Portacaval Shunting and Hyperammonemia Stimulate the Uptake of l-[3H]Arginine but Not of l-[3H]Nitroarginine into Rat Brain Synaptosomes

Abstract: Elevated activities of nitric oxide synthase (NOS) have been reported previously in the brains of portacaval-shunted (PCS) rats, a model of chronic hepatic encephalopathy (HE). As l -arginine availability for nitric oxide synthesis depends on a specific uptake mechanism in neurons, we studied the kinetics of l -³H]-arginine uptake into synaptosomes prepared from the brains of PCS rats. Results demonstrate that l -arginine uptake is significantly increased in cerebellum (60%; p < 0.01), cerebral cortex (42%; p < 0.01), hippocampus (56%; p < 0.01), and striatum (51%; p < 0.01) of PCS rats compared with sham-operated controls. Hyperammonemia in the absence of portacaval shunting also stimulated the transport of l -³H]arginine; kinetic analysis revealed that the elevated uptake was due to increased uptake capacity ( V _max) without any change in affinity ( K _m). Incubation of cerebellar synaptosomes with ammonium acetate for 10 min caused a dose-dependent stimulation of l -³H]arginine uptake. Neither portacaval shunting nor hyperammonemia had any significant effect on the synaptosomal uptake of N ^G-nitro- l -³H]arginine. These studies demonstrate that increased NOS activity observed in experimental HE may result from increased availability of l -arginine resulting from a direct stimulatory effect of ammonia on l -arginine transport.