Somatostatin (growth hormone-release inhibiting hormone)

Somatostatin response to a mixed meal in normals and in type I diabetics

Somatostatin has been proposed as a regulatory peptide of nutrient entry and fuel homeostasis because of its ability to inhibit the release of substances involved in food digestion and metabolism. The aim of the study was to evaluate the somatostatin response to a test meal in type I diabetics at the clinical onset of the disease and after two months of intensive insulin therapy. Normal subjects and diabetics in good metabolic control showed a characteristic biphasic somatostatin rise after a test meal; this response was lacking in diabetics at the onset of the disease. The response of somatostatin to a mixed meal in normals confirms its involvement in nutrient digestion and metabolism. The lacking somatostatin response in newly diagnosed type I diabetics might be related to deficient GIP response to the test meal or to other factors such as the insulinopenia or metabolic derangement characteristic of the clinical onset of the disease.

Studies on the mechanisms of somatostatin release after insulin induced hypoglycaemia in man

Insulin-induced hypoglycaemia, which stimulates gastric acid secretion, is associated with an increase in circulating somatostatin levels in man. In order to assess the mechanisms involved in this rise, six normal volunteers connected to a Biostator for continuous glucose monitoring were studied, on three separate occasions. On each occasion after basal blood sampling, 0.15 i.u./kg body weight of insulin was administered i.v. and further samples were obtained intermittently over 150 min. On one occasion, dextrose was infused by the Biostator to prevent hypoglycaemia, while on the other two, a constant infusion of either normal saline or the specific H2 antagonist cimetidine was administered. Insulin plus dextrose caused no significant changes in circulating somatostatin levels, whereas insulin plus saline was associated with a marked, sustained and significant rise in all subjects; insulin plus cimetidine also produced a rise but it was delayed; the area under the curve was significantly (P less than 0.05) greater with insulin plus saline than with insulin plus cimetidine. These results show that in man insulin itself does not stimulate somatostatin secretion directly, but indirectly via hypoglycaemia. Further, the inhibition of gastric acid secretion with cimetidine reduces somatostatin release during insulin-induced hypoglycaemia. This suggests that gastric acid may mediate somatostatin secretion associated with insulin-induced hypoglycaemia.

Growth hormone neuroregulation and the clinical relevance of somatostatin

The hypothalamus controls GH secretion from the anterior pituitary using two peptides; somatostatin inhibits GH, but physiologically the most important appears to GHRF, the structure of which has recently been discovered by two groups. This exciting development has not only given us further insight into the control of GH secretion, but also posed interesting questions as to the cause of the abnormal GH responses to various stimuli seen in patients with acromegaly. The other hypothalamic peptide controlling GH secretion, somatostatin has been the subject of intensive research in the last ten years. It is widely distributed and has important physiological actions including those involved in GH secretion and its action as a hypothalamic hormone. The secretion and synthesis of these two hypothalamic hormones is in turn modulated by a number of neurotransmitters, the most important of which appears to be dopamine. Knowledge gained in these studies has enabled the development of useful tools in the diagnosis of growth hormone deficiency as well as the only effective medical treatment for acromegaly. Much remains to be learnt of the physiology of growth hormone releasing factor and as a result further patients will benefit in the future.

Somatostatin-28 and somatostatin-14 suppression of arginine-, insulin-, and TRH-stimulated GH and PRL secretion in man

GH and PRL responses to arginine infusion and to the combined pituitary stimulation test (0.1 u/kg of insulin, 200 micrograms of TRH, 100 micrograms of LHRH) were compared in five normal men during infusions of somatostatin-14 (SS-14) and somatostatin-28 (SS-28), and during control infusions of vehicle alone. SS-14 and SS-28 were infused at a rate of 1.8 nmol/kg over 210 min. Arginine (0.5 g/kg) was infused from 30 to 60 min and the combined pituitary stimulation test commenced at 120 min. Arginine (0.5 g/kg) infused from 30 to 60 min induced an increase in GH secretion in all subjects and this increase was completely abolished in these same subjects when infused with SS-14 and SS-28. Arginine-induced hyperglycaemia was significantly greater during infusion of SS-14 and further enhanced by infusion of SS-28. A small increase in PRL secretion occurred after arginine infusion and this was not inhibited by SS-14 or SS-28. Insulin and TRH administration induced marked increases in both GH and PRL secretion. The mean GH increase was significantly inhibited by SS-14 and SS-28 up to 165 min but not thereafter. The PRL increase was significantly inhibited by SS-28 but not by SS-14 and this greater efficacy was also indicated by administering different doses of SS-28 to one subject. Taken together with the demonstration that SS-28 is released from the median eminence, these findings indicate that SS-28 has a hormonal role in the regulation of GH and PRL secretion.

