MECHANISM AND REGULATION OF SWELLING-ACTIVATED INOSITOL EFFLUX IN BRAIN GLIAL-CELLS

Rat C6 glioma cells chronically acclimated to hypertonic media accumulate large quantities of inositol. When returned to isotonic conditions, the cells swell and lose inositol slowly via a four- to fivefold increase in the rate of passive inositol efflux. The inositol efflux pathway is a Na+-independent transport mechanism with low affinity for inositol and is inhibited by quinidine, quinine, various anion transport blockers, and cis-unsaturated fatty acids. Ionomycin-induced elevation of intracellular Ca2+ (Ca(i)2+) had no effect on basal or swelling-induced inositol efflux. Inositol efflux was not inhibited by chelation of Ca(i)2+ with 1,2-bis(2-aminophenoxy)-N,N',N'-tetraacetic acid. In addition, Ca(i)2+ measured with fura 2 did not change during cell swelling, indicating that increases in Ca(i)2+ do not regulate inositol efflux. Exposure of C6 cells to 20 nM phorbol 12-myristate 13-acetate, 0.5 mM adenosine 3',5'-cyclic monophosphate (cAMP), or 50 muM forskolin had no effect on basal inositol efflux but stimulated swelling-induced inositol loss by 2.6-, 2.2-, and 3.4-fold respectively. Exposure to the protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine or staurosporine or downregulation of protein kinase C (PKC) activity, however, had no inhibitory effect on inositol efflux, and cellular cAMP levels were not altered by cell swelling. Taken together, these results indicate that stimulation of PKC and protein kinase A modulates the activity of the efflux pathway but is not required for swelling-induced activation. Ketoconazole, cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate, and gossypol, inhibitors of lipoxygenase enzymes, blocked both basal and swelling-induced inositol efflux, suggesting indirectly that lipoxygenase metabolites may be responsible for swelling-induced activation of the efflux mechanism. The characterisitics of inositol efflux in C6 cells are similar to those described for volume regulatory sorbitol and taurine efflux in a number of cell types, suggesting the existence of a common transport mechanism.