Beta-adrenoceptor activity change after prolonged treatment with growth hormone and somatostatin.

1. The effect of 10 days treatment with growth hormone (GH) (1 mg/kg body wt/day) and somatostatin (SRIF) (0.25 mg/kg body wt/day) subcutaneously on the activity of beta-adrenoceptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [3H]dihydroalprenolol ([3H]DHA) as radioligand. 2. The administration of GH significantly increased the beta-adrenoceptor binding affinity and the administration of SRIF decreased the beta-adrenoceptor binding capacity in the hypothalamus. 3. In the pituitary the beta-adrenoceptor binding affinity was significantly decreased after both hormonal applications. 4. In the cerebral cortex the beta-adrenoceptor binding affinity was significantly decreased after the GH treatment and increased after the SRIF treatment. 5. The present study provides direct evidence for GH and SRIF effects on the activity of rat beta-adrenoceptors and supports the view about the involvement of beta-adrenergic mechanisms in the neurotransmitter regulation of GH secretion in the rat.     

Chronic central leptin infusion modifies the response to acute central insulin injection by reducing the interaction of the insulin receptor with IRS2 and increasing its association with SOCS3

Leptin and insulin have overlapping intracellular signaling mechanisms and exert anorexigenic actions in the hypothalamus. We aimed to determine how chronic exposure to increased leptin affects the hypothalamic response to a rise in insulin. We analyzed the activation and interactions of components of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in the hypothalamus of rats treated icv for 14 days with leptin followed by a central injection of insulin and killed 15 min later. Insulin increased glycemia and chronic leptin reduced this insulin induced rise in glucose. Leptin decreased the association between the insulin receptor beta chain (IRβ) and insulin receptor substrate 2 (IRS2), augmented the association between Janus kinase 2 and IRS2, increased levels of the catalytic subunit of PI3K and pAkt-Ser473 and decreased forkhead box O number 1 levels. Insulin reduced the association between suppressor of the cytokine signaling 3 and IRβ, increased IRβ-IRS2 association and pAkt-Thr308 levels, with chronic leptin exposure blunting these effects. In conclusion, chronic exposure to leptin decreases the central response to insulin by increasing suppressor of the cytokine signaling 3 association to IR, which inhibits insulin signaling at the level of interaction of its receptor with IRS2 and activates PI3K by promoting Janus kinase 2-IRS2 association. Thus, these results suggest that this mechanism could be a target for the treatment of insulin resistance.     

Beta-adrenoceptor activity change after prolonged treatment with growth hormone and somatostatin

1. The effect of 10 days treatment with growth hormone (GH) (l mg/kg body wt/day) and somatostatin (SRIF) (0.25 mg/kg body wt/day) subcutaneously on the activity of β-adrenoceptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [³H]dihydroalprenolol ([³H]DHA) as radioligand.2. The administration of GH significantly increased the β-adrenoceptor binding affinity and the administration of SRIF decreased the β-adrenoceptor binding capacity in the hypothalamus.3. In the pituitary the β-adrenoceptor binding affinity was significantly decreased after both hormonal applications.4. In the cerebral cortex the β-adrenoceptor binding affinity was significantly decreased after the GH treatment and increased after the SRIF treatment.5. The present study provides direct evidence for GH and SRIF effects on the activity of rat β-adrenoceptors and supports the view about the involvement of β-adrenergic mechanisms in the neurotransmitter regulation of GH secretion in the rat.     

Multiple subcutaneous injections of somatostatin induce tachyphylaxis of the suppression of plasma insulin, but not glucagon, in the rat

The time course of pancreatic effects of somatostatin was studied over a period of 2 h in unanesthetized unrestrained rats after administration of the peptide by intravenous infusion and by single and multiple subcutaneous injections. During infusion of 10 and 30 micrograms/kg per min, somatostatin continuously suppressed plasma insulin and plasma glucagon. Plasma glucose was significantly increased at the lower dose, but not affected at the higher dose. Single subcutaneous injections of 0.3 and 3 mg/kg decreased plasma insulin and glucagon dose-dependently for 20-60 min without affecting plasma glucose. Multiple subcutaneous injections of somatostatin (one to four doses of 3 mg/kg, administered at intervals of 30 min) caused an initial decrease of plasma insulin (at 30 min), a rebound-increase at 60 and 90 min, and a final return to control values by 120 min. Plasma glucagon remained continuously suppressed. Plasma glucose increased significantly at 60 and 90 min and tended to return towards control values thereafter. In conclusion, pancreatic B cells - but not A cells - of the rat develop tachyphylaxis to somatostatin within 2 h after multiple subcutaneous injections of the peptide. By this mode of administration, 'selective' suppression of plasma glucagon can be achieved with somatostatin in the rat.     

