Abstract

The effects of intravenous somatostatin-28 (S28) infusion on glucose-stimulated and glucagon-like peptide-1(7–36)amide (GLP-1)-augmented insulin secretion were studied in sheep. S28 was infused via a jugular catheter for 15 min at a rate of 1·1 pmol/kg/min either alone or together with GLP-1 and/or glucose. S28 infusion did not significantly lower circulating basal insulin concentrations in fed sheep. Glucose-stimulated insulin secretion was significantly inhibited by S28 infusion, serum concentrations decreasing from about 200 to 150 pmol/l. GLP-1 significantly augmented glucose-stimulated insulin secretion, serum concentrations increasing from about 230 to 280 pmol/l. S28 completely counteracted this effect of GLP-1. S28 infusion also significantly decreased the circulating concentrations of glucose-dependent insulinotrophic polypeptide (GIP) and GLP-1 in fed sheep (from about 110 to 45 pmol/l for GIP and from about 25 to 15 pmol/l for GLP-1). The physiological implications of these observations are discussed with particular reference to the ruminant. It is concluded that S28 may have an important endocrine role in the control of insulin secretion and regulation of nutrient partitioning.

Journal of Endocrinology (1996) 151, 107–112

Abstract

We have previously demonstrated that chicken embryonic somatotropes are responsive to GHRH shortly after they differentiate. In contrast, relatively little is known about the regulation of GH secretion by somatostatin (SRIF) and IGF-I during chicken embryonic development. In the present study, anterior pituitary cells were isolated from day 16, 18 and 20 embryos and subjected to reverse hemolytic plaque assays (RHPAs) for chicken GH to assess the effect of SRIF and IGF-I on basal GH release and SRIF on GHRH-stimulated GH secretion. We found that all three ages responded to SRIF, under both basal and stimulated conditions. SRIF inhibition of basal GH release was evident for day 18 and 20 cells by 9 h, while 18 h were required for day 16 cells. After 18 h, 10⁻¹¹ m SRIF depressed basal GH secretion by day 16 and 18 cells, while 10⁻⁹ m SRIF was required to depress GH plaque percentages by day 20 cells. GHRH stimulated GH release from all ages tested. After 2 h, SRIF partially suppressed GHRH-stimulated GH release by day 20 cells. After 6 h, day 18 cells responded to SRIF, reducing the percentage of plaque-forming cells under GHRH stimulated conditions. After 18 h, the percentage of day 16 cells forming GH plaques was reduced for cells treated with GHRH and SRIF, compared with cells treated with GHRH alone. All ages examined also responded to IGF-I. After 36 h, GH release by day 16 and 18 cells was decreased when exposed to IGF-I. While IGF-I decreased the relative amount of GH secreted per somatotrope on day 20, a paradoxical increase in the percentage of cells secreting GH was noted. These results indicate that anterior pituitary somatotropes are responsive to SRIF and IGF-I during late embryonic development of chickens.

Journal of Endocrinology (1997) 154, 303–310

ABSTRACT

The effects of plasma free fatty acids (FFA) and somatostatin-14 (S-14) on concentrations of plasma GH, glucagon and insulin were investigated in juvenile ducks. Oleic acid, S-14 or both were infused into 4- to 7-week-old birds and plasma GH, glucagon-like immunoreactivity (GLI), immunoreactive insulin (IRI) and FFA were measured.

An increase in plasma GH and a decrease in GLI but no change in IRI was observed after infusion of 9 mg oleic acid/kg per min. A decrease in plasma GH, FFA and IRI and an increase in plasma GLI was seen after infusion of 800 ng S–14/kg per min. These effects of S-14 on IRI and GLI were abolished when S-14 was infused simultaneously with oleic acid.

It is concluded that FFA have a direct stimulatory effect on GH secretion and an inhibitory effect on glucagon secretion. Somatostatin-14 directly inhibits the secretion of GH and its stimulatory effect on the secretion of glucagon is mediated by a depression in concentrations of plasma FFA. Finally, S-14 has no effect on plasma insulin when basal levels of plasma FFA are maintained.

