INTRASYNAPTOSOMAL FREE CALCIUM AND NITRIC-OXIDE METABOLISM IN CENTRAL-NERVOUS-SYSTEM OXYGEN-TOXICITY

Background: Central nervous system (CNS) oxygen (O-2) toxicity is comp lex, and the etiology of its most severe manifestation, O-2 convulsion s, is yet to be determined. A role for nitric oxide (NO) has been prop osed, although recent data have indicated that NO is synthesized from L-arginine by an enzyme, NO synthase (NOS). The enzyme is dependent on free calcium (Ca2+) concentration, therefore increases in intracellul ar Ca2+ may constitute the physiological and pathophysiological mechan isms for stimulating the synthesis of NO. Methods: In this study, the intrasynaptosomal free calcium concentration ([Ca2+](i)) was measured by the fluorescence of fura-2/AM, and cGMP (as an indirect marker of N O levels) was by radioimmunoassay (RIA) in the rat hippocampus after h yperbaric oxygen (HBO) exposure. We also investigated the effects of d aurisoline (DSL, calcium channel blocker) and N-nitro-L-arginine (LNNA , NOS inhibitor) on the above biochemical parameters and the developme nt of oxygen toxicity. Results: The results show that when the rats we re exposed to HBO at 0.5 MPa the intrasynaptosomal Ca2+ and cGMP level s increased by two and three times, respectively, whereas with the use of DSL prior to HBO, the accumulation of [Ca2+](i) and cGMP dropped t o 56% and 60%, correspondingly. In the rats medicated with LNNA prior to HBO. [Ca2+](i) and cGMP levels dropped to 70% and 36% of the HBO gr oup. At the same time, the appearance of CNS oxygen toxicity was delay ed and the survival rate increased. The protective effects of LNNA wer e reversed by L-arginine pretreatment. These findings suggest that the neuronal Ca2+ overload during HBO exposure is a major factor in the p athogenesis of CNS O-2 toxicity, and cGMP-NO pathways may be directly involved in HBO-induced seizures.