Immunoreactive Somatostatin Levels Research Articles

Metabolic half-life of somatostatin and peptidase activities are altered in Alzheimer's disease.

Several reports have described decreased immunoreactive somatostatin levels in specific regions of post-mortem brain tissue from patients diagnosed with senile dementia of the Alzheimer type (SDAT). In an attempt to determine if the metabolism of somatostatin is also altered as a result of SDAT, we examined the regional metabolic half-life of somatostatin-28 (SS-28) and somatostatin-14 (SS-14). The activity of the following peptidases was also determined: neutral endopeptidase E.C. 3.4.24.11; metalloendopeptidase E.C. 3.4.24.15; carboxypeptidase E (E.C. 3.4.17.10); and trypsin-like serine protease. The metabolic half-life of SS-28 was significantly reduced in post-mortem Brodmann Area 22 of SDAT tissue. This decrease in SS-28 metabolic half-life was correlated with a significant increase in trypsin-like serine protease activity in the same SDAT brain region. The formation rate of SS-14 from SS-28 incubated with Brodmann Area 22 homogenates was also increased in SDAT tissues as compared to controls. A regional variation in neutral endopeptidase E.C. 3.4.24.11 was also noted in both controls and SDAT samples. Although postmortem intervals of samples varied significantly, no effect was seen on any biochemical parameter measured. Results from this study provide evidence that a correlation can be made between changes in metabolic half-life somatostatin and alterations in neuropeptidase activities due to SDAT. As these data show alterations in both proteolytic metabolism and peptidase activities, many other biologically active peptide substrates could also be affected in SDAT.

Inhibition of Rat Pancreatic Exocrine Secretion by Neuropeptide Y

Neuropeptide Y (NPY) is a unique 36 amino acid peptide with strong sequence homology to pancreatic polypeptide and peptide YY. In the rat pancreas, NPY-positive fibers have been demonstrated in close association with exocrine structures, suggesting a regulatory role for the peptide. In conscious rats with pancreatic ductal cannulas, amylase output stimulated by cholecystokinin octapeptide (CCK-8: 0.2 microgram kg - 1 h -1) was dose-dependently inhibited by intravenous NPY infusion (20, 40, and 80 micrograms kg-1 h-1). Inhibitory effects were rapid in onset but reversed with cessation of NPY infusion. With continuous NPY infusion (40 microgram kg-1 h-1), prolonged inhibition of amylase output by the vagal stimulant 2-deoxyglucose (100 mg kg-1) was observed (greater than 50% inhibition in each of none consecutive 10-min periods). In contrast, NPY infusion in doses of 20, 40, or 80 micrograms kg-1 h-1 produced no alteration in immunoreactive somatostatin levels. In vitro, NPY incubation (10(-13)-10(-8) M) produced no change in basal amylase release from dispersed, purified acinar cells. In addition, co-incubation of NPY (10(-8)-10(-6) M) with CCK-8 (10(-13)-10(-8) M) produced no inhibition of CCK-stimulated amylase release from dispersed acini. In contrast, NPY (10(-6) M) produced significant inhibition of amylase release from pancreatic lobules that had been stimulated by 75 mM potassium (135 +/- 11% versus 177 +/- 18% of basal level) or by 25 microM veratridine (196 +/- 19% versus 398 +/- 152%).(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo somatostatin, vasopressin, and oxytocin synthesis in diabetic rat hypothalamus

