Richard E. Dobbs

Release of immunoreactive somatostatin from the pancreas in response to glucose, amino acids, pancreozymin-cholecystokinin, and tolbutamide.

The effects of glucose, amino acids, pancreozymin-cholecystokinin, and tolbutamide upon the release of immunoreactive somatostatin (IRS) from the isolated perfused pancreas were studied. In seven experiments in which glucose was perfused either at a concentration of 100 or 350 mg/dl or at 25 mg/dl, IRS levels were significantly greater at the higher glucose concentrations. In three dose-response experiments in which the perfusing glucose concentration was increased at 30-min intervals from an initial concentration of 25 mg/dl to a final concentration of 300 mg/dl, progressive increases in IRS release were noted at glucose concentrations of 100 mg/dl and above. Perfusion of a 20 mM mixture of 10 amino acids also elicited a prompt and significant biphasic IRS rise in each of six experiments. In five experiments, 20 mM leucine evoked a similar response in mean IRS. Perfusion with 0.075 Ivy U/ml of pancreozymin-cholecystokinin, with or without the presence of a 1 mM 10-amino acid mixture, elicited a prompt rise in IRS with a pattern resembling that of insulin in a total of six experiments. Tolbutamide (0.75 mg/min) also stimulated IRS release in five of six challenges. The IRS responses to nutrients and to pancreozymin and their similarity to the insulin responses raise the possibility that, like insulin, pancreatic somatostatin may have an endocrine role related to nutrient homeostasis.

Somatostatin-induced changes in insulin and glucagon secretion in normal and diabetic dogs.

In conscious dogs intravenously infused somatostatin (3.3 mug per min for 1 h) caused prompt and sustained declines in mean plasma insulin and glucagon, even during alanine infusion and intraduodenal casein hydrolysate feeding; plasma glucose declined, but not significantly. 6.7 mug per min of somatostatin significantly lowered pancreatoduodenal vein glucagon and insulin within 2.5 min and profoundly suppressed their secretion throughout the infusion. Consistent bihormonal suppression occurred at rates as low as 24 ng per kg per min, but was variable at 12 and 2.4 ng per kg per min. When somatostatin-induced (3.3 mug per min) hypoglucagonemia was corrected by exogenous glucagon, hyperglycemia occurred. In dogs with long-standing insulin-requiring alloxan diabetes 3.3 mug per min of somatostatin suppressed glucagon to 55 pg per ml throughout the 30-min infusion and lowered glucose by 36.4+/-6.1 mg per dl, about 1 mg per dl per min. Glucagon suppression was maintained despite alanine infusion, and glucose, which rose 29 mg per dl during alanine infusion without somatostatin, declined 58 mg per dl in the somatostatin-treated diabetic dogs despite alanine. Continuous infusion of somatostatin for 24 h in five insulin-requiring alloxan-diabetic dogs suppressed glucagon and lowered glucose significantly, usually to below normal. It is concluded that in normal dogs pharmacologic doses of somatostatin virtually abolish insulin and glucagon secretion in the basal state and during hyperaminoacidemia. Hyperglycemia occurs during somatostatin-induced insulin lack only if hypoglucagonemia is corrected. Somatostatin suppresses glucagon in diabetic dogs and lowers their plasma glucose approximately 1 mg per dl per min, even when the gluconeogenic substrate alanine is abundant. Glucagon suppression can be maintained for several hours in such dogs and hyperglycemia is thereby reduced.

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of cholecystokinin upon immunoreactive somatostatin release by the perfused canine pancreas.

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of pancreozymin-cholecystokinin on immunoreactive somatostatin release were studied in the isolated perfused dog pancreas. Gastrin at a concentration of 65 ng/ml and the octapeptide of pancreozymin-cholecystokinin at a concentration of 25 ng/ml produced a prompt, but transient statistically significant, twofold rise in mean somatostatin concentration. Secretion at a concentration of 0.3 U/ml and gastric inhibitory polypeptide concentration of 58 ng/ml produced a prompt two- to threefold rise in mean somatostatin release, which persisted throughout the perfusion period. With all four polypeptides the pattern of the somatostatin response resembled that of insulin. It appears that pancreatic somatostatin release is stimulated by gastrointestinal hormones that influence the secretion of insulin and glucagon.

