concentration (1.8 mM) revealed two current components with fast and slow inactivating kinetics. The fast inactivating component was present when cells were held at very negative potentials (−80 mV). It was insensitive to the dihydropyridine (DHP) derivative, nifedipine. In contrast, the slow inactivating component was present at less negative holding potentials. It was blocked by nifedipine. (3) The two current components were found to have closely similar voltage dependencies for activation. (4) These results suggest that the fast inactivating decay of the Ca current was mediated not only by the entry of Ca into the cell but also by a voltage-dependent process via a different type of Ca channel with fast inactivating kinetics. (5) Recordings from cell-attached membrane patches with 100 mM external Ba clearly showed the existence of multiple types of Ca channels with different conductances. (6) The large conductance channels (30 pS) activated at more positive potentials (0 mV) and their averaged current decayed much more slowly. The DHP Ca antagonist, nifedipine, inhibited the large conductance channels increasing the proportion of blank sweeps and reducing the averaged current. On the other had, the DHP Ca-agonist, BayK 8644, increased the averaged current by increasing the mean open-times of the large conductance channels. The presence of micromolar Cd in the patch pipettes produced a flickering block of the large conductance channels. (7) The small conductance channels (7 pS) activated at more negative potentials (−40 mV ∼30 mV) and the averaged current decayed rapidly within 100 ms. They showed no sensitivity to nifedipine and Cd ions. (8) In summary, we have identified at least two distinct types of Ca channels with different conductances, different pharmacological sensitivities, and different voltage- and time-dependent kinetics in smooth muscle cells isolated from guinea-pig taenia coli. The large and small conductance channels could be classified asl- andt-type Ca channels, respectively, which have been described in neuronal and heart cells. The contribution of those channels to the macroscopic component of whole-cell Ca current is also discussed.