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B. SHAPIRO, M. SHEPPARD, S. KRONHEIM and B. L. PIMSTONE

Immunoreactive somatostatin is present in the brain, gut and pancreas of the South African clawed toad, but is absent from the skin, a rich source of many other brain–gut peptides.

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MICHEL RIEUTORT

An in-vitro study of GH secretion by rat fetal and neonatal pituitary glands was conducted using a perifusion system. After a 2 h period the GH content of the effluent was constant. Theophylline, thyrotrophin releasing hormone (TRH) and rat stalk median eminence extract (SME) were effective stimuli of GH release from the pituitary glands of the 19·5-day-old fetuses. Somatostatin, added to the medium (10 μg/ml), had no inhibitory effect on GH release (basal or stimulated by either theophylline or SME) before day 4 after birth. After postnatal day 5, somatostatin always inhibited GH secretion. These findings were consistent with the results of experiments in vivo. In rats tested within 4 days of birth, sodium pentobarbitone-stimulated plasma GH levels were not reduced by somatostatin; on day 4 and thereafter somatostatin depressed the response to pentobarbitone injection.

These results indicate a postnatal maturation of the regulation of GH release by the hypothalamo–hypophysial system in the rat.

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S.-K. Lam, S. Harvey, T. R. Hall and G. S. G. Spencer

ABSTRACT

The influence of somatostatin on thyroid function has been examined in immature domestic fowl passively immunized with somatostatin antiserum. Plasma thyroxine (T4) and tri-iodothyronine (T3) concentrations were markedly increased within 10 min of antisomatostatin administration and remained raised for at least 5 h. The increases in the T3 and T4 concentrations following somatostatin immunoneutralization were directly related to the volume of antisera administered. The increase in the T3 concentration exceeded the increase in the T4 concentration, resulting in a T3: T4 ratio greater than unity. While the raised T4 concentration began to decline 30 min after antisomatostatin administration, raised T3 concentrations were sustained for at least 2 h, and further increased the plasma T3: T4 ratio.

These results demonstrate that somatostatin immunoneutralization stimulates thyroid function in fowl. The magnitude and rapidity of the thyroidal responses to somatostatin immunoneutralization suggests that they occur independently of the hypothalamic-pituitary-thyroid axis. Somatostatin appears to exert a tonic inhibitory control on avian thyroid function, possibly by effects mediated at the thyroid gland to inhibit T4 release and by peripheral effects to suppress the conversion of T4 and T3.

J. Endocr. (1986) 110, 127–132

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D Olchovsky, J F Bruno and M Berelowitz

Abstract

Growth hormone-releasing factor (GRF) mRNA expression in male rats occurs predominantly in the hypothalamus (mainly in the arcuate nucleus), and among extraneural sites primarily in the testis. Hypothalamic GRF is the physiological tropic stimulus to growth hormone secretion. However, the role of GRF in the testis is unknown. We have shown previously that hypothalamic GRF mRNA expression is significantly reduced in streptozotocin (STZ)-diabetic rats. This reduction is confined to the arcuate nucleus and probably accounts for the suppression of growth hormone pulsatility.

The present studies were performed to evaluate GRF expression in the testis of streptozotocin (STZ)-diabetic rats. Diabetes was induced by injection of STZ (100 mg/kg i.p.). Seventeen to twenty days later diabetic rats were hyperglycemic compared with vehicle-injected controls and demonstrated growth failure. Insulin treatment reduced the glycemia and increased body weight towards normal. Total RNA was extracted from the hypothalamus and testis, and GRF mRNA levels estimated by solution hybridization/nuclease protection assay. Levels of hypothalamic somatostatin mRNA were measured to serve as control values. GRF mRNA was significantly (P<0·001) decreased in the hypothalamus of STZ-diabetic rats (0·2 ± 0·07 mean relative densitometric units, n=8) compared with controls (1·0 ± 0·19, n=8) with no change in somatostatin mRNA expression. In contrast, testicular GRF mRNA was increased 70% (P<0·05) in STZ-diabetic rats. Insulin treatment resulted in normalization of hypothalamic GRF mRNA levels (1·1 ± 0·17, n=5) with no effect on testicular GRF mRNA expression.

In conclusion GRF gene expression is discordantly regulated in tissues of male STZ-diabetic rats. While reduced GRF expression may account for the low growth hormone state in this model, increased testicular GRF mRNA (with the previously reported reduction of insulin-like growth factor-I mRNA) resembles the response seen in growth hormone-sensitive tissue (especially the hypothalamus) to this growth hormone-deficient state.

