INTRASYNAPTOSOMAL FREE MG2-FURA-2 - MODULATION BY NA+ GRADIENT AND BYEXTRASYNAPTOSOMAL ATP( CONCENTRATION MEASURED WITH THE FLUORESCENT INDICATOR MAG)

Synaptosomes can be loaded with mag-fura-2 without significant perturb ation of their ATP content by incubation for 10 min at 37 degrees C wi th 10 mu M mag-fura-2 acetoxymethyl ester in Hanks'-HEPES buffer (pH 7 .45). The intrasynaptosomal free Mg2+ concentration ([Mg2+](i)) was fo und to be dependent on external Mg2+ concentration, increasing from 0. 8 to 1.25 mM when the concentration of Mg2+ in the incubation medium i ncreased from 1 to 8 mM. Dissipation of the Na+ gradient across the pl asma membrane of synaptosomes by treatment with the Na+ ionophore mone nsin (0.2 mM) or with veratridine (0.2 mM) and ouabain (0.6 mM) produc ed a moderate increase of [Mg2+](i), from 1.0 to 1.2-1.3 mM in an incu bation medium containing 5 mM Mg2+, Plasma membrane depolarization by incubation of synaptosomes in a medium containing 68 mM KCl and 68 mM NaCl had no effect on [Mg2+](i). Reversal of the Na+ gradient by incub ation of synaptosomes in a medium in which external Na+ was replaced b y choline increased [Mg2+](i) up to 1.6 and 2.2 mM for extrasynaptosom al Mg2+ concentrations of 1 and 8 mM, respectively. We conclude that a Na+/Mg2+ exchange operates in the plasma membrane of synaptosomes. In the presence of Mg2+ in the incubation medium, extrasynaptosomal ATP, but not ADP or adenosine, increased [Mg2+](i) from 1.1 +/- 0.1 up to 1.6 +/- 0.1 mM. The nonhydrolyzable ATP analogue adenosine 5'-(beta ga mma-imido)triphosphate antagonized the effect of ATP, but had no effec t by itself on [Mg2+](i). It is concluded that Mg2+ transport across t he plasma membrane of synaptosomes is modulated by the activity of an ecto-ATPase or an ecto-protein kinase.