Evidence of an electrogenic sodium pump in molluscan radular muscle cells

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.

Effects of monensin on the response to low potassium in a molluscan muscle, the radular protractor of Busycon canaliculatum

1. A sucrose gap technique was used to study the effect of a sodium ionophore on the potential changes which occur during superfusion with potassium-free solution. Crucial values were checked with a microelectrode technique. 2. Potassium-free solution induces a complex response consisting of a hyperpolarizing phase (HP) and then a depolarization phase (DP) during exposure to zero K+, followed by a transient extra hyperpolarizing phase (EHP) on readmission of K+. 3. The sodium ionophore, monensin, has the effect of increasing the amplitude of the DP, in perfusion medium of normal Na+ content. This is similar to the effect of treatment with neurohumors and to the after-effect of direct electrical stimulation. The effects of ACh and monensin are additive. 5. These effects are consistent with an action of monensin in increasing Na+ flux in the direction of the concentration gradient and support the hypothesis that neurohumors stimulate the sodium pump by increasing Na+ influx.