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Background: In skeletal muscle and adipocytes, insulin-stimulated glucose transport has been known to occur through the translocation of glucose transporter (GLUT) 4 from the intracellular pool to the plasma membrane. The Tsumura Suzuki obese diabetic (TSOD) mouse, a new genetic animal model of type 2 diabetes, develops moderate degrees of obesity and diabetes that are especially apparent in animals more than 11 weeks old. A defect in insulin stimulation of GLUT4 translocation also contributes to the characteristics of type 2 diabetes., Objective: To characterize this mouse further, we examined the alteration in insulin-stimulated GLUT4 translocation in the skeletal muscle and adipose tissue., Methods: For glucose and insulin tolerance tests, the mice were given glucose or insulin and blood samples were collected. After isolation of low-density microsomal membrane and plasma membrane from skeletal muscle and adipose tissue, insulin-stimulated translocation of GLUT4 in these TSOD mice was examined by Western blot., Results and Conclusions: TSOD mice showed a significant increase in blood glucose after the glucose load, and exhibited a significantly attenuated decrease in blood glucose concentrations after administration of insulin, compared with that in control Tsumura Suzuki non-obese (TSNO) mice. The insulin-stimulated translocation of GLUT4 from low-density microsomal membranes to plasma membrane was significantly reduced in both skeletal muscle and adipose tissue of TSOD mice. These results indicate that the reduced insulin sensitivity in diabetic TSOD mice is presumably due, at least in part, to the impaired GLUT4 translocation by insulin in both skeletal muscle and adipocytes.

Fetal pancreatic islets show a deficiency in insulin secretion in response to glucose, but the underlying mechanism is disputed. By isolating pancreatic islets from 21-day pregnant rats and culturing them with 2.8 or 11.1 mM glucose for 7 days, we attempted to clarify the involvement of low glucokinase activity in the poor glucose response in islets cultured with 2.8 mM glucose relative to the response obtained from those cultured with 11.1 mM glucose. The insulin secretion induced by 10 mM glyceraldehyde or 15 mM leucine, but not that induced by 20 mM glucose, from islets cultured with 2.8 mM glucose was higher than the basal insulin secretion, suggesting that the defect in glucose stimulation in fetal islets may be localized somewhere before the glyceraldehyde 3-phosphate step in the glycolytic pathway. When islets cultured with 11.1 mM glucose as distinct from those cultured with 2.8 mM glucose were incubated with glucose, the glycolytic intermediate contents were increased in a concentration- and time-dependent manner. Utilization of glucose at 20 mM, but not at 5 mM, in islets cultured with 11.1 mM glucose was higher than that in islets cultured with 2.8 mM glucose. The Vmax value of glucokinase, but not that of hexokinase or aldolase, in islets cultured with 11.1 mM glucose was higher by 150% than that in islets cultured with 2.8 mM glucose. The results suggest that the poor secretion of insulin in response to glucose can be explained by insufficient glucose metabolism due to the low glucokinase activity.

The effects of D-glyceraldehyde and glucose on islet function were compared in order to investigate the difference between them in the mechanism by which they induce insulin secretion. The stimulation of insulin secretion from isolated rat islets by 10 mM glyceraldehyde was not completely inhibited by either 150 microM diazoxide (an opener of ATP-sensitive K(+) channels) or 5 microM nitrendipine (an L-type Ca(2+)-channel blocker), whereas the stimulation of insulin secretion by 20 mM glucose was completely inhibited by either drug. The insulin secretion induced by glyceraldehyde was less augmented by 100 microM carbachol (a cholinergic agonist) than that induced by glucose. The stimulation of myo-inositol phosphate production by 100 microM carbachol was more marked in islets incubated with the hexose than with the triose. The content of glyceraldehyde 3-phosphate, a glycolytic intermediate, in islets incubated with glyceraldehyde was far higher than that in islets incubated with glucose, whereas the ATP content in islets incubated with the triose was significantly lower than that in islets incubated with the hexose. These results suggest that glyceraldehyde not only mimics the effect of glucose on insulin secretion but also has the ability to cause the secretion of insulin without the influx of Ca(2+ )through voltage-dependent Ca(2+) channels. The reason for the lower potency of the triose than the hexose in stimulating insulin secretion is also discussed.

