Effect of Ca2+ deprivation on short-term desensitization of isolated smooth muscle cells showing an all-or-none response to acetylcholine.

1. Short-term desensitization of the contractile response of isolated smooth muscle cells from guinea pig taenia caecum to acetylcholine was examined in the absence of external Ca2+. 2. Isolated smooth muscle cells contracted to 10(-7)-10(-6) M acetylcholine, showing an all-or-none response. 3. Desensitization was induced by incubating the cells with 10(-4) M acetylcholine for 10 min in Ca2+-free solution. 4. Desensitized cells showed an all-or-none response but required a higher concentration of acetylcholine for induction of contraction, indicating that the desensitization was due to a change in the threshold concentration. 5. Desensitization is suggested to be induced before massive influx of Ca2+ into the cells.