Reduction in blood and urine glucose levels in streptozotocin and alloxan diabetes by phenazine methosulfate

Summary The purpose of this investigation was to ascertain the ability of phenazine methosulfate (PMS) to improve the streptozotocin (STZ) and alloxan induced diabetic conditionin vivo as determined by changes in blood and urine glucose levels and by alteration in the secretion of insulin by isolated islets. STZ and alloxan diabetes was induced in male albino rats (200–250 g body weight). A single injection of PMS (6.0 mg/kg) or nicotinamide (500 mg/kg) simultaneously with diabetic doses of either STZ or alloxan caused a significant reduction in blood and urine glucose levels three days after the injection. The reduction in glycemic levels was greater with PMS than with nicotinamide. Daily PMS (0.5 mg/kg) injection, initiated 5, 10, 20 or 30 days after the development of STZ- and alloxan-diabetes, caused a significant decrease in blood and urine glucose levels and also increased body weight determined 60 days after STZ or alloxan administration. These effects were observed even if the injections were initiated 20 or 30 days after the onset of the diabetic syndrome. Glucose stimulated insulin secretion was significantly inhibited by pre-incubation of isolated islets for one hour at 37 °C with either STZ or alloxan. However, insulin secretion was induced by PMS in the STZ or alloxan pretreated islets. Nicotinamide neither protected nor induced insulin secretion under similar conditions. The level of insulin secretion induced by PMS whether in the normal islets or in islets previously exposed to the B-cytotoxic agents were comparable in quantity to glucose (17 mM)-stimulated insulin secretion. Data presented in this study are suggestive evidence for the potentiality of a reactive proton donor as an insulin secretagoguein vivo andin vitro.     

Evidence that hyperglycemia increases muscarinic binding in pancreatic islets of the rat

We studied the effects of fasting and of diabetes on binding of [3H]methylscopolamine to pancreatic islets of the rat. In nondiabetic rats, fasting for 36 h decreased binding of the muscarinic antagonist by 33% (P less than 0.05). Fasting also abolished the insulin response to 10 microM acetylcholine. Diabetes was induced by injecting streptozotocin (STZ) neonatally in rats. At the time of the experiments (6-10 weeks of age) these rats exhibited hyperglycemia (12.6 +/- 1.0 vs. 7.1 +/- 0.3 mM blood glucose in nondiabetics) but had a normal weight. Relative to islets from age- and sex-matched nondiabetic rats, islets from STZ rats were smaller (0.7 +/- 0.1 vs. 1.9 +/- 0.2 nl islet volume) and contained less insulin (218 +/- 33 vs. 1390 +/- 71 microU/islet). When calculated per islet volume, binding of [3H]methylscopolamine to STZ islets was enhanced by 80% in comparison to binding to normal islets (P less than 0.001). Scatchard analysis indicated that enhanced binding was due to increased number of binding sites. Cholinergic-induced insulin release, as assessed by carbamylcholine, was 37% higher in STZ than in normal islets (P less than 0.05) when expressed per islet volume and 3- to 4-fold enhanced in STZ islets when expressed per islet insulin content. Insulin treatment of STZ rats for 3 days lowered blood glucose, diminished binding of [3H]methylscopolamine, and abolished carbamylcholine-induced insulin secretion. We conclude that the level of glycemia in vivo participates in the regulation of the number of muscarinic receptors in the pancreatic islet and that such regulation is associated with changes in cholinergic-induced insulin secretion.     

Interaction of cyclic AMP and alloxan on insulin secretion in isolated rat islets perifused in vitro.

The interaction of alloxan and cyclic AMP on glucose-induced insulin secretion was studied with the use of isolated rat islet perifusion. Simultaneous perifusion with cyclic AMP and alloxan did not protect the islets against the effect of alloxan. However, addition of dibutyryl cyclic AMP to the alloxan solution produced 40% protection of glucose-induced insulin secretion. Partial protection was obtained with either theophylline (41%) or caffeine alone (54%), and the addition of 1 mg/ml glucose to the theophylline or caffeine solution provided greater than 68% protection. The levels of islet tissue cyclic AMP were more than 2.1 times that of islets not protected against the alloxan effect, when partial or nearly complete reversal of the inhibitory action of alloxan on glucose-induced insulin secretion was effected by theophylline or caffeine. These results suggest that cyclic AMP affords partial protection against the effect of alloxan on glucose-induced insulin secretion.     

