AMMONIA AND MANGANESE INCREASE ARGININE UPTAKE IN CULTURED ASTROCYTES

Recent work has suggested a possible role for nitric oxide (NO) in the development of hepatic encephalopathy (HE). In this study, we examine d the effect of ammonia and manganese, factors implicated in the patho genesis of HE, on the transport of arginine (a precursor of NO) into p rimary cultures of astrocytes. Treatment with 5 mM ammonia for 1-4 day s produced a maximal (53%) increase in L-arginine uptake at 3 days whe n compared to untreated cells. Kinetic analysis following 4-day treatm ent with 5 mM ammonia revealed an 82% increase in the V-max and a 61% increase in the K-m value. similar analysis with 100 mu M manganese sh owed a 101% increase in V-max and a 131% increase in the K-m value. Th ese results suggest that both manganese and ammonia alter L-arginine u ptake by modifying the transporter for arginine. A decrease of 32% in the non-saturable component of L-arginine transport was also observed following treatment with ammonia. When cultures were treated separatel y with 5 mM ammonia and 100 mu M manganese for 2 days, the uptake of L -arginine increased by 41% and 57%, respectively. Combined exposure le d to no further increase in uptake. Our results suggest that ammonia a nd manganese may contribute to the pathogenesis of HE by influencing a rginine transport and thus possibly NO synthesis in astrocytes.