Increased blood somatostatin concentration in streptozotocin diabetic rats

[The role of glucose and insulin in the metabolic regulation of growth hormone secretion]

The exact physiological basis for the suppression of growth hormone secretion by oral glucose intake remains unknown, despite the widespread use of the oral glucose tolerance test in endocrinology. Lack of growth hormone suppression by glucose occurs in about a third of patients with acromegaly, as well as in other disorders. It is currently known that the secretion of growth hormone is affected by various factors, such as age, gender, body mass index, and the redistribution of adipose tissue. There is also evidence of the impact of overeating as well as being overweight on the secretion of growth hormone. It is known that both of these conditions are associated with hyperinsulinemia, which determines the possibility of its predominant role in suppressing the secretion of growth hormone. The purpose of this review is to discuss the accumulated data on the isolated effects of hyperglycemia and hyperinsulinemia on growth hormone secretion, as well as other metabolic regulators and conditions affecting its signaling. Understanding of the pathophysiological basis of these mechanisms is essential for further research of the role of glucose and insulin in the metabolic regulation of growth hormone secretion. However, the studies in animal models are complicated by interspecific differences in the response of growth hormone to glucose loading, and the only possible available model in healthy people may be the hyperinsulinemic euglycemic clamp.

Growth Hormone Response to Oral Glucose Load: From Normal to Pathological Conditions

The exact physiological basis of acute growth hormone (GH) suppression by oral glucose is not fully understood. Glucose-mediated increase in hypothalamic somatostatin seems to be the most plausible explanation. Attempts to better understand its underlying mechanisms are compromised by species disparities in the response of GH to glucose load. While in humans, glucose inhibits GH release, the acute elevation of circulating glucose levels in rats has either no effect on GH secretion or may be stimulatory. Likewise, chronic hyperglycemia alters GH release in both humans and rats nonetheless in opposite directions. Several factors influence nadir GH concentrations including, age, gender, body mass index, pubertal age, and the type of assay used. Besides the classical suppressive effects of glucose on GH release, a paradoxical GH increase to oral glucose may be observed in around one third of patients with acromegaly as well as in various other disorders. Though its pathophysiology is poorly characterized, an altered interplay between somatostatin and GH-releasing hormone has been suggested and a link with pituitary ectopic expression of glucose-dependent insulinotropic polypeptide receptor has been recently demonstrated. A better understanding of the dynamics mediating GH response to glucose may allow a more optimal use of the OGTT as a diagnostic tool in various conditions, especially acromegaly.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Regulation of rat pancreatic CCKB receptor and somatostatin expression by insulin

The cholecystokinin B receptor (CCK(B)R) is localized on pancreatic endocrine somatostatin delta-cells. Pancreatic somatostatin content was increased in diabetic rats. The mechanisms involved in this phenomenon are unknown, and we believe insulin is involved. In this study, four groups of rats were used: controls, streptozotocin-induced diabetic, streptozotocin-induced diabetic with insulin, and streptozotocin-induced diabetic with insulin and its cessation. Rats were killed after 7-28 days of treatment for diabetes, and somatostatin mRNA expression and pancreatic somatostatin content, CCK(B)R mRNA and protein expression evaluation in total pancreas and purified islets, and the cellular localization of somatostatin and CCK(B)R in islets was measured. Data indicate that diabetes is established after 7 days, is controlled by insulin, and reappears after treatment cessation. Pancreatic somatostatin mRNA expression and somatostatin content were increased during diabetes, normalized during insulin treatment, and reaugmented after treatment cessation. Gland and islet CCK(B)R mRNA and protein almost disappeared during diabetes; CCK(B) mRNA reappeared in response to insulin, but the protein did not. Confocal microscopy confirmed data obtained on somatostatin and CCK(B)R as established biochemically in the course of the treatments. In conclusion, these data strongly suggest that insulin can negatively control pancreatic somatostatin mRNA and hormone content and positively control CCK(B)R mRNA; the CCK(B)R protein appears to be delayed.

