Donna J. Koerker

Somatostatin: Hypothalamic Inhibitor of the Endocrine Pancreas

Somatostatin, a hypothalamic peptide that inhibits the secretion of pituitary growth hormone, inhibits basal insulin secretion in fasted cats and rats. In fasted baboons both basal and arginine-stimulated secretion of insulin and glucagon are inhibited. Somatostatin appears to act directly on the endocrine pancreas. The action is dose-related, rapid in onset, and readily reversed.

Elimination of the action of glucagon-like peptide 1 causes an impairment of glucose tolerance after nutrient ingestion by healthy baboons.

Glucagon-like peptide 1 (GLP-1) is an insulinotropic hormone released after nutrient ingestion which is known to augment insulin secretion, inhibit glucagon release, and promote insulin-independent glucose disposition. To determine the overall effect of GLP-1 on glucose disposition after a meal we studied a group of healthy, conscious baboons before and after intragastric glucose administration during infusions of saline, and two treatments to eliminate the action of GLP-1: (a) exendin-[9-39] (Ex-9), a peptide receptor antagonist of GLP-1; or (b) an anti-GLP-1 mAb. Fasting concentrations of glucose were higher during infusion of Ex-9 than during saline (4.44 +/- 0.05 vs. 4.16 +/- 0.05 mM, P < 0.01), coincident with an elevation in the levels of circulating glucagon (96 +/- 10 vs. 59 +/- 3 ng/liter, P < 0.02). The postprandial glycemic excursions during administration of Ex-9 and mAb were greater than during the control studies (Ex-9 13.7 +/- 2.0 vs. saline 10.0 +/- 0.8 mM, P = 0.07; and mAb 13.6 +/- 1.2 vs. saline 10.6 +/- 0.9 mM, P = 0.044). The increments in insulin levels throughout the absorption of the glucose meal were not different for the experimental and control conditions, but the insulin response in the first 30 min after the glucose meal was diminished significantly during treatment with Ex-9 (Ex-9 761 +/- 139 vs. saline 1,089 +/- 166 pM, P = 0.044) and was delayed in three of the four animals given the neutralizing antibody (mAb 946 +/- 262 vs. saline 1,146 +/- 340 pM). Thus, elimination of the action of GLP-1 impaired the disposition of an intragastric glucose meal and this was at least partly attributable to diminished early insulin release. In addition to these postprandial effects, the concurrent elevation in fasting glucose and glucagon during GLP-1 antagonism suggests that GLP-1 may have a tonic inhibitory effect on glucagon output. These findings demonstrate the important role of GLP-1 in the assimilation of glucose absorbed from the gut.

Correlations of In Vivo β-Cell Function Tests With β-Cell Mass and Pancreatic Insulin Content in Streptozocin-Administered Baboons

In vivo β-cell function tests are used increasingly in humans during the preclinical phase of insulin-dependent diabetes mellitus (IDDM), but the severity of the β-cell loss responsible for the abnormalities seen in these tests is unknown. We have measured several physiological β-cell function tests—fasting plasma glucose, glucose disappearance constant, fasting insulin, acute insulin responses to arginine (AIRarginine) and glucose (AIRglucose), and glucose potentiation of AIRarginine (∆ AIRarginine/∆G) and two direct objective measurements (pancreatic insulin content [PIC] and quantitative β-cell mass)—in adolescent male baboons (Papio anubis/cyanocephalus). We have correlated in vivo measurements obtained within 3 days after the animals were killed with in vitro estimates of PIC and β-cell mass in 15 animals, (2 nondiabetic requiring insulin treatment and 13 after varying doses of streptozocin to induce degrees of β-cell damage ranging from normoglycemia to severe hyperglycemia). There was a strong linear correlation between β-cell mass and PIC (r = 0.79, P < 0.001). Physiological measures of β-cell function were significantly correlated with both PIC and β-cell mass. The correlations between physiological measures and β-cell mass were linear and intercepted the β-cell mass axis at 0.15–0.2 g, suggesting that in vivo measures of β-cell function approach 0 when there is still ∼40–50% of the β-cell mass detectable histologically. With PIC, the linear correlations intercepted the axes close to 0. These findings provide considerable validity to the measurements of β-cell function used in preclinical IDDM in humans. Our data suggest that such physiological measurements give an accurate valid reflection of changes in β-cell mass and pancreatic insulin content.

