Insulin secretion in the conscious mouse is biphasic and pulsatile

Islets in most species respond to increased glucose with biphasic insulin secretion, marked by a sharp first-phase peak and a slowly rising second phase. Mouse islets in vitro, however, lack a robust second phase. To date, this observation has not been extended in vivo. We thus compared insulin secretion from conscious mice with isolated mouse islets in vitro. The arterial plasma insulin response to a hyperglycemic clamp was measured in conscious mice 1 wk after surgical implantation of carotid artery and jugular vein catheters. Mice were transfused using clamps with blood from a donor mouse to maintain blood volume, allowing frequent arterial sampling. When plasma glucose in vivo was raised from approximately 5 to approximately 13 mM, insulin rose to a first-phase peak of 403+/-73% above basal secretion (n=5), followed by a rising second phase of mean 289+/- 41%. In contrast, perifused mouse islets ( approximately 75 islets/trial) responded with a similar first phase of 508+/- 94% (n=4) but a smaller and virtually flat second phase of 169+/- 9% (n=4, P<0.05). Furthermore, the slope of the second-phase response differed significantly from zero in mice (2.63+/-0.39%/min, P<0.01), in contrast to perifused islets (0.18+/- 0.14%/min, P>0.30). Mice also displayed pulsatile patterns in insulin concentration (period: 4.2+/- 0.4 min, n=8). Conscious mice thus responded to increased glucose with biphasic and pulsatile insulin secretion, as in other species. The robust second phase observed in vivo suggests that the processes needed to generate second-phase insulin secretion may be abrogated by islet isolation.     

Retardant effect of hyperglycemia on the rise in plasma fatty acids following insulin withdrawal in man

The present study was undertaken to examine the influence of hyperglycemia in retarding the rise in circulating FFA noted after acute insulin withdrawal in man. The arterial FFA response to somatostatin administration was measured in the presence of (a) euglycemia and (b) hyperglycemia. In seven normal men who received somatostatin (0.9 mg/h) with euglycemia maintained by exogenous glucose infusion plasma insulin levels fell to levels 4 uU/ml and plasma FFA concentrations rose from 659 +/- 123 to 2057 +/- 268 uEq/l. When somatostatin was infused with hyperglycemia maintained at approximately 230 mg/dl, plasma insulin levels were again maintained at levels 4 uU/ml. Despite similar insulinopenia plasma FFA concentrations rose from 510 +/- 56 to only 1125 +/- 180 uEq/l, significantly less than in the previous protocol (p less than 0.01). These data indicate that hyperglycemia per se significantly attenuates the rise in circulating FFA caused by acute insulin withdrawal in man.     

Somatostatin-induced inhibition of insulin secretion from isolated islets of rat pancreas in presence of glucagon.

In order to study the oeffect of somatostatin on the endocrine pancreas directly, islets isolated from rat pancreas by collagenase were incubated for 2 hrs 1) at 50 and 200 mg/100 ml glucose in the absence and presence of somatostatin (1, 10 and 100 mg/ml) and2) at 200 mg/100 ml glucose together with glucagon (5 mug/ml), with or without somatostatin (100 ng/ml). Immunologically measurable insulin was determined in the incubation media at 0, 1 and 2 hrs. Insulin release was not statistically affected by any concentration stomatostatin. On the other hand, somatostatin exerted a significant inhibitory action on glucagon-potentiated insulin secretion (mean +/- SEM, mu1/2 hrs/10 islets: glucose and glucagon: 1253 +/- 92; glucose, glucagon and somatostatin: 786 +/- 76). The insulin output in th epresence of glucose, glucagon and somatostatin was also significantly smaller than in thepresence of glucose alone (1104 +/- 126) or of glucose and somatostatin (1061 +/- 122). The failure of somatostatin to affect glucose-stimulated release of insulin from isolated islets contrasts its inhibitory action on insulin secretion as observed in the isolated perfused pancreas and in vivo. This discrepancy might be ascribed to the isolation procedure using collagenase. However, somatostatin inhibited glucagon-potentiated insulin secretion in isolated islets which resulted in even lower insulin levels than obtained in the parallel experiments without glucagon. It is concluded that the hormone of the alpha cells, or the cyclic AMP system, might play a part in the machanism of somatostatin-induced inhibition of insulin release from the beta-cell.     