Inhibitors of phosphatidylinositol 3-kinase amplify insulin release from islets of lean but not obese mice

We examined the effects of phosphatidylinositol 3-kinase (PI3K) inhibition by wortmannin or LY294002 on glucose-induced secretion from mouse islets. Islets were collagenase isolated and perifused or subjected to Western blot analyses and probed for insulin receptor-signaling components. In agreement with previous studies, mouse islets, when compared with rat islets, were minimally responsive to 10 mM glucose stimulation. The inclusion of 50 nM wortmannin or 10 microM LY294002 significantly amplified 10 mM glucose-induced release from mouse islets. The effect of wortmannin was abolished by the calcium channel antagonist nitrendipine or by lowering the glucose level to 3 mM. Wortmannin had no effect on 10 mM alpha-ketoisocaproate-induced secretion. In contrast to its potentiating effect on islets from CD-1 mice, wortmannin had no effect on 10 mM glucose-induced release from ob/ob mouse islets. Western blot analyses revealed the presence of the insulin receptor, insulin receptor substrate proteins 1 and 2 and PI3K in CD-1 islets. These results support the concept that a PI3K-dependent signaling pathway exists in beta-cells and that it may function to restrain glucose-induced insulin secretion from beta-cells. They also suggest that, as insulin resistance develops in peripheral tissues, a potential result of impaired PI3K activation, the same biochemical anomaly in beta-cells promotes a linked increase in insulin secretion to maintain glucose homeostasis.     

A link between insulin resistance and hyperinsulinemia: inhibitors of phosphatidylinositol 3-kinase augment glucose-induced insulin secretion from islets of lean, but not obese, rats

Wortmannin (5-100 nM), a specific phosphatidyinositol 3-kinase inhibitor, augmented 8 mM glucose-induced insulin secretion from control Sprague Dawley rat islets in a dose-dependent manner. This effect persisted after its removal from the perifusion medium; however, this augmenting effect was reduced by the calcium channel inhibitor nitrendipine or by lowering the glucose level to 3 mM. Wortmannin amplified insulin release induced by the combination of 6-8 mM glucose plus 1 microM carbachol; however, it had no effect on phorbol ester- or alpha-ketoisocaproate-induced insulin secretion. The potentiating action of wortmannin on 8 mM glucose-induced release was duplicated by LY294002. Wortmannin had no effect on glucose usage rates or inositol phosphate accumulation in [3H]inositol-prelabeled islets. Of particular significance, although 50 nM wortmannin potentiated 8 mM glucose-induced secretion from islets of lean Zucker control rats, the fungal metabolite had little effect on 8 mM glucose-induced release from islets of insulin-resistant Zucker fatty rats. These findings support the concept that the same biochemical process, inhibition ofphosphatidyinositol 3-kinase, that causes peripheral tissue insulin resistance enhances beta-cell sensitivity to glucose and produces a compensatory increase in insulin secretion from these cells. The efficacy of wortmannin depends on the in vivo status of the donor's insulin signaling pathways. This elegant biochemical control mechanism in beta-cells ensures the maintenance of glucose homeostasis despite a reduction in insulin action on peripheral tissues.     

Effects of porcine pancreastatin on secretion and biosynthesis of insulin and glucose oxidation of isolated rat pancreatic islets

