Involvement of alpha 1-adrenergic receptors in the inhibitory effect of catecholamines on the thyrotropin-induced release of thyroxine by the mouse thyroid.

TSH enhanced the release of T4 from the mouse thyroid incubated in vitro. Norepinephrine, a nonspecific alpha-adrenergic agonist, and methoxamine, an alpha 1-agonist, inhibited the TSH-stimulated release of T4 at 10(-4) and 10(-5) M, whereas clonidine, an alpha 2-agonist, exerted a weak inhibition. The inhibitory effect of 10(-5) M norepinephrine on the T4 release stimulated by TSH was prevented by prazosin, an alpha 1-antagonist, at concentrations higher than 10(-7) M, whereas yohimbine, an alpha 2-antagonist, exerted weak activity in antagonizing the inhibition induced by norepinephrine. Norepinephrine, methoxamine, and clonidine did not significantly reduce the cAMP accumulation stimulated by TSH in the mouse thyroid incubated in vitro. These findings in the mouse thyroid indicate that catecholamines act by way of alpha 1-adrenergic receptors to suppress TSH-stimulated release of T4 without reducing the cAMP levels stimulated by TSH in the mouse thyroid.