ABSTRACT

The new mutant GH-deficient dwarf (Dw) rat was used to study the effects of GH-releasing factor (GRF) or somatostatin (SRIF) on GH release. In anaesthetized adult Dw female rats, i.v. injections of GRF (0·031–2·0μg) elicited a dose-dependent release of GH. Although the peak plasma GH responses to maximal GRF doses were much lower in adult Dw rats compared with normal rats of this strain (AS), the responses largely reflected their relative pituitary GH contents (140±17 μg vs 2·9±0·4 μg, AS vs Dw (means ± s.e.m.), P < 0·001). Except at 20 days of age, normal AS rats were more sensitive to GRF than Dw rats despite their larger body weight. Peak GH responses to injection of 31·25 ng GRF increased nine-fold in normal rats between 20 and 40 days, whereas the GH responses to this GRF dose diminished in Dw rats over this age range, and their pituitary GH content was only 2–5% of that of age-matched AS rats. Treatment with human GH (200 μg/day for 7 days) stimulated growth in 40-day-old Dw rats and slightly increased the GH response to a low dose of GRF. Basal GH levels in adult Dw animals were sevenfold lower than in AS rats (2·4±0·3 vs 17·6±3·3 μg/l P < 0·001) and were further suppressed by i.v. infusion of SRIF (25 μg/h). As in normal rats, a rebound GH secretion occurred in Dw rats after stopping SRIF, which was blocked by injection of anti-GRF serum. The disappearance rate of ¹²⁵I-labelled rat GH from plasma was identical in AS and Dw rats. We conclude that dwarf rats show GH deficiency as early as 20 days of age; they respond to GRF, but release only small amounts of GH due to their reduced pituitary GH content. Although basal GH release is reduced in Dw rats, the levels are higher than would be expected from the 50-fold reduction in pituitary stores, and may reflect a chronic reduction in SRIF and/or increase in GRF release induced by prolonged GH deficiency in the Dw rat.

Journal of Endocrinology (1990) 127, 69–75

ABSTRACT

Changes in somatostatin-like immunoreactivity (SLI) were examined in the retina and peripheral blood of diabetic rats treated with streptozotocin (STZ) and insulin. There was no change in retinal SLI content at 4 and 11 days after administration of STZ but, thereafter, SLI increased progressively in the diabetic animals by 220% at 18 days and 300% at 27 days. Plasma SLI levels increased by 500% at 11 days and maintained similar levels thereafter. Diabetic animals treated with insulin (3–5 i.u. daily) for 27 days showed a significant (P < 0·01) decrease of retinal and plasma SLI levels compared with untreated diabetic animals. It is concluded that there is a significant increase of retinal and plasma SLI levels in diabetic rats which tends to normalize after several days of insulin treatment.

J. Endocr. (1987) 114, 363–367

ABSTRACT

The effect of GH-releasing factor(1–44) (GRF) alone, or together with somatostatin (SRIF), dopamine (DA), vasoactive intestinal peptide (VIP) or cycloheximide was studied in a total of ten human somatotrophinomas using a static cell culture system. Growth hormone-releasing factor (2·0×10⁻⁸ mol/l) significantly (P<0·05) stimulated GH release from nine out of ten tumours over 4-h incubations, and a dose-related effect (2·0×10⁻¹⁰−2·0×10⁻⁸ mol/l) was observed in five tumours thus studied.

Repeated GRF (2·0×10⁻⁸ mol/l)-mediated GH release was seen during 96% (n = 25) of experiments performed on six tumours over 4 h and up to 27 days in culture. Growth hormone-releasing factor (2·0×10⁻⁸ mol/l) also stimulated GH release from five out of seven somatotrophinomas during 60-min incubations.

Somatostatin (6·1×10⁻⁹ mol/l) completely inhibited GRF-induced GH secretion from four tumours studied over 4 h, but in each case there was significant (P<0·05) 'rebound' of GH release from cultures exposed to both GRF and SRIF during a subsequent recovery period. Dopamine suppressed basal GH release from two out of four tumours, but in each case had a greater inhibitory effect on GRF-mediated GH release. Vasoactive intestinal peptide directly stimulated GH release from two out of three tumours, and the effects were additive to maximal stimulatory doses of GRF. Cycloheximide significantly (P< 0·01) enhanced GRF-stimulated release of GH during a 60-min incubation, but inhibited both basal and GRF-stimulated release over 4 and 8 h. We conclude that a static cell culture system can be used successfully to study the action of GRF on human somatotrophinoma tissue for up to 27 days in culture. Human somatotrophinomas seem to be only rarely unresponsive to the stimulatory action of GRF, which is itself significantly modulated by SRIF, DA and VIP. De-novo protein synthesis appears to be necessary for the full expression of GRF activity on human somatotrophinoma tissue.