J. Endocr. (1985) 106, 21–25

ABSTRACT

Binding of ¹²⁵I-labelled [Tyr¹]-somatostatin (¹²⁵I-[Tyr¹]-SRIF) to pituitary caudal lobe membranes was suppressed in immature chickens 1 and 2 h after i.v. administration of unlabelled SRIF at concentrations of 1–100 μg/kg. In-vitro preincubation of chicken pituitary glands for 0·5–4·0 h with 0·1 μmol SRIF/l similarly reduced the binding of ¹²⁵I-[Tyr¹]-SRIF to caudal lobe membrane preparations. After a 4-h incubation in 0·1 mmol SRIF/l, the withdrawal of SRIF from the incubation media was accompanied 4 h later by a partial recovery in the binding of ¹²⁵I-[Tyr¹]-SRIF to pituitary membranes. Passive immunoneutralization of endogenous SRIF resulted in a prompt (within 1 h) and sustained (for at least 24 h) suppression of ¹²⁵I-[Tyr¹]-SRIF binding to pituitary membranes. The i.m. administration of cysteamine (300 mg/kg) to 12-week-old birds depleted hypothalamic SRIF stores and decreased the density of ¹²⁵I-[Tyr¹]-SRIF-binding sites in the caudal and cephalic lobes of the chicken pituitary gland. The reduction in SRIF content and in SRIF-binding sites occurred within 1 h of cysteamine administration and was maintained for at least 24 h. In 6-week-old birds, cysteamine (300 mg/kg) administration suppressed pituitary binding of ¹²⁵I-[Tyr¹]-SRIF for at least 5 days. Circulati concentrations of GH were markedly decreased 1 and 4 h after cysteamine injection, but not after 24 h.

Pituitary binding sites for ¹²⁵I-[Tyr¹]-SRIF were not affected by pretreatment of pituitary glands for 2–12 h in vitro with thyroxine or oestradiol-17β (1 nmol/l–10 μmol/l) or with ovine GH or recombinant DNA-derived chicken GH (1–100 μg/ml in vitro and 100–1000 μg/kg in vivo). Ovine prolactin, at concentrations of 1–100 μg/ml was also without effect on ¹²⁵I-[Tyr¹]-SRIF binding to pituitary membranes following a 2- or 4-h incubation with pituitary glands. Pituitary binding sites for ¹²⁵I-[Tyr¹]-SRIF were, however, increased after a 24-h incubation with 1 μmol tri-iodothyronine (T₃)/l in vitro and 4 and 24 h after the administration of T₃ (100–1000 μg/kg) in vivo. Although T₃ had no direct inhibitory effect on ¹²⁵I-[Tyr¹]-SRIF binding to pituitary membranes, binding was suppressed 1 and 2 h after the in-vivo administration of T₃ at concentrations of 100–1000 μg/kg.

These results therefore demonstrate homologous and heterologous regulation of SRIF-binding sites in the chicken pituitary gland.

Journal of Endocrinology (1990) 127, 417–425

ABSTRACT

Secretion of GH occurs in episodic bursts under the dual control of two hypothalamic peptides, GH-releasing factor (GRF) and GH-inhibiting factor (somatostatin, SRIF). Recent studies in rats suggest that episodic GH secretion is generated by the periodic release of GRF, which is associated with the simultaneous withdrawal of SRIF secretion. To test the possibility that GRF discharge is functionally coupled with the withdrawal of SRIF, we investigated whether acute withdrawal of SRIF can induce GRF release by the rat hypothalamus using highly specific antisera against SRIF and rat GRF. In conscious unrestrained rats, i.v. administration of SRIF antiserum at the period of the GH trough induced a rapid onset of the GH secretory surge with a peak value of 309 ± 67 μg/l (mean ± s.e.m.) 30 min after injection. Pretreatment with antiserum to rat GRF resulted in a ∼83% suppression of the GH surge induced by SRIF antiserum without affecting the trough GH values. GRF antiserum also significantly inhibited the spontaneous GH surge. In urethane-anaesthetized rats, as in conscious rats, an acute phasic GH release was caused by SRIF antiserum despite the interference of anaesthesia with spontaneous GH secretion. A further surge-like GH secretion was not restored during the next several hours, however, with the GH secretory profile being characterized by a tonic increase in the baseline levels of GH. In the anaesthetized rat antiserum to rat GRF, having no effect on basal GH levels, similarly inhibited by ∼66% the acute GH surge induced by SRIF antiserum and decreased by about 30% the later sustained rise in GH.