Insulin Binding in Four Regions of the Developing Rat Brain

Specific insulin binding has been demonstrated in partially purified membranes prepared from four regions of the developing rat brain. Insulin binding to brain membranes demonstrated kinetics and hormonal specificity that were quite similar to those reported for traditional insulin target tissues (e.g., liver and adipose tissue), and binding was significantly correlated with receptor concentration. Binding in the olfactory bulbs, cerebrum, cerebellum, and hypothalamus all reached highest values at 15 days of postnatal life, with the olfactory bulbs generally showing the greatest binding at all ages studied. A temporal relationship was found between insulin binding to brain membranes in the postnatal rat and plasma membrane protein synthesis, especially in the cerebellum and olfactory bulbs.     

Effects of hypophysectomy and acute growth hormone treatment upon glucose metabolism in adipose tissues and isolated adipocytes of rats.

Glucose oxidation and incorporation into lipid were measured in epididymal adipose tissues and isolated adipose cells of normal and hypophysectomized rats in an effort to determine whether the acute hypoglycemic effect of a systemic growth hormone (GH) injection was related to alterations in the glucose metabolism of adipose tissue. The rats were fed rat chow or a high sucrose diet and received 100 mug GH intraperitoneally 30 minutes or three and one-half hours before sacrifice. Hypophysectomized rats showed a lower plasma glucose as compared with normal rats on both diets. Thirty minutes after a GH injection there was a further decrease of the plasma glucose which, however, was not present in those rats receiving GH three and one-half hours before sacrifice. Adipose tissues from hypophysectomized rats fed the high sucrose diet showed a blunted insulin sensitivity as compared with normal rats on a similar diet. The insulin sensitivity of these tissues was further decreased 30 minutes after a GH injection. Basal glucose metabolism of isolated adipocytes from hypophysectomized rats, as compared with normal rats, was depressed if they were fed rat chow, was at normal levels if they were fed the high sucrose diet and was increased if they were fed the sucrose diet and received triiodothyronine and cortisone supplements. No manipulations of diet or hormonal treatments made the isolated adipocyte from hypophysectomized rats sensitive to insulin either 30 minutes or three and one-half hours after a GH injection. Since basal glucose utilization is not enhanced by GH injection and both the blunted insulin sensitivity of adipose tissue and the absent insulin sensitivity of adipopocytes would be expected to produce hyperglycemia rather than hypoglycemia, it is concluded that immediate systemic effects of a GH injection on carbohydrate metabolism are not related to changes in glucose metabolism of the peripheral adipose tissues.     

Insulin and the way the brain handles glucose.

—In living rats the concentration of insulin in the circulating blood was raised and independently of this the glucose concentration in the blood plasma was varied from hyperglycaemic to hypoglycaemic levels. Hyperglycaemia increased the influx of glucose into the brain and it also, for a limited period, increased the glucose gain by the brain. Insulin, on the other hand, did not affect influx but significantly increased the gain of glucose by the brain. It is suggested that although both hyperglycaemia and insulin can increase glucose gain by the brain they do so in entirely different ways.     

Insulin affects glucose uptake by muscle and mammary tissues of lactating ewes

Effects of insulin on exchanges of glucose across skeletal muscle and mammary tissue were measured in short-term studies in lactating ewes. Insulin secretion was suppressed by a primed/continuous infusion of somatostatin, then insulin was administered by continuous intravenous infusion of doses that were increased, in a step-wise manner, from 0 to 2 U h-1. Plasma glucose was maintained essentially constant by frequent monitoring and intravenous administration of exogenous glucose. Somatostatin suppressed but did not completely inhibit insulin secretion as shown by maintenance of plasma concentration of C-peptide. As plasma insulin was increased, while arterial glucose was maintained stable, uptake of glucose by skeletal muscle increased and glucose uptake by the mammary gland decreased. These observations confirm the role of insulin in regulating glucose uptake by skeletal muscle and raise the possibility that insulin also regulates glucose uptake by the mammary gland.     