J. Endocr. (1985) 105, 269–276

ABSTRACT

Recently, data have been presented showing that muscarinic cholinergic agonists or antagonists can modulate, in opposite ways, GH-releasing hormone (GHRH)-induced GH release in man. The aim of the present study was, first, to confirm these findings in the rat and, secondly, if confirmed, to investigate the mechanism(s) subserving the effect of cholinergic drugs.

In adult male rats bearing chronic indwelling atrial cannulae, pretreatment with the cholinergic antagonists pirenzepine (0·5 mg/kg, i.v.) or atropine (0·5 mg/kg, i.v.) significantly reduced the rise in plasma GH induced by GHRH (2 μg/kg, i.v.), while pretreatment with the cholinergic agonist pilocarpine (3 mg/kg, i.v.) potentiated it. In rats with hypothalamic somatostatin (SRIF) depletion, i.e. rats with anterolateral deafferentation of the mediobasal hypothalamus or rats treated with cysteamine, the modulatory action of cholinergic drugs on the neuroendocrine effect of GHRH was completely lacking. In these two experimental models, an antiserum raised against SRIF failed to elicit a rise in plasma GH and measurement of hypothalamic SRIF content revealed a clear-cut reduction of the neuropeptide. Atropine (1 μmol/l) and pilocarpine (1 μmol/l), added to pituitary cells in vitro, failed to alter GHRH-induced GH release. The present results indicate that muscarinic cholinergic agonists and antagonists modulate GHRH-induced GH release in the rat and suggest that the effect of cholinergic modulation takes place through SRIF.

J. Endocr. (1986) 111, 271–278

ABSTRACT

The effects of endogenous hypothalamic neurohormones and activators of second messenger signalling systems on the secretion of GH and on cell content of GH mRNA of cultured bovine adenohypophysial cells were studied. Synthetic bovine GH-releasing factor (bGRF; 100 nmol/l) increased secretion of GH by bovine adenohypophysial cells five-fold relative to control. Forskolin (an adenyl cyclase activator; 10 μmol/l) and the synthetic cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP; 1 mmol/l) increased secretion of GH by 1·9- and 1·7-fold respectively, relative to control. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA), provided at 1 μmol/l or 10 nmol/l, increased GH secretion by 6·6- and four-fold respectively, relative to control. Somatostatin-14 (SRIF-14) attenuated basal, bGRF-, forskolin- and dbcAMP-stimulated secretion of GH by 40, 49, 47 and 67% respectively, but did not, however, diminish PMA-stimulated GH secretion. The content of GH mRNA in cultured bovine adenohypophysial cells increased 2·2-, 1·7- and 3·2-fold by administration of bGRF, forskolin and PMA respectively, relative to control. Although GH mRNA content was unchanged by SRIF-14 treatment relative to control, SRIF-14 did reduce bGRF-stimulated bGH mRNA content by 67%. This study demonstrates that mechanisms subserving GH secretion in bovine adenohypophysial cells (e.g. adenyl cyclase and protein kinase C) may be coupled with mechanisms which regulate expression of the GH gene or with factors affecting message stability.

Journal of Endocrinology (1990) 125, 109–115

Abstract

The physiological role of endogenous circulating GHreleasing hormone (GHRH) and somatostatin (SRIH) on spontaneous pulsatile and neostigmine-induced secretion of GH was investigated in adult rams actively immunized against each neuropeptide. All animals developed antibodies at concentrations sufficient for immunoneutralization of GHRH and SRIH levels in hypophysial portal blood. In the anti GHRH group, plasma GH levels were very low; the amplitude of GH pulses was strikingly reduced, although their number was unchanged. No stimulation of GH release was observed after neostigmine administration. The reduction of GH secretion was associated with a decreased body weight and a significant reduction in plasma IGF-I concentration. In the antiSRIH group, no changes in basal and pulsatile GH secretion or the GH response to neostigmine were observed as compared to controls. Body weight was not significantly altered and plasma IGF-I levels were reduced in these animals. These results suggest that in sheep, circulating SRIH (in the systemic and hypophysial portal vasculature) does not play a significant role in pulsatile and neostigmine-induced secretion of GH. The mechanisms of its influence on body weight and production of IGF-I remain to be determined.