The in vivo labeling of somatostatin-14, somatostatin-28, arginine vasopressin, and oxytocin was studied in rat hypothalamus after third ventricular administration of [35S]cysteine to streptozotocin-diabetic and normal rats. Immunoreactive somatostatin levels in hypothalamus were unaffected by diabetes, as was the incorporation of [35S]cysteine into hypothalamic somatostatin-14 and somatostatin-28. In contrast, immunoreactive vasopressin levels in hypothalamus and posterior pituitary (and oxytocin levels in posterior pituitary) were below normal in diabetic rats. Moreover, [35S]cysteine incorporation into hypothalamic vasopressin and oxytocin (probably mainly in the paraventricular nucleus because of its proximity to the third ventricular site of label injection) was significantly above normal. The increments in vasopressin and oxytocin labeling were reversed by insulin administration. In vivo cysteine specific activity and the labeling of acid-precipitable protein did not differ between normal and diabetic animals; effects of diabetes on vasopressin and oxytocin labeling were therefore not caused by simple differences in cysteine specific activity. These results suggest that diabetes 1) does not influence the production of somatostatin peptides in hypothalamus but 2) stimulates the synthesis of vasopressin and oxytocin. For vasopressin at least, the increase in synthesis may be a compensatory response to the known increase in its secretion that occurs in uncontrolled diabetes.

Regional distribution of immunoreactive somatostatin in the bovine pineal gland.

Regional distribution of immunoreactive somatostatin (IRS) and melatonin were investigated in the bovine pineal gland. The total IRS and melatonin content ranged from 0.26 to 2.28 pmol, and from 19.4 to 42.7 pmol, respectively, per bovine pineal. Reverse phase liquid chromatography of pineal extracts demonstrated that more than 90% of IRS coeluted with synthetic somatostatin-14 and somatostatin-28. While the IRS content was shown to vary considerably throughout the gland, with a constant and marked maximal concentration at the proximal end of the pineal, the maximal melatonin concentration appeared in the central part of the gland, coinciding with the total protein distribution. The existence of the highest levels of pineal IRS near the habenular commissure, where the afferent fibers of the central pinealopetal innervation enter the gland, suggests that pineal somatostatin may be, at least in part, of neural origin.

Identification of immunoreactive somatostatin in the rat Harderian gland: Regulation of its content by growth hormone, beta-adrenergic agonists and calcium channel blockers

Immunoreactive somatostatin (IRS) was identified in the male rat Harderian gland (HG) by radioimmunoassay. Tissue was extracted and a displacement curve performed; there were no significant differences between values obtained with serial dilutions of extracted tissue and those from purified somatostatin standard used in the radioimmunoassay. Basal values of HG-IRS were found to be in the nanomolar range (10.8±3.5 ng IRS/mg protein). Hypophysectomy did not change the HG-IRS but, in vivo growth hormone (GH) treatment led to a dramatic increase (6–7-fold) in the levels of IRS in the HG. Isoproterenol, a β-adrenergic agonist, when administered in vivo significantly decreased the HG-IRS content. The effect of two different calcium channel blockers on the isoproterenol-induced decrease of HG-IRS was studied; no changes were observed with nifedipine but verapamil, injected one hour after isoproterenol administration, prevented the drop in HG-IRS levels. These data demonstrate the existence of IRS in a new location, the rat Harderian gland, and support a classical endocrine regulation for its tissue concentration.

Biochemical changes in rhesus monkey during the first days after streptozotocin administration are indicative of selective β cell destruction

Hormonal and glycemic changes in 22 rhesus monkeys were characterized during the first days after treatment with streptozotocin (STZ) (45 to 55 mg/kg, administered intravenously [IV]. Almost half ( 10 22 ) of the monkeys developed insulin-dependent diabetes mellitus (STZ-IDDM) within five days following injection. Four of the remaining monkeys did not become insulin dependent for at least 6 months after STZ treatment, during which time they were considered non-insulin-dependent, and eight monkeys never required exogenous insulin. In the STZ-IDDM group, plasma immunoreactive c-peptide (IRC-P) levels fell by three hours after STZ from a mean ± SEM of 252 ± 82 to 101 ± 45 pg/mL, as glucose and immunoreactive glucagon (IRG) levels increased from 65 ± 3 and 120 ± 37, respectively, to 336 ± 43 mg/dL and 234 ± 52 pg/mL, respectively. Between six and 30 hours after treatment, IRC-P increased to a peak of 1,561 ± 360 pg/mL before falling permanently to <60 pg/mL by 66 hours. During this period, glucose and IRG responded in a reciprocal fashion by falling and then increasing to levels above 300 mg/dL and 300 pl/mL, respectively, by 66 hours. In the non-insulin-dependent diabetes mellitus (STZ-NIDDM) group, no clear reciprocal relationship between IRC-P and glucose and IRG was obtained. In nine additional monkeys subjected to total pancreatectomy (Px), IRC-P and IRG levels fell immediately and permanently by >90% and 75%, respectively. Levels of immunoreactive somatostatin increased steadily over the initial 96 hours following STZ, but did so in both STZ-IDDM and Px monkey groups. Plasma lipid hydroperoxide levels measured in two monkeys that developed STZ-IDDM were unchanged during the first 96 hours after STZ administration. In conclusion, the glycemic and hormonal pattern observed with STZ-IDDM rhesus monkeys during the first days following treatment is indicative of selective pancreatic β-cell destruction resulting from a direct action of STZ.

