Effects of α2-adrenergic action on cyclic AMP levels in canine thyroid slices

The elevation of cyclic AMP levels induced by TSH in canine thyroid slices was markedly inhibited by norepinephrine. Yohimbine (100 μM), an α₂-adrenergic blocker, completely abolished the inhibitory effect of norepinephrine. The inhibition by yohimbine was dose-related. Phentolamine, a mixed α-adrenergic blocker, showed similar effects, while prazosin, an α₁-adrenergic blocker exhibited no such effects.These results indicate that there exist α₂-adrenergic receptors in canine thyroid glands, which may be involved in thyroid gland functions.