Neurohumoral control of ionic pumping in molluscan muscle: III. Cholinergic control of the response to low potassium levels.

1. Isolated radular protractor muscles (RPM) of Busycon canaliculatum show a complex response to potassium-free solution. 2. While exposed to a bathing solution lacking K+, the RPM begins to respond with a hyperpolarizing phase. Within minutes, this gives way to a depolarizing phase (DP). After readmission of a bathing solution containing K+, there is an extra hyperpolarizing phase. 3. Acetylcholine over a range of concentrations from 10(-8)M to 10(-3)M progressively enhances the DP. 4. The action of acetylcholine is sodium dependent.