Effect of activators and blockers of ligand-regulated ion channels on the activity of the Cl-stimulated Mg2+-ATPase of the plasma membrane fraction from bream (Abramis brama L.) brain.

Abstract

Effects of GABA, glycine, acetylcholine, and glutamate (agonists of the GABAa/benzodiazepine, glycine, choline, and glutamate receptors, respectively) at concentrations in the range 10(-8)-10(-4) M on the activity of "basal" Mg(2+)-ATPase of the plasma membrane fraction from bream brain and on its activation by Cl(-) were investigated. GABA and glycine activated "basal" Mg(2+)-ATPase activity and suppressed its activation by Cl(-). Acetylcholine and glutamate activated "basal" Mg(2+)-ATPase to a lesser extent and did not suppress the activation of the enzyme by Cl(-). The activation of "basal" Mg(2+)-ATPase by neuromediators was decreased by blockers of the corresponding receptors (picrotoxin, strychnine, benztropine mesylate, and D-2-amino-5-phosphonovaleric acid). In addition, picrotoxin and strychnine eliminated the inhibiting effect of GABA and glycine, respectively, on the Cl(-)-stimulated Mg(2+)-ATPase activity. Agonists of the GABAa/benzodiazepine receptor--phenazepam (10(-8)-10(-4) M) and pentobarbital (10(-6)-10(-3) M)--activated the "basal" Mg(2+)-ATPase activity and decreased the Cl(-)-stimulated Mg(2+)-ATPase activity. The dependence of both enzyme activities on ligand concentration is bell-shaped. Moreover, phenazepam and pentobarbital increased the "basal" Mg(2+)-ATPase activity in the presence of 10(-7) M GABA and did not influence it in the presence of 10(-4) M GABA and 10(-6) M glycine. The data suggest that in the fish brain membranes the Cl(-)-stimulated Mg(2+)-ATPase interacts with GABAa/benzodiazepine and glycine receptors but not with m-choline and glutamate receptors.