CARBACHOL MODULATES VOLTAGE SENSITIVITY OF CALCIUM CHANNELS IN BRONCHIAL SMOOTH-MUSCLE OF RATS

The role of voltage-dependent Ca channels in carbachol (CCh)-induced contraction of rat bronchus was investigated. Membrane depolarization and BAY K 8644, a Ca channel opener, significantly enhanced CCh-induced contractions. Nisoldipine, an organic Ca channel blocker, significantly inhibited the contractions. Cadmium, an inorganic Ca channel blocker, completely inhibited maintained contractions caused by CCh. These results suggested that the voltage-dependent Ca channels play an important role in sustained cholinergic contractions. This hypothesis was tested further by investigating the properties of single Ca channels of rat bronchus smooth muscle cells. We used 10 mM Ba as the charge carrier and BAY K 8644 to increase open times. The single-channel conductance was 16.8 pS. Steady-state open probability (NP(o)) increased steeply with membrane depolarization (e-fold for 4 mV). The primary effect of CCh (10-mu-M) on Ca channels was to shift the membrane potential at which NP(o) was half maximal from -34 to -43 mV without changing the steepness factor or maximal NP(o). This CCh-induced increase in NP(o) was not caused by depolarization, because the single-channel current amplitude was unchanged by CCh. We conclude that one of the mechanisms by which CCh opens Ca channels of rat bronchus smooth muscle is by shifting the activation curve in the hyperpolarized direction.