These results indicate that an acute rather than a chronic withdrawal of SRIF can trigger the phasic discharge of GRF by the hypothalamus, and raise the possibility that SRIF, originally isolated as a hypothalamic hypophysiotrophic GH-inhibiting factor, also acts as a GRF-inhibiting factor within the hypothalamus. We suggest that the episodic GH secretion may be initiated by an acute disinhibition of GRF secretion caused by the sudden withdrawal of SRIF.

J. Endocr. (1988) 117, 245–252

Abstract

The GH-releasing activity of the α₂-adrenergic agonist clonidine has been extensively studied in the rat, but the mechanism(s) by which clonidine stimulates GH release remains controversial. In the present study, we examined the effects of various doses of clonidine on spontaneous pulsatile GH secretion in conscious rats, and tested the hypothesis that the GH-releasing activity of clonidine is mediated primarily by an inhibition of hypothalamic somatostatin (SRIF) release. In the first experiment, free-moving adult male rats were given either saline or various doses of clonidine i.v. (30, 50 and 125 μg/kg) at times of spontaneous peaks (1100 h) and troughs (1300 h) in the GH rhythm. Clonidine, at all doses tested, failed to stimulate GH release when administered at the time of a spontaneous peak. In contrast, injection of clonidine at trough times (when SRIF tone is high) consistently augmented plasma GH levels (mean ± s.e.m. integrated GH release; 30 μg/kg, 1843·0±484·0; 50 μg/kg, 1469·0± 490·3; 125 μg/kg, 1675·6 ± 513·4 vs 201·3 ± 100·1 ng/ml per 45 min in saline-injected controls; P<0·05 or less). No significant regression was observed between increasing doses of clonidine and GH release. In the second experiment, i.v. administration of 30 μg clonidine/kg during a GH trough period, 30 min prior to GH-releasing factor (GRF) challenge, significantly potentiated the GH response to GRF compared with rats given saline (7218·7 ±806·6 vs 4206·9 ± 1068·1 ng/ml per 30 min; P<0·05). Clonidine treatment, at all doses tested, resulted in hyperglycaemia and behavioural effects. These results showed that: (1) clonidine is not a potent GH secretagogue in the rat; (2) when administered i.v., clonidine exerts a maximal GH-releasing activity already at the dose of 30 μg/kg; and (3) clonidine-induced GH release in the rat occurs mainly through an inhibition of hypothalamic SRIF release rather than by stimulating GRF secretion.

Journal of Endocrinology (1994) 141, 259–266

ABSTRACT

Human pancreatic GH-releasing factor (hpGRF) increased the concentrations of plasma GH when infused i.v. into immature ducks. A dose-dependent increase in plasma GH was observed within 10 min of the start of infusion and was maintained during the 30-min infusion period. Simultaneous infusion of somatostatin S-14 prevented the increase in plasma GH induced by hpGRF, but when the infusion had finished there was a rebound increase in plasma GH. Infusion of the highest dose of hpGRF (800 ng/kg per min) in adult ducks had no significant effect on plasma GH.

Plasma somatostatin concentrations were reduced during the infusion of hpGRF in young but not in adult ducks. This observation suggests that the stimulatory effect of hpGRF on GH secretion may be partly due to its inhibitory effect on somatostatin secretion. Infusion of hpGRF in ducklings also increased the concentrations of glucagon and decreased levels of insulin in the plasma. Peripheral plasma glucagon and insulin levels in adult ducks were unaffected by hpGRF infusion. These results indicate that in ducklings, hpGRF increases plasma GH and glucagon concentrations and lowers plasma somatostatin and insulin levels. In the adult, these hormonal responses to hpGRF are not maintained. The highly stimulatory effect of hpGRF on GH secretion in ducklings may explain why plasma GH concentrations are high in these birds.

