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 suitable for specific interaction with fish tissues. Here we report that in isolated catfish hepatocytes, yohimbine, usually thought to act as a specific antagonist for AR of the alpha2 subtype, at high concentrations, increases adenylyl cyclase activity and synergistically enhances the forskolin-induced enzyme stimulation. Such effects are counteracted by the beta-AR antagonist propranolol, but not by the alpha-AR antagonist phentolamine. Moreover, yohimbine seems to antagonize both alpha1- and alpha2-adrenergic ligand-binding in catfish liver membrane in a manner somewhat different from the mammalian systems. Together with previous evidence that yohimbine blocks the rise of intracellular calcium induced by epinephrine via alpha1-AR, the present results seem to indicate that this compound is not a suitable tool for studying alpha2-AR in fish liver.