Several lines of evidence suggest that insulin resistance and/or hyperinsulinemia may play an important role in the pathogenesis of hypertension. We studied the effect of pregnancy on insulin metabolism in spontaneously hypertensive rats (SHRs) and in Wistar-Kyoto rats (WKYs) as a control. Pregnancy markedly reduced blood pressure in both strains of rats, but insulin resistance as determined by the hyperinsulinemic glucose clamp (10 mU/kg/min) increased in SHRs and was unchanged in WKYs. The plasma insulin response to an intravenous glucose challenge in SHRs was low and did not change with pregnancy. Therefore, it is suggested that the regulation of blood pressure in these animals is linked to an unknown factor rather than to insulin resistance and hyperinsulinemia. Fetuses from SHRs had a lower body weight and plasma glucose level and higher plasma insulin and pancreatic insulin levels than those from WKYs. Thus, fetal hyperinsulinemia in the SHR may be linked to the development of hypertension in adulthood.

Insulin secretory activity was compared in the spontaneously hypertensive rat (SHR) versus the normotensive Wistar-Kyoto (WKY) rat and Wistar rat. When the isolated pancreas was perfused with 16.7 mmol/L glucose, insulin release was significantly greater in the SHR versus the other groups. On the other hand, there was no difference in arginine (19 mmol/L)-induced insulin secretion among the three groups. To determine insulin biosynthesis during glucose stimulation, the pancreas was perfused with 16.7 mmol/L glucose for 180 minutes. Insulin secretion was greater in SHR versus WKY and Wistar rats, but the net increase in insulin content was not different between the three groups. These results strongly suggest that in vivo hyperinsulinemia in the SHR is associated with increased in vitro insulin secretion in response to glucose. The mechanisms by which enhanced glucose-induced insulin secretion is linked to hypertension in the SHR remain unclear.

The effects of vanadate on B-cell function and replication in rats after 90% partial pancreatectomy (Px) were compared with insulin therapy. At the age of 4 weeks, male Wistar rats were subjected to sham operation or Px. Vanadate (0.2 mg/ml) was given in drinking water for 3 weeks starting at 2 weeks after surgery. Regular insulin (2.4 units/day) was administered as a continuous subcutaneous infusion through an osmotic pump. Plasma glucose levels were significantly higher in the Px rats than in the sham rats from 1 week after surgery. Vanadate lowered plasma glucose levels to near normal values in the Px rats as early as 2 days. The effect was sustained throughout the experiment. The hypoglycemic effect of insulin was less than that of vanadate. During an i.p. glucose tolerance test, plasma glucose levels were decreased in the Px rats treated with vanadate or insulin, while plasma insulin levels were not affected. The insulin content in the Px rats treated with vanadate was significantly (P < 0.01) greater than in the insulin-treated Px rats. Histological examination showed fibrotic degeneration in the enlarged islets of Px rats, whereas the normal structure was retained in most islets of the Px rats treated with vanadate and insulin. In addition, B-cell areas within the islet were restored to normal levels not only in the insulin-treated Px rats but in the vanadate-treated Px rats. However, both vanadate and insulin failed to stimulate proliferative activity of the B-cells. These data suggest that vanadate is a new therapeutic option to ameliorate the diabetic state after Px.

We assessed the effect of CS-045, a new hypoglycemic agent, on B-cell function in partially pancreatectomized rats. At the age of 4 weeks, male Wistar rats were subjected to 90% pancreatectomy (Px). For 2 weeks starting at 6 weeks after surgery the Px rats were treated with CS-045 (CS rats) mixed with chow pellets in a proportion of 0.2% (w/w). To compare the efficacy of CS-045 with that of insulin therapy, an osmotic pump was implanted to release insulin (1.2 units/day) into the intraperitoneal cavity of the Px rats (Is rats). Plasma glucose levels in the CS and Is rats were significantly lower than in the control Px rats; however, no marked improvement in plasma glucose or insulin levels was observed in glucose tolerance test (2 g/kg, i.p.) in the CS rats. Insulin secretion by the isolated perfused pancreas in response to 16.7 mM glucose showed a biphasic pattern, but was slightly reduced in the Px and CS rats compared with the Is rats. Insulin secretion induced by 19 mM arginine was unaffected by the treatment. The insulin content of the CS rats was significantly greater than in the Px and Is rats. Histological observations suggested regranulation of the pancreatic islets of the CS rats. B-cell areas within the islet were restored to normal levels in the Cs and Is rats. These findings indicate that the hypoglycemic effect of CS-045, which is not mediated by insulin secretion from the residual pancreas, prevents destruction of the islet.