Androgenic control of immunoreactive somatostatin in the Harderian gland of the Syrian hamster.

Harderian glands of Syrian hamsters contained measurable levels of immunoreactive somatostatin. After an extraction procedure, serial dilutions of tissue were assayed and showed parallelism in the displacement curve with dilutions of purified somatostatin standard in the radioimmunoassay. Somatostatin concentrations were higher in female hamsters (10.0 +/- 2.1 ng/mg protein) than in males (2.6 +/- 0.4 ng/mg protein). Castrated males had somatostatin values in the range of females (12.4 +/- 2.3 ng/mg protein) at 1 month after gonadectomy. Testosterone implants prevented the rise of Harderian gland somatostatin in castrated males. Gonadectomized males had lower somatostatin content in the gland than did control males (1.0 +/- 0.2 ng/mg protein) at 2 months after castration. Somatostatin values in females were unaffected by gonadectomy, but there were variations during the oestrous cycle, with a nadir detected at dioestrus-1 and maximal values coincident with the day of the ovulation.

Raised Thyrotrophin‐Releasing Hormone, Pyroglutamylamino Peptidase, and Proline Endopeptidase Are Present in the Spinal Cord of Wobbler Mice but Not in Human Motor Neurone Disease

The Wobbler mouse (wr) is a mutant that exhibits loss of anterior horn cells in the spinal cord and brainstem and subsequent muscle wasting, particularly of the forelimbs and neck. The wr mice, 2-3 months of age, were found to have increased levels of immunoreactive-thyrotrophin-releasing hormone (ir-TRH) in the spinal cord and pons and medulla, but not in other CNS areas. This increase was observed in dorsal and ventral cord and at cervical, thoracic, and lumbar levels and was confirmed by HPLC to be authentic TRH. The levels of immunoreactive-somatostatin, -neurotensin, and -substance P were not raised in the CNS of wr mice. The activities of two peptidases capable of degrading TRH, pyroglutamylaminopeptidase (PGAP, EC 3.4.11.8) and proline endopeptidase (PEP, EC 3.4.21.26), and the level of 5-hydroxyindoleacetic acid were also raised in the spinal cord of 2-3-month-old wr mice although the activities of alanine aminopeptidase and lactate dehydrogenase and the level of 5-hydroxytryptamine were not. Increased spinal cord levels of ir-TRH and PGAP and PEP activities were not observed in the 1-month-old wr mice. In addition, a pilot study using spinal cord obtained at autopsy from three patients with motor neurone disease and 12 control subjects indicated no increase in spinal cord ir-TRH, PGAP, or PEP in human motor neurone disease.