Heterogeneity of plasma glucagon immunoreactivity in normal, depancreatized, and alloxan-diabetic dogs☆☆☆

Filtration of basal plasma from normal, alloxan-diabetic, and depancreatized dogs on Bio Gel P-10 yielded four glucagon-immunoreactive fractions. One of them appeared in the true glycagon area with the glucagon-125I (3500 mol vt). Of the other three, one appeared in the void volume (greater than 20000 mol wt), another just before the insulin-125I (congruent to 9000 mol wt), and the last one close to the salt peak (less than 2000 mol wt). The increase of total plasma glucagon immunoreactivity observed in depancreatized and alloxan diabetic dogs was mainly due to an increase in the 3500 and 9000 molecular-weight fractions. Arginine infusion in depancreatized dogs caused an increase in the 3500 molecular-weight fraction. Somatostatin or insulin infusion in depancreatized and alloxan-diabetic dogs resulted in disappearance of the 3500 molecular-weight fraction.

The Effects of Triglyceride Absorption upon Glucagon, Insulin, and Gut Glucagon-Like Immunoreactivity

The effects of a fat meal upon plasma insulin, glucagon, and glucagon-like immunoreactivity (GLI) have been studied in conscious dogs and in human volunteers. In dogs the intraduodenal instillation of 10 g/kg of peanut oil was accompanied by increases in the mean plasma levels of all three polypeptides that averaged 5 muU/ml, 107 pg/ml, and 2.1 ng/ml, respectively. 3 g/kg of peanut oil, when emulsified with egg yolk, elicited a much greater response of the three hormones, and a physiologic dose of 1 g/kg in emulsified form also caused a significant rise in glucagon and GLI. The islet cell hormone response was not ascribable to chylomicronemia since intravenous infusion of canine chyle failed to stimulate glucagon secretion; moreover, in dogs with a thoracic duct fistula in which chyle was excluded from the circulation, the intraduodenal administration of a fat meal elicited the normal islet cell hormone response, as well as a rise in GLI. 10 g/kg of medium-chain triglycerides failed to elicit these same responses. In six human volunteers the oral administration of 3 g/kg peanut oil was accompanied by increments of 2 muU/ml, 26 pg/ml, and 1.5 ng/ml in the mean levels of insulin, glucagon, and GLI. The changes in insulin and glucagon in man were neither statistically significant nor biologically impressive. It is concluded that in dogs fat absorption is accompanied by prompt and substantial increases in plasma glucagon and GLI and a small transient rise in insulin. The evidence favors an enterogenic signal to the islets of Langerhans rather than their stimulation by chylomicrons. Pancreozymin is qualified to serve as such a signal. The physiologic implications of this study are considered.

Immunoreactive somatostatin levels in plasma of normal and alloxan diabetic dogs

The recent discovery by Luft and associates [l] and by Dubois [2] of somatostatin-containing islet cells, subsequently identified as D-cells [3-51, has opened a new phase in the physiology and pathophysiology of the endocrine pancreas. It is,now recognized that diabetes resulting from a deficiency of insulin-secreting cells, such as human juvenile type diabetes and streptozotocin diabetes in rats, is characterized by an apparent increase in somatostatincontaining D-cells per islet [6] and that the content of immunoassayable somatostatin in the islets of such rats is high [7]. However, it has not been determined if these abnormalities in the islets are reflected by hypersomatostatinemia indeed, it is not known if under normal circumstances somatostatin is released into the circulation from those tissues in which its presence has been demonstrated [8,9]. In this study, we attemped to determine, first, if the levels of immunoreactive somatostatin-like material (IRS) in the venous plasma of organs with a substantial somatostatin content are higher than in peripheral venous plasma and, second, if hypersomatostatinemia is present in dogs with alloxan-induced deficiency of B-cells.

The effect of somatostatin on the response of GLI to the intraduodenal administration of glucose, protein, and fat

SummaryThe effects of somatostatin on GLI release during the absorption of intraduodenally administered glucose, casein hydrolysate and longchain triglycerides were studied in conscious dogs. Whereas, after an intraduodenal glucose load, GLI rose promptly in saline-infused control experiments to a peak of 5 ng/ml (SEM ± 4) in 60 minutes, significantly lower values were observed during somatostatin infusion (P<0.025 – 0.05). A similar reduction in the magnitude of the GLI response to intraduodenally administered casein hydrolysate (P<0.05) and fat (p<0.05) was observed.