Gene regulation of somatostatin receptors in rats

Using the rat as a model, the present article summarises the spatial, temporal and hormonal regulation of the somatostatin receptor subtypes and their mRNAs in brain and periphery and attempts to provide a molecular basis for somatostatin receptor gene regulation by the structural and functional analyses of their promoters.

Time course of hypothalamic growth hormone-releasing hormone and somatostatin content in streptozocin diabetic rats: evidence for early changes in hypothalamic regulation

Growth hormone secretion is markedly suppressed early in streptozocin induced diabetes mellitus of the rat. Our studies were designed to delineate early changes in hypothalamic regulation by growth hormone-releasing hormone (GHRH) and somatostatin (SS) with the aim of determining the best time period for hypothalamic secretion studies. Although hypothalamic GHRH content (ng/hypothalamus) and SS concentration (ng/mg wet weight) were unchanged at 17 to 20 days in previous studies, we anticipated changes earlier in the time course from transient imbalances in release and synthesis. We examined hypothalamic GHRH content and SS concentration in control, diabetic, and insulin treated diabetic rats (n = 5-13; streptozocin 100 mg/kg i.p.) at 0, 2, 4, 7, 10 and 21 days. In diabetic rats GHRH content was greater at day 2 (142 +/- 9% of control-same day, P < 0.05) and day 4 (139 +/- 17%, P < 0.05), but was less at day 10 (67 +/- 4%, P < 0.01). GHRH content of insulin treated diabetic rats was elevated at day 2 (158 +/- 10%, P < 0.05), but subsequently was unchanged from control. In diabetic rats SS concentration was decreased at day 4 (78 +/- 5%, P < 0.01) and at day 21 (91 +/- 3%, P < 0.05). Our results show earliest changes compared to control in GHRH content at 2 days and in SS concentration at 4 days. These findings support early changes in hypothalamic secretion, define a time period of 1 to 10 days for further studies of release and gene expression, and suggest complex relationships of gene expression, peptide synthesis, and peptide release.

Growth Hormone neuroregulation in diabetes mellitus

GH secretion is markedly altered in diabetic rats and humans. Diabetes in the rat, whether occurring spontaneously or after streptozotocin administration, results in depressed GH secretion. This defect is likely caused by an increase in hypothalamic somatostatin tone and decreased pituitary GH. The effects of diabetes in humans depend upon the etiology of the disease. In type-1 diabetes, GH secretion is increased and in type 2 it is decreased. Again, these changes are hypothesized to be due to opposite alterations in hypothalamic somatostatin. Current evidence suggests that GH hypersecretion in human type-1 diabetes may be relevant to important metabolic and angiopathic complications of the disease.

Effects of streptozotocin-induced diabetes on lymphocyte POMC and growth hormone gene expression in the rat

Diabetes in the rat is associated with a change in the profiles of several neuroendocrine hormones resulting in poor growth and decreased immune function. Since lymphocytes can also serve as a source of neuroendocrine hormones, we have examined whether the change in hormone profiles are accompanied by an impairment of lymphocyte GH and POMC gene expression in the immune system. Diabetes was induced by the administration of streptozotocin (STZ; 10 mg/100 g body weight) and 3 days later GH and ACTH protein and mRNA were determined. The results show a modest diminution of GH RNA in the spleen of diabetic animals whereas the expression of POMC mRNA and ACTH by the thymus was enhanced. The expression of POMC in the spleen appeared unaltered while the increase of POMC RNA in the thymus was evident after the first day of STZ treatment. STZ had no direct effect on GH or POMC expression in the spleen or thymus cells in vitro. Insulin does not appear to be involved in the expression of lymphocyte GH or POMC. The administration of insulin to the diabetic animals had no significant effect on the expression of GH or POMC by the immune cells. In addition, lymphocytes do not appear to serve as a source of insulin or are the expression of genes for lymphocyte GH or ACTH altered by insulin in vitro. Taken together, the findings are the first to report on the expression of neuroendocrine genes in lymphocytes during diabetes. The mechanism for the inhibition of GH and stimulation of POMC expression by lymphocytes in diabetic animals is unknown, but it is tempting to speculate an important role in the development of the autoimmunity that characterizes this complex disease.

