Involvement of Endogenous Noradrenaline Release in Methylene Blue-Induced Contraction of Isolated Rabbit Aorta

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Abstract
The vasocontractile response to methylene blue (Meb) was investigated in isolated rabbit aorta. Meb (1-100 .mu.M) induced a slowly developing contraction after a long latency in rabbit aortic strip. The maximal contraction was obtained by 50 .mu.M Meb, which corresponds to 1 .mu.M noradrenaline (NA)-induced contraction. Once the maximal contraction was induced by Meb, the strip completely lost the contractile response to a further application of Meb. The usual NA-induced contraction, however, could be observed in such a Meb-insensitive aortic strip. Meb-induced contractions were not affected by atropine, diphenhydramine, methysergide, indomethacin, nordihydroguaiaretic acid and removal of endothelial cells from the aortic strip, but they were abolished by prazosin. In aortic strips from rabbits pretreated with reserpine (3.0 mg/kg, i.m.) for a day, Meb failed to induce contraction. Meb evoked the [3H] release from [3H]NA-preloaded aortic strips. In high performance liquid chromatographic analysis, a considerable amount of NA was found in the bathing fluid of the aortic strip in the presence of Meb. In addition, Meb pretreatment inhibited [3H]NA uptake by the aortic strip and abolished the contractile response to an electrical stimulation of adrenergic nerve terminals. Although Meb decreased the basal level of cyclic GMP in the aortic strip, Meb-induced [3H]NA release from the aortic strip was not affected by 8-bromo cyclic GMP. These results suggest that Meb-induced contraction of rabbit aorta is due to the release of endogenous NA from its storage pools of intramural adrenergic nerves through an independent mechanism of its cyclic GMP lowering effect. In addition, incubation of aortic strips with Meb resulted in depleting the storage NA and blocking the nerve function, suggesting that Meb might be useful for a pharmacological tool as an adrenergic neuron blocking agent in vitro.

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