Previous exposure to glucose enhances somatostatin secretion from the isolated perfused rat pancreas

Summary Previous exposure to glucose enhances insulin and depresses glucagon secretion by the pancreas. We have investigated whether secretion of somatostatin is also influenced by a glucose priming effect. In perfused rat pancreas from 36 h fasted rats a 5 min pulse of arginine (8 mmol/l) rapidly elicited a peak of somatostatin release. A similar somatostatin response was evoked by a second, identical, pulse of arginine after perfusion with basal glucose (3.9 mmol/l) for 45 min. On the other hand when 27.7 mmol/l D-glucose, was administered for 20 min between arginine pulses, there was significant stimulation of somatostatin secretion. When arginine was re-introduced 15 min after the cessation of the pulse of elevated glucose the magnitude of the arginine-induced peak (min 0–2 of stimulation) was increased from 16.2±4.1 to 33.1±4.7 pg/2 min, p<0.01, relative to the first stimulation with arginine. None of these effects of glucose could be reproduced by Dgalactose. The somatostatin response to arginine was higher in pancreata from fed than from 36 h fasted animals as was also basal release (22.8±5.0 vs 9.0±2.0 pg/min). In the fed state the response to the second pulse of arginine was however reduced by 50% after perfusion with basal glucose. This decrease in responsiveness was counteracted by perfusion with 27.7 mmol/l glucose for 20 min between the arginine pulses. It is concluded that previous exposure to an elevated concentration of glucose enhances D-cell responsiveness to arginine in the fasted as well as the fed state.     

Glucose-induced somatostatin secretion by the rat pancreas is not potentiated by glucose

The effect of previous exposure to glucose on subsequent glucose-stimulated insulin and somatostatin secretion has been investigated using the isolated perfused rat pancreas. As expected, when the pancreases of non-diabetic rats were exposed to 16.7 mM glucose on two occasions, 20 min apart, insulin secretion during the second period of exposure to high glucose was greater than that during the first period. By contrast, there was no potentiation of somatostatin secretion during the second glucose stimulation with respect to that of the first. Indeed, when the basal glucose concentration was low (1.4 or 2.8 mM) somatostatin secretion during the second glucose stimulation was lower than that during the first. Since exogenous insulin is known to inhibit glucose-induced somatostatin secretion, it seemed possible that lack of visible potentiation of glucose-induced somatostatin secretion by glucose could have been due to partial D cell inhibition by simultaneously augmented insulin secretion during the second glucose stimulation. In an attempt to exclude such an interaction between B and D cells, somatostatin secretion was also studied in the pancreases of spontaneously diabetic, Wistar (BB) rats (these animals are insulin deficient and are maintained by daily injections of insulin). However, even though insulin secretion was not detectable from these pancreases, glucose potentiation of glucose-induced somatostatin secretion did not occur. Although the pancreatic B and D cells are known to respond in a similar manner to many secretagogues the present results show that glucose potentiation of glucose-stimulated somatostatin secretion is not found under circumstances where potentiation of insulin secretion does occur. In addition, the absence of potentiated somatostatin secretion could not be attributed to partial inhibition of the D cell by insulin.     

Glucagon and somatostatin secretion from the perfused splenic bulb of duck pancreas

Glucagon, somatostatin and insulin secretions were evaluated in a new type of perfusion preparation: the naturally A and D cell rich splenic bulb of duck pancreas. Stable basal levels were observed with 11 mM glucose, corresponding to normoglycaemia, and all secretions were stimulated by 1 mM 3-isobutyl-1-methyl-xanthine and by 10 mM arginine, demonstrating the technique's validity. In the absence of aminoacids in the perfusion medium, A cell blindness to glucose was corrected by physiological levels of insulin (2 ng/ml); insulinodependency of A cells, and unresponsiveness of D cells to glucose, probably not ruled by insulin, were observed. However, in the presence of aminoacids, glucagon was inhibited and somatostatin secretion stimulated by glucose (33 mM), independently of insulin (2 ng/ml). Aminoacids greatly influenced pancreatic hormone release.     

