| Description |
Glucagon and PGEs stimulated adenylate cyclase from plasma membrane enriched fractions of whole rat liver. Significant stimulation by PGEs is dependent on the presence of guanosine-5'-triphosphate (GTP). Guanosine-5'-(beta,y-mino)-triphosphate (GMPPNP) stimulated adenylate cyclase itself, but PGE1 did not stimulate in the presence of optimal or suboptimal concentrations of this GTP analog. In contrast, glucagon caused additional stimulation when optimal concentrations of GMPPNP were included. Prostaglandins F1a, and F2a had no effect on adenylate cyclase activity with or without the inclusion of GTP. Hepatocytes and endothelial cells were separated from rat liver after perfusion with digestive enzymes. Hepatocyte adenylate cyclase was totally stimulated by glucagon but was relatively insensitive to PGE1. Adenylate cyclase from endothelial cells was stimulated by both glucagon and PGE1. The subcellular localization of guanylate cyclase was examined in rat liver. About 85% of the enzyme activity of homogenates was found in the soluble fraction (104,000 x g supernatant). Particulate guanylate cyclase was localized in plasma membranes, mitochondria, and microsomes. Triton X-100 stimulated particulate enzyme activity 3.7-fold; whereas it stimulated soluble enzyme activity 1.5-fold. Optimal MnCL2 concentration was 5 mM when substrate GTP concentration was 1.0 mM. Sodium azide stimulated the particulate and soluble enzyme approximately 5-fcid and 23-fold, respectively. AMP, GDP, ADP, and ATP Inhibited soluble guanylate activity with increasing order of potency, respectively. Neither PGE1, nor PGF1a had any effect on enzyme activity. PGE1, or PGF1a, also did not reverse guanylate cyclase inhibition by ATP. Guanylate cyclase from whole homogenates of isolated rat liver cells was studied. Guanylate cyclase from all cell fractions was stimulated by sodium azide but was insensitive to acetylcholine, CaCL2, and secretin. The hepatocyte wash" fraction (phagocytically inactive endothelial cells, erythrocytes, and other cell debris) contained the highest basal and sodium azide stimulated enzyme activities, compared with enzyme activity from hepatocytes and Kupffer cells. Glucagon stimulated both cyclic AMP accumulation and glycogenolysis in isolated hepatocytes, liver slices, and perfused liver. PGE1, had no effect on cyclic AMP and glucose production in hepatocytes but stimulated cyclic AMP accumulation in liver slices and perfused liver with no apparent effect on glycogenolysis. PGE1, arachidonic acid, and indomethacin did not effect glucagon mediated increases in cyclic AMP and glucose production in hepatocytes. Arachidonic acid increased PGE levels 1 .5-fold in isolated hepatocytes, and this increase was inhibited by the addition of cyclic AMP or Indomethacin, Glucagon, by itself or with arachidonic acid, did not effect PGE biosynthesis in hepatocytes. The data are interpreted to indicate that PGEs stimulate adenylate cyclase from endothelial cells, with no apparent effect on glucose and cyclic AMP production in hepatocytes. Also, non-parenchymal cells contain high adenylate and guanylate cyclase specific activities, as compared to enzymes from hepatocytes. PGE biosynthesis in hepatocytes may be regulated by cyclic AMP at a point below substrate (arachidonic acid) availability, since cyclic AMP inhibited PGE biosynthesis in both the absence and presence of exogeneous arachidonic acid. |
