Carbachol-induced oscillations in membrane potential and [Ca2+]i in guinea-pig ileal smooth muscle cells

1. Cytosolic free Ca2+ concentration ([Cl2+](i)) and membrane potential were simultaneously recorded from single smooth muscle cells of guinea-pig ileum, using a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques. 2. Carbachol (CCh, 2 mu M) produced. oscillatory changes in [Ca2+](i) and membrane potential which coincided well in time with each other, and peaks of membrane potential oscillations reached a saturated level of around -7 mV. Thapsigargin (1 mu M) abolished these effects of 2 mu M CCh. La3+ (3 mu M) immediately prevented the discharge of spike potentials, but allowed both on-going oscillatory responses to persist for a while. 3. CCh (0.25-0.75 mu M) caused membrane potential and [Ca2+](i) to oscillate in some 20% of cells studied. Every membrane potential oscillation was preceded by the discharge of single or multiple spike potentials. The effects of CCh were readily abolished by La3+ (3 mu M). 4. In cells exhibiting no oscillatory response to 0.25-0.75 mu M CCh, an electrically evoked action potential usually generated changes in [Ca2+](i) and membrane potential similar to those following spontaneously evoked action potentials, and sometimes it did so only after [Ca2+], or InsP(3) had been slightly elevated by repeatedly evoking action potentials or by increasing CCh concentration in the bath medium. 5. The results suggest that in ileal smooth muscle cells,the oscillations of [Ca2+](i) and membrane potential arising from muscarinic stimulation result from release of Ca2+ from internal stores and that there is a Ca2+-induced potentiation of coincidently elicited cation channel openings. Under weak muscarinic stimulation, Ca2+ entry upon action potential discharge can trigger such a release of stored Ca2+, resulting in synchronous generation of a large rise in [Ca2+], and a slow, large membrane depolarization.