Synergistic insulin release is observed in combination with muscarinic agonist and glucose. In order to define the role of changes in the cytoplasmic free Ca2+ level ([Ca2+]i), the effect of combination with carbachol and glucose on both phosphoinositide breakdown in rat pancreatic islets and [Ca2+]i dynamics in single fura 2-loaded B-cells was investigated. When the islets were prelabeled with myo-[2-3H]inositol, phosphoinositide breakdown was obtained and inositol trisphosphate levels were increased in the presence of carbachol (0.5 mM). Inositol trisphosphate levels induced by carbachol were further increased by co-stimulation with 16.7 mM glucose. [Ca2+]i in single-B-cells was increased by 16.7 mM glucose. A prompt, transient rise in [Ca2+]i was induced by carbachol. However, carbachol failed to augment the [Ca2+]i response in the presence of 16.7 mM glucose. These data suggest that a synergistic release of insulin occurs without an amplification of [Ca2+]i regardless of an increase in inositol trisphosphate.

The effect of maternal hypoglycemia on early organogenesis was studied in normal and diabetic rats. Female Wistar rats were made diabetic by an intravenous injection of streptozotocin (45 mg/kg) 2-3 weeks before conception. On day 9.5 or 10.5 of embryo development, both control and diabetic dams received saline or Actrapid human insulin (400 mU/rat) intraperitoneally after 19-h starvation. The fasting plasma glucose levels in diabetic dams decreased from approximately 23 to 8 mM. Hypoglycemia as low as 3.5 mM was maintained for 60 min in insulin-treated mother rats. Pregnancy was terminated on day 11.6 of embryo development. A significant growth retardation was found in diabetic embryos as compared with normal embryos. Maternal hypoglycemia lowered the DNA content in normal but not diabetic embryos, while the teratogenic effect of maternal hypoglycemia was not pronounced in either normal or diabetic embryos. These data may suggest that maternal hypoglycemia in vivo in early pregnancy influences the embryogenesis but not teratogenesis of rat embryos.

The association between phosphoinositide hydrolysis and insulin release in fetal islets was investigated. Islets from 21.5-day-old fetal rats were cultured for 7 days in inositol-free RPMI 1640 containing 11.1 mM glucose and labeled with 20 microCi/ml [3H]inositol for the final 2 days. The labeled islets were then perifused under various conditions for 60 min. Glucose (16.7 mM) caused a modest increase in [3H] efflux from labeled islets, but there was a subsequent fall-off in [3H] efflux. Fetal islets showed the first phase of insulin release without the continued rising second phase. When islets were perifused with 5 mM LiCl, the glucose-induced efflux of [3H] was greatly reduced, whereas glucose-induced insulin release was not affected. A pronounced effect of LiCl was an increase in inositol monophosphate, indicating increased phospholipase C activity. Although marked release of [3H] from labeled islets occurred in the presence of 5 mM inositol, the decreases in radioactive inositol lipid and insulin release induced by glucose were not changed. These data suggest that the fall-off in the second phase of insulin release from fetal islets may be partly due to blunted phosphoinositide hydrolysis.

We compared insulin secretory activity from rat pancreatic islets with that from a hamster beta-cell line (HIT-T). Insulin release from HIT cells was maximally stimulated with glucose at a concentration of 11.1 mM, and rather inhibited by higher concentrations of glucose. Blunted insulin release induced by glucose was shown in the isolated rat islets exposed to either streptozotocin (2.2 mM) for 30 min or alloxan (2 mM) for 5 min, whereas glucose-induced insulin release from HIT cells was not affected by pre-treatment with such beta-cell toxin. When pancreatic islets and HIT cells were cultured for 7 days with 10 mM nicotinamide, an inhibitor of poly (ADP-ribose) synthetase, glucose-induced insulin release from the islets was inhibited, whereas insulin release from HIT cells was rather enhanced. Insulin release induced by either arginine or leucine was also enhanced in HIT cells cultured with nicotinamide. These findings indicate that the multiple components of insulin stimulus-secretion coupling in HIT cells are considerably different from those in rat pancreatic islets.