Somatostatin blockade of acute and chronic stimuli of the endocrine pancreas and the consequences of this blockade on glucose homeostasis.

The nature and extent of somatostatin-induced inhibition of pancreatic endocrine secretion were studied by administration of a number of stimuli of either glucagon or insulin to over night fasted baboons with and without an infusion of linear somatostatin. The stimuli for acute-phase insulin release were intravenous pulses of glucose, tolbutamide, isoproterenol, and secretin. When given 15 min after the start of a somatostatin infusion, these agents were essentially unable to stimulate insulin secretion. Chronic insulin secretion was stimulated by infusions of either glucose or glucagon. Within 10 min of the start of a super-imposed infusion of somatostatin, insulin levels fell to less than 40 percent of prestimulus control and remained suppressed for the duration of the somatostatin infusion. Stimulation of glucagon secretion by insulin-induced hypoglycemia was also blocked by somatostatin. Plasma glucose decreased during somatostatin infusions except when superimposed upon an infusion of glucagon. Somatostatin had no effect on glucose production in a rat liver slice preparation. We conclude: (a) Somatostatin is a potent and so far universally effective inhibitor of both acute and chronic phases of stimulated insulin and glucagon secretion (b) The inhibitory effect is quickly reversible and the pattern of recovery of secretion is appropriate to prevailing signals; (c) Present evidence suggests that the effect of somatostatin on blood glucose is mediated through its effect on blood glucagon; (d) In the overnight-fasted baboon both in the basal state and 45 min into a 4-mg/kg-min glucose infusion, a somatostatin-induced fall in serum insulin levels appears to be unable to prevent a decrease in hepatic glucose production.

Effects of Somatostatin on Hemostasis in Baboons

Because some baboons repeatedly infused with somatostatin died we reviewed available autopsy material. All six animals chronically treated with somatostatin displayed gross or microscopical pulmonary hemorrhage and increased hemosiderin in lung and liver whereas only one of six untreated animals had a similar abnormality. We therefore examined the hemostatic system in living baboons. Thrombocytopenia (mean platelet count of 84,000 per microliter) was noted in six of seven baboons chronically treated with somatostatin; platelet survival was normal. Clotting factors were unaffected. Fibrinogen concentration and survival were unchanged. The acute effects of intravenous somatostatin (0.8 micrograms per kilogram per minute for two hours) in previously untreated animals transiently decreased platelet count, reduced retention of platelets on glass-bead columns and inhibited aggregation induced by ADP, collagen and epinephrine. Bleeding times were not prolonged. Somatostatin added to platelet-rich plasma in vitro was without effect. These data suggest that prolonged administration of somatostatin should be undertaken with caution.

Decreased Insulin- and Glucagon-Pulse Amplitude Accompanying β-Cell Deficiency Induced by Streptozocin in Baboons

The effect of β-cell deficiency on the spontaneous pulsatile secretory pattern of the islets of Langerhans was studied in the baboon. Measures of β-cell function were correlated with the secretory pattern before and at intervals after streptozocin administration. The degree of insulin deficiency was variable and ranged from mild to moderate. Highly regular pulses were less prevalent in baboons compared with rhesus monkeys and humans, but the mean frequency was similar and was not affected by treatment. The principal effect of β-cell destruction was to proportionately reduce the pulse amplitude of insulin (−39%, P < .003) without detectable change in pulse frequency, interhormonal phase relationship, or the regularity of pulses. Glucagon-pulse amplitude also fell (−19%, P < .09), but not significantly. However, glucagon-pulse amplitude was strongly correlated with insulin-pulse amplitude (r = −.59, P < .002), whereas mean fasting plasma concentrations of insulin and glucagon were not significantly changed after treatment. Because streptozocin affects only the β-cell, the data indicate a major influence of the insulin pulse on the α-cell secretory pulse. The data do not support the presence of a separate pacemaker for the α-cell but do not eliminate this possibility. The strong correlation of reduction in insulin-pulse amplitude with increasing fasting glucose and decreasing glucose disappearance lends support to growing evidence that the pattern of insulin secretion is an important determinant of normal glucose homeostasis.

Effect of nicotinic acid-induced insulin resistance on pancreatic B cell function in normal and streptozocin-treated baboons.