Journal of Endocrinology (1996) 148, 189–192

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K M Fairhall, A Mynett and I C A F Robinson

Abstract

Growth hormone (GH) release is stimulated by a variety of synthetic secretagogues, of which growth hormone-releasing hexapeptide (GHRP-6) has been most thoroughly studied; it is thought to have actions at both pituitary and hypothalamic sites. To evaluate the central actions of this peptide, we have studied GH release in response to direct i.c.v. injections in anaesthetized guinea pigs. GHRP-6 (0·04–1 μg) stimulated GH release >10-fold 30–40 min after i.c.v. injection. The same GH response required >20-fold more GHRP-6 when given by i.v. injection. GH release could also be elicited by a non-peptide GHRP analogue (L-692,585, 1 μg i.c.v.), whereas a growth hormone-releasing factor (GRF) analogue (human GRF 27Nle(1–29)NH2, 2 μg, i.c.v.) was ineffective. A long acting somatostatin analogue (Sandostatin, SMS 201–995, 10 μg i.c.v.) (SMS) given 20 min before 200 ng GHRP-6 blocked GH release. This was unlikely to be due to a direct effect of SMS leaking out to the pituitary, since central SMS injections did not affect basal GH release, nor did they block GH release in response to i.v. GRF injections. We conclude that the hypothalamus is a major target for GHRP-6 in vivo. Since the GH release induced by central GHRP-6 injections can be inhibited by a central action of somatostatin, and other data indicate that GHRP-6 activates GRF neurones, we suggest that somatostatin may block this activation via receptors known to be located on or near the GRF cells themselves. Somatostatin may therefore be a functional antagonist of GHRP-6 acting centrally, as well as at the pituitary gland.

Journal of Endocrinology (1995) 144, 555–560

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H. Sugihara, S. Minami and I. Wakabayashi

ABSTRACT

To examine the characteristics of GH secretion following the termination of the infusion of somatostatin, unrestrained adult female Wistar rats were subjected to repeated infusions of somatostatin separated by 30-min control periods. When somatostatin was infused for 150 min at a dose of 3, 30 or 300 μg/kg body wt per h, the magnitude of the rebound GH secretion increased in a dose-dependent manner. The infusion of somatostatin at a dose of 300 μg/kg body wt per h for 60, 150 or 240 min progressively augmented the size of the rebound GH secretion. When an antiserum to rat GH-releasing factor (GRF) was injected i.v. 10 min before the end of the infusion, the peak amplitude of the rebound GH secretion (300 μg/kg body wt, 150 min) was reduced to less than 20% of that of control rats. The rebound GH secretion (300 μg/kg body wt per h, 150 min) was augmented by a bolus injection of human GRF (1 μg/kg body wt). The combined effect of the end of infusion of somatostatin and a bolus injection of GRF on the amount of GH secreted was additive. The plasma GH response to GRF was completely inhibited when human GRF (3 μg/kg body wt per h) and somatostatin (300 μg/kg body wt per h) were infused simultaneously for 150 min. The magnitude of the rebound GH secretion following the termination of the co-administration was larger than that following the somatostatin infusion alone, but this rebound was not enhanced by a bolus injection of human GRF. Moreover, the amount of GH secreted was significantly less than that after the termination of somatostatin infusion plus a bolus injection of human GRF in the absence of preceding GRF administration.

These results suggest that at least part of the influence of somatostatin on GH secretion is exerted at the level of the hypothalamus through modulating the release of GRF. In addition, it is inferred that the simultaneous infusion of GRF and somatostatin induces the attenuation of the GH response to GRF through a receptor effect.

Journal of Endocrinology (1989) 122, 583–591

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R. N. Clayton and L. C. Bailey

ABSTRACT

The effect of somatostatin on GH-releasing factor (GRF)-induced desensitization of somatotrophs was studied in vitro. Primary cultures of rat anterior pituitary cells pretreated for 4 or 18 h with GRF(1–40) (100 nmol/l) showed a 50% or greater reduction in maximal GH release when rechallenged with 10 nmol GRF/l. Rechallenge GRF dose–response curves were either very flat, making accurate measurement of the dose giving 50% maximum stimulation (ED50) impossible, or the ED50 concentration was increased from 0·3 nmol/l (untreated) to 2 nmol/l (GRF pretreated). Although GRF pretreatment reduced cellular GH content by 40–50%, correction for this did not restore GRF responsiveness measured in terms of maximal GRF-stimulated/unstimulated GH release (maximal/basal ratio), or the GRF ED50 concentration. Maximal/basal GH release per 4 h from GRF-pretreated cells was reduced when cells were rechallenged with forskolin (5 μmol/l) or calcium ionophore (A23187; 10 μmol/l), to the same extent as when rechallenged with 10 nmol GRF/l. Although this might be explained by a reduction in the pool of releasable GH, an alternative explanation is that pretreatment with GRF disrupts the GH release mechanism(s) at a common step(s) beyond cyclic AMP generation and Ca2+ influx.

