The adenylate cyclase stimulator forskolin increases intracellular cyclic AMP (cAMP) in rat FRTL-5 cells within minutes and, after a lag phase of 20-24 h, an increase of cells in metaphase is seen. The dose-response relationships were similar in both systems, with significant increases in the number of metaphases observed at approximately 0.1 mumol/l and a doubling of cAMP levels at 1 mumol/l, whilst doses of 0.1 mmol/l and above proved cytotoxic. An involvement of intracellular cAMP as a positive intermediate in cell division was further suggested by the finding that a low dose of forskolin (0.1 mumol/l) potentiated TSH stimulation of mitosis. Isobutyl methyl xanthine (IBMX), a phosphodiesterase inhibitor, also acted as a mitogen and potentiated TSH action. Moreover, the simultaneous inclusion of low doses of IBMX and forskolin additionally potentiated TSH stimulation of mitosis. An analogue of cAMP, dibutyryl cAMP, also stimulated mitosis and acted over a restricted dose range, with maximal stimulation at 1 mmol/l. We conclude that cAMP may act as a positive signal for FRTL-5 thyroid cell proliferation.

The i.c.v. administration of 0.1 or 10 micrograms ovine (o)GH to 12- to 16-week-old hypothyroid chickens of a sex-linked dwarf (SLD) strain suppressed the basal plasma GH concentrations, measured 24 h afterwards. The GH response of the oGH-injected SLDs to TRH was suppressed, in a dose-related way, in comparison with that induced by TRH in birds given control injections (10 micrograms) of bovine serum albumin (BSA). Basal circulating concentrations of GH in euthyroid K strain birds of the same age were even lower than in the SLDs following injection of 10 micrograms oGH, and were not further reduced by oGH administration. The GH response to TRH in the K strain birds injected i.c.v. with 0.1 or 10 micrograms oGH was, nevertheless, suppressed in comparison with the BSA-injected K strain controls. The i.c.v. administration of oGH also suppressed circulating concentrations of LH and the LH response to TRH in the K strain birds. Twenty-four hours after i.c.v. administration of oGH (10 micrograms), the somatostatin (SRIF) content in the medial basal hypothalamus of 8-week-old euthyroid cockerels was greater than that in BSA (10 micrograms)-injected controls. At the same time, the binding of [3H]3-methyl-histidine2-TRH to the pituitary caudal and cephalic lobes of GH-injected birds was less than that in the controls. These results suggest that GH regulation in avian species is partly mediated by an inhibitory short-loop mechanism (mediated by hypothalamic SRIF and a down-regulation of pituitary TRH-binding sites) that suppresses basal and secretagogue-induced GH release.

Intracerebroventricular (i.c.v.) administration of GH-releasing factor (GRF) (at 1 or 10 micrograms) to anaesthetized immature (6- to 8-weeks-old) or adult (greater than 24-weeks-old) domestic fowl had no effect on basal GH concentrations in peripheral plasma, but suppressed (after 20 min) the acute GH response to exogenous (i.v.) thyrotrophin-releasing hormone (TRH) (1 micrograms/kg). The i.c.v. injection of GRF also reduced the content of somatostatin (SRIF) and dopamine (DA) in the hypothalamus, while increasing the concentration of the DA metabolite 3,4-dihydroxyphenyl acetic acid (DOPAC) and the DOPAC/DA ratio. The release of SRIF from hypothalamic tissue was stimulated in vitro by 100 nmol GRF/l. The inhibitory effect of i.c.v. GRF on TRH-induced GH secretion was blocked when it was simultaneously injected i.c.v. with SRIF antiserum. These results demonstrate central effects of GRF on avian hypothalamic function and suggest an inhibitory role for this peptide in GH regulation, possibly mediated through increased SRIF secretion.

Peripheral plasma concentrations of GH in adult chickens were increased, in a dose-related manner, between 5 and 30 min after the intracerebroventricular (i.c.v.) injection of 0.1 or 10 micrograms TRH. In contrast, i.v. administration of comparable doses of TRH had no significant effect on circulating GH concentrations. [3H]3-methyl-histidine2-TRH [( 3H]Me-TRH) was located in the pituitary gland and peripheral plasma within 5 min of its i.c.v. administration, although in amounts that were unlikely to affect directly pituitary function. [3H]Me-TRH rapidly accumulated in the hypothalamus following its i.c.v. administration (but not after i.v. injection), and the central effect of TRH on GH secretion in birds is therefore likely to be induced by effects at hypothalamic sites.

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