Abstract: | Acetylcholine (AC) in concentrations of 10?9–10?7 g/ml increases the smooth-muscle tone of the guinea pig taenia coli by increasing the permeability of the cell membranes to inward flows of Ca45 ions. In concentrations of 10?6 g/ml or higher AC induces the liberation of membrane calcium and, in concentrations of 10?5–10?3 g/ml, it significantly increases the membrane permeability of the smooth-muscle cells to Na22 ions, causing depolarization and an increase in action potential frequency. It is postulated that the mechanism coupling the cholinergic stimulus with the final effect (muscle contraction) includes three components: increased entry of Ca++ into the smooth-muscle cells, liberation of membrane calcium, and a spike mechanism. |