Muscarinic receptor binding and Ca2+ influx in the all-or-none response to acetylcholine of isolated smooth muscle cells.

Isolated smooth muscle cells from guinea pig taenia caecum were prepared by collagenase digestion. Isolated single smooth muscle cells showed an all-or-none response to acetylcholine (ACh) under our experimental conditions. A Scatchard plot of the specific binding of [3H]quinuclidinyl benzilate (QNB) to the cells gave a straight line, and the KD and Bmax values were calculated as 0.18 +/- 0.03 nM and 1.11 +/- 0.10 pmol/mg protein, respectively. ACh competitively inhibited the specific binding of [3H]QNB in a concentration-dependent manner. Thus, although the cells showed an all-or-none response, ACh bound to the receptor concentration dependently. The contraction of the cells in response to ACh was inhibited in a concentration-dependent manner, by nicardipine suggesting that the contraction of smooth muscle cells in response to ACh depended on the influx of extracellular Ca2+. The ACh-stimulated increase of the Ca2+ influx was very rapid and correlated well with the contraction of the cells; it decreased after reaching a maximum. The all-or-none response of the cells could be due to a rapid influx of Ca2+. These results suggest that ACh binds to its receptor in a concentration-dependent manner, and that when the binding reaches a certain threshold, a rapid influx of Ca2+ occurs and the cells show an all-or-none response, followed by an efflux of Ca2+.