Characterisation of immunoreactive somatostatin in human fetal hypothalamic tissue

Levels of immunoreactive somatostatin (IRS) were measured in extracts of hypothalamic tissue from human fetuses of 12–26 weeks gestation. The IRS contents (0.7–22.5 ng) and concentrations (2.7–118.0 pg/mg wet weight tissue) both increased slightly with gestation. Sephadex G-50 chromatography of 11 extracts showed up to four peaks of IRS, one co-eluting with synthetic somatostatin-14 (S14), a second co-eluting with synthetic somatostatin-28 (S28) and two other peaks having approximate molecular weights of 6000 and 10 000, respectively. The levels of S14 and S28 increased significantly during gestation, while the levels of 6000 molecular weight IRS decreased with age. We suggest that the increase in S14 and S28 levels may be the cause of the fall in circulating growth hormone (GH) in the fetus in later gestation.     

Inhibition by antiserum to rat growth hormone-releasing factor of growth hormone secretion induced by a met5-enkephalin analog, FK33-824, in rats

A Met5-enkephalin analog, FK33-824 (5, 10 and 20 micrograms/100 g body wt, iv) caused a dose-related increase in plasma growth hormone (GH) in urethane-anesthetized male rats. Pretreatment with cysteamine (30 mg/100 g body wt, sc), a depletor of hypothalamic somatostatin, increased the plasma GH response to FK33-824 (10 micrograms/100 g body wt, iv). Antiserum specific for rat GH-releasing factor (GRF) (0.5 ml/rat, iv) blunted GH release induced by FK33-824 (10 micrograms/100 g body wt, iv) in rats with or without cysteamine pretreatment. These results suggest that GH secretion induced by the opioid peptide is mediated, at least in part, by hypothalamic GRF in the rat.     

Human, but not rat, IRS1 targets to the plasma membrane in both human and rat adipocytes

Adipocytes are primary targets for insulin control of metabolism. The activated insulin receptor phosphorylates insulin receptor substrate-1 (IRS1), which acts as a docking protein for downstream signal mediators. In the absence of insulin stimulation, IRS1 in rat adipocytes is intracellular but in human adipocytes IRS1 is constitutively targeted to the plasma membrane. Stimulation of adipocytes with insulin increased the amount of IRS1 at the plasma membrane 2-fold in human adipocytes, but >10-fold in rat adipocytes, with the same final amount of IRS1 at the plasma membrane in cells from both species. Cross-transfection of rat adipocytes with human IRS1, or human adipocytes with rat IRS1, demonstrated that the species difference was due to the IRS1 protein and not the cellular milieus or posttranslational modifications. Chimeric IRS1, consisting of the conserved N-terminus of rat IRS1 with the variable C-terminal of human IRS1, did not target the plasma membrane, indicating that subtle sequence differences direct human IRS1 to the plasma membrane.     

Metabolic and hormonal parameters after insulin-induced hypoglycemia in man, comparison between biosynthetic human insulin and purified pork insulin

Although clinically undistinguishable, some authors have found important differences in the counterregulatory response between Biosynthetic Human Insulin (BHI) and Purified Pork Insulin (PPI). To reassess the problem 10 healthy volunteers of both sexes underwent paired iv insulin tolerance test with both BHI and PPI (0.10 U/kg b.w.). To check the humoral response the variations of glucose, free fatty acids (FFA), prolactin, growth hormone, ACTH and plasma renin activity were evaluated. Blood glucose depression and further recovery by BHI and PPI administration paralleled each other, so were, prolactin, FFA, and plasma renin activity. A slight section of ACTH, and GH was observed under BHI challenge. There were not statistically significant differences between both insulins on any of the six parameters studied. The data do not confirm earlier published reports indicating hormonal and metabolic differences between human and porcine insulin.     

Deficiency of immunoreactive somatostatin in the median eminence of Snell dwarf mice

