Octopamine-mediated elevation of cyclic AMP in haemocytes of the American cockroach, Periplaneta americana L.

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Abstract
The injection of 10 μL of 5 × 10−3 M octopamine into the haemocoel of adult male Periplaneta americana results in a 20 × increase in haemolymph cyclic AMP (cAMP) levels within 3 min. Synephrine also causes a marked increase in haemolymph cAMP, and less pronounced elevations were obtained following the injection of tyramine, dopamine, norepinephrine, 5-hydroxytryptamine, phenylethanolamine, β-phenylethylamine, and L-phenylephrine. The octopamine effect is time dependent for at least 10 min and dose dependent with the EC50 for the injected dose calculated to be about 2.5 × 10−3 M. These results indicate that the octopamine response is receptor mediated and studies on isolated haemocytes suggest that the octopamine-sensitive receptors are located on haemocytes. Incubation of whole haemocytes in medium containing octopamine results in a dose-dependent elevation of cAMP with the EC50 calculated at about 7.5 × 10−6 M. Synephrine, tyramine, and dopamine also elevate cAMP levels in incubated haemocytes, and the activator of adenylate cyclase, forskolin, causes a marked increase in cAMP. The octopamine-mediated response is blocked by mianserin, phentolamine, and cyproheptadine but not by the β-adrenergic blocking agents propranolol and dichloroisoproterenol.
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