Two types of calcium channels in single smooth muscle cells from rabbit ear artery - studied with whole cell and single channel recordings
AB Freshly dispersed rabbit ear artery cells were studied using patch-clamp techniques to measure membrane currents in whole cells and single-channel currents in membrane patches. Whole-cell calcium currents recorded at physiologic extracellular calcium concentrations were small (~10 pA). Recordings with 110 mM external barium gave much larger currents and revealed two current components with properties similar to those in other vascular smooth muscle preparations and to the calcium currents designated as T and L in heart cells and neurons. T current was activated with weak depolarizations and inactivated rapidly, while L current was activated with relatively strong depolarizations and inactivated more slowly. L current was Increased by dihydropyridine agonists and decreased by dihydropyridine antagonists, while T current was unaffected. Recordings from cell-attached and outside-out membrane patches with 110 mM external barium showed unitary calcium channel currents with conductances of 8 and 25 pS. The small conductance channels had kinetic properties that accounted for T current in the whole-cell recordings, while the 25-pS channels showed the voltage dependence, the time dependence, and the dihydropyridine sensitivity expected for L-type channels. We conclude that vascular smooth muscle cells contain two types of calcium channels with properties very similar to those described for T- and L-type calcium channels in other cell types; the L current appears to be the predominant current component in whole-cell recordings.