The role of glucagon in the pathogenesis of the endogenous hyperglycemia of diabetes mellitus

The effect of glucagon suppression by somatostatin upon endogenous hyperglycemia was studied in three forms of experimental insulin deficiency in dogs: alloxan diabetes, total pancreatectomy, and diazoxide administration. In six insulin-requiring alloxan-diabetic dogs deprived of insulin for 24 hr, mean plasma glucose declined to 77% +/- 6% of the baseline level of 350 +/- 41 mg/dl during 3 hr of glucagon suppression, significantly below the unsuppressed saline controls (p less than 0.01-0.05). When somatostatin was discontinued, glucagon rose and glucose increased 21% (p less than 0.05) in 30 min. Significant correlation between maximal changes in glucagon and glucose was observed (r = 0.81; p less than 0.001). Even during a 1-hr alanine infusion in such dogs, glucose declined an average of 36 +/- 9 mg/dl, instead of rising 51 +/- 7 mg/dl as in unsuppressed controls. Maximal changes in glucagon and glucose were correlated (r = 0.85; p less than 0.01). In eight depancreatized dogs pretreated intravenously with continuous insulin and glucose infusions, withdrawal of insulin was followed by a rise in extrapancreatic glucagon; mean plasma glucose rose from 212 +/- 43 to 415 +/- 80 mg/dl 270 min after the end of the insulin infusion. However, when glucagon was suppressed after insulin withdrawal, glucose remained below 240 mg/dl, significantly less than the controls (p less than 0.005); when somatostatin was stopped, glucagon rose and glucose increased 88 +/- 19 mg/dl within an hour. The rises in glucagon and glucose were significantly correlated (r = 0.68; p less than 0.05). Glucagon suppression by somatostatin during diazoxide-induced blockade of insulin secretion in four normal dogs reduced hyperglycemia significantly but did not prevent it. The results support the hypothesis that a relative or absolute excess of glucagon, as well as a relative or absolute deficiency of insulin, is etiologically important in the development of endogenous hyperglycemia in diabetes mellitus, the hyperglucagonemia probably mediating the glucose overproduction.

Response of pancreatic immunoreactive somatostatin to arginine

Abstract Perfusion of isolated dog pancreases with arginine (20 mM) was associated with a prompt and sustained increase in immunoreactive somatostatin (IRS) in the venous effluent while insulin and glucagon rose promptly but soon receded from their peak levels. These results are compatible with a postulated feedback relationship between somatostatin-, glucagon-, and perhaps insulin-secreting cells of the islets in which somatostatin, stimulated by local glucagon, restrains glucagon secretion and perhaps glucagon-mediated insulin release as well. The demonstration that D-cells of the pancreatic islets contain immunoreactive somatostatin (1, 2, 3) which is probably biologically active (4), and are situated topographically between the A-cells and B-cells in the heterocellular region of the islet (5) has suggested a functional role for these components of the islet of Langerhans (6). In view of the inhibitory action of somatostatin upon both insulin and glucagon secretion (7, 8, 9), it was postulated that the D-cell might serve to restrain glucagon and/or insulin secretion (6). We have since reported that the release of IRS from the isolated dog pancreas increases promptly during the perfusion of high concentrations of glucagon whereas high concentrations of insulin do not appear to stimulate IRS release (10). In this study we examine the effect of perfusion with arginine, a potent stimulus of both glucagon and insulin secretion, upon pancreatic IRS release.

Studies of the physiology and pathophysiology of the pancreatic D cell

Abstract The recent discovery by Luft and associates 1 and by Dubois 2 of somatostatin-containing islet cells, subsequently identified as D cells, 3–6 has aroused speculation concerning their physiologic and pathophysiologic roles. The ability of somatostatin to inhibit insulin and glucagon secretion has suggested that the function of the D cell may be to restrain the secretory activity of its neighboring cells, perhaps via local or paracrine action. 7–10 The possibility that pancreatic somatostatin is a hormone secreted into the circulation to act upon remote target tissues has received little attention. The present communication will review ongoing studies in our laboratories that point to an endocrine role for somatostatin in nutrient homeostasis and suggest that D cell function may be abnormal in certain forms of diabetes.