To study the interaction between insulin secretion and insulin action in maintaining glucose homeostasis, we induced experimental insulin resistance in eight normal baboons, in six baboons treated with 40 mg/kg streptozocin (STZ-40), and in six baboons treated with 200 mg/kg streptozocin (STZ-200). Insulin resistance was induced by a 20-d continuous intravenous infusion of nicotinic acid (NA). Normal animals showed compensatory increases in several measures of insulin secretion (fasting insulin [FI], acute insulin response to arginine [AIRarg], acute insulin response to glucose [AIRgluc], and glucose potentiation slope [delta AIRarg/delta G]), with no net change in fasting plasma glucose (FPG) or glycosylated hemoglobin (HbAtc). STZ-40 animals showed compensatory increases in FI, AIRarg, and AIRgluc, but delta AIRarg/delta G failed to compensate. Although FPG remained normal in this group during NA infusion, HbA1c rose significantly. STZ-200 animals failed to show compensatory changes in both AIRgluc and delta AIRarg/delta G, with both HbA1c and FPG rising. These animals showed a paradoxical inhibition of insulin secretion in response to intravenous glucose during NA infusion, at a time when they were hyperglycemic. These data indicate that a significant degree of insulin resistance does not cause hyperglycemia in the presence of normal B cell function but, in animals with reduced B cell mass and superimposed insulin resistance, the degree of hyperglycemia is proportional to the degree of pancreatic B cell dysfunction.

Assessment of obesity in pigtailed monkeys (Macaca nemestrina).

Obesity was studied in a colony of 873 Macaca nemestrina to establish tools for epidemiologic studies, to examine a genetic form of obesity, to document age/sex relationships to obesity, and to compare metabolic profiles of obese and normal monkeys. Age/weight growth curves were analyzed to select the most obese monkeys and age- and sex-matched normal controls. Degree of adiposity was determined using tritiated water for estimation of lean body mass. Body weight, anterior trunk height, and abdominal and triceps skinfolds were measured. Fasting insulin, fasting free fatty acids, and glucose disappearance rate were determined. The results give evidence of similarities between macaque and human obestiy.

Evidence for a biological role of somatostatin in the Pacific hagfish, Eptatretus stouti.

Immunoreactive somatostatin was found in acetic acid extracts of the intestine (10–11 pg/mg wet wt) and the islet-organ (approximately 10 ng/mg wet wt) of the Pacific hagfish, Eptatretus stouti. Hagfish islet-organs were incubated in vitro in a modified Krebs-Ringer bicarbonate medium, and the concentration of insulin released into the medium was measured with a specific radioimmunoassay for hagfish insulin. The amount of insulin released by islet-organs incubated in medium containing 20 mg of glucose/dl did not differ significantly from that released by tissues incubated in medium containing 120 mg of glucose/dl. Release of insulin decreased markedly when tissues were incubated in medium containing 120 mg of glucose/dl and 100 ng of cyclic somatostatin/ml. Subsequent incubation of the somatostatintreated islet-organs in medium without somatostatin resulted in recovery of insulin secretion to the expected rate. The results of this study provide evidence that somatostatin may function in the regulation of insulin secretion in the cyclostomes.

Somatostatin biosynthesis and release in the hypothalamus and pancreas of the rat

Abstract Somatostatin has been localized in several cells of neuroectodermal origin by immunocytochemistry and radioimmunoassay and is presumably synthesized and secreted locally, thereby fulfilling some of the characteristics of a neurotransmitter or neuromodulator.1,2 Recent studies in perfused dog pancreas and isolated rat islets indicate that somatostatinlike immunoreactivity (SLI) is secreted from D cells and its presence in the human cerebral spinal fluid implies release from peptidergic neurons.3–6 That peptide biosynthesis occurs by ribosomal mechanisms followed by posttranslational enzyme-mediated cleavage eventuating in peptides destined for secretion has been well established.7 Although nonribosomal synthesis of conventional neurotransmitters occurs and has been implicated in the synthesis of thyrotropin-releasing hormone and luteinizing hormone-releasing hormone,8 most evidence favors their biosynthesis through cleavage of large molecular weight precursors.9,10 The mechanisms of biosynthesis and release of somatostatin have not been elucidated. We present preliminary data in the rat consistent with the notion that hypothalamic neurons and the pancreatic D cell synthesize and release somatostatin through posttranslational scission of precursor polypeptides.