Role of somatostatin in the acute immobilization stress-induced GH decrease in rat

In the present work we have investigated to what extent somatostatin (SRIF) release from median eminence (ME) is affected by stress immobilization (IMO) in unanesthetized rats stereotaxically implanted with a push-pull cannula (PPC). One week after implantation, the ME was perfused with artificial cerebrospinal fluid for 1 hour in basal, stress and recovery conditions respectively. Samples were collected every 15 min and SRIF was measured by RIA. In another group of animals, a jugular cannula was inserted the day before and plasma samples were taken off simultaneously with the ME perfusate for GH and SRIF analysis respectively. SRIF release from the ME is rapidly (15 min) and significantly increased (58 +/- 11 vs 28 +/- 5 pg/15 min; n = 7; P < 0.01) in rats bearing only PPC. Intriguingly, animals bearing a jugular catheter plus a PPC showed no increase in SRIF release during the first 15 min of IMO in spite of a striking decrease of plasma GH (27.2 +/- 3.8 vs 3.6 +/- 1.3 ng/ml; n = 6; P < 0.001) observed at this time. However, in spite that the animals responded with a significant increase in SRIF, the response was later and more reduced than in animals without jugular cannula. Since our two rat groups--as result of jugular cannula surgery 24 hours before--showed differences such as a food intake, body weight gain, plasma GH levels and basal SRIF release, we think that these differences could explain the modifications in the regulatory mechanisms involved in GH control under acute stress.

Effect of growth hormone-releasing stimuli in streptozotocin diabetic rats

The dynamics of growth hormone (GH) secretion in response to different GH secretagogues has been studied in adult freely moving male rats one month after induction of diabetes by single i.v. injection of streptozotocin (60 mg/kg). Baseline plasma GH concentrations and pituitary GH content were not different in streptozotocin-diabetic (St-D) rats and controls. Clonidine (0.15 mg/kg i.v.), an alpha 2-adrenergic agonist, failed to evoke GH release in St-D rats. Substitution therapy with insulin (1 IU/100 g b.wt.daily) delivered through subcutaneously implanted minipumps, allowed re-institution of a normal GH responsiveness to clonidine. At odds with clonidine, FK 33-824 (0.1 mg/kg i.v.), a potent analog of the opioid peptide Met-enkephalin, induced a similar rise in plasma GH levels in control and St-D rats. Finally, administration of a synthetic replicate of a GH-releasing hormone of human pancreatic origin, hpGRF-40 (2.5 micrograms/kg i.v.) elicited a higher GH response in St-D rats than in controls. These data indicate that in St-D rats: (1) an impaired function of noradrenergic pathways controlling GH release is present; (2) contrary to previous beliefs, an alpha 2-adrenergic mechanism is not involved in the GH-releasing effect of opioid peptides; and (3) pituitary GH responsiveness to hpGRF is increased.

Pancreatic somatostatin

This is a review of pancreatic somatostatin which is limited in its scope and therefore focuses upon some selected issues. Throughout the entire review the same basic questions recur: Why do islets contain somatostatin? What is the physiological role of somatostatin and what does this peptide have to do with diabetes? Clear answers to these questions do not emerge, but a number of hunches are explored. The review provides a very brief look at somatostatin secretion, a discussion of the potential interactions which islet D cells might have with other islet cell types, consideration of how knowledge of islet anatomy may help us understand the D cell, and finally some comments about what happens to the D cell in diabetes and fasting.