Plasma Immunoreactive Somatostatin Response to Arginine After Glycemic Control With Continuous Subcutaneous Insulin Infusion in Type I Diabetics

The effects of 3 wk of near normoglycemia by continuous subcutaneous insulin infusion (CSII) on plasma immunoreactive somatostatin (IRS) responses to arginine (0.5 g X kg-1 X 30 min-1) in seven patients with insulin-dependent diabetes mellitus (IDDM) were compared with the same patients in poor glycemic control during conventional insulin therapy (CIT) and with seven normal controls. After 3 wk of CSII treatment, mean daily blood glucose and HbA1 decreased to mean (+/- SE) values of 129 +/- 6 mg/dl and 8.0 +/- 0.1%, respectively. Plasma free-insulin levels in IDDM patients 30 min before a meal during CSII were significantly higher than those during CIT or in normal controls. Fasting mean plasma IRS levels of the IDDM patients during CSII (7.3 +/- 1.4 pg/ml) were not different from those during CIT (8.4 +/- 1.3 pg/ml) and in normal controls (5.9 +/- 0.8 pg/ml). Arginine elicited a rise in plasma IRS during CIT in all seven IDDM patients during CIT and in the normal controls, with peak values of 18.2 +/- 4.1 and 12.5 +/- 1.8 pg/ml, respectively. However, no significant increase in plasma IRS was observed in all seven IDDM patients during CSII. The integrated values of plasma IRS during the arginine-infusion study of the IDDM patients treated with CIT were significantly higher than those of the normal controls. The increased integrated values of plasma immunoreactive glucagon response to arginine observed during CIT became normalized after CSII. These results suggest that glycemic control with CSII in IDDM patients suppresses the increased plasma IRS response to arginine that occurs during CIT.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in lmmunoreactive Somatostatin Levels in Hypothalamic and Gastroenteropancreatic Tissue as a Consequence of Neonatal Treatment with Monosodium Glutamate

The present study was conducted to evaluate the effects of neonatal treatment of male rats with monosodium-L-glutamate (MSG) on levels of immunoreactive somatostatin (IRS) in specific regions of the gastroenteropancreatic (GEP) system, in discrete hypothalamic areas, and in peripheral blood. In two identical experiments, IRS concentrations measured in the arcuate nucleus and median eminence of adult, MSG-treated rats were significantly reduced in comparison to IRS levels measured in control littermates. Levels of IRS in the preoptic-periventricular nucleus were significantly reduced only in one experiment. In contrast, neonatal MSG treatment resulted in a twofold increase of IRS levels in the pancreas and antral region of the stomach. Peripheral plasma IRS concentrations were significantly elevated in MSG-treated rats only in one experiment. Since MSG-treated rats have a deficiency in growth hormone (GH) secretion, an additional experiment was performed to determine if GH replacement therapy could reverse some or all of the changes in IRS concentrations induced by MSG treatment. With the exception of a further increase in antral IRS levels, replacement with rGH failed to restore IRS levels in other tissues or plasma of MSG rats to levels measured in control rats. These results show that neonatal MSG treatment not only affects IRS levels in the hypothalamus and blood as previously reported, but in parts of the GEP system as well. Further, effects on hypothalamic IRS levels are opposite to those on GEP IRS levels. The significance of these findings may relate to the altered metabolic state of these animals as a consequence of perturbations in the secretion of other hormones from the hypothalamus, anterior pituitary gland, and possibly the GEP system.

Somatostatin release from the median eminence of unanesthetized rats: lack of correlation with pharmacologically suppressed growth hormone secretion.

We describe a push-pull perfusion technique to investigate the role of somatostatin in the pharmacological suppression of growth hormone (GH) secretion. Immunoreactive somatostatin (IRS) released from the median eminence (ME) was studied in chronically cannulated, unanesthetized male rats. In control rats which received vehicle injection, plasma GH levels showed a normal ultradian rhythm and relatively stable levels of IRS (around 30 pg/15 min) appeared in the perfusate during the 6-h perfusion. Intracerebroventricular (i.c.v) administration of human growth hormone (40 micrograms), neurotensin (2 micrograms), glucagon (25 micrograms) or intravenous (i.v.) injection of oxymetazoline (50 micrograms/kg b.w.), an alpha-adrenergic agonist, or endotoxin (150 micrograms/kg b.w.) suppressed subsequent GH surges. In these rats, however, IRS levels in the ME perfusate failed to change significantly compared to control rats. These results suggest that changes in somatostatin release may play a minor role in the suppression of plasma GH levels caused by these substances, and that major regulatory effects may be achieved via the suppression of growth hormone-releasing factor release.

