Coexistence of α1and β Adrenergic Receptors in the Liver of the FrogRana esculenta,the ToadBufo bufo,the LizardPodarcis sicula campestris,and the TurtlePseudemys picta elegans ☆

Abstract In mammals and birds the characteristics of α 1 adrenergic receptors and their biological role in liver metabolism have been clearly described, although the predominance of receptor subtypes varies with species. In contrast, the actual presence of hepatic α 1 adrenergic receptors in fish, amphibians, and reptiles has been questioned. Only recently has their existence been demonstrated in some fish species and also in the wood frog Rana sylvatica. The present study assessed the presence of α 1 adrenergic binding sites on hepatic membranes of frogs, toads, lizards, and turtles using the specific α 1 adrenergic receptor antagonist [ 3 H]prazosin; for comparison, the binding of the specific β adrenergic receptor antagonist [ 3 H]CGP-12177A was evaluated in the same preparations. α 1 Adrenergic receptors are indeed present in the liver of the ectotherms examined. Specific binding is saturable, reversible, and linear as a function of tissue concentration. The binding data indicated the presence of two classes of binding sites displaying high and low affinities with K d s in the nanomolar and micromolar ranges, respectively. The present study provides the first evidence for the presence of α 1 adrenergic receptors in the liver of toad, lizard, and turtle while confirming their existence in another species of frog, Rana esculenta.