Inhibition of glucagon and insulin secretion from the perfused rat pancreas by a B-cell-selective somatostatin analog

The isolated perfused rat pancreas was used to study the effects of somatostatin and the analog des-Asn⁵-[D-Trp⁸, D-Ser¹³]-somatostatin on arginine-stimulated insulin and glucagon secretion. Even though the analog was found to inhibit glucagon secretion at concentrations of 10 and 100 ng/ml, it had a relatively more inhibitory effect on the B cell than on the A cell than did somatostatin itself. These data suggest that the A- and B-cell receptors for these two peptides differ.     

Effect of insulin on glucose- and arginine-stimulated somatostatin secretion from the isolated perfused rat pancreas

The effects of exogenous insulin on somatostatin secretion from the isolated perfused rat pancreas have been investigated in the presence of 5.6 mM glucose and when somatostatin secretion was stimulated by either glucose (16.7 mM) or arginine (20 mM). Insulin (15 mU/ml) significantly and rapidly suppressed glucose- and arginine-stimulated somatostatin release. However, at 5.6 mM glucose and, in the absence of other stimulators of somatostatin release, insulin had no effect on the somatostatin secretion rate.     

The influence of somatostatin on glucagon and pancreatic polypeptide secretion in the isolated perfused human pancreas

Summary The current study was undertaken to determine whether intraislet somatostatin regulates glucagon or pancreatic polypeptide (PP) secretion in the human pancreas. A high-affinity, high-specificity monoclonal somatostatin antibody (CURE.S6) was used to immunoneutralize somatostatin in the isolated, perfused human pancreas. Single-pass perfusion was performed in pancreata obtained from cadaveric organ donors using a modified Krebs media with either 3.9 or 12.9 mM glucose. Sequential test periods separated by basal periods were performed with infusion of either exogenous somatostatin-14 (SS-14), CURE.S6, or a combined infusion. Infusion of SS-14 did not significantly alter glucagon or PP secretion during low-glucose or high-glucose perfusion. Immunoneutralization of intraislet somatostatin with CURE.S6 resulted in a significant increase of glucagon secretion under low-glucose conditions (ΔX=15±3 pM) (p<0.05), but did not significantly effect glucagon secretion under high-glucose conditions (ΔX=−2±3 pM) (p=NS). PP secretion remained unchanged during CURE.S6 infusion. Combined infusion of SS-14 and CURE.S6 did not significantly alter glucagon or PP secretion. The data suggest that intraislet somatostatin may have an inhibitory role in the regulation of glucagon secretion during low-glucose conditions and that intraislet somatostatin does not regulate PP secretion in the isolated, perfused human pancreas.     

Differential inhibition of insulin and islet amyloid polypeptide secretion by intraislet somatostatin in the isolated perfused human pancreas.

Islet amyloid polypeptide (IAPP) and insulin are co-stored and generally secreted in parallel; however, studies have demonstrated that the IAPP/insulin molar secretory ratio may be altered in response to certain stimuli. Because we previously demonstrated that intraislet somatostatin is an inhibitory regulator of basal insulin secretion in the isolated perfused human pancreas, this study was designed to determine the relative influence on the regulation of IAPP versus insulin secretion. Single-pass perfusion was performed in pancreata obtained from cadaveric organ donors with continuous perfusion of a modified Krebs media with the glucose level maintained at constant 3.9 mM. Intraislet somatostatin was immunoneutralized by the infusion of either a highly sensitive monoclonal somatostatin antibody (SAb) or its FAb fragment (SFAb). Sequential test periods separated by basal periods were performed by infusion of either of the following: glucose, SAb, SFAb, or appropriate controls. IAPP/insulin molar secretory ratio decreased by 33% in response to infusion of either SAb or the SFAb, respectively (p < 0.01), and decreased by 67% in response to glucose infusion (p < 0.01). An alteration of the IAPP/insulin secretory ratio is seen in response to infusion of exogenous glucose or in response to the neutralization of intraislet somatostatin.     

