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

M. Kohda, S. Komori, T. Unno and H. Ohashi

1. Cytosolic free Ca²⁺ concentration ([Ca²⁺]_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 µM) 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 µM) abolished these effects of 2 µM CCh. La³⁺ (3 µM) 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 µM) 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 µM).

4. In cells exhibiting no oscillatory response to 0·25-0·75 µM 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⁺]_i or InsP₃ 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⁺]_i and a slow, large membrane depolarization.

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