Importance of magnesium ions in development of tolerance to ethanol: studies on cultured cerebral vascular smooth muscle cells, type-2 astrocytes and intact rat brain.

This study was designed to examine the roles of intracellular free magnesium ion concentration (Mg(2+)) in ethanol-induced intoxication and development of tolerance in cultured canine cerebral vascular smooth muscle cells and astrocytes as well as intact rat brain. The basal, resting level of Mg(2+) in cerebrovascular cells was 732.5 +/- 82.4 microM. Exposure of cultured canine cerebral vascular smooth muscle cells to ethanol (10 and 25 mM) for 24 h reduced the concentrations of Mg(2+) to 521.1 +/- 59.6 microM, and 308.2 +/- 37.8 microM, respectively. However, exposure of these cultured vascular cells to the same concentrations of ethanol, after initial pretreatment with ethanol for 24 h, failed to interfere with the levels of Mg(2+). Measurement of Mg(2+) at 48 h and 72 h indicated that the decreased levels of Mg(2+) induced by ethanol at 24 h treatment returned toward baseline. Similar experiments were performed in cultured type-2 astrocytes isolated from neonatal rat brain. The basal level of Mg(2+) in type-2 astrocytes was about 125 microM. Incubation of these cells with 10 mM ethanol for 10 min resulted in a 27% reduction in the level of Mg(2+), whereas incubation with 25 mM ethanol resulted in almost a 50% reduction in Mg(2+). The decreased levels of Mg(2+) lasted around 30 min, until the measurement finished. Continuous incubation of these cultured astrocytes, with ethanol (either 10 mM or 25 mM), for more than 24 h, indicated that the concentrations of Mg(2+) in type-2 astrocytes were equivalent to those at basal, resting levels. In vivo 31P-NMR spectroscopy, performed on intact rat brains, indicated that an initial administration of 4 mg/kg ethanol ( approximately 20-25 mM blood alcohol level) resulted (after 20-40 min of exposure) in severe deficits in whole brain Mg(2+) (550 +/- 33 microM to 358 +/- 24 microM). Repeated injections of ethanol (4 mg/kg) over the next 24-72 h resulted in progressively diminishing effects on brain Mg(2+). These experimental data indicate that chronic ethanol treatment can induce a tolerance to depletion of Mg(2+) in cerebrovascular smooth muscle cells, type-2 astrocytes as well as intact rat brain. The results suggest that Mg(2+) might play a major role in alcohol-induced tolerance in the brain.