Somatostatin and the cerebral cortex.

A unique subset of interneurons which are rich in immunoreactive somatostatin (IRS) exists in the cerebral cortex. The regulation of IRS secretion by these cells is reviewed. Acetylcholine, glutamic acid and several neuropeptides including VIP, CCK, and metenkephalin have been identified as IRS secretagogues. The types of molecules which stimulate IRS release, the electrophysiologic effects of somatostatin, and the recognition of abnormal IRS levels in human CNS diseases were all used to formulate a working model of the role of the somatostatinergic cell in ongoing cerebral cortical function.

Changes in immunoreactive somatostatin in brain following lidocaine-induced kindling in rat

The kindling phenomenon has become a useful model for studying epileptogenesis. The present authors have previously reported increased levels of immunoreactive somatostatin (IR-SRIF) in various regions of the brain of electrically-amygdaloid kindled (EAK) rats. In this study, an examination was made of immunoreactive somatostatin in pharmacologically-kindled (PK) rats. Sixteen male Sprague-Dawley rats were injected intraperitoneally (i.p.) with a subthreshold dose of lidocaine (60 mg/kg), once daily. Once the kindling phenomenon was established, kindled rats (7), non-kindled rats (9) and controls (6) were sacrificed by microwave irradiation. Another group of 5 rats was injected with a single suprathreshold dose of lidocaine (110 mg/kg) and killed 10 min after the resultant seizure. Various brain areas were removed and assayed for immunoreactive somatostatin in kindled rats. Immunoreactive somatostatin was significantly greater than in controls in the amygdala (56%; P < 0.02), entorhinal + piriform cortex (50%; P < 0.05) and hypothalamus (29%; P < 0.02). In non-kindled rats, immunoreactive somatostatin increased only in the amygdala (58%; P < 0.02). No difference was found in the immunoreactive somatostatin content of rats injected with an suprathreshold dose of lidocaine compared to controls. The alteration of immunoreactive somatostatin, in both lidocaine-kindled and electrically-amygdaloid kindled rats suggests a possible role of this neuropeptide in kindling.

A stimulating effect of glucose on somatostatin release is impaired in noninsulin-dependent diabetes mellitus.

The effect of oral glucose (1 g/kg BW) on levels of immunoreactive somatostatin (SLI) in peripheral venous plasma was investigated in young and older nondiabetic subjects (mean ages, 25.8 and 56.7 yr, respectively) as well as in subjects with decreased iv glucose tolerance (K value less than 1.0) and in diet-treated diabetic patients with fasting hyperglycemia (blood glucose, greater than 7.0 mmol/liter). SLI was assayed after extraction of plasma on silica glass beads. In control experiments (glucose omitted), SLI levels tended to decline. Ingestion of glucose was followed by a moderate (52% or less) increase in SLI levels in subjects with normal or decreased iv glucose tolerance. The stimulating effect was sustained for 90 or 120 min after glucose ingestion and the increase in plasma SLI was significant (P less than 0.05-0.02) whether in relation to prestimulatory values or control experiments. In contrast, in overtly diabetic patients, glucose ingestion was not followed by increased SLI levels. It is concluded that oral glucose stimulates SLI secretion in individuals of different ages and with varying degrees of glucose tolerance, but the response is impaired in type 2 diabetic patients with fasting hyperglycemia.

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.

Fetal and maternal plasma levels of immunoreactive somatostatin at delivery: evidence for its increase in the umbilical artery and its arterio-venous gradient in the feto-placental circulation.