Snell dwarf mice (dw/dw) are characterized by a genetically determined, congenital lack of pituitary GH, TSH and prolactin. Given that hypothalamic somatostatin is involved in the regulation of pituitary GH and TSH release, it was decided to investigate the content of immunoreactive somatostatin (IRS) in the median eminence of dw/dw and phenotypically normal mice of the same strain. The content of IRS in the pyloric antrum and pineal gland of these animals was also examined. The effects of ovariectomy and of hyperprolactinemia (induced by a pituitary graft under the kidney capsule) on the median eminence content of IRS were also studied in both normal and dwarf mice. Median eminence IRS content was significantly lower in the dw/dw (23.6 +/- 1.8 ng) than in normal mice (57.4 +/- 7.1 ng); no difference was found in the pyloric IRS content of dw/dw (16.9 +/- 1.6 ng/mg of protein) and normal animals (13.8 +/- 1.9 ng/mg of protein), nor in the pineal content of IRS (639.4 +/- 64.4 pg/gland in the dw/dw; 732 +/- 265 pg/gland in normals). Neither ovariectomy nor hyperprolactinemia were found to affect the IRS content in the tissues studied in normal or dwarf mice. Treatment of an additional group of 9 dwarf mice with L-thyroxine (L-T4 2 micrograms/48 h. s.c. for 2 weeks) significantly increased the animals weight (10.2 +/- 0.4 g versus 7.4 +/- 0.3 g) and produced maturation of facial features; however, it did not change the IRS content in any of the tissues studied. It is concluded that the content of IRS in the median eminence of mice with a congenital lack of GH, TSH and prolactin is significantly reduced and that this is unlikely to be related to the deficiency of thyroid hormones in these animals.     

The effect of ventromedial hypothalamic nuclei destruction on somatostatin and insulin release from the isolated perfused rat pancreas

The effect of electrolytic lesions in the ventromedial hypothalamic nuclei (VMH) on somatostatin and insulin release was studied using the isolated perfused rat pancreas. Obesity gradually developed in the rats after placement of the VMH lesions, and fasting insulin levels determined immediately before the isolation of the pancreas were significantly higher than those in sham-operated controls. In the presence of 4.4 mM glucose, both perfusate somatostatin and insulin responses to arginine were significantly greater than in the controls, suggesting that VMH lesions cause not only hypersecretion of insulin but hypersecretion of somatostatin as well.     

Effect of pentobarbital and urethane on the release of hypothalamic somatostatin and pituitary growth hormone

Hypothalamic somatostatin release was investigated in the rat to elucidate the mechanism of anesthetic action on growth hormone (GH) release from the pituitary. Intraperitoneal injection of sodium pentobarbital (5 mg/100 gm B.W.) significantly elevated serum GH levels and increased hypothalamic somatostatin concentration from basal values of 0.98 +/- 0.01 to 1.21 +/- 0.06 ng/mg wet wt. In contrast, urethane (150 mg/100 gm B.W., IP) administration lowered serum GH levels and hypothalamic somatostatin concentration (0.64 +/- 0.04 ng/mg wet wt.). However, the mean concentration of pancreatic somatostatin showed no change in either case. In rats receiving passive immunization with 0.5 ml rabbit antiserum to somatostatin (SRIF-AS), serum GH levels were significantly increased (67.5 +/- 12.3 ng/ml) and did not differ from those in the group treated with normal rabbit serum (NRS) plus pentobarbital (101.3 +/- 18.5 ng/ml). However, serum GH levels in rats injected with SRIF-AS plus pentobarbital were increased to higher values than in rats given SRIF-AS alone. When urethane was administered to rats after passive immunization with SRIF-AS, urethane-induced suppression of serum GH levels was markedly inhibited (5.5 +/- 2.0 vs. 33.5 +/- 7.5 ng/ml). These results suggest a possibility that the changes in serum GH levels observed with pentobarbital or urethane administration may be induced at least in one part by somatostatin released from the hypothalamus.     

Studies on the hormonal regulation of fuel metabolism in the human newborn infant undergoing anaesthesia and surgery

Little is known of the endocrine and metabolic milieu in preterm and term neonates exposed to surgical stress. In order to define the effects of anaesthesia and surgery on the hormonal regulation of intermediary metabolism, the levels of plasma insulin, glucagon, adrenaline and noradrenaline were measured in addition to blood glucose, lactate, pyruvate, alanine, acetoacetate, hydroxybutyrate, glycerol and plasma-free fatty acids in 38 neonates (23 term, 15 preterm) undergoing surgery. Blood samples were drawn pre-operatively, at the end of surgery, and at 6, 12 and 24 h post-operatively. Plasma levels of adrenaline and noradrenaline increased significantly in response to surgery. In term neonates, plasma insulin concentrations were unaltered at the end of surgery, but were significantly increased throughout the post-operative period; plasma glucagon levels were unchanged at the end of surgery but had significantly decreased by 24 h after surgery. Insulin levels in preterm neonates remained unchanged during surgery as well as in the post-operative period. All neonates developed a significant peri-operative hyperglycaemia which persisted up to 12 h after surgery. Blood lactate and pyruvate increased during surgery, accompanied by significant increases in plasma free fatty acids, total ketone bodies and glycerol concentrations by the end of surgery. Blood glucose concentrations were significantly correlated with plasma adrenaline levels at the end of surgery and with plasma glucagon at 6 h post-operatively. The insulin/glucose ratio was significantly decreased at the end of surgery in term and preterm neonates. Further analysis showed that total parenteral nutrition given just before surgery and thiopentone anaesthesia given during surgery significantly augmented the peri-operative hyperglycaemic response of term neonates. Thus, stress-related hormonal changes in preterm and term neonates may precipitate a catabolic state characterized by glycogenolysis, gluconeogenesis, lipolysis and mobilization of gluconeogenic substrates in the post-operative period. Prevention of these metabolic derangements by anaesthetic or hormonal manipulation may possibly help to improve the clinical outcome of neonates undergoing surgery.     