The effects of porcine pancreastatin were studied on insulin secretion induced by glucose and nonnutrient stimuli, insulin biosynthesis, and glucose oxidation of cultured rat islets. Pancreastatin (100 nM) significantly suppressed, by 32-52%, the insulin response to 27, 16.7, 11, and 5.5 mM but not to 50 mM glucose, whereas 10 nM pancreastatin inhibited insulin release significantly only at 11 and 5.5 mM glucose. Pancreastatin (10 and 100 nM) also suppressed release induced by 20 mM arginine (by 26 and 30%) as well as by 1 microgram/ml of glibenclamide (by 56 and 72%, respectively). Pancreastatin (10 and 100 nM) furthermore inhibited insulin release induced by 0.1 mM 3-isobutyl-1-methylxanthine (IBMX) (by 40 and 61%, respectively) and 1.0 mM IBMX (by 44 and 76%, respectively). Neither glucose oxidation nor overall insulin biosynthesis in islets was significantly affected by pancreastatin, although a slight but significant enhancement of biosynthesis was noted at 1.7 mM glucose in the presence of 100 nM pancreastatin. In conclusion, these data demonstrate that porcine pancreastatin suppresses glucose-induced insulin response from isolated rat islets in a competitive manner. This effect seems not to be exerted through a suppression of (pro)insulin biosynthesis or glucose metabolism in the islets, and thus the effect mediated by pancreastatin must be on a step distal to the coupling between islet glucose metabolism and insulin secretion. The relatively strong inhibition by the peptide of IBMX-induced insulin release suggests that it acts on the cAMP system of islet B cells.     

A 48-hour lipid infusion in the rat time-dependently inhibits glucose-induced insulin secretion and B cell oxidation through a process likely coupled to fatty acid oxidation.

Short- and long-term effects of hyperlipidemia with elevated FFA on insulin secretion were investigated. Male Sprague-Dawley rats were fed ad libitum and additionally infused with Intralipid 10%, 1.0 ml/h. After 3 h of Intralipid the response to 27 mM glucose in isolated perfused pancreas was enhanced by 86%, P less than 0.02. After 6 h of Intralipid enhancement had subsided. After 48 h of Intralipid glucose-induced insulin release was inhibited by 49%, from 1950 +/- 177 microU/min after saline to 1003 +/- 232 microU/min after Intralipid, P less than 0.02. Inhibition was glucose-selective since responses to other secretagogues (1 mM 3-isobutyl-1 methylxanthine, 10 mM octanoate, or 5 mM alpha-ketoisocaproic acid) were unaffected as were pancreatic contents of insulin (2284 +/- 111 mU/pancreas after saline, 2566 +/- 131 mU/pancreas after Intralipid). In isolated islets from 48 h lipid infused rats production of [14-C]CO2 from D[U-14-C]glucose was decreased (P less than 0.02) in parallel with the insulin response to 27 mM glucose. Glucose-induced secretion was partially normalized by in vitro exposure to a carnitine palmitoyl-transferase I inhibitor (Etomoxir). Effects of a 48 h lipid infusion were also tested during hyperglycemia. Rats were infused with glucose, and hyperglycemia was enhanced by dexamethasone (25 micrograms/24 h). Hyperglycemia depressed glucose-induced secretion from perfused pancreas from 2072 +/- 22 microU/min after saline + dexamethasone to 1185 +/- 155 microU/min after glucose + dexamethasone, P less than 0.01). Intralipid, added to the latter protocol, further inhibited glucose-induced secretion to 437 +/- 87 microU/min, P less than 0.005. Hyperlipidemia is concluded to be associated with short term stimulation but long term inhibition of glucose-induced insulin secretion. Evidence indicates that inhibition depends on fatty acid oxidation, is coupled to decreased glucose oxidation and operates both during normo- and hyperglycemia.     

Irreversible loss of normal beta-cell regulation by glucose in neonatally streptozotocin diabetic rats

Summary Animals with NIDDM display abnormal glucose regulation of insulin secretion and biosynthesis. We tested reversibility of abnormal regulation by normoglycaemia using an islet transplantation technique. Inbred non-diabetic and neonatally STZ diabetic rats (n-STZ) were used. Transplantations insufficient to normalize the blood glucose levels (200 islets under kidney capsule) were performed from diabetic to normal (D-N) and from diabetic to diabetic (D-D), as well as from normal to normal (N-N) and from normal to diabetic (N-D) rats. Four weeks after transplantation, graft bearing kidneys were isolated and perfused with Krebs-Henseleit bicarbonate buffer to measure insulin secretion in response to 27.8 mmol/l glucose and 10 mmol/l arginine. Four weeks of normoglycaemia failed to restore glucose-induced insulin secretion from n-STZ islets (glucose induced increment:-1.7±2.5 fmol/min in D-N, 1.2±7.1 fmol/min in D-D). In contrast to normal islets, normoglycaemia reduced insulin mRNA contents (60±24 in D-N, 496±119 in D-D; O.D.-arbitrary units). However, arginine-induced secretion was markedly enhanced by diabetic environment in both normal and n-STZ islet grafts. These results indicate that selected aspects of glucose recognition are irreversibly damaged by a long-term diabetic state or, alternatively, by a lasting effect of STZ administration.Abbreviations NIDDM non-insulin-dependent diabetes mellitus - STZ streptozotocin - O.D. optical density - IRI immunoreactive insulin     