Co-incubation of cells with somatostatin and GRF (100 nmol/l) partially reversed the desensitizing action of GRF during both 4- and 18-h pretreatments in a dose-dependent manner, with 1 μmol somatostatin/l being most effective. Maximal GRF (100 nmol/l)-stimulated/basal GH release was 4·4 ± 1·0 (mean ± s.e.m., n = four experiments), 1·55 ± 0·09 and 2·43 ± 0·1 for control, GRF-pretreated (4 h) and GRF plus somatostatin-pretreated cells respectively. Comparable values for cells pretreated for 18 h were 3·66 ± 0·44 (n = three experiments), 1·78 ± 0·28 and 3·04 ± 0·04 for control, GRF- and GRF plus somatostatin-pretreated cells. Somatostatin reduced the 50% depletion of cellular GH caused by GRF pretreatment to 15–20%, as well as attenuating GH released during the pretreatment period by 40 ± 5% (mean ± s.e.m., n = seven experiments). Somatostatin restored somatotroph sensitivity of GRF-desensitized cells indicating that, in addition to reversing depletion of the releasable pool of GH, the counter-regulatory hormone also prevents disruption of post-receptor cellular biochemical events which remain to be identified. These results add to the list of GRF actions inhibited by somatostatin and suggest a potentially important role for somatostatin in vivo to maintain somatotroph responsiveness to GRF.

J. Endocr. (1987) 112, 69–76

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A. Grossman and S. Tsagarakis

In the halcyon days when life was simple, many thought that pituitary hormones were under the control of single hypothalamic factors which regulated their synthesis and release. Matters became a little more complex when the search for growth hormone (GH)-releasing hormone was punctuated by the discoveries of somatostatin, which inhibited both GH and thyrotrophin (TSH), by the co-release of TSH and prolactin by thyrotrophin-releasing hormone, and then by the substantiation of other prolactin-releasing factors such as vasoactive intestinal peptide. It has since become increasingly clear that pituitary peptides are regulated by a whole series of hypothalamic factors, both stimulatory and inhibitory, and are also subject to intrapituitary paracrine modulation.

There has, however, been slow acceptance of the concept that the release of adrenocorticotrophin (ACTH) too may be finely tuned by an inhibitory factor. There is clearly a predominant role for a stimulatory factor to ACTH release, the earliest candidate for

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YS Huang, K Rousseau, N Le Belle, B Vidal, E Burzawa-Gerard, J Marchelidon and S Dufour

Insulin-like growth factor (IGF)-I has been suggested as a potential signal linking growth and puberty in mammals. Using the juvenile European eel as a model, we employed a long-term, serum-free primary culture of pituitary cells to study the direct effect of IGF-I on gonadotrophin (GtH-II=LH) production. IGF-I increased both cell content and release of GtH-II in a time- and dose-dependent manner. IGF-I and IGF-II had similar potencies but insulin was 100-fold less effective, suggesting the implication of an IGF type 1 receptor. Other growth and metabolic factors, such as basic fibroblast growth factor and thyroid hormones, had no effect on GtH-II production. IGF-I did not significantly increase the number of GtH-II immunoreactive cells, indicating that its stimulatory effect on GtH-II production does not result from gonadotroph proliferation. Comparison of IGF-I and somatostatin (SRIH-14) effects showed that both factors inhibited growth hormone (GH) release but only IGF-I stimulated GtH-II production by eel pituitary cells. This indicates that the effect of IGF-I on gonadotrophs is not mediated by the reduction of GH released by somatotrophs into the culture medium. This study demonstrates a specific stimulatory effect of IGF-I on eel GtH-II production, played out directly at the pituitary level. These data obtained in a primitive teleost suggest that the role of IGF-I as a link between body growth and puberty may have been established early in the evolution of vertebrates.

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J. R. E. Davis and N. Hoggard

Pituitary adenomas are a common form of endocrine neoplasia in man, and cause clinical problems resulting from syndromes of hormone hypersecretion, hypofunction of the residual normal pituitary gland, or from mass effects from the tumour bulk itself. They can now be treated by surgery, by irradiation or by endocrine therapies such as dopamine or somatostatin agonists, but none of these options has proved entirely satisfactory. After intense scrutiny of pituitary physiology and biochemistry, only now are some of the causes of pituitary tumour formation becoming understood, and this short review will discuss some recent advances in the field.

Pituitary tumours generally arise from a single differentiated cell type expressing its appropriate mature pituitary hormone product (such as prolactin, growth hormone (GH), adrenocorticotrophin (ACTH) or thyroid-stimulating hormone (TSH), and the hormone hypersecretion often leads to a clinically recognized syndrome. About 25% of adenomas are clinically 'non-functioning', but most of these in