Non-selectivity of yohimbine for adrenergic receptors in fish liver

Most studies on adrenergic receptors (AR) have been performed on mammalian tissues, but the adrenergic ligands routinely utilized seem not always suit able for specific interaction with fish tissues. Here we report that in iso lated catfish hepatocytes, yohimbine, usually thought to act as a specific antagonist for AR of the a, subtype, at high concentrations, increases aden ylyl cyclase activity and synergistically enhances the forskolin-induced en zyme stimulation. Such effects are counteracted by the beta-AR antagonist p ropranolol, but not by the alpha-AR antagonist phentolamine. Moreover, yohi mbine seems to antagonize both alpha(1)- and alpha(2)-adrenergic ligand-bin ding in catfish liver membrane in a manner somewhat different from the mamm alian systems. Together with previous evidence that yohimbine blocks the ri se of intracellular calcium induced by epinephrine via alpha(1)-AR, the pre sent results seem to indicate that this compound is not a suitable tool for studying alpha(2)-AR in fish liver. (C) 1999 Elsevier Science Inc. All rig hts reserved.