COEXISTENCE OF ALPHA(1) AND BETA-ADRENERGIC RECEPTORS IN THE LIVER OFTHE FROG RANA-ESCULENTA, THE TOAD BUFO-BUFO, THE LIZARD PODARCIS-SICULA CAMPESTRIS, AND THE TURTLE PSEUDEMYS PICTA ELEGANS

In mammals and birds the characteristics of alpha(1) adrenergic recept ors and their biological role in liver metabolism have been clearly de scribed, although the predominance of receptor subtypes varies with sp ecies. In contrast, the actual presence of hepatic alpha(1) adrenergic receptors in fish, amphibians, and reptiles has been questioned. Only recently has their existence been demonstrated in some fish species a nd also in the wood frog Rana sylvatica. The present study assessed th e presence of oil adrenergic binding sites on hepatic membranes of fro gs, toads, lizards, and turtles using the specific alpha(1), adrenergi c receptor antagonist [H-3]prazosin; for comparison, the binding of th e specific beta adrenergic receptor antagonist [H-3]CGP-12177A was eva luated in the same preparations. alpha(1) Adrenergic receptors are ind eed present in the liver of the ectotherms examined. Specific binding is saturable, reversible, and linear as a function of tissue concentra tion. The binding data indicated the presence of two classes of bindin g sites displaying high and low affinities with K(d)s in the nanomolar and micromolar ranges, respectively. The present study provides the f irst evidence for the presence of alpha(1) adrenergic receptors in the liver of toad, lizard, and turtle while confirming their existence in another species of frog, Rana Esculenta. (C) 1997 Academic Press.