Abnormal regulation by glucose of somatostatin secretion in the perfused pancreas of NIDDM rats

We have investigated the influence of non-insulin-dependent diabetes on the regulation of somatostatin secretion from the pancreatic D cell. These results were compared with the concomittantly measured secretory responses from A and B cells. Rats were rendered non-insulin-dependent diabetic by neonatal injection of streptozotocin (STZ). Secretion was studied in perfused pancreas at 6-10 weeks of age. At this age, STZ rats were mildly hyperglycemic, their nonfasting blood glucose being 9.0 +/- 0.8 vs. 5.6 +/- 0.2 mM in control rats. In perfused pancreas from the latter rats, high glucose, i.e., 16.7 mM, stimulated somatostatin secretion but completely failed to do so in STZ rats. Arginine (in the presence of low glucose, i.e., 3.3 mM) moderately stimulated somatostatin secretion in controls but fourfold more in STZ rats. Preperfusion with high glucose markedly potentiated subsequent arginine-induced somatostatin secretion in controls but failed to do so in STZ rats. Basal glucagon release was inhibited by ambient high glucose in control and STZ rats alike. Arginine-induced glucagon release was profoundly inhibited both by ambient and previous exposure to glucose in controls but only slightly and nonsignificantly in STZ rats. The insulin response to high glucose in controls was reduced by 90% in STZ. The insulin response to arginine (in the presence of low glucose) was 3.3-fold enhanced in STZ. Ambient and previous high glucose markedly enhanced arginine-induced insulin secretion in controls but only moderately so in STZ rats. We conclude that already mild hyperglycemia is associated with marked D-cell insensitivity to glucose that is qualitatively similar to A- and B-cell insensitivity.     

Increased insulin binding to pancreatic islets of aged rats

In rat pancreatic islets, the effect of old age (24-month-old) on [125I]insulin binding, glucose-induced insulin release and inhibition of insulin secretion by exogenous insulin were studied. The results were compared with corresponding data obtained from young (3-month-old) rats. Specifically bound [125I]insulin in islets of old rats was increased by 40% (P less than 0.02) compared to that in young rats. Scatchard plots of displacement studies indicated an increase in receptor number rather than receptor affinity. The insulin-releasing capacity of 16.7 mM glucose did not differ between islets of old and young rats when medium insulin was bound to added antiinsulin serum. In the presence of 16.7 mM glucose (without the addition of antiinsulin serum), insulin secretion was less in islets of old rats compared to that in young rats (283 +/- 38 vs. 528 +/- 29 microU/ml; P less than 0.001). Exogenous insulin inhibited glucose (16.7 mM)-induced insulin release more in islets of old rats than in those of young rats. In conclusion, the present in vitro results may be interpreted to reflect increased insulin binding to islets of aged rats and, consequently, increased inhibition of glucose-mediated insulin secretion due to increased feedback of insulin.     

Loss of insulin response to glucose but not arginine during the development of autoimmune diabetes in BB/W rats: relationships to islet volume and glucose transport rate.

