Adrenergic signaling and second messenger production in hepatocytes of two fish species

The activation of cAMP and inositol 1,4,5-trisphosphate (IP3) transduction pathways by epinephrine (EPI) has been studied in hepatocytes and hepatic membranes of two teleost species, the American eel (Anguilla rostrata) and the black bullhead (Ictalurus melas). EPI at 10 microM increased both cAMP and IP3 in a dose- and species-dependent manner. The activation of both systems was the greatest in the eel, and the IP3 system was activated at lower EPI concentrations than the cAMP system. The individual transduction pathways were identified by the EPI-induced increase in cAMP being blocked by propranolol (PROP) but not phentolamine (PHT) and the increase in IP3 by PHT but not PROP. alpha1-Adrenoceptors were characterized by the specific binding of [3H]prazosin (PRZ) to purified hepatic membranes: Kd and Bmax values were 3.0 and 2.2 nM and 108 and 178 fmol x mg-1 protein for eel and bullhead, respectively. PRZ binding was displaced by nonradioactive PRZ and PHT but not by PROP. [3H]IP3 bound specifically to microsomal membranes from hepatic tissues of both species and this binding could best be explained by a two-site binding model; significant differences in binding characteristics were seen between eel and bullhead membranes. This study demonstrates that EPI may act on hepatocytes isolated from the American eel and black bullhead through both signal transduction pathways, inducing changes in glucose production and Ca2+ mobilization previously reported in these species. The quantitative differences between the two species in aspects of this system may reflect differences at the level of the coupling between receptor and messenger production, or at the degradation of the messenger, aspects of the transduction pathways which may be either true species differences or related to the conditions under which the species were held.