We studied the effect of nicotinamide, a poly (ADP-ribose) synthetase inhibitor, on B-cell functions in 7-week-old male Wistar rats after 90% pancreatectomy (Px). Nicotinamide (0.5 g/kg body wt.) was injected once a day i.p. from 1 week before the surgery, and the injection was continued for 12 weeks after Px. Plasma glucose levels were significantly lower in Px rats treated with nicotinamide (NA rats) than in Px rats without treatment. In in vitro experiments with perfusion of the isolated pancreas, insulin secretion induced by 16.7 mM glucose was blunted in Px rats whereas insulin secretion induced by 19 mM arginine was increased. Nicotinamide treatment restored glucose-induced insulin secretion to normal and partly suppressed the hyperresponse of insulin to arginine. In addition, nicotinamide partially restored the priming effect induced by 28 mM glucose. The insulin content of the perfused pancreas was slightly but significantly greater in NA rats than in Px rats. Histological examination revealed fibrotic degeneration and degranulation in the islets of Px rats, whereas the normal structure was retained in most islets of NA rats. These results suggest that nicotinamide ameliorates islet function in partially depancreatized rats.

Effects of age and glucose levels on insulin secretion and synthesis were studied in the perfused pancreas of young (2-month-old) and older (10-month-old) female Wistar rats. Insulin secretion induced by 16.7 mM glucose showed a triphasic pattern: an early spike and fall (first phase, 0-6 min), followed by a sustained gradual increase (second phase, 7-120 min) and a gradual decreased release thereafter (third phase, 121-360 min) during the perfusion period of 360 min. First and second phase insulin secretion, but not third phase, were lower in older rats than in young rats. Insulin synthesis in old rat pancreas perfused with 16.7 nM glucose for 360 min was much greater than that of young rats. Second phase insulin secretion was restored to comparable levels by 28 mM glucose in older rats. Repeated pulses of 28 mM glucose potentiated subsequent insulin secretion in young rats, but not in older rats. These findings provide further evidence that sensitivity to glucose in pancreatic B cells is altered by aging.

Glucose-stimulated insulin release from rat pancreas is known to be blunted by aging. In the present study, we examined the effect of aging on insulin release induced by various secretagogues using the isolated perfused pancreas of female rats. Insulin release from the perfused pancreas in response to 16.7 mM glucose in 8-month-old rats (older rats) was much less than that in 2-month-old rats (young rats). The first phase of insulin release after glucose stimulation was attenuated in older rats. The addition of 0.1 mM 3-isobutyl-1-methylxanthine (IBMX) potentiated glucose-induced insulin secretion in both groups of rats. However, the second phase of insulin secretion in older rats was lower than that in younger rats. The phorbol ester 12-O-tetradecanoyl phorbol ester (TPA, 200 nM) enhanced both the first and the second phases of insulin release induced by glucose in both groups of rats. The amount of first phase insulin release induced by TPA with glucose in young rats was greater than that in older rats, whereas the second phase of insulin release was similar in both groups of rats. On the other hand, tolbutamide (200 uM) similarly stimulated the first phase of insulin release in both age groups of rat. In addition, the amount of cumulative insulin secretion induced by tolbutamide during the second phase was slightly but significantly greater in older rats than in young controls. Insulin content in the pancreas was significantly greater in older rats than in young rats and increased after the stimulation with TPA and tolbutamide.(ABSTRACT TRUNCATED AT 250 WORDS)

We studied the effect of maternal hypoglycemia on skeletal growth in the offspring of nondiabetic and diabetic rats. Female Wistar rats were injected with streptozocin (30 mg/kg i.v.) 2-3 wk before mating, and diabetes was confirmed by an intraperitoneal glucose tolerance test. On postconception day 9.5 or 10.5, both control and diabetic dams received saline or Actrapid human insulin (400 mU/rat i.p.). Hypoglycemia (approximately 2.8 mM) was induced for 120 min in the insulin-treated mothers. Pregnancy was terminated on gestational day 20. Fetal bones and cartilage were double-stained with alizarin red S and alcian blue 8GS. Insulin-induced hypoglycemia caused delayed ossification in the fetuses of the control dams. The number of malformations, e.g., costal fusion waves, increased greatly. These effects were more striking in the fetuses of dams that had received insulin on day 10.5 rather than on day 9.5 of embryo development. This type of insulin-induced hypoglycemia further delayed ossification of the fetal bones in diabetic dams. The influence of maternal hypoglycemia on skeletal malformations and/or variations was greater in the fetuses of diabetic dams than in the fetuses of control dams. These data suggest that maternal hypoglycemia in early pregnancy has a striking effect on skeletal growth and malformations in fetuses. In addition, mild glucose intolerance in dams may amplify these hypoglycemic effects.