Splanchnic and hepatic metabolism of somatostatin: a study in cirrhotic patients with a portacaval shunt

Experimental data suggest that somatostatin is metabolized by both liver and kidneys. Results in humans are conflicting. By studying a group of cirrhotic patients with surgically induced end-to-side portacaval shunts, basally and during a somatostatin infusion, we have been able to analyze separately the hepatic and splanchnic metabolism of this peptide. After catheterization, samples were obtained from the pulmonary artery, portal and hepatic veins. Basal pulmonary artery immunoreactive somatostatin (IRS) was significantly higher (p less than 0.001) in the cirrhotic patients (96 +/- 11 pg per ml) than in a sex- and age-matched control group (31.4 +/- 5.8 pg per ml). During the infusion of exogenous somatostatin, IRS values were higher in arterial (12,269 +/- 1,198 pg per ml) than in hepatic venous blood (7,648 +/- 1,234 pg per ml), indicating hepatic extraction of the peptide; but there was also a substantial splanchnic extraction demonstrated by higher arterial (12,269 +/- 1,532 pg per ml) than portal values (6,754 +/- 1,040 pg per ml) of IRS. During the somatostatin infusion, at very high circulation IRS levels, the liver was able to extract 38% of the peptide. This suggests that the high basal IRS levels found in liver cirrhosis are not likely to be due to hepatic failure. Possible mechanisms may involve increased somatostatin secretion, predominance of high molecular weight moieties of IRS which may not be as effectively removed by the liver, and/or portal-systemic shunting.

Somatostatin-28 inhibits LHRH-stimulated gonadotrophin secretion in man

The LH and FSH responses to 100 micrograms LHRH in a combined pituitary stimulation test were compared to five normal men during infusions of somatostatin-28 (SS-28) and somatostatin (SS-14) and during control infusions of haemacel. SS-28 and SS-14 were administered in equimolar concentrations of 1.8 nmol/kg body weight over 210 min and LHRH was injected as a bolus at 120 min after commencement of the infusion. SS-28 significantly reduced both LH and FSH responses to LHRH compared with control infusions. SS-14 did not significantly alter the FSH response but caused a significant reduction in the LH concentration 75 and 90 min after LHRH injection. These findings demonstrate that SS-28 has distinct biological activity in addition to its role as a putative precursor of SS-14. Inhibition of gonadotropin hormone secretion by SS-14 and SS-28 suggest that these hypothalamic peptides may play a part in the regulation of LH and FSH secretion.

Circulating somatostatin after food and glucose in man

Using a recently validated radioimmunoassay, changes in circulating somatostatin have been measured in normal subjects after food (a standard breakfast), and oral and intravenous glucose. After the standard breakfast, a clear and sustained rise in plasma somatostatin was seen in all subjects from a mean value (+/-1 SE) of 28 +/- 7 pg/ml to a mean peak value, at 60 min of 57 +/- 11 pg/ml. When glucose was taken by mouth a significant but smaller rise was seen, but intravenous glucose caused no significant change in plasma somatostatin. A rise in circulating somatostatin after feeding has not previously been demonstrated in normal man and it is suggested that somatostatin may have an important endocrine role in the gut.

Immunoreactive somatostatin changes during insulin-induced hypoglycaemia and operative stress in man

Little is currently known about the factors controlling somatostatin secretion. A radioimmunoassay has been developed that is sufficiently specific and sensitive to be used for physiological studies of circulating levels in man. During insulin-induced hypoglycaemia a rise in plasma somatostatin was seen in each of ten subjects studies. Although this paralleled the rise in circulating glucagon and growth hormone, no individual relationships were found either between these variables or to any change in cortisol or insulin C-peptide. In contrast no rise in somatostatin was seen during surgical stress. Thus, contrary to expectation, circulating somatostatin levels can be altered by metabolic stimuli. It seems likely that this peptide may serve an endocrine as well as a paracrine role since its modulating effects may occur not only near to but also at a distance from the site of secretion. It is not yet clear whether the somatostatin measured comes from the hypothalamus, any other part of the central nervous system or the gastrointestinal tract.

Development and validation of a specific radioimmunoassay for somatostatin in human plasma

Little is known about the factors controlling somatostatin secretion in man, and data are not available on the changes in circulating levels in various human physiological or pathophysiological states. This is mainly a consequence of the technical difficulties involved in measuring somatostatin in plasma. In the presence of plasma, binding of somatostatin tracer to antibody was consistently decreased by about 20%, and this could not be abolished by the addition of EDTA and aprotinin or by the use of specially prepared somatostatin-free plasma. Furthermore, in the presence of plasma, endogenous somatostatin does not dilute in parallel with synthetic cyclic somatostatin standard. We have, therefore, developed and validated a radioimmunoassay for somatostatin using prior extraction of the peptide onto leached silica glass. Tyrosine-II somatostatin was iodinated using lactoperoxidase and purified on ODS silica. This method is superior to iodination using chloramine-T with CMC cellulose purification, and gives a highly purified preparation with a shelf-life of at least eight weeks. Using this tracer and a specific antiserum, the limit of sensitivity of the assay was 10 pg/ml, with an intra-assay coefficient of variation of 12% (n = 16) and inter-assay coefficient of variation of 15% (n - 10). Parallelism has been demonstrated between standard synthetic cyclic somatostatin and all extracted plasma samples. The mean recovery of exogenous somatostatin from plasma was 78%. The fasting level of immunoreactive somatostatin at 0,900 hours in 40 normal subjects ranged from 17 to 81 pg/ml. Care is needed, however, when comparing these values with those obtained from other laboratories since standard preparations of somatostatin vary considerably in their immunopotency.