Inhibition of fast- and slow-phase depolarization-dependent synaptosomal calcium uptake by ethanol

Uptake of 45Ca++ by synaptosomes isolated from cerebral cortex, cerebellum, midbrain and brain stem of male, Sprague-Dawley rats was measured at 1-, 3-, 5-, 15-, 30- and 60-sec time periods. At 1 sec, the Ca++ uptake rate by cerebrocortical synaptosomes was 1.45 mumol/sec/g of protein, whereas the 60-sec rate was 0.03 mumol/sec/g of protein. In vitro addition of ethanol, 80 mM, inhibited depolarization-dependent (65 mM KCl) 45Ca++ uptake by synaptosomes but the time-response relationships varied depending upon the brain region studied. In cerebrocortical synaptosomes, ethanol significantly inhibited only the fast-phase component of 45Ca++ uptake (1 and 3 sec). Ethanol inhibited 45Ca++ uptake by midbrain synaptosomes at all measurement times studied (1, 3, 5 and 15 sec), whereas in cerebellum and brain stem ethanol inhibited 45Ca++ uptake at 3- and 5-sec time periods. Ethanol at concentrations of 25, 50, 100 and 150 mM inhibited 45Ca++ uptake by 9.0, 15.9, 24.8 and 30.7%, respectively, in cerebrocortical synaptosomes. In vitro ethanol, 80 mM, added to cerebrocortical synaptosomes isolated from rats fed a nutritionally adequate liquid ethanol diet did not significantly inhibit depolarization-dependent 45Ca++ uptake. The results of this study show that pharmacologically relevant ethanol concentrations inhibit voltage-dependent 45Ca++ uptake into synaptosomes. This inhibitory action may, at least in part, underlie some of the intoxicating effects of ethanol. In addition, chronic administration of ethanol resulted in an apparent adaptive response such that addition of ethanol no longer blocked 45Ca++ uptake. This adaptive response involving the calcium channel may represent a cellular mechanism for functional tolerance development.