Components coupling cholinergic excitation with contraction of smooth muscles of guinea pig taenia coli

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.