MODULATION OF MAXI-K-DEPENDENT CA2+ CHANNELS AND METHACHOLINE IN SINGLE AIRWAY MYOCYTES( CHANNELS BY VOLTAGE)

The role of Ca2+ influx through voltage-dependent Ca2+ channels and th e inhibitory effects of methacholine on large-conductance Ca2+-activat ed K+ (K-Ca) channels (maxi-K+ channels) were studied in voltage-clamp ed (nystatin), fura 2-loaded airway smooth muscle cells. Spontaneous t ransient outward currents (STOCs) were strongly coupled to voltage-dep endent Ca2+ channel activity; activity was suppressed by nisoldipine a nd Cd2+ and increased by BAY K 8644 within seconds. Moreover, release of intracellular Ca2+ by caffeine or cyclopiazonic acid only partially suppressed STOCs, and the remainder were almost completely blocked by nisoldipine. Methacholine suppressed STOCs but also significantly dec reased the mean outward current. Whole cell current inhibition was obs erved in the presence of 4-aminopyridine but not in the presence of ch arybdotoxin. Caffeine inhibited STOCs but macroscopic outward currents were not altered. In the continued presence of caffeine, methacholine abolished the remaining STOCs and decreased the mean K+ current. We c onclude that STOCs are activated by influx of Ca2+ through plasmalemma l voltage-dependent Ca2+ channels, as well as by release of Ca2+ from intracellular stores, and muscarinic stimulation depresses the mean K- Ca current via a pathway independent of the depletion of intracellular Ca2+ stores.