HORMONAL-REGULATION OF CALCIUM CURRENT IN FRESHLY ISOLATED AIRWAY SMOOTH-MUSCLE CELLS

Single freshly isolated smooth muscle cells of adult bovine trachea were voltage clamped, and the calcium inward current was separated from K+ currents by blocking the large outward currents with intra- and extracellular Cs+ and extracellular tetraethylammonium chloride. Isoproterenol stimulated peak calcium current (I(Ca)) in a dose-dependent manner through the beta-adrenergic receptor. The isoproterenol effect was not mediated or caused by the stimulation of a K+ or Na+ current, a decrease in the intracellular concentrations of Ca2+ or H+, the stimulation of the Na+-H+ or the Na+-Ca2+ exchanger. Neither basal nor isoproterenol-stimulated I(Ca) was affected by internal dialysis of the cell with adenosine 3',5'-cyclic monophosphate (cAMP), cAMP analogues, or the catalytic subunit of cAMP-kinase. Internal dialysis of the cells with guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) blocked the stimulation of isoproterenol whereas dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) induced an isoproterenol-like maximal increase of I(Ca). These results show that the beta-adrenergic receptor stimulates the L-type calcium current of isolated tracheal smooth muscle cells independent of cAMP and cAMP-kinase through a GTP/GDP regulated protein.