Evidence for α-adrenergic receptors acting through the guanylate cyclase system in human cultured thyroid cells

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Abstract
In order to investigate the presence of .alpha.-adrenergic receptors in human thyroid, the effect of .alpha.-adrenergic agonists and antagonists on cGMP cellular content of human thyroid cells was studied in primary culture. Epinephrine as well as TSH were not able to modify the cGMP cellular levels, while norepinephrine significantly increased cGMP accumulation already at 10 nM, a dose inactive on cAMP accumulation. A non-selective .alpha.-adrenergic antagonist, phentolamine, significantly inhibited cGMP accumulation induced by norepinephrine. Norepinephrine-induced cGMP accumulation was unaffected by prazosin, an .alpha.1-adrenergic antagonist, but was abolished by yohimbine, an .alpha.2-adrenergic antagonist. Phenylephrine, an .alpha.-adrenergic agonist, produced an increase of cellular cGMP levels without modifying cAMP content. In the presence of TSH, the cGMP response to norepinephrine was not modified; however, the increase of cAMP levels was inhibited by norepinephrine at doses inactive on cAMP accumulation, but active on cGMP levels. The present results demonstrate the existence in human thyroid cells of .alpha.2-adrenergic receptors, regulating the guanylate cyclase system. The counter-regulation exerted by .alpha.-adrenergic agonists on the response to TSH may operate on the TSH-dependent adenylate cyclase.