Role of glucose and insulin in the dynamic regulation of glucagon release by the perfused rat pancreas

The effect of glucose upon the release of glucagon and insulin from the perfused rat pancreas in vitro was studied by varying both the concentration of glucose (from 3.3 to 4.6, 8.5, or 11.1 mmol/l) and the time of exposure to an elevated concentration of the sugar (5, 10 or 23 min). The results suggest that the amount of insulin released during the early period of stimulation could contribute to both the speed and extent of the inhibition in glucagon release. The rate of recovery from inhibition in the A cell, however, appeared to be independent of insulin and was related, in a dose-dependent and time-dependent manner, only to the glucose stimulus. It is suggested that a direct effect of glucose upon the A cell is involved in the physiological regulation of glucagon secretion. An indirect effect of glucose, as mediated via insulin release, may contribute to the rapidity and magnitude of inhibition in A cell secretory activity.     

Modulation by glucose and D-glyceraldehyde of glucagon and insulin secretion in the isolated perfused streptozotocin-treated rat pancreas

Summary In order to compare the effects of D-glyceraldehyde or glucose on glucagon secretion in insulin deficiency, the isolated streptozotocin-treated rat pancreas was perfused with arginine alone and arginine plus either glucose or D-glyceraldehyde. The glucagon secretion induced by arginine alone was not modified by pretreatment with streptozotocin, but the glucagon secretion induced by arginine plus either glucose or D-glyceraldehyde was less inhibited in the streptozotocin-treated pancreas. We conclude, therefore, that insulin deficiency may interfere with the metabolism of D-glyceraldehyde as well as glucose in the pancreatic A-cells, thus interfering with the inhibitory effect of glucose and D-glyceraldehyde of glucagon secretion.     

The effect of age and glucose concentration on insulin secretion by the isolated perfused rat pancreas

Age changes in the beta-cell's sensitivity to glucose as well as in its overall capacity to secrete insulin may play a part in the glucose intolerance of aging. The isolated perfused rat pancreas preparation was used to study the effect of age and glucose level on insulin secretion. Overnight-fasted male Wistar 12- and 23-month-old rats had basal plasma glucose levels of 106 +/- 4 (SE) and 100 +/- 4 mg/dl. Perfusate glucose levels were raised from 80 mg/dl to either 150, 220, or 360 mg/dl for 50 min (n = 6 to 8 in each group). Insulin secretion followed the typical biphasic pattern of an early spike and fall, followed by a sustained gradual increase at both ages. First-phase (0-10 min) insulin secretion in the old rats was significantly lower at 150 (184 vs. 524 microU/min, P less than 0.05) and 220 mg/dl (327 vs. 644 microU/min, P less than 0.05), while it was nearly identical at 360 mg/dl. Although lower in the old rats, second-phase (11-50 min) insulin secretion was not statistically significantly different for each glucose level. When first- and second-phase insulin secretion rates were combined, the old rats' insulin secretion was only lower at the 150 mg/dl level (248 vs. 426 microU/min, P less than 0.05). Thus, at the more physiological glucose level, old rats showed a significantly lower response, while at the higher levels insulin secretion was similar. This diminishing age effect with increasing glucose dose suggests a defect in islet sensitivity to glucose rather than a diminished capacity to secrete insulin.     

Plasma from BB/Wor rats increases insulin secretion by perfused rat pancreas

Diabetes-prone (DP) BB rats develop spontaneous autoimmune diabetes mellitus in the context of multiple abnormalities of humoral and cellular immunity. Diabetes-resistant (DR) BB rats have phenotypically normal immune systems and rarely become spontaneously hyperglycemic, but can be rendered diabetic by in vivo immune elimination of T cells that express the RT6 surface alloantigen. To determine if humoral factors in these animals influence beta-cell function, we studied the effect of BB rat plasma on glucose-induced insulin secretion from the isolated perfused rat pancreas. We found that plasma dialyzed to remove molecules less than 14 kD from nondiabetic DR and DP BB rats significantly enhanced total insulin secretion [4806 +/- 711 ng (+/- SEM; n = 6) and 4968 +/- 1235 ng (n = 7), respectively] from perfused pancreata when compared with the effects of either plasma from Wistar-Furth rats (2585 +/- 336 ng; n = 9) or medium containing no plasma (1862 +/- 92 ng; n = 38). The presence of chemically induced diabetes was also associated with enhanced insulin secretion [3276 +/- 414 ng (n = 8) using alloxan and 3956 +/- 470 ng (n = 7) using streptozocin], but the greatest degree of enhancement was observed with plasma from spontaneously diabetic BB rats (6521 +/- 751 ng; n = 17). The enhancement of insulin secretion by BB rat plasma, both diabetic and nondiabetic (DR and DP), was characterized by preservation of first and second phase hormone release. Heat inactivation of acutely diabetic BB rat plasma did not affect its ability to stimulate insulin secretion. We conclude that the plasma of BB rats, both before and after the onset of autoimmune diabetes mellitus, contains a factor other than complement of greater than or equal to 14 kD that enhances insulin secretion in vitro from the isolated perfused pancreas.     