Decreased nutrient-stimulated insulin secretion in chronically hypoglycemic late-gestation fetal sheep is due to an intrinsic islet defect

We measured in vivo and in vitro nutrient-stimulated insulin secretion in late gestation fetal sheep to determine whether an intrinsic islet defect is responsible for decreased glucose-stimulated insulin secretion (GSIS) in response to chronic hypoglycemia. Control fetuses responded to both leucine and lysine infusions with increased arterial plasma insulin concentrations (average increase: 0.13 +/- 0.05 ng/ml leucine; 0.99 +/- 0.26 ng/ml lysine). In vivo lysine-stimulated insulin secretion was decreased by chronic (0.37 +/- 0.18 ng/ml) and acute (0.27 +/- 0.19 ng/ml) hypoglycemia. Leucine did not stimulate insulin secretion following acute hypoglycemia but was preserved with chronic hypoglycemia (0.12 +/- 0.09 ng/ml). Isolated pancreatic islets from chronically hypoglycemic fetuses had normal insulin and DNA content but decreased fractional insulin release when stimulated with glucose, leucine, arginine, or lysine. Isolated islets from control fetuses responded to all nutrients. Therefore, chronic late gestation hypoglycemia causes defective in vitro nutrient-regulated insulin secretion that is at least partly responsible for diminished in vivo GSIS. Chronic hypoglycemia is a feature of human intrauterine growth restriction (IUGR) and might lead to an islet defect that is responsible for the decreased insulin secretion patterns seen in human IUGR fetuses and low-birth-weight human infants.     

Sulfatide is associated with insulin granules and located to microdomains of a cultured beta cell line

Previous studies using pancreas from various mammals and freshly isolated islets from rat pancreas have provided evidence supporting possible involvement of the glycosphingolipid sulfatide in insulin processing and secretion. In this study, sulfatide expression and metabolism in the beta cell line RINr1046-38 (RIN-38), commonly used as a model for beta cell functional studies, were investigated and compared with previous findings from freshly isolated islets. RIN-38 cells expressed similar amounts (2.7 +/- 1.1 nmol/mg protein, n = 19) of sulfatide as isolated rat islets and also followed the same metabolic pathway, mainly through recycling. Moreover, in agreement with findings in isolated islets, the major species of sulfatide isolated from RIN-38 cells contained C16:0 and C24:0 fatty acids. By applying subcellular isolations and electron microscopy and immunocytochemistry techniques, sulfatide was shown to be located to the secretory granules, the plasma membrane and enriched in detergent insoluble microdomains. In the electron microscopy studies, Sulph I staining was also associated with mitochondria and villi structures. In conclusion, RIN-38 cells might be an appropriate model, as a complement to isolated islets where the amount of material often limits the experiments, to further explore the role of sulfatide in insulin secretion and signal transduction of beta cells.     

Plasma insulin and glucagon and their release from pancreatic islet in hyperosmolar diabetic rat.

In order to investigate the metabolic abnormalities in hyperosmolar diabetes from the viewpoint of insulin or glucagon, experimental hyperosmolar diabetes was produced by a combination of cortisol injection and water deprivation or only by the latter in streptozotocin-induced moderately hyperglycemic rat. They had a high blood glucose level and high plasma osmotic pressure. Fasting plasma insulin tended to decrease in the dehydrated state whether diabetic or not. Fasting plasma glucagon was increased to 0.047 +/- 0.009 nmol/l (P less than 0.05) in the non-diabetic dehydrated state (normal 0.026 +/- 0.004 nmol/l), and a similar high level of plasma glucagon was observed in the dehydrated diabetic rat (0.052 +/- 0.020 nmol/l), especially after cortisol treatment. In isolated rat islet, insulin released from the dehydrated diabetic rat at a high concentration of glucose was to some extent lower than that of diabetic rat, and released IRG vice versa. The insulin:glucagon ratio in the presence of high glucose was significantly lower in the dehydrated diabetic rat than in the normal rat (P less than 0.01). In the diabetic rat this ratio was not significantly different. This finding was also consistent with the results of in vivo experiments. Thus more catabolic hormonal changes were found in in vivo and in vitro studies in the hyperosmolar diabetic rat.     