The concentrations of immunoreactive somatostatin (IR-SRIF) in plasma samples taken from the umbilical artery (UA), umbilical vein (UV), and maternal antecubital vein (MV) were measured in 23 cases of normal delivery at term. High concentrations of IR-SRIF were detected in the plasma from the UA (mean +/- SD, 73.2 +/- 40.6 pg/ml), the value being about 2.5 times that in the UV (29.5 +/- 17.5 pg/ml; P less than 0.001). An arterio-venous gradient was observed in all 23 subjects. The plasma level of IR-SRIF in the MV (10.9 +/- 4.6 pg/ml) was significantly lower than those in the UA (P less than 0.001) and UV (P less than 0.001) and was almost the same as that in nonpregnant women (12.7 +/- 5.4 pg/ml). Chromatographic analysis of an extract of plasma from the UA gave only one peak of immunoreactive material, which was eluted in the same position as synthetic SRIF-14. A similar result was obtained in nonpregnant women. No correlation was found between the plasma level of IR-SRIF and that of GH, insulin, glucagon, or gastrin in the UA. The present findings suggest that the high concentration of IR-SRIF in the feto-placental circulation originates from the fetus and reflects a particular role of this peptide in the process of functional maturation of the neuroendocrine system in early human life.

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.

Hypothalamic obesity induced by monosodium glutamate (MSG) in rats: changes in the endocrine pancreas in the course of and after induction obesity

It has been reported that the neonatal administration of monosodium glutamate (MSG) in rodents produces lesions of the arcuate nucleus and the ventromedial nucleus of the hypothalamus and results in obesity in adulthood. Though there are a large number of reports using MSG-treated animals, most of them deal with neuronal and neuroendocrinological pathophysiology within the central nervous system. The present study, therefore, was designed to investigate the time-course of metabolic and hormonal changes in relation to the development of hypothalamic obesity induced by MSG in female Wister rats. In addition, responses of insulin and somatostatin to glucose loading were examined in rats with hypothalamic obesity. MSG, 2mg per g of body weight, was subcutaneously injected for 5 consecutive days after birth. Body weight, plasma glucose and immunoreactive insulin (IRI) were measured at one- to four-week intervals in rats aged from 6 days to 11 months. The Lee index, plasma triglyceride and immunoreactive glucagon (IRG) were measured at one-month intervals from one to 11 months. Immunoreactive somatostatin (IRS) and gastrin (IRGa) were determined at the 7th month or later. All parameters in plasma described above were assayed on samples drawn from the jugular vein. In addition, IRI and IRS in the pancreas were measured until 6 months. Further, responses of glucose, IRI and IRS to intragastric glucose loading (3g/kg body weight) were examined in portal plasma at the 11th month. All parameters in the MSG rats were compared with those in age-matched female controls which received the vehicle alone. Plasma IRI was elevated by 54% in the MSG rats even at the 6th day. Thereafter, the MSG rats showed a progressive increase in plasma IRI which was 96% and 237% higher at the 1st and 3rd month, respectively. Parallel with the change in plasma IRI, there was a constant 1.5-fold elevation of pancreatic IRI in the MSG rats aged between the 21st day and the 6th month, though pancreatic IRI decreased by 20% at 6 days. Pancreatic IRS also increased at the 21st day, but thereafter decreased by 48% at the 6th month. A progressive increase in plasma TG which was 37,139,203 and 279% higher at 2, 3, 4 and 5 months, respectively, was followed by a 2- to 3-fold elevation during following 6 months. The MSG rats showed a constant 20% reduction in body weight for the first 2 months, but thereafter showed a progressive increase throughout the observation which was 17, 39 and 55% higher at 4, 7 and 11 months, respectively. The Lee index of obesity also progressively increased from 337 ± 4 at the 3rd month to 390 ± 6 at the 8th month, but thereafter maintained a constant 1.2-fold elevation. In contrast with the marked changes in plasma IRI, no significant differences in plasma glucose were found between the two groups for the entire period of observation. Plasma IRG also showed a constant 1.2- to 2.0-fold elevation irrespective of the duration of obesity. Rats with obesity induced by MSG had markedly elevated levels of IRI and IRS not only in jugular plasma but also in portal plasma. In addition, responses of the two hormones to intragastric glucose loading were exaggerated in portal plasma, whereas response of plasma glucose was unchanged. Plasma IRGa was also observed to be increased by 38 - 81%. These findings indicate that hyperinsulinemia is the primary factor in the development of hypothalamic obesity induced by MSG. Hypothalamic obesity induced by neonatal administration of MSG in the rat is characterized by hypersecretion of glucagon and somatostatin as well as insulin, and may provide a readily available and valuable animal model for studying the hypothalamico-pancreatic axis.