Effect of ventromedial hypothalamic lesions on the secretion of somatostatin,insulin, and glucagon by the perfused rat pancreas

Somatostatin, insulin, and glucagon secretion by the perfused pancreas were studied in adult female rats 10 days after ventromedial hypothalamic (VMH) lesions and in sham operated controls to assess the role of their hypothalamic control. Insulin secretion was significantly greater in VMH-lesioned rats both under basal conditions and after stimulation by theophylline and arginine plus theophylline. Basal glucagon secretion was greater in VMH-lesioned rats as was the glucagon response to theophylline alone and in combination with arginine. Basal somatostatin secretion was similar in VMH and control rats but somatostatin secretion induced by theophylline and by arginine plus theophylline was significantly increased in VMH-lesioned rats. Both the pancreatic content and concentration of somatostatin were increased in VMH-lesioned rats. These results indicate the presence of hyperresponsiveness of A, B, and D cells following VMH destruction and provide new evidence for a role of the hypothalamus in the regulation of pancreatic somatostatin secretion.     

Possible involvement of the cholinergic system in hormonal secretion by the perfused pancreas from ventromedial-hypothalamic lesioned rats

Summary Total arginine-induced secretion of insulin, glucagon and somatostatin was studied during a 20 min period in isolated perfused pancreases from control and non-hyperphagic ventromedial hypothalamic (VMH) lesioned rats. Compared to controls pancreases from VMH-lesioned rats secreted more insulin (82±13ng vs 36±9ng) and more glucagon (130±23ng vs 73±14ng) but less somatostatin (0.58±0.18ng vs 1.12±0.14ng). These abnormalities were restored to normal by perfusion with atropine (25 mol/l). Pancreases of both groups were perfused with the cholinergic agonist methacholine (100 mol/l). Again pancreases from VMH-lesioned rats secreted more insulin (157±19ng vs 33±6ng) and more glucagon (95±13 ng vs 57±9 ng) but less somatostatin (0.80±0.15 ng vs 1.30±0.18 ng). These results support the concept that, in pancreases isolated from VMH-lesioned rats increased cholinergic activity may prevail via increased release of endogenous acetylcholine from islet-postsynaptic ganglion cells together with increased numbers of muscarinic receptors on postsynaptic ganglion cells as well as on endocrine cells.     

Effect of ammonia on glucagon secretion from the perfused pancreas of cirrhotic rats

Effects of ammonia on glucagon and insulin secretion from the perfused pancreas of cirrhotic rats were investigated to clarify the occurring mechanism of hypersecretion of pancreatic glucagon in liver cirrhotics. The results were as follows: During ammonia loading, insulin secretion was inhibited in a dose-related manner, whereas glucagon secretion was gradually increased at high concentrations of ammonia (2 mM) in control rats; this tendency was augmented in the presence of alpha-ketoglutarate in cirrhotic rats. On cessation of ammonia loading, a transient but definite increase in glucagon and insulin secretion was observed. Basal plasma glucagon and ammonia levels as well as basal glucagon secretion from the perfused pancreas of cirrhotic rats were significantly higher than in control rats. Basal insulin secretion from the perfused pancreas of cirrhotic rats was not different in spite of high levels of plasma insulin. Glucagon secretory response to glucose and arginine from the perfused pancreas of cirrhotic rats was higher than in the control pancreas, whereas insulin secretion was lower. In these cirrhotic rats, an increase in the number of islet cells, particularly A cells, was observed. These data suggested that hypersecretion of pancreatic glucagon which was responsible for hyperglucagonemia in cirrhotic rats might be attributed to high levels of ammonia and alpha-ketoglutarate in blood as well as to the fluctuation of abnormal ammonia concentration in blood and to the hypertrophy of islets, particularly of the A cell group due to hypersecretion.