Effects of endothelin-1 and nitric oxide on glucokinase activity in isolated rat hepatocytes

To test the hypothesis that endothelin-1 (ET-1) and nitric oxide (NO) influ ence glucokinase (GK) activity in an opposite manner, we evaluated the effe cts of ET-1, L-NAME, an inhibitor of NO synthase, and L-arginine, a substra te for NO synthase, on GK activity and glycogen content in isolated rat hep atocytes. Moreover, to understand the receptor involved in the process, the effects of BQ 788, a specific antagonist of ETB receptor, and PD 142893, a n antagonist of ETA-ETB receptors, were also evaluated. GK activity, cyclic guanosine monophosphate (cGMP), and glycogen intracellular content were me asured on isolated hepatocytes, while glucose levels and NO as NO2-/NO3- we re determined in the medium. High ET-1 levels induced a 20% decrease of NO2 -/NO3- levels and cGMP intracellular content, followed by a 49% reduction o f GK activity and a 15% decrease of glycogen. In parallel, a 10% increase o f glucose in the medium was observed. In the presence of L-NAME, GK activit y and glycogen levels showed analogous decrements as observed with ET-1. Al so in this case, a significant decrease of the intracellular content of cGM P was observed. No synergistic effects of ET-1 and L-NAME were observed. L- Arginine was able to counteract the inhibitory effect of ET-1 on cGMP and G K activity. Glycogen content was slightly but not significantly reduced, an d under those conditions, a significant decrease of glucose in the medium w as observed. When hepatocytes were incubated with ET-1 plus BQ 788 or ET-1 plus PO 142893, GK activity was unchanged. Interestingly, no changes were o bserved in NO2-/NO3- levels and the intracellular content of cGMP was not m odified when the antagonists of ET-1 receptors were added to the medium. In conclusion, the present study shows that the NO pathway seems to be an imp ortant regulator of GK activity and glycogen content through cGMP activity. In addition, ET-1 seems to be not active per se, but its activity seems me diated by a simultaneous decrease of NO levels. Copyright (C) 2000 by W.B. Saunders Company.