Fetal rat islet insulin deficiency following maternal administration of streptozotocin

Pancreatic islets were isolated from the fetuses of normal rats and rats made diabetic by the iv administration of streptozotocin (STZ) on either Day 3 or 5 of pregnancy. Of the rats made diabetic on Day 3, one group also received insulin injections at the appearance of glucosuria. Maternal blood glucose on Day 20 of gestation was significantly different in the diabetic rats (405 +/- 27 mg/dl) from the normal (97 +/- 1 mg/dl) and insulin-treated diabetic rats (69 +/- 9 mg/dl). While fetal weight was significantly decreased in the STZ-treated rats (2.64 +/- 0.13 g vs 3.52 +/- 0.05 g for the control group, P less than 0.005), fetal glucose was significantly higher in the STZ-treated than in normal pups (342 +/- 11 vs 35 +/- 1 mg/dl, P less than 0.005). Both fetal weight and glucose were normalized by insulin treatment: 3.16 +/- 0.18 g and 31 +/- 7 mg/dl, respectively. Insulin release from fetal islets of diabetic dams was blunted after a week in culture both in basal and stimulated conditions. After 2 weeks in culture, there was partial recovery in the insulin response to glucose but it did not equal to that measured in fetal islets from the normal and insulin-treated diabetic rats. These data suggest maternal hyperglycemia severely impairs fetal weight and insulin release from fetal rat islets in vitro, and correction of the hyperglycemia by insulin treatment not only improves fetal weight and glucose concentrations, but it also normalizes insulin release from fetal rat islets in vitro.     

The pancreatic glucagon and C-peptide secretion during hyperinsulinemia in euglycemic glucose clamp with or without somatostatin infusion in normal man

6 normal subjects received two times of 2 hr euglycemic glucose clamp studies (insulin infusion rate 40 mU/M2/min) one with and the other without somatostatin (SRIF) infusion (500 microgram/hr). Serum C-peptide and glucagon levels were measured during clamp to study the sensitivity of pancreatic alpha and beta cells to the suppressive effects of exogenous hyperinsulinemia during normoglycemia in normal subjects and to find whether SRIF had any modulative effects on endocrine pancreas secretion at the status of hyperinsulinemia. The results showed that in normal man the degree of suppression of pancreatic glucagon secretion by hyperinsulinemia (approximately 100 uU/ml) during euglycemic glucose clamp without SRIF infusion was less than that of C-peptide with mean value of 62 +/- 4% of basal glucagon remained at the end of clamp study; while only about 30 +/- 2% of basal C-peptide concentrations remained. But during SRIF infused glucose clamp studies (SRIF was infused from 60 to 120 min), 32 +/- 2% of mean basal C-peptide concentrations and 38 +/- 6% of mean basal glucagon concentrations left at the end of 2 hr clamp studies when serum insulin level was about 100 uU/ml. For the glucose infusion rate (M value), it was significantly greater in our normal subjects in response to insulin + SRIF as compared to insulin alone (12.0 + 0.9 vs 8.8 +/- 1.4; P less than 0.01). We concluded: during hyperinsulinemia (100 uU/ml), the sensitivity of pancreatic alpha cells to insulin seems less than that of beta cells in normal man at normoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of L-asparaginase on insulin secretion from isolated rat islets of Langerhans.