Journal of Endocrinology (1995) 144, 83–90

ABSTRACT

Despite causing marked inhibition of somatic growth, glucocorticoids enhance both the response to GH-releasing hormone (GHRH) and the amplitude of naturally occurring GH secretory pulses in the male rat. The relative contribution of the two major hypothalamic regulatory factors for GH (somatostatin and GHRH) to these observed effects remains speculative. In the present studies, we have investigated endogenous and stimulated GH release in rats pretreated with glucocorticoid or vehicle, and the effects of passive immunoneutralization of somatostatin or GHRH.

In an initial study, four groups of eight rats were treated with either saline or various doses of a depot preparation of betamethasone: low dose, 0·85 mg; medium dose, 1·7 mg; high dose, 3·4 mg. All doses significantly suppressed body weight gain, total adrenal weight and concentrations of both plasma corticosterone and pituitary ACTH. Seven days after betamethasone treatment, GH responses to an i.v. injection of 1 μg human GHRH(1–29) were evaluated during pentobarbitone anaesthesia. Compared with saline-treated controls (peak GH concentration of 506·0±68·5 μg/l), peak GH levels were enhanced by the low dose (704·4±47·8 μg/l, P<0·05), unaltered by the medium dose (543±65·8 μg/l) and suppressed by the high dose (312·7±55·2 μg/l, P<0·05) of betamethasone. Similarly, the area under the secretory curves was increased by 46% following the low dose (P<0·01), unaltered by the medium dose and reduced by 33% after the high dose of betamethasone.

In a second study, rats were pretreated for 7 days before blood sampling with either the medium dose of betamethasone or saline. On day 5, 48 h before blood sampling, an indwelling venous catheter was fitted enabling sampling of conscious rats. On the day of study, blood samples were taken at 30-min intervals over an initial 2-h period (10.00–12.00 h). Following the sample at 12.00 h, rats were given the reconstituted and dialysed immunoglobulin fraction from either control sheep serum (NSIgG), sheep anti-rat GHRH serum (GHRHab) or sheep anti-somatostatin serum (SRIHab), and samples were taken for a further 90 min (12.30–14.00 h). Directly after the sample at 14.00 h, GH stimulation was effected in all rats using 1 μg human GHRH(1–29) with samples taken at 5, 10, 20 and 40 min following stimulation. During the initial sampling period, mean GH levels were significantly (P<0·005) higher in steroidpretreated animals than in saline-pretreated controls (29·3±5·8 vs 13·2±1·6 μg/l), with a higher amplitude secretory pulse occurring at 11.30 h (80·7±18·6 vs 26·4±4·1 μg/l, P < 0·01). Administration of GHRHab to saline-pretreated animals did not alter mean GH levels when compared with animals receiving control NSIgG (saline plus NSIgG, 9·3±1·1; saline plus GHRHab, 8±1·1 μg/l, P = NS). In contrast, the raised mean GH levels seen in betamethasone-pretreated rats receiving NSIgG (12·3 ±1·1 μg/l) were reduced by GHRHab administration (7·6±1·1 μg/l); these levels were not different from those of the saline-pretreated group suggesting that the observed permissive effect of glucocorticoids on GH secretion is mediated through enhanced GHRH activity. SRIHab increased mean basal GH levels to a similar extent in both saline- and betamethasone-pretreated groups (17·4±1·2 μg/l and 19·3 ±1·1 μg/l respectively, P<0·01 vs comparable NSIgG group). Administration of the various immunoglobulin fractions had no effect on GHRH-stimulated GH secretion except when SRIHab was given to betamethasone-pretreated animals, resulting in a significantly increased peak response (1467±93 μg GH/l, P<0·001) when compared with either saline- or betamethasone-pretreated rats given NSIgG (643±95 and 791±92 μg/l respectively). This enhancement following SRIHab administration was not seen in saline-pretreated animals (893±180 μg GH/l).

These results imply that glucocorticoid treatment increases basal GH levels through a GHRH-dependent mechanism and also increases pituitary sensitivity to exogenous GHRH when inhibitory somatostatin tone is blocked.

