IN-VIVO AND IN-VITRO EFFECTS OF PHENYTOIN (PHT) ON ATPASES AND [C-14]PHT BINDING IN SYNAPTOSOMES AND MITOCHONDRIA FROM RAT CEREBRAL-CORTEX

The effect of phenytoin (PHT) on Na+-K+-ATPase and Mg2+-ATPase activit ies and on [C-14]-PHT binding in vitro to synaptosomal and mitochondri al subcellular fractions from rat cerebral cortex was studied after ch ronic PHT treatment. Synaptosomal and mitochondrial fractions were cha racterized with plasma membrane and mitochondrial enzymatic markers. S ynaptosomal Na+-K+-ATPase was not affected in vitro by PHT 1-200 mu M or by chronic treatment with 2-50 mg/kg/day of the unlabeled drug for 8 days. Mitochondrial Mg2+-ATPase was significantly stimulated by PHT after chronic treatment with 5 mg/kg/day for 8 days; reaching maximal effect (76%), at 10-25 mg/kg. PHT had no effect on mitochondrial Mg2+- ATPase when added in vitro. [C-14]-PHT binding in vitro to the subcell ular fractions was determined by dialysis to assess in vivo binding of the unlabeled PHT during chronic treatment. Indeed, [C-14]-PHT bound to synaptosomes was significantly reduced by chronic PHT treatment fro m 218 +/- 10 to 119 +/- 11 pmol/mg protein after 1 week of treatment; a similar effect was obtained after 2-3 weeks with 10 mg/kg/day. Mitoc hondrial fraction bound 117 +/- 10 pmol/mg protein labeled PHT. Chroni c treatment with unlabeled PHT also reduced the amount of [C-14]-PHT b ound to 19.9 +/- 2.2 pmol/mg protein. These results show slow reversib le PHT in vivo binding to synaptosomes and mitochondrias from rat cere bral cortex, supporting the idea that the modulatory action of PHT on Na+ and Ca2+ permeabilities are mediated through these slow reversible binding proteins. The data also suggest a possible role of intrasynap tosomal mitochondria in [Ca2+](i) buffering.