The effects of L-asparaginase were evaluated on glucose-induced insulin release from isolated rat islets of Langerhans. Islets were obtained by enzymatic digestion of pancreas from Sprague-Dawley rats. The study of L-asparaginase effects on insulin secretion was performed in a static incubation of islets. Insulin secretion was measured at 60 min of incubation with different secretagogues with and without L-asparaginase. L-Asparaginase at concentrations from 310 to 5,000 U/ml could inhibit the glucose-induced insulin secretion in a dose-dependent manner. This effect was not recovered after incubation in the absence of the drug for another 2 h. The half-maximal inhibitory effect of the enzyme on insulin secretion was observed at L-asparaginase concentrations of 1,000 U/ml. Tolbutamide (200 microM) and ketoisocaproic acid (20 mM) did not induce insulin secretion in the presence of moderately high L-asparaginase concentrations. L-Asparaginase did not inhibit glucose-induced insulin secretion in the presence of isobutyl-methyl-xanthine (IBMX) (20 microM) or forskolin (20 microM). L-Asparaginase promoted a decrease in total c-AMP in isolated rat islets at concentrations from 500 to 1,500 U/ml when they were stimulated by glucose. If islets were treated with IBMX or forskolin, L-asparaginase did not inhibit the glucose-induced total c-AMP levels in islets.     

Changes in glucose-stimulated insulin secretion after long-term treatment of C57BL mice with glibenclamide

An insulin response to increased glucose concentrations could not be found in vivo and in vitro after long-term treatment of C57BL/KsJ and C57BL/6J mice with Glibenclamide. This missing stimulation of insulin secretion was not the result of an exhaustion of the islets or a disturbed (pro)insulin biosynthesis as demonstrated by measurements of insulin content of the islets and by in vitro (pro)insulin biosynthesis experiments. In the presence of glucose (15 mmol/l) theophylline increased the insulin secretion of isolated islets of Glibenclamide-treated mice to values similar to control islets. The insulin response to an i.p. glucose loading was found to be normal in comparison with control mice 1-2 weeks after the Glibenclamide treatment had been finished.     

Heat shock restores insulin secretion after injury by nitric oxide by maintaining glucokinase activity in rat islets

Heat shock protein (hsp), including hsp70, has been reported to restore the glucose-induced insulin release suppressed by nitric oxide (NO). However, the mechanism underlying this recovery remains unclear. In the present study, we examine the effects, in rat islets, of heat shock on insulin secretion inhibited by a small amount of NO and also on glucose metabolism, the crucial factor in insulin release. Exposure to a higher dose (15 U/ml) of interleukin-1beta (IL-1beta) abolished the insulin release by stimulation of glucose or KCl in both control and heat shocked islets. In rat islets exposed to a lower dose (1.5 U/ml) of IL-1beta, insulin secretion in response to glucose, but not to glyceraldehydes (GA), ketoisocaproate (KIC), or KCl, was selectively impaired, concomitantly with lower ATP concentrations in the presence of 16.7 mM glucose, while such suppression of insulin secretion and ATP content was not observed in heat shock-treated islets. NO production in islets exposed to 1.5 U/ml IL-1beta was significantly, but only partly, decreased by heat shock treatment. The glucose utilization rate measurement using [5-3H]-glucose and [2-3H]-glucose and the glucokinase activity in vitro were reduced in islets treated with 1.5 U/ml IL-1beta. In heat shock-treated islets, glucose utilization and glucokinase activity were not affected by 1.5 U/ml IL-1beta. These data suggest that heat shock restores glucose-induced insulin release inhibited by NO by maintaining glucokinase activity and the glucose utilization rate in islets in addition to reducing endogenous NO production.     

Insulin sensitivity of rates of glycolysis and glycogen synthesis in soleus,stripped soleus,epitrochlearis, and hemi-diaphragm muscles isolated from sedentary rats

The effect of insulin concentrations on the rates of glycolysis and glycogen synthesis in four different in vitro rat muscle preparations (intact soleus, stripped soleus, epitrochlearis, and hemi-diaphragm) were investigated: the concentrations of insulin that produced half-maximal stimulation of the rates of these two processes in the four muscle preparations were similar - about 100 muunits/ml. This is at least 10-fold greater than the concentration that produced half-maximal inhibition of lipolysis in isolated adipocytes. Since 100 muunits/ml insulin is outside the normal physiological range in the rat, it is suggested that, in vivo, insulin influences glucose utilization in muscle mainly indirectly, via changes in the plasma fatty acid levels and the 'glucose/fatty acid cycle'. Consequently the view that insulin stimulates glucose utilization in muscle mainly by a direct effect on membrane transport must be treated with caution.     