In order to elucidate the possible role of C-kinase in exaggerated insulin release in pregnancy, the effects of phorbol ester TPA and a C-kinase inhibitor H-7 were investigated using the isolated perfused pancreas from nonpregnant and pregnant rats. At the termination of perfusion, the insulin content of the perfused pancreas was determined to estimate insulin biosynthesis. Insulin release from the perfused pancreas was markedly augmented by 20 nM TPA in the presence of 4.4 mM glucose in pregnant rats, but not in nonpregnant rats. When glucose concentrations in the perfusate were raised to 16.7 mM, insulin release from the perfused pancreas was profoundly enhanced in pregnant rats. TPA further augmented insulin release, but the insulin content was not affected by TPA. In contrast to the considerable effect of TPA in the presence of 4.4 mM glucose, the potentiating effect of TPA on insulin release was rather weaker in pregnant than in non-pregnant rats in the presence of 16.7 mM glucose. The release of insulin induced by 16.7 mM glucose was inhibited by the addition of 100 microM H-7 in nonpregnant rats, whereas insulin release from pregnant rat pancreases was not altered. Thus, the effect of TPA and H-7 on insulin release can be more clearly observed in the beta-cells of nonpregnant rats than those of pregnant ones when maximal concentrations of glucose are used as a stimulant. Exaggerated insulin release caused by glucose in pregnancy may be due to already fully activated C-kinase in the beta-cells.

The effects of chemical diabetes and fasting on fuel metabolism and insulin secretory activity in late pregnancy were investigated. Female Wistar rats were made chemically diabetic (CD) by intravenous injection of streptozotocine (30 mg/kg) 2 weeks before conception. When CD pregnant rats were fed, plasma glucose and insulin levels were not significantly different from those of normal pregnant rats. Ketone body levels, however, were higher in CD pregnant rats than in normal pregnant rats, indicating insulin resistance in CD rats. Insulin secretion from the perfused pancreas caused by arginine or glucose was markedly decreased in CD pregnant rats. The pregnant rats were fasted for 2 days, from day 19 to 21 of gestation. Plasma glucose and insulin concentrations decreased similarly in the two groups, whereas ketone body concentrations in CD pregnant rats were significantly higher than those in normal pregnant rats. Glucose-induced insulin secretion by the perfused pancreas was markedly attenuated by fasting and was not significantly different in normal and CD pregnant rats. These observations suggest that diabetes mellitus accelerates starvation in late gestation, due to increased insulin resistance and poor insulin secretion, and that fasting in diabetic pregnancy amplifies ketogenesis.

The effect of 16- and 48-h fasting on pancreatic somatostatin, insulin, and glucagon secretion was studied, using the isolated perfused rat pancreas. In the presence of 4.4 mM glucose, basal somatostatin and insulin concentrations in the perfusate were significantly lower in 48-h fasted rats than in fed animals, whereas basal glucagon secretion was significantly elevated in fasted rats. The infusion of 19 mM arginine significantly augmented secretion of somatostatin and glucagon and attenuated insulin secretion in 48-h fasted rats. It is concluded that fasting causes a decrease in basal pancreatic somatostatin secretion in vitro, although the response to arginine is rather exaggerated. Insulin and glucagon secretion also changed during the fasting. These results suggest that not only insulin and glucagon, but also somatostatin contribute to nutrient homeostasis.

To study the mechanism of imparied insulin secretion from rat fetal islets, the insulin responsiveness of islets from fetuses (day 21.5 of gestation) to a variety of secretagogues was compared with that of adult rat islets. Forskolin (30 microM)-induced insulin release from fetal and adult islets was 2.7-and 2.5-fold higher, respectively, than that from islets treated with 5.6 mM glucose alone. The effects of 12-O-tetradecanoyl phorbol-13-acetate (TPA) (200 nM) were also similar in fetal and adult islets. Thus, the responsiveness to forskolin and TPA showed no significant difference in adult and fetal islets. A synergistic effect of combinations of various insulin secretagogues was observed in adult islets; however, a weak synergistic effect was present with gliclazide plus TPA only in fetal islets. After islets were cultured in RPMI 1640 (containing 11.1 mM glucose), gliclazide-, forskolin-, and TPA-induced insulin release reached the levels obtained in adult islets. However, the synergistic effect of gliclazide and TPA disappeared after culture of the islets. These results suggest that the poor insulin secretion from fetal islets is not due to a defect in the activating system of either cAMP or C-kinase, but to the immaturity of the interaction of those messenger systems.

