Targets of octopamine action in the lobster: cyclic nucleotide changes and physiological effects in hemolymph, heart and exoskeletal muscle.

We have examined three potential targets of octopamine action of the lobster--hemolymph, heart and exoskeletal muscle--and found that each tissue responds to the amine with increases in cyclic adenosine 3': 5'-monophosphate (cAMP) and changes in measurable physiological phenomena. A highly specific octopamine-stimulated increase in cAMP is observed in hemolymph, and this effect is probably due to the presence of a unique octopamine receptor on hematocytes. Pharmacological studies show that synephrine is a potent agonist of the hemolymph receptor for octopamine and that the alpha adrenergic receptor blockers phentolamine and dibenamine and the dopamine receptor blocker, chlorpromazine, are antagonists. Addition of octopamine to lobster hemolymph increases the rate of changes the nature of the clotting reaction. AMP levels in lobster heart are elevated by both octopamine and serotonin. Both amines also increase the rate and the amplitude of the heart beat. Octopamine, serotonin and dopamine all increase cAMP levels in lobster exoskeletal muscle preparations, and each amine causes changes in muscle tension and the strength of nerve evoked contractions. In none of the tissues studied have we been able to establish a direct relationship between amine-stimulated increases in cAMP levels and the physiological responses.