MODULATION OF CARBACHOL-INDUCED [CA2-MUSCLE CELLS(](I) OSCILLATIONS BY CA2+ INFLUX IN SINGLE INTESTINAL SMOOTH)

1 Oscillations of cytosolic Ca2+ concentration ([Ca2+](i)) evoked by c arbachol (CCh; 2 mu M), a muscarinic agonist, were detected as oscilla tory changes of muscarinic receptor-coupled cationic current (I-cat) i n guinea-pig ileal smooth muscle cells by the whole cell patch-clamp t echnique. 2 Reduction of extracellular Ca2+ from 2 mM to 0.2 or 0.05 m M, during CCh-induced I-cat oscillations, caused them to disappear or to decrease markedly in frequency. A return to 2 mM Ca2+ concentration restored the initial I-cat oscillations. 3 Application of nifedipine (1-3 mu M) or D600 (2-5 mu M) to block the voltage-gated Ca2+ channel (VGCC) decreased the frequency of the ongoing I-cat oscillations in th e cells held at -20 mV, but it was without effect in cells held at -60 mV. 4 Displacement of the holding potential of -20 mV to -60 mV to de activate VGCC produced a decrease, an increase or no noticeable change in the frequency of the I-cat oscillations in different cells. Displa cement to 20 mV to inactivate VGCC invariably produced a decrease in t he frequency. In nifedipine-treated cells, the I-cat oscillations vari ed in frequency voltage-dependently in a reverse and linear way within the range -80 to 40 mV. 5 Application of thapsigargin (1 or 2 mu M), an inhibitor of Ca2+-ATPase in the membrane of internal Ca2+ stores, c aused CCh-induced I-cat oscillations to disappear with a progressing p hase during which their amplitude, but not frequency, declined. 6 The results suggest that membrane Ca2+ entry has a crucial role to play in regulation of the frequency of CCh-induced [Ca2+](i) oscillations in addition to persistence of their generation, and that the effect is br ought about by a potential mechanism independent of Ca2+ store repleni shment. They also provide evidence that two types of Ca2+ permeant cha nnels, VGCC and an as yet unidentified channel, are involved in the Ca 2+ entry responsible for modulation of [Ca2+](i) oscillations.