Characterization of adrenergic receptors and related transduction pathways in the liver of the rainbow trout

Epinephrine (EPI) is thought to act by stimulating adenylyl cyclase (ACase) and cAMP production through beta-adrenoceptors in the liver of more primitive vertebrates, Recent observations, however, point to an involvement of alpha(1)-adrenoceptors in EPI action, at least in some fish species, The role of the alpha(1)- and beta-adrenergic transduction pathways has been investigated in rainbow trout (Oncorhynchus mykiss) hepatic tissue, Radioligand-binding assays with the beta-adrenergic antagonist H-3-CGP-12177 using hepatic membranes purified on a discontinuous sucrose gradient confirmed the presence of beta-adrenoceptors (K-d 0.36 nM, B-max 8.61 fmol . mg(-1) protein). We provide the first demonstration of alpha(1)-adrenoceptors in these same membranes; analysis of binding data with the alpha(1)-adrenergic antagonist H-3-prazosin demonstrated a single class of binding sites with a K-d of 15.4 nM and a B-max of 75.2 fmol . mg(-1) protein. There is a straight correlation between beta-adrenoceptor occupancy, ACase activation and cAMP production. On the contrary, the role of inositol 1,4,5-trisphosphate (IP3) has to be elucidated; in fact, despite the presence of specific microsomal binding sites for IP3 (K-d 6.03 nM, B-max 90.2 fmol . mg(-1) protein), its cytosolic concentration was not modulated by EPI. On the other hand, we have previously shown in American eel and bullhead hepatocytes that alpha(1)-adrenergic agonists are able to increase intracellular concentrations of IP3 and Ca2+ and to activate glycogenolysis. These data suggest a marked variation in the liver of different fish both in terms of alpha(1)-binding sites affinity and of alpha(1)-adrenoceptor/IP3/Ca2+ transduction systems.