Response of circulating immunoreactive somatostatin to nutritional stimuli in normal subjects

We have previously reported that in normal subjects plasma immunoreactive somatostatin levels rise after a mixed meal. The contribution of individual nutrients to this rise, and the molecular nature of the somatostatin immunoreactivity measured, have now been studied. Six normal healthy subjects received, on separate occasions, isocaloric (520 calories,) and isovolumetric (260 ml) quantities of carbohydrate, protein, and fat. The mean fasting plasma somatostatin level was 29 ± 5 pg/ml. After carbohydrate a peak of 48 ± 7 pg/ml was reached at 30 min, and after protein and fat there were more sustained rises with peak levels of 74 ± 8 pg/ml and 80 ± 9 pg/ml, respectively. Sephadex G50 chromatography of extracts of fasting peripheral plasma showed two main peaks of somatostatin immunoreactivity, one coeluting with cyclic somatostatin and a larger peak of approximately 3500 molecular weight (mol wt). Levels of both 1600 and 3500 mol wt somatostatin were increased 60 min after a mixed meal. Approximately 80% of the 3500 mol wt form of somatostatin could be converted to the 1600 mol wt form by treatment with dithiothreitol (an agent which reduces disulphide bonds). It is concluded that: (a) in normal subjects fat and protein are potent stimuli for somatostatin release; (b) somatostatin in normal peripheral plasma exists in multiple forms, and that both 1600 and 3500 mol wt forms of somatostatin immunoreactivity are stimulated by feeding; (c) the 3500 mol wt form could represent a dimer of somatostatin, or a somatostatin molecule linked to a second peptide chain by disulphide bonds.

Stimulation by bombesin of immunoreactive somatostatin release into rat hypophysial portal blood.

Bombesin was injected into the cerebral ventricle of male rats anesthetized with urethane to study its effect on plasma levels of immunoreactive somatostatin (IRS) in hypophysial portal and jugular blood. An intraventricular injection of bombesin (0.2 and 2 micrograms/rat) caused a significant and dose-related increase in plasma IRS in hypophysial portal blood but not in jugular blood. Although bombesin placed into the cerebral ventricle is known to stimulate glucagon and epinephrine release, an iv injection of glucagon (100 micrograms/100 g BW) or epinephrine (2.5 micrograms/100 g BW) did not cause any significant changes in plasma IRS levels in hypophysial portal and jugular blood, suggesting that these substances do not mediate bombesin stimulation of portal IRS release. Pretreatment with naloxone (75 micrograms/100 g BW, iv) failed to affect the portal IRS release induced by bombesin (2 micrograms/rat), indicating that the opiate receptor is not likely to be involved in this reaction. To ascertain whether IRS released by bombesin into hypophysial portal blood is biologically active, the effect of bombesin on the plasma GH level was then examined. Bombesin (2 micrograms/rat) injected intraventricularly completely suppressed the rise of plasma GH after the intraventricular injection of beta-endorphin (1 microgram/rat) or the iv injection of prostaglandin E1 (5 micrograms/100 g BW). Bombesin thus appears to stimulate the secretion of IRS, and probably biologically active somatostatin as well, from the hypothalamus into hypophysial portal blood, thereby inhibiting GH release from the anterior pituitary.