Effects of antisomatostatin and antiglucagon sera on insulin secretion were studied in isolated pancreatic islets of rats. Addition of antisomatostatin or antiglucagon serum to the incubation medium containing 5.5 mM glucose significantly increased insulin secretion from the islets. In contrast, these effect were not apparent when the glucose concentration was elevated to 20 mM. In the presence of antiglucagon serum the free glucagon concentration in the medium was decreased, while the total glucagon was markedly increased, suggesting that the antiglucagon serum caused an increase in glucagon secretion. These results suggest that A- and D-cells may play an important role in regulating insulin release at physiologic glucose concentration.

Glucose stimulation of [32P]-prelabelled pancreatic islets induced an increased incorporation of radioactivity into phosphatidylinositol. Glucagon and agents that increases cAMP in the islet did not affect phosphatidylinositol turnover, in spite of increased release of insulin. Furthermore, the potent inhibitor of insulin release, somatostatin did not alter phospholipid metabolism. Colchicine inhibited glucose-stimulated turnover of phosphatidylinositol as well as insulin release. These results may suggest that: (1) cAMP and phosphatidylinositol turnover are involved in different transmembrane control system for regulating insulin release; and (2) the function of microtubules modulate phospholipid metabolism.

Pregnancy is thought to create a metabolic condition of accelerated starvation. To clarify this idea, the effect of fasting on pregnant rats (day 21 of gestation) and their fetuses was examined. Although pregnancy significantly increased plasma insulin, plasma ketone body concentrations in fed pregnant rats were higher than those of age-matched fed virgin rats. After 48 hr fasting (i.e., fasting during days 19-21 of gestation), plasma insulin was markedly decreased in virgin rats compared with term pregnant rats, while ketone bodies were significantly higher in pregnant rats than in virgin rats. Body weight was lower in fetuses from fasted mothers than those from fed mothers. Starvation also markedly diminished the insulin response to glucose in isolated, perfused pancreases in both virgin and pregnant rats. The amount of insulin released during glucose stimulation was greater in pregnancy, and the inhibitory effect of 48 hr fasting on insulin release was greater in virgin rats than in pregnant rats. It is possible, therefore, that in term pregnant rats a decrease in insulin release caused by fasting may cause more profound catabolism than in nongravid rats.

In order to elucidate the effect of glucagon antiserum on the endocrine pancreas, the release of somatostatin, glucagon, and insulin from the isolated perfused rat pancreas was studied following the infusion of arginine both with and without pretreatment by glucagon antiserum. Various concentrations of arginine in the presence of 5.5 mM glucose stimulated both somatostatin and glucagon secretion. However, the responses of somatostatin and glucagon were different at different doses of arginine. The infusion of glucagon antiserum strongly stimulated basal secretion in the perfusate total glucagon (free + antibody bound glucagon) and also enhanced its response to arginine, but free glucagon was undetectable in the perfusate during the infusion. On the other hand, the glucagon antiserum had no significant effect on either insulin or somatostatin secretion. Moreover, electron microscopic study revealed degrannulation and vacuolization in the cytoplasm of the A cells after exposure to glucagon antiserum, suggesting a hypersecretion of glucagon, but no significant change was found in the B cells or the D cells. We conclude that in a single pass perfusion system glucagon antiserum does not affect somatostatin or insulin secretion, although it enhances glucagon secretion.

We have previously reported occurrence of Ca2+-activated, phospholipid-dependent protein kinase (referred as protein kinase C) in the rat pancreatic islets. It has been suggested that unsaturated diacylglycerol which results from hydrolysis of phosphatidylinositol by phospholipase C-like enzyme activates protein kinase C. Therefore, we studied the effect of exogenous phospholipase C on insulin release from isolated islets of rat pancreas. Bacterial phospholipase C enhanced insulin release induced by glucose in a dose dependent manner. The effect, however, was decreased in the islets pretreated with colchicine. Both phospholipase C and glucose caused an increase in 32p incorporation into phosphatidylinositol. These results indicate that phospholipid metabolism is linked to the insulin release mechanism.