Biphasic effect of thyrotropin-releasing factor (TRH) on α-melanotropin secretion from frog intermediate lobe in vitro

Abstract

The kinetics of a-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (10 −8 M), the secretion rate of α-MSH displayed two phases. During the first phase, secretion of α-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released α-MSH occurring at about 100 min. Exposure of NIL to another TRH (10 −8 M) pulse 90 min later induced a normal stimulation of α-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10 −5 M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced α-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (10 −6 M), the biphasic response to prolonged infusion of TRH (10 −8 M) was still observed. Administration of a short pulse of TRH (10 −7 M) during the declining part of the first phase or during the second phase of prolonged TRH (10 −8 M) infusion induced a significant enhancement of α-MSH stimulation. From these results we conclude that (1) prolonged TRH infusion causes α-MSH release in a biphasic manner; (2) attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of α-MSH; (3) the biphasic response depends neither on de novo biosynthesis nor on intracellular transit of the prohormone from the endoplasmic reticulum to the Golgi complex. Thus, we propose that prolonged exposure of frog NIL to TRH may cause transconformational changes of membrane TRH receptors from high to low affinity states.