Plasma levels of growth hormone, corticosterone and insulin, during induced hepatocyte synchronization in young rats

A wave of synchronous hepatocytes entering the cell cycle can be obtained in vivo after a subcutaneous injection (e.g. of casein) in rats at around Post-natal Day 10, when plasma growth hormone (GH) levels reach a low plateau (40 +/- 2 ng/ml) and liver cell proliferation rate is high. The present work reports the following changes in plasma hormone concentrations after synchronization of 20% of the hepatocyte population: (1) during the G1 phase (i.e. 6-12 hr after the mitogenic trigger), plasma GH concentration has dropped further (25 +/- 1.5 ng/ml). It was back to 90% of control levels during the S phase, mitosis and the following response including a transitory decrease in labelling index below control values. Injected together with the irritating mitotic trigger, a single dose of rat GH reduced the cell synchronization and post-synchronization effects by 50%. (2) Plasma corticosterone levels varied inversely to those of GH, increasing to twice the control values during G1 and were back to physiological levels when synchronized hepatocytes entered the S phase. (3) Variations in insulin levels were similar to that of corticosterone, with narrower ranges and reduced amplitudes. Our data suggest a possible correlation between the observed variations in plasma hormone levels and the induced synchronous hepatocyte response.     

Strain-dependent difference in inducible nitric oxide synthesis (iNOS) expression in rat pancreatic islets correlates with interferon regulating factor 1 (IRF-1) and heat shock protein 70 (HSP70) expression

Nitric oxide (NO) may be a necessary but not sufficient mediator of cytokine-mediated, selective beta-cell destruction. Previously, we have described a difference in NO-dependent IL-1beta sensitivity in vivo and in vitro of pancreatic islets from two rat strains, Brown Norway (BN) and Wistar Kyoto (WK), the latter being the more sensitive strain. Here we investigated whether strain-dependent, differential islet iNOS expression was associated with differences in islet expression of the IL-1 receptor type 1(IL-1RI) or interferon regulating factor 1 (IRF-1), and/or caused differences in HSP70 expression, a marker of cell defence against oxidative stress. Methods: isolated islets from both rat strains were exposed to increasing concentrations of IL-1beta (0-150 pg/ml) for 24 hours or for varying culture periods (4-48 hours) to 15 pg/ml of IL-1beta. Measurements: accumulated insulin and nitrite release into incubation medium, and islet mRNA and protein expression of iNOS, IL-1RI, IRF-1 and HSP70 by semi-quantitative RT-PCR and Western blotting. Results: Higher insulin and lower nitrite release into the incubation medium were seen for BN compared to WK rats islets in both dose- and time-response experiments. IRF-1 expression preceded iNOS expression and was more pronounced in WK than in BN islets. No strain differences were observed for islet expression of IL-1RI. A strain-dependent HSP70 expression in response to IL-1beta with the highest levels in WK rat islets following iNOS expression was seen. Conclusion: There was a strain-dependent difference in iNOS expression which was associated with IRF-1 and HSP70 expression.     

D-Glyceraldehyde causes production of intracellular peroxide in pancreatic islets, oxidative stress, and defective beta cell function via non-mitochondrial pathways

