Mg 2+-dependent phosphatase as an inhibitory mediator of the nonselective cation current induced by aluminum fluoride in guinea-pig chromaffin cells

Abstract

Internal administration of the G protein activator, guanosine-5′- o-(3-thiotriphosphate) (GTPγS) or aluminum fluoride (AlF) complex, produce an inward nonselective cation current ( I NS) at −55 mV. This current was rapidly diminished under conditions of high intracellular Mg 2+ ([Mg 2+] = 979 μM), the half decay time ( T 1 2 ) being 80 to 100 s. As [Mg 2+] in AlF solutions decreased from 400 to 12 μM, the maximum amplitude of AlF-induced I NS became larger and the current was diminished more slowly. The AlF I NS in the presence of 12 μM Mg 2+ reversed polarity at about + 9 mV, irrespective of the extent of decline. Bath application of muscarine produced a sustained I NS was rapidly diminished. Addition of vanadate (0.5 mM) to 979 μM Mg 2+-containing AlF solution mimicked the effects of low Mg 2+ solution. Inversely, addition of alkaline phosphatase (40 units/ml) to 12 μM Mg 2+ AlF solution reproduced the effects of high Mg 2+ solution. It is suggested that AlF complex deactivates I NS through facilitating an apparent activity of Mg 2+-dependent phosphatase.