beta-adrenergic receptor subtypes in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus

The subtype of beta-adrenergic receptors in melanophores of the marine gobi es Tridentiger trigonocephalus and Chasmichthys gulosus was studied. Pigmen t of denervated melanophores in isolated, split caudal fins was preliminari ly aggregated by incubating the specimens in a physiological saline contain ing 10 mu M phentolamine and 30-100 mu M verapamil or 2-10 nM melatonin, an d the responses of the melanophores to a beta-adrenergic agonist added to t he incubating medium were recorded photoelectrically. The beta-adrenergic a gonists noradrenaline, adrenaline, isoproterenol, salbutamol and, dobutamin e were all effective in evoking a dispersion of melanophore pigment in the presence of phentolamine and verapamil or melatonin. The pigment-dispersing effect of noradrenaline (beta(1)-selective agonist) was inhibited by metop rolol (beta(1)-selective antagonist), propranolol, and butoxamine. Whereas, the effect of salbutamol (beta(2)-selective agonist) was hardly inhibited by metoprolol, though it was considerably inhibited by propranolol and ICI- 118551. It was estimated that beta(1)- and beta(2)-adrenergic receptors coe xist at ratios of 8.6:91.4, in the melanophore of Tridentiger trigonocephal us, and 25:75, in the melanophore of Chasmichthys gulosus, through the anal yses of Hofstee plots of the effects of the beta-adrenergic drugs. It was s uggested that the relation between the pigment-dispersing effect of a beta- adrenergic agonist on the melanophores and the concentration of the drug fo llows mass action kinetics, when the effect is mainly caused by the activat ion of beta(2)-adrenergic receptors of the melanophores, However, when it i s mainly caused by the activation of beta(1)-adrenergic receptors of the me lanophores, the relation does not follow mass action kinetics.