Effect of neonatal hypoxia on leptin, insulin, growth hormone and body composition in the rat.

The purpose of the present study was to evaluate the effect of exposure to hypoxia from birth to 7 days of age on leptin, insulin, growth hormone (GH), insulin-like growth factor-1 (IGF-1), glucose, corticosterone, body weight, and body composition in rats studied at 7 days of age and then after return to normoxia. Hypoxia for the first 7 days of life resulted in a significant decrease in plasma leptin, body weight, and an increase in corticosterone and insulin with no change in plasma glucose, GH or IGF-1. There was no significant effect of hypoxia on % lean body mass, but a small but significant increase in % body fat. Bone mineral density (BMD) was lower in 7-day-old hypoxic rats as compared to normoxic controls. All hormonal variables and BMD had normalized by 7 days after return to normoxia. However, body weight remained lower even 5 weeks after return to normoxia. We conclude that leptin is decreased during neonatal hypoxia despite no change in adiposity. Furthermore, insulin is increased probably to overcome the effects of increased counterregulatory hormones (such as corticosterone).     

Effects of fructose and glucose on plasma leptin, insulin, and insulin resistance in lean and VMH-lesioned obese rats

To determine the influence of dietary fructose and glucose on circulating leptin levels in lean and obese rats, plasma leptin concentrations were measured in ventromedial hypothalamic (VMH)-lesioned obese and sham-operated lean rats fed either normal chow or fructose- or glucose-enriched diets (60% by calories) for 2 wk. Insulin resistance was evaluated by the steady-state plasma glucose method and intravenous glucose tolerance test. In lean rats, glucose-enriched diet significantly increased plasma leptin with enlarged parametrial fat pad, whereas neither leptin nor fat-pad weight was altered by fructose. Two weeks after the lesions, the rats fed normal chow had marked greater body weight gain, enlarged fat pads, and higher insulin and leptin compared with sham-operated rats. Despite a marked adiposity and hyperinsulinemia, insulin resistance was not increased in VMH-lesioned rats. Fructose brought about substantial insulin resistance and hyperinsulinemia in both lean and obese rats, whereas glucose led to rather enhanced insulin sensitivity. Leptin, body weight, and fat pad were not significantly altered by either fructose or glucose in the obese rats. These results suggest that dietary glucose stimulates leptin production by increasing adipose tissue or stimulating glucose metabolism in lean rats. Hyperleptinemia in VMH-lesioned rats is associated with both increased adiposity and hyperinsulinemia but not with insulin resistance. Dietary fructose does not alter leptin levels, although this sugar brings about hyperinsulinemia and insulin resistance, suggesting that hyperinsulinemia compensated for insulin resistance does not stimulate leptin production.     

Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids in rabbits

Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids were studied in fasted rabbits by infusing epinephrine, norepinephrine, isoproterenol, phentolamine (an adrenergic alpha-receptor blocking drug) and propranolol (an adrenergic beta-receptor blocking drug). The adrenergic effects on the plasma levels of insulin, glucose and free fatty acids were similar to those found in other species. The plasma levels of insulin were increased by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) and decreased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine). The plasma levels of glucose were increased by both alpha- and beta-receptor stimulation, and the epinephrine-induced hyperglycaemia was only blocked by combined infusions with phentolamine and propranolol. The plasma levels of free fatty acids were increased by saline and further increased by beta-receptor stimulation (isoproterenol), while epinephrine and norepinephrine gave variable results. Alpha-receptor stimulation (propranolol + epinephrine) slightly decreased the plasma levels of free fatty acids. The plasma levels of glucagon, however, were mainly increased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine) and increased only to a minor extent by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) in rabbits. This is in contrast to results reported for humans, where beta-receptor stimulation seems to be most important in stimulating glucagon release.