J. Endocr. (1987) 114, 25–32

ABSTRACT

Plasma and pituitary GH content, in-vitro GH release and somatostatin-like immunoreactivity (SLI) in the stalk-median eminence were studied up to 7 days after making an anterolateral cut (ALC) around the medial-basal hypothalamus. Plasma GH concentration increased within 15 min to a very high level, then fell to a high level which was unchanged for several hours. The GH concentration then steadily decreased between days 2 and 7. The SLI content in the stalk-median eminence decreased to 3·5% of the control value within 3 days. The GH content of the anterior pituitary gland was 58·8% of the control value by 1 week after the operation but the in-vitro sensitivity to somatostatin of the GH cells failed to change. Pentobarbitone injection stimulated GH release in the sham-operated controls but decreased it in the rats with an ALC.

These findings suggest that transection of somatostatin-containing fibres is followed by a rapid rise and a lasting high concentration of plasma GH which slowly returns towards lower levels in parallel with a marked depletion of pituitary GH content. In rats with transected somatostatin innervation of the median eminence, sodium pentobarbitone probably decreases GH secretion by depressing the secretion of GH-releasing hormone.

J. Endocr. (1985) 104, 121–127

ABSTRACT

Secretion of GH in the rat has been shown to be dependent upon age and sex. Using rat hypothalamic explants in vitro, we have studied the release and hypothalamic content of GH-releasing hormone (GHRH) and somatostatin in male and female Wistar rats at four different ages (10, 30 and 75 days, and 14 months). Basal release of GHRH was not significantly different between male and female rats, but at all ages males released more GHRH in response to stimulation by both 28 and 56 mmol potassium/l than female rats (P<0·05). Neither basal nor potassium-stimulated release of GHRH altered with age. In contrast, both basal and potassium-stimulated secretion of somatostatin increased significantly (P<0·01) with age, but was the same in the two sexes. Hypothalamic GHRH content, as assessed by the extractable tissue content following incubation, was significantly (P<0·01) lower in 10-day-old rats compared with older rats, but remained constant after 30 days of age. Somatostatin content, in contrast, increased progressively with age (P<0·01). The hypothalamic content of the two peptides was the same in both sexes.

In conclusion, our findings demonstrate that male rats release more GHRH in vitro than female rats, possibly reflecting the increased pulse amplitude of GH seen in males in vivo; the progressive fall in secretion of GH previously reported during ageing appears to parallel the progressive increase in somatostatin release and content seen in our in-vitro system.

Journal of Endocrinology (1989) 123, 53–58

ABSTRACT

Hypothalamic GH-releasing hormone (GHRH) and somatostatin contents were determined in male rats at pre- and mid-pubertal stages following acute (0·15 mg/kg) or chronic (0·1 mg/kg twice daily for 7 days) administration of clonidine. Hypothalamic GHRH content increased markedly during transition from pre- to mid- and late-pubertal stages (2·05±0·17vs 3·13±0·26 and 3·46±0·20 ng/hypothalamus respectively, means ±s.e.m.). Hypothalamic GHRH content was markedly decreased 1 h after clonidine administration in mid-pubertal but not in prepubertal rats (3·67±0·23 vs 2·65±0·22 ng/hypothalamus). Hypothalamic somatostatin content, on the other hand, decreased in both age groups, although the decrease in the mid-pubertal group was more pronounced (51% and 38% respectively). In addition, the GH responsiveness to clonidine was higher at mid-puberty than at prepuberty. Determination of hypothalamic somatostatin and GHRH content in the chronic experiment 4 h after the last drug administration revealed a marked decrease in GHRH in the mid-pubertal rats and a slight decrease in the prepubertal rats (1·78±0·32 vs 3·15±0·31 and 2·01±0·30 vs 2·35±0·15 ng/hypothalamus; P < 0·005 and P<0·05 respectively) whereas somatostatin levels were not altered. It is suggested that the clonidineinduced GH secretion is modulated by a chain of events which involves primary stimulation of GHRH release resulting in increased GH secretion which, via a negative-feedback mechanism, triggers an enhancement of somatostatin release which ultimately normalizes the system. This mechanism is age-dependent and reaches full maturity only at the onset of puberty.

Journal of Endocrinology (1991) 131, 381–385