D-Glyceraldehyde (D-GLYC) is usually considered to be a stimulator of insulin secretion but theoretically can also form reactive oxygen species (ROS), which can inhibit beta cell function. We examined the time- and concentration-dependent effects of D-GLYC on insulin secretion, insulin content, and formation of ROS. We observed that a 2-h exposure to 0.05-2 mM D-GLYC potentiated glucose-stimulated insulin secretion (GSIS) in isolated Wistar rat islets but that higher concentrations inhibited GSIS. A 24-h exposure to 2 mm D-GLYC inhibited GSIS, decreased insulin content, and increased intracellular peroxide levels (2.14 +/- 0.31-fold increase, n = 4, p < 0.05). N-Acetylcysteine (10 mM) prevented the increase in intracellular peroxides and the adverse effects of d-GLYC on GSIS. In the presence of 11.1 but not 3.0 mm glucose, koningic acid (10 microM), a specific glyceraldehyde-3-phosphate dehydrogenase inhibitor, increased intracellular peroxide levels (1.88 +/- 0.30-fold increase, n = 9, p < 0.01) and inhibited GSIS (control GSIS = p < 0.001; koningic acid GSIS, not significant). To determine whether oxidative phosphorylation was the source of ROS formation, we cultured rat islets with mitochondrial inhibitors. Neither rotenone or myxothiazol prevented D-GLYC-induced increases in islet ROS. Adenoviral overexpression of manganese superoxide dismutase also failed to prevent the effect of D-GLYC to increase ROS levels. These observations indicate that exposure to excess D-GLYC increases reactive oxygen species in the islet via non-mitochondrial pathways and suggest the hypothesis that the oxidative stress associated with elevated D-GLYC levels could be a mechanism for glucose toxicity in beta cells exposed chronically to high glucose concentrations.     

铃蟾肽对四氧嘧啶引起的大鼠糖尿病的预防作用

本工作从三个不同的层次对铃蟾肽防止胰岛 B 细胞损伤的作用进行了研究:(1)在整体水平,预先注射铃蟾肽(50μg/kg,iv)可明显抑制单独给予四氧嘧啶(200mg/kg,s.c.)引起的大鼠血糖升高和血浆胰岛素水平下降的趋势。(2)在离体胰腺灌流实验发现,在四氧嘧啶之前预灌流铃蟾肽(10~(-2)mmol/L)可使胰腺对高糖刺激产生反应性分泌;而仅以四氧嘧啶灌流时,胰腺对高糖刺激无反应。(3)在离体胰岛水平,初步研究了在四氧嘧啶引起胰岛 B 细胞功能改变时,铃蟾肽对胰岛内胰岛素、胰高血糖素和生长抑素分泌的影响。结果表明,铃蟾肽可防止四氧嘧啶引起的胰岛素和生长抑素分泌的抑制及胰高血糖素分泌的增加趋势。     

Effect of chronic psychological stress on insulin release from rat isolated pancreatic islets

Despite documented studies, the exact role of stress on diabetes is still unclear. The present study investigates the effect of chronic psychological stress on insulin release from isolated rat pancreatic islets. Male Wistar rats were divided into two groups of control and stressed (n=8/group). The animals of the stressed group were exposed to restraint stressors (1 h twice daily) for 15 or 30 consecutive days. At the beginning and end of the experimental periods, the animals were weighed and blood samples taken to determine the fasting plasma levels of glucose, insulin and corticosterone. On the following day the pancreatic islets of 5/group of the above animals were isolated and the static release of insulin in the presence of different glucose concentrations (2.8, 5.6, 8.3, 16.7 mM) was assessed. The results showed that in the stressed group, fasting plasma glucose levels were increased significantly on the 15th day as compared to the control group. However there was no significant increase on the 30th day. Fasting plasma insulin was significantly decreased on the 15th and 30th days of the experiment in the stressed group. Stressed rats showed significantly higher fasting plasma corticosterone levels, only on the 15th day, as compared to the control rats. In response to increasing concentrations of glucose, insulin release from islets of the stressed group was increased significantly on the 30th day of the experiment as compared to the control group. We conclude that chronic psychological stress could increase responsiveness of pancreatic beta cells to glucose, in vitro, and thus, low insulin levels of the stressed animals, in vivo, may be due to reason(s) other than the reduction of insulin releasing capacity of pancreatic beta cells.     

Effect of interferon and double-stranded RNA on B-cell function in mouse islets of Langerhans.

The direct effects of alpha- and beta-interferons on isolated mouse pancreatic islets were investigated in vitro and found to be similar. After 7 h incubation with interferon concentrations above 350 units/ml, glucose-stