Abstract

At concentrations higher than that required to produce maximal vasoconstriction, tyramine caused concentration-dependent relaxation of rat aortic strips contracted maximally by tyramine, norepinephrine, phenylephrine, 5-hydroxytryptamine, prostaglandin F2 alpha, endothelin, angiotensin II and potassium; isoproterenol did not relax potassium-contracted strips. The vasorelaxant effect of tyramine was not antagonized by propranolol, pindolol or nadolol, all of which markedly antagonized the effects of isoproterenol. The vasorelaxant activity of tyramine was endothelium-independent and not inhibited by hemoglobin, methylene blue or L-NG-nitro arginine; it did not exhibit tachyphylaxis and was neither inhibited by cocaine, guanethidine, reserpine and chemical sympathectomy nor by alpha adrenoceptor, dopamine receptor, 5-hydroxytryptamine receptor, histamine receptor and adenosine receptor antagonists. Inhibition of cyclooxygenase, lipoxygenase and monamine oxidase activities did not decrease the vasorelaxant activity of tyramine. The vasorelaxant effect of tyramine was not decreased by altering external calcium from 0.25 to 4 mM nor was it potentiated by nifedipine. Phenylethylamine was the minimum structural requirement for this propranolol-resistant vasorelaxant activity; beta-carbon and 3-ring hydroxylation abolished this activity, but N-methylation partly overcame the effect of beta-hydroxylation. The vasopressor effect of tyramine in anesthetized rat was reversed to vasodepressor effect by phenoxybenzamine plus propranolol. The propranolol-resistant vasorelaxant effect of tyramine was also confirmed on isolated human placenta arteries. At vasorelaxant concentrations, tyramine did not increase cyclic GMP and cyclic AMP but inhibited inositol-1,4,5-triphosphate. It is suggested that at high concentrations tyramine and related phenylethylamines cause endothelium- and beta-2 adrenoceptor independent vasorelaxation either via specific tyramine receptors or nonselectively.