Mg2+-dependent phosphatase as an inhibitory mediator of the nonselective cation current induced by aluminum fluoride in guinea-pig chromaffin cells.

Internal administration of the G protein activator, guanosine-5'-o-(3-thiotriphosphate) (GTP gamma S) or aluminum fluoride (AIF) complex, produced an inward nonselective cation current (INS) at -55 mV. This current was rapidly diminished under conditions of high intracellular Mg2+ ([Mg2+] = 979 microM), the half decay time (T1/2) being 80 to 100 s. As [Mg2+] in AlF solutions decreased from 400 to 12 microM, the maximum amplitude of AlF-induced INS became larger and the current was diminished more slowly. The AlF INS in the presence of 12 microM Mg2+ reversed polarity at about +9 mV, irrespective of the extent of decline. Bath application of muscarine produced a sustained INS in the absence of AlF complex, but in its presence, the overall current comprising a spontaneously developed INS and muscarine-induced INS was rapidly diminished. Addition of vanadate (0.5 mM) to 979 microM Mg2+ -containing AlF solution mimicked the effects of low Mg2+ solution. Inversely, addition of alkaline phosphatase (40 units/ml) to 12 microM Mg2+ AlF solution reproduced the effects of high Mg2+ solution. It is suggested that AlF complex deactivates INS through facilitating an apparent activity of Mg2+ -dependent phosphatase.