Contribution of the pancreas to circulating somatostatin-like immunoreactivity in the normal dog

These studies were performed to assess the contribution of the pancreas to the somatostatin-like immunoreactivity (SLI) circulating in arterial and portal venous plasma. Basal SLI concentrations in arterial, pancreatic venous, and portal venous plasma were 95 +/- 9, 277 +/- 32, and 130 +/- 12 pg/ml, (means +/- SEM), respectively. Measurement of pancreatic and portal venous blood flow (5 +/- 1 vs. 365 +/- 46 ml/min) and hematocrit allowed calculation of net, base-line SLI output from the right lobe of the pancreas (521 +/- 104 pg/min) and from the gastrointestinal tract (8,088 +/- 1,487 pg/min), which suggested that the contribution of the pancreas to circulating SLI was minor when the D cells were not stimulated. To stimulate the secretion of SLI from both pancreatic and nonpancreatic sources, isoproterenol, a beta-adrenergic agonist, was infused intravenously for 1 h into six anesthetized dogs. Arterial SLI increased by 52 +/- 9 pg/ml; superior pancreatico-duodenal venous SLI increased by 380 +/- 95 pg/ml; portal venous SLI increased by 134 +/- 14 pg/ml. Pancreatic venous blood flow remained unchanged at 5 +/- 1 ml/min, but portal venous blood flow increased to 522 +/- 62 ml/min. SLI output from the right lobe of the pancreas increased by 684 +/- 227 pg/min and that from the gastrointestinal tract increased by 23,911 +/- 3,197 pg/min, again suggesting that the pancreas was a minor source of circulating SLI even when the D cells were stimulated. We conclude that the measurement of arterial-venous SLI concentrations, in the absence of measurements of organ blood flow, can give a false impression of the organ's contributions of circulating SLI. To verify that the contribution of the pancreas was negligible, six dogs received an acute pancreatectomy and then an intravenous infusion of isoproterenol at the same rate. In these dogs, both the base-line level of SLI in arterial plasma (109 +/- 12 pg/ml) and the increment during isoproterenol (56 +/- 8 pg/ml) were similar to those of normal dogs. Likewise, in pancreatectomized dogs both the base-line level of SLI in portal venous plasma (129 +/- 16 pg/ml) and the increment during isoproterenol (174 +/- 34 pg/ml) were similar to those of normal dogs. We conclude that, in normal dogs, the pancreas makes a negligible contribution to the basal and stimulated level of SLI in arterial and portal venous plasma and therefore that these levels should not be used as an index of secretory activity of the pancreatic D cells.

Alterations in somatostatin and other islet cell functions in the spontaneously diabetic BB Wistar rat: biochemical and morphological characterization

In the present study we have attempted to assess the functional status of somatostatin cells in relation to the function of the other islet cell types (B, A, and PP cells) in the BB Wistar rat. Somatostatin-like immunoreactivity (SLI), glucagon, and insulin were measured in extracted plasma obtained from the hepatic portal vein (PV) and inferior vena cava (IVC) of acutely diabetic untreated rats, insulin-treated diabetic rats and nondiabetic controls. Extracts of the pancreas were assayed for SLI, glucagon, and insulin, and the pancreatic populations of A, B, D, and PP cells were evaluated by morphometry. Extrapancreatic somatostatin changes were assessed by measurement of SLI in extracts of the whole gut, hypothalamus, and retina. Direct studies of SLI, glucagon, and insulin secretion in response to glucose, arginine, and theophylline were carried out using the isolated perfused pancreases of two separate groups of untreated diabetic and nondiabetic rats. Our results showed that in the severely insulin deficient BB Wistar rat (1) pancreatic concentrations of SLI, glucagon, and insulin were reduced; (2) the B cells are virtually eliminated and the D cells severely reduced early in diabetes; A and PP cells are resistant initially but eventually sustain major losses as observed in terminal islets; (3) retinal SLI is reduced, but SLI in gut and brain appears unchanged; (4) the secretion of SLI, glucagon, and insulin from the perfused pancreas is diminished 60%, 36%, and 99%, respectively; (5) PV and IVC blood levels of SLI and glucagon are elevated despite decreased pancreatic secretion; (6) The trans-hepatic gradient of SLI is reduced; and (7) Insulin treatment normalizes the elevated PV and IVC levels of SLI and glucagon. It is concluded that the elevated PV and IVC levels of SLI are secondary to insulin deficiency and result from increased SLI secretion most probably from the gut and from diminished hepatic metabolism. The origin of the hyperglucagonemia is less certain, but as in the case of SLI, important contributions from extra-pancreatic secretion appears likely.