Thyrotrophin-releasing hormone stimulates polyphosphoinositide metabolism in the frog neurointermediate lobe

Abstract

Previous studies have demonstrated that TRH is a potent stimulator of alpha-MSH secretion from frog pituitary melanotrophs. In order to determine the intracellular events responsible for TRH-evoked alpha-MSH release, we have investigated the effect of TRH on polyphosphoinositide breakdown in frog pars intermedia. Neurointermediate lobes were labelled to isotopic equilibrium with myo-[3H]inositol. TRH stimulated the rate of incorporation of [3H]inositol into the phospholipid fraction. The effect of TRH was concentration-dependent; half-maximal stimulation of alpha-MSH release and inositol incorporation occurred at 12 and 28 nmol TRH/l respectively. In prelabelled neurointermediate lobes, lithium (10 mmol/l) enhanced the radioactivity in inositol monophosphate, bisphosphate (IP2) and trisphosphate (IP3). LiCl (10 mmol/l) induced a 38% inhibition of alpha-MSH release from perifused neurointermediate lobes but did not impair TRH-induced alpha-MSH secretion. In the presence of LiCl, TRH (1 mumol/l) induced a transient increase of the radioactivity in IP3, which was evident by 30 s and maximal by 1 min (+100%). TRH treatment also increased the radioactivity in IP2, which reached a plateau after 5 min (+100%). The increase in radioactivity in IP3 induced by TRH was closely paralleled by a rapid loss of [3H]phosphatidylinositol bisphosphate (PIP2), which was maximal by 1 min (-70%). These results indicate that, in frog pars intermedia, TRH-evoked alpha-MSH secretion is coupled to breakdown of PIP2. The data suggest that, in amphibian melanotrophs, as previously shown in GH3 tumour cells and in rat pituitary mammotrophs, TRH causes rapid stimulation of polyphosphoinositide-hydrolysing phospholipase C.

RETRACTED: Functional characterization of a nonclassical nicotine receptor associated with inositolphospholipid breakdown and mobilization of intracellular calcium pools.

“…Measurement of membrane phospholipid breakdown was performed as described (16). The NILs were prelabeled in Leibovitz medium with myO_[3H]inositol (100 ,uCi/ml; 1 uCi = 37 kBq) for 18 hr at 21°C.…”

Adenosine Inhibits Alpha‐Melanocyte‐Stimulating Hormone Release from Frog Pituitary Melanotrophs. Evidence for the Involvement of A1 Adenosine Receptors Negatively Coupled to Adenylate Cyclase

“…the actions of acetylcholine and muscarine wcre markedly reduced. It has recently been shown that TRH-evoked x-MSH release was mediated through activation of polyphosphoinositidc metabolism (42). In contrast, the mcchanism of action of acetylcholine has not yet been determined.…”

Sauvagine and TRH Differentially Stimulate Proopiomelanocortin Biosynthesis in the <i>Xenopus laevis </i>Intermediate Pituitary

“…The transient TRH ef fects, at the levels of both biosynthesis and secretion, indi cates that TRH acts via a different second-messenger sys tem. In most systems studied, including the Rana melano trope cell [28], signal transduction by TRH is assumed to act through activation of phospholipase C with produc tion of inositol triphosphate and diacylglycerol [41], Fur thermore, PKC involvement in POMC gene expression was found in porcine intermediate lobe cells [42], In that study, PMA reduced the POMC mRNA level after 8 and 24 h of treatment. In neurointermediate lobes of blackadapted Xenopits, a 24-hour treatment with 10-7 M PMA can also reduce POMC biosynthesis [C.H.…”

“…It stimulates a-MSH release in the frog R. ridibunda [27] and in Xenopus [15]. In the Rana pars intermedia, TRH-evoked a-MSH secretion has been coupled to breakdown of phosphatidylinositol 4,5-biphosphate and generation of the second messenger inositol 1,4,5-triphosphate [28]. In Xenopus melanotropes, involvement of the inositol phospholipid metabolism in protein biosynthesis is feasible because of the fact that neurointermediate lobes of black-adapted animals have a higher capacity to produce inositol phos phates than those of white-adapted animals [29].…”