The insulin and glucagon responses to 10 mM glucose and 10 mM arginine were studied in pancreata isolated from nondiabetic diabetes-prone and diabetes-resistant BB/W rats at 60, 80, and 140 days of age and in diabetic BB/W rats on the 1st and 14th days of their diabetes. In the former group the insulin response to glucose declined progressively with age (r = -0.575; P less than 0.01) and at 140 days was significantly below age-matched diabetes-resistant controls (P less than 0.05). The insulin response to arginine did not decline with age in either group. For diabetic rats, on the first day of the diabetes, the insulin response to glucose was absent but the response to arginine did not differ from nondiabetic controls. On day 14 responses to glucose and arginine were both absent. The glucagon response to arginine showed no trend despite a decline in baseline glucagon secretion. Endocrine tissue in nondiabetic diabetes-prone rats made up 0.8 +/- 0.2% of the pancreas at 60 days of age and 0.52 +/- 0.22% at 140 days of age; the latter was significantly less than in 140-day-old diabetes-resistant controls (P less than 0.05). In diabetic rats on the 1st and 14th days of diabetes endocrine tissue was 0.2 +/- 0.1% and 0.07 +/- 0.02%, respectively. The glucose transport rate in islets isolated on the first day of diabetes was profoundly reduced compared to age-matched nondiabetic diabetes-prone controls. Thus, a population of arginine-responsive, glucose-unresponsive islets with low glucose transport rates is present at the onset of overt diabetes in BB/W rats.     

Differential effects of beta-adrenergic agonists on insulin secretion from pancreatic islets isolated from rat and man

The selective beta 2-adrenergic agonist clenbuterol was ineffective as a stimulus for insulin secretion when isolated rat pancreatic islets were incubated with glucose at concentrations between 4 and 20 mM. Inclusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine led to potentiation of glucose-induced insulin secretion, but did not facilitate stimulation by clenbuterol. Furthermore, maintenance of isolated rat islets for up to 3 days in tissue culture also failed to result in the appearance of a secretory response to beta-agonists. By contrast, clenbuterol induced a dose-dependent increase in insulin release from isolated human islets incubated with 20 mM glucose. Clenbuterol did not increase the basal rate of insulin secretion (4 mM glucose) in human islets. Under perifusion conditions, the secretory response of human islets to clenbuterol was rapid, of similar magnitude to that seen under static incubation conditions and could be sustained for at least 30 min. The increase in insulin secretion induced by clenbuterol was inhibited by propranolol, indicating that the response was mediated by activation of beta-receptors. In support of this, a similar enhancement of glucose-induced insulin secretion was elicited by a different beta 2-agonist, salbutamol, in human islets. The results indicate that the B cells of isolated rat islets are unresponsive to beta-agonists, whereas those of human islets are equipped with functional beta-receptors which can directly influence the rate of insulin secretion.     

Nutrient control of insulin secretion in perifused adult pig islets

OBJECTIVES: Xenotransplantation of pig islets is a potential solution to the shortage of human islets, but our knowledge of how these islets secrete insulin in response to nutrients is still fragmentary. This was the question addressed in the present study. METHODS: After 24 h culture adult pig islets were perifused to characterize the dynamics of insulin secretion. Some responses were compared to those in human islets. RESULTS: Increasing glucose from 1 to 15 mM weakly (approximately 2x) stimulated insulin secretion, which was potentiated (approximately 12x) by the cAMP-producing agent, forskolin. The effect of glucose was concentration-dependent (threshold at 3-5 mM and maximum at approximately 10 mM). The pattern of secretion was biphasic with a small first phase and an ascending second phase, and a paradoxical increase when the glucose concentration was abruptly lowered. Diazoxide abolished glucose-induced insulin secretion and tolbutamide reversed the inhibition. Glucose also increased secretion when islets were depolarized with tolbutamide or KCl. Insulin secretion was increased by leucine+glutamine, arginine, alanine or a mixture of amino acids, but their effect was significant only in the presence of forskolin. Upon stimulation by glucose alone, human islets secreted approximately 10x more insulin than pig islets, and the kinetics was characterized by a large first phase, a flat second phase, and rapid reversibility. CONCLUSIONS: Compared with human islets, in vitro insulin secretion by adult pig islets is characterized by a different kinetics and a major quantitative deficiency that can be corrected by cAMP.     

