MUSCARINIC RECEPTOR-BINDING AND CA-2+ 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 inhiibited in a concentration-dependent manner, by nicardipine suggesting that the contraction of smooth muscle cells in response to ACh depended on the inlux 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+.