In vitro study of frog (Rana ridibunda Pallas) neurointermediate lobe secretion by use of a simplified perifusion system. II. Lack of action of thyroxine on TRH-induced alpha-MSH secretion.

Thyrotropin-releasing hormone (TRH) stimulates alpha-melanocyte-stimulating hormone (alpha-MSH) secretion in amphibia as well as thyrotropin-stimulating hormone (TSH) and prolactin secretions in mammals. Since thyroid hormones regulate the stimulatory effect of TRH on TSH and prolactin, the possible role of thyroxine (T4) in the control of alpha-MSH secretion in amphibia, has been investigated. Neurointermediate lobes of Rana ridibunda were perifused in amphibian culture medium for 7 hr and the amounts of alpha-MSH released into the effluent perfusate were measured by radioimmunoassay. In vivo treatment with T4 (0.5 mg/kg twice a day for 9 days) did not modify the in vitro response of the neurointermediate lobes to TRH (10(-9) to 10(-7) M). In addition, prolonged infusion of T4 in vitro did not alter spontaneous and TRH-induced alpha-MSH release. In spite of the inhibitory effect of T4 on TRH-induced TSH and prolactin secretions in mammals, the present data show that, in frogs, thyroid hormone does not modulate the stimulation of alpha-MSH secretion induced by TSH.