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Influence of thyroid hormone on the positive inotropic effects mediated by alpha- and beta-adrenoceptors in isolated guinea pig atria and rabbit papillary muscles.

Effects of thyroxine treatment for 7--11 days on the positive inotropic effects mediated by alpha- and beta-adrenoceptors were studied in isolated guinea pig atria and rabbit papillary muscles. In guinea pig atria, the thyroxine treatment inhibited the positive inotropic effect of lower concentrations of phenylephrine (PHE), and attenuated the inhibitory effect of phentolamine on the PHE response. The effect of isoproterenol (ISO) was potentiated by the thyroxine treatment. In rabbit papillary muscles, the thyroxine treatment shifted the dose--response curve for PHE to the right and attenuated the inhibitory effect of phentolamine on the PHE response. Propranolol, in both guinea pig atria and rabbit papillary muscles, inhibited the PHE response more effectively in preparations from thyroxine-treated animals than in controls. In guinea peg atria, the attenuation of the PHE response mediated by alpha-adrenoceptors was observed after the thyroxine treatment for only 2 days, whereas the potentiation of the ISO response required the thyroxine treatment for a longer period. It was concluded that the thyroxine treatment attenuated the positive inotropic effect mediated by alpha-adrenoceptors and potentiated that mediated by beta-adrenoceptor-mediated positive inotropic effects due to the thyroxine treatment may be independent of each other.