Coexistence of alpha1 and beta adrenergic receptors in the liver of the frog Rana esculenta, the toad Bufo bufo, the lizard Podarcis sicula campestris, and the turtle Pseudemys picta elegans

Abstract

In mammals and birds the characteristics of alpha1 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 alpha1 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 alpha1 adrenergic binding sites on hepatic membranes of frogs, toads, lizards, and turtles using the specific alpha1 adrenergic receptor antagonist [3H]prazosin; for comparison, the binding of the specific beta adrenergic receptor antagonist [3H]CGP-12177A was evaluated in the same preparations. alpha1 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 Kds in the nanomolar and micromolar ranges, respectively. The present study provides the first evidence for the presence of alpha1 adrenergic receptors in the liver of toad, lizard, and turtle while confirming their existence in another species of frog, Rana esculenta.