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

This study was designed to examine the roles of intracellular free magnesiu m ion concentration ([Mg2+](i)) in ethanol-induced intoxication and develop ment 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+](i) in cerebrovascular cells was 732.5 +/- 82.4 muM. Exposure of culture d canine cerebral vascular smooth muscle cells to ethanol (10 and 25 mM) fo r 24 h reduced the concentrations of [Mg2+](i) to 521.1 +/- 59.6 muM, and 3 08.2 +/- 37.8 muM, respectively. However, exposure of these cultured vascul ar cells to the same concentrations of ethanol, after initial pretreatment with ethanol for 24 h, failed to interfere with the levels of [Mg2+] Measur ement of [Mg2+](i) at 48 h and 72 h indicated that the decreased levels of [Mg2+](i) induced by ethanol at 24 h treatment returned toward baseline. Si milar experiments were performed in cultured type-2 astrocytes isolated fro m neonatal rat brain. The basal level of [Mg2+](i) in type-2 astrocytes was about 125 muM. Incubation of these cells with 10 mM ethanol for 10 min res ulted in a 27% reduction in the level of [Mg2+](i), whereas incubation with 25 mM ethanol resulted in almost a 50% reduction in [Mg2+]i. The decreased levels of [Mg2+](i) 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+](i) in type-2 astrocytes, were equivalent to those at basal, resting lev els. In vivo P-31-NMR spectroscopy, performed on intact rat brains, indicat ed that an initial administration of 4 mg/kg ethanol (similar to 20-25 mM b lood alcohol level) resulted (after 20-40 min of exposure) in severe defici ts in whole brain [Mg2+](i) (550 +/- 33 muM to 358 +/- 24 muM). Repeated in jections of ethanol (4 mg/kg) over the next 24-72 h resulted in progressive ly diminishing effects on brain [Mg2+](i). These experimental data indicate that chronic ethanol treatment can induce a tolerance to depletion of [Mg2 +](i) in cerebrovascular smooth muscle cells, type-2 astrocytes as well as intact rat brain. The results suggest that [Mg2+](i) might play a major rol e in alcohol-induced tolerance in the brain. (C) 2001 Elsevier Science Inc.