Contractile agonists activate voltage-dependent calcium channels in airway smooth muscle cells

To determine whether agents that cause contraction of airway smooth muscle affect sarcolemmal calcium channel activity, unitary calcium channel currents (using Ba2+ as the charge carrier) were recorded in on-cell configuration from acutely dissociated (dog, pig, and ferret) and cultured (human) airway smooth muscle cells. Addition of the contractile agonists methacholine or bradykinin increased the open-state probability of the large-conductance calcium channel 37.2- and 45-fold, respectively. The increase in open-state probability was not due to cellular depolarization because increases occurred in the absence of depolarization. Channel activation by the agonist was determined to result in the favoring of a long (16.5 +/- 5.0 ms) open lifetime for the channel, which was not observed under control conditions, in the absence of BAY K 8644. We also report the unitary calcium channel currents from a second, smaller conductance calcium channel. This channel was present in all cell types and had a mean conductance of 9.5 +/- 0.8 pS (80 mM Ba2+). Exposure of cells to agonist also resulted in an increase in the open-channel probability of the small-conductance calcium channel (10.4-fold), which did not result from cellular depolarization. These experiments demonstrate that the molecular pathways exist between contractile agonist receptors and sarcolemmal calcium channels in airway smooth muscle cells. Because membrane patches were not directly exposed to agonist, receptor-channel linkage probably occurs via a second messenger-coupling pathway.