Journal of Endocrinology (1991) 131, 75–86

A reduction in the availability of oxygen and nutrients across the placenta in the last trimester of pregnancy may lead to intrauterine growth retardation (IUGR) which, in turn, may cause a persistent postnatal growth failure. However, it is unknown whether this persistent growth retardation is centrally mediated through alterations in the components of the growth hormone (GH)-axis. We tested the hypothesis that alterations in the development of the central components of the GH-axis contribute to the persistent growth failure observed after experimentally induced IUGR or early postnatal food restriction (FR) in the rat. Using semi-quantitative in situ hybridization, we compared somatostatin (SS), GH-releasing hormone (GHRH) and neuropeptide Y (NPY) mRNA levels in adult rats experimentally subjected to IUGR or FR. We report that IUGR increased the expression of SS mRNA in the periventricular nucleus (PeN) of adult male and female rats by 128% and 153% respectively, did not alter the expression of GHRH mRNA in the arcuate nucleus (ARC) and decreased the NPY mRNA expression in the ARC by 73% in males and 61% in females, whereas in the FR group no changes in the expression of these mRNAs were observed. These data show that the timing of malnutrition or the presence of the placenta is important for the long-term alterations since the effects only occurred in the prenatally induced growth retardation and not in the early postnatally induced growth retardation group.

Thyrotropin-releasing hormone (TRH) and somatostatin (SRIH) concentrations were determined by RIA during both embryonic development and posthatch growth of the chicken. Both TRH and SRIH were already detectable in hypothalami of 14-day-old embryos (E14). Towards the end of incubation, hypothalamic TRH levels increased progressively, followed by a further increase in newly hatched fowl. SRIH concentrations remained stable from E14 to E17 and doubled between E17 and E18 to a concentration which was observed up to hatching. Plasma GH levels remained low during embryonic development, ending in a steep increase at hatching. Plasma TSH levels on the other hand decreased during the last week of the incubation. During growth, TRH concentrations further increased, whereas SRIH concentrations fell progressively towards those of adult animals. Plasma TSH levels increased threefold up to adulthood; the rise in plasma GH levels during growth was followed by a drop in adults. In conclusion, the present report shows that important changes occur in the hypothalamic TRH and SRIH concentration during both embryonic development and posthatch growth of the chicken. Since TRH and SRIH control GH and TSH release in the chicken, the hypothalamic data are compared with plasma GH and TSH fluctuations.

Abstract

The effects of somatostatin (SRIF) on prolactin (PRL) synthesis and release were examined in primary cultured pituitary cells derived from normal and estradiol (E₂)-primed male rat pituitaries. The cells were continuously incubated in a pulse medium containing [³H]leucine with or without 10⁻⁶ mol/l SRIF for a period of 15, 30, 60, 180 or 360 min. Following incubation, the medium was recovered and the cells were fractionated into cytosolic and granular fractions. PRL was isolated by SDS-PAGE and newly synthesized PRL ([³H]PRL) was identified by coincident peaks of tritium activities and PRL contents. The specific activity (SA, c.p.m./ng), a ratio of [³H]PRL to total PRL, was determined for the granular, cytosolic and medium fractions.

In control and SRIF-treated groups of non-primed pituitary cells, SAs of all three fractions significantly increased during the 6-h incubation. Cytosolic and granular SAs showed similar profiles of increasing rate in comparison to control. Medium SAs showed a significantly higher value in the SRIF-treated group than in the control group only at 180 min. These observations indicate that, in the non-primed condition, PRL synthesis is not inhibited by SRIF. Medium SAs in the E₂-primed group were significantly higher than SAs in the non-primed control cells during the initial 3 h of incubation, and cytosolic and granular SAs were significantly higher than those of the non-primed control during the 3- to 6-h incubation period. These observations demonstrate that E₂ enhances PRL synthesis and secretion of newly synthesized PRL. SRIF treatment of E₂-primed lactotrophs resulted in a significant decrease in SAs of all three fractions as compared with those of the E₂-primed control. Our results indicate that in normal male rat pituitary cells SRIF does not inhibit PRL synthesis but effectively inhibits PRL synthesis in E-primed lactotrophs. This suggests that the inhibitory action of SRIF on PRL synthesis is estrogen dependent.

Journal of Endocrinology (1996) 148, 69–76