Effects of prior 5-hydroxytryptamine exposure on rat islet insulin secretory and phospholipase C responses

Glucose-induced insulin secretion is inhibited by 5-hydroxytryptamine (5HT). In the present studies the specificity of 5HT inhibition of release and the potential biochemical mechanisms involved were investigated. Dose-dependent inhibition of 15 mM glucose-induced secretion was induced by a prior 3 h incubation with 5HT. At the highest 5HT concentration (500 microM) employed, both first and second phase responses to 15 mM glucose were reduced 50-60%. In addition, this level (500 microM) of 5HT virtually abolished 10 mM glucose-induced secretion. In contrast, secretion in response to the protein kinase C activator phorbol 12-myristate 13-acetate (500 nM) was immune to 500 microM 5HT pre-treatment. Glucose usage rates were comparable in both control and 500 microM 5HT-pretreated islets. However, the generation of inositol phosphates and the efflux of 3H-inositol from 3H-inositol-prelabeled islets in response to stimulatory glucose were impaired in parallel with insulin secretion. Based on these observations the following conclusions were reached: (1) 5HT impairs glucose-induced insulin release by altering glucose-induced activation of phospholipase C. (2) Biochemical events distal to phospholipase C remain intact despite this proximal biochemical lesion. (3) Amperometric analysis of 5HT release from 5HT-pretreated islets must take into consideration its profound adverse impact on glucose-induced insulin secretion.     

Prolactin regulates adenylyl cyclase and insulin secretion in rat pancreatic islets

A role for prolactin (PRL) in the regulation of adenylyl cyclase (AC), cyclic AMP (cAMP) formation and insulin secretion was studied in isolated rat pancreatic islets cultured for 4 days at 5.5 mM glucose in the absence (control) or presence of PRL (500 ng/ml). In PRL-treated islets, stimulation by glucose (8 mM), carbamylcholine chloride (CCh) and phorbol dibutyrate increased cAMP levels 40, 89, and 151%, respectively, above similarly stimulated control islets without PRL. Moreover, insulin secretion in PRL-treated islets was more than doubled in response to 8 mM glucose plus glucagon-like peptide 1 compared with control islets. PRL also increased protein kinase C (PKC) activity in cultured islets. When islets were cultured at an insulin secretion desensitizing concentration of glucose (11 mM) for 4 days, there was a decrease in forskolin-stimulated cAMP production. However, the presence of PRL with 11 mM glucose prevented the glucose-induced decrease in cAMP production. Insulin secretion in response to 17 mM glucose was also higher (P<0.02) in islets cultured with 11 mM glucose plus PRL compared with islets cultured with 11 mM glucose alone. Islet AC types -III, -V, and -VI mRNA levels increased relative to 18s rRNA following PRL treatment. In contrast, culture at 11 mM glucose decreased relative AC-III, -V and -VI mRNA levels by as much as 50%. Culture with PRL prevented the decrease in AC expression during islet culture with 11 mM glucose, and the mRNA levels remained similar to control islets cultured at 5.5 mM glucose. Thus, PRL not only increased islet AC expression and activity and insulin secretory responsiveness, but also protected islets from chronic glucose-induced inhibition of these beta-cell activation parameters.     

Selective impairment of pancreatic A cell suppression by glucose during acute alloxan-induced insulinopenia: in vitro study on isolated perfused rat pancreas

This study was performed in order to investigate the role of insulin in the modulation of pancreatic A cell response to glucose. The isolated perfused rat pancreas model was used: intraislet insulinopenia was induced in vitro by 0.56 mM alloxan infusion over 15 min. Alloxan caused a transitory insulin release but did not affect glucagon secretion. Exposure to alloxan completely abolished insulin response to 20 mM arginine, 1.6 mM glucose, and 11.1 mM glucose. Glucagon response to 20 mM arginine and 1.6 mM glucose was unchanged by alloxan pretreatment compared to control pancreata not treated with alloxan. However, the suppression of glucagon release by 11.1 mM glucose was abolished in the alloxan experiments. Twenty milliunits per ml of insulin infused during 11.1 mM glucose infusion restored glycemic suppression of glucagon release, but it produced only a slight inhibitory effect on A cell function in the presence of 3.9 mM glucose. Our study indicates that glucose is the physiological suppressor of the pancreatic A cell and that, in this regard, insulin exerts only a permissive effect.     

Effect of alloxan on insulin and glucagon secretion in perifused isolated islets

Summary The effect of alloxan on insulin and glucagon secretion was investigated in perifused isolated rat islets. Five minutes of exposure to 1.4 mmol/l alloxan in a low-glucose medium (5.6 mmol/l) abolished subsequent leucine stimulated insulin and glucagon secretion. In a medium containing 19 mmol/l arginine and 3.3 mmol/l glucose, insulin secretion was only slightly diminished by alloxan pretreatment, whereas glucagon secretion was reduced to about 60% of controls. Exposure to alloxan in a high glucose medium (27.8 mmol/l) did not effect insulin or glucagon secretion.     

Changes in islet blood flow in rats with NIDDM

Summary Islet blood flow was quantified in NIDDM rats either of the GK strain on after neonatal injection of STZ (n0-STZ), using the non-radioactive microsphere technique. In the basal state, there was a good correlation between plasma insulin level and islet blood flow, i.e. both were increased or decreased in comparison to those of control rats in GK and n0-STZ rats, respectively. The increased islet blood flow and plasma insulin levels observed in the GK rats were abolished by bilateral subdiaphragmatic vagotomy. During a glucose challenge, whereas plasma insulin and islet blood flow were doubled in control rats, these parameters were not modified in the diabetic rats. These data demonstrate an alteration in the islet blood flow of diabetic rats during a glucose challenge which could participate in the abnormal glucose-induced insulin secretion previously described in these two models.Abbreviations NIDDM non-insulin-dependent diabetes mellitus - STZ streptozotocin     

Genetically obese rats with (SHR/N-cp) and without diabetes (LA/N-cp) share abnormal islet responses to glucose.

To assess the effect of hyperglycemia on the function of islets obtained from obese rats, the behavior of isolated islets from LA/N-corpulent (nondiabetic obese) and SHR/N-corpulent (diabetic obese) male rats was examined and compared. Islets from both genetic models showed a left-shifted glucose dose-response curve for insulin release (concentrations for half-maximal release, 5 to 6 mmol/L v 12 to 13 mmol/L in LA/N lean littermates and 3 mmol/L v 10 mmol/L in lean SHR/N). When insulin release was expressed per unit islet volume, the fourfold to fivefold enlarged islets from both obese diabetic and obese nondiabetic rats showed decreased insulin secretory response in high (16.5 to 28 mmol/L) glucose concentrations, although the decrease was more severe in the diabetic rats. Glucose-stimulated insulin release by islets from both models was relatively resistant to inhibition by 1.2 mmol/L mannoheptulose (eg, 82% +/- 3% inhibition in LA/N lean v 16% +/- 8% in LA/N obese), although nearly complete inhibition was observed with 16 mmol/L mannoheptulose (96% v 85%, NS). Islets of obese diabetic rats were also resistant to the calcium-channel blocker, verapamil, suggesting an abnormal pathway of stimulus-secretion coupling for glucose. Glucose oxidation to carbon dioxide was increased in both obese models at all glucose concentrations when expressed per islet. In data expressed per unit volume, the larger islets from the obese-nondiabetic rats showed a left-shifted dose-response curve with an unchanged maximum rate of glucose oxidation at high (16.5 mmol/L) glucose concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Melatonin inhibits insulin secretion and decreases PKA levels without interfering with glucose metabolism in rat pancreatic islets

The effect of melatonin (0.1 microM) on freshly isolated islets from adult rats was investigated. Melatonin caused a marked decrease of insulin secretion by islets in response to glucose. The mechanism involved was then examined. Melatonin did not interfere with glucose metabolism as indicated by the measurement of glucose oxidation. However, the content of the protein kinase A (PKA) catalytic alpha-subunit was significantly decreased in islets exposed to melatonin for 1 hr in the presence of 8.3 mM glucose, whereas that of the protein kinase C (PKC) alpha-subunit remained unchanged. Melatonin also inhibited forskolin-induced insulin secretion, a well known activator of adenylate cyclase (AC) activity. This may explain the low content of insulin found in islets incubated in the presence of melatonin for 3 hr. In fact, 3',5' -cyclic adenosine monophosphate (cAMP), a product of AC activity, stimulates insulin synthesis. These findings led us to postulate that a down-regulation of the PKA signaling pathway may be the mechanism involved in the melatonin inhibition of the process of glucose-induced insulin secretion.