Effects of D- and L-octopamine and of pharmacological agents on the metabolism of locust flight muscle.

The monoamine octopamine has been shown to be widely distributed in the nervous tissues of invertebrates (see Robertson & Juorio, 1976), and some of its effects resemble those of catecholamines in vertebrates. Previous experiments (Candy, 1978) have shown that the oxidation of substrates by perfused locust flight muscle is stimulated by DL-octopamine. However, it was not established whether the octopamine receptors were specific for the Dor the L-form of the amine. Since locust haemolymph has now been shown to contain the o-isomer (Goosey & Candy, 1980), it was important to establish whether the muscles respond to D-octopamine. Results of experiments designed to test this are reported in the present communication. Octapamine also has a modulatory action on the myogenic rhythm of locust leg muscle (Evans & OShea, 1978) and can stimulate adenylate cyclase from lobster blood cells (Batelle & Kravitz, 1978) and cockroach brain (Harmer & Horn, 1977). The receptors of these tissues resemble the a-adrenergic receptors of mammals, as shown by effects of drugs that are known to selectively block aor 8-type responses in mammalian systems. We now report the effects of such drugs on the oxidative metabolism of locust flight muscle. The oxidation of [U-14Clglucose to 14C0, by working locust (Schlstocerca americana gregaria) thoracic muscle was followed by using methods previously described (Candy, 1970, 1978). A mixed substrate of 0 . 0 8 ~ [ ~ ~ C ] g ~ U o s S e and 0.1 Mbutyrate (unlabelled) was used, and perfusion was performed until the rate of appearance of 14C0, reached a steady state (60-80min). This was taken as the control rate of oxidation. The compound under test was then added to the perfusion medium, and the oxidation rate was followed for a further 30 min. Where appropriate, D-octopamine was then added and the perfusion was continued for 30min. The method of Kappe & Armstrong (1964) was used to obtain o-octopamine and L-octopamine. The effects of different concentrations of D-octopamine on the perfused muscle were tested. A half-maximum response was obtained with approx. 0 .7p~-~-octoparnine. At 0.1 5 p~ (corresponding to the concentration of D-octopamine in haemolymph after a few minutes’ flight; Goosey & Candy, 1980) the control oxidation rate of 0.37 f 0.02pmol of glucose oxidized/min per g of muscle increased to 0.51 fO.03 after addition of D-octopamine (mean f s.E.M., n = 9, P < 0.001). This establishes that physiological concentrations of D-octopamine are capable of stimulating oxidative metabolism in flight muscle. The L-isomer of octopamine also stimulated oxidation of glucose by the muscles, but approximately 10-fold as much L-octopamine was required to give the same stimulation as a given concentration of D-octopamine. The octopamine receptor therefore appears to show stereospecificity for D-octopamine, in keeping with the presence of the D-isomer in locust haemolymph. Harmer & Horn (1977) have shown that the adenylate cyclase from cockroach brain is stimulated by D-octopamine to a much greater extent than L-octopamine. The /.?-blockers propranolol and dichloroisoproterenol do not markedly inhibit octopamine stimulation of oxidation (Table 1). but the a-blocker phentolamine almost completely inhibits the action of octopamine. However, phenoxybenzamine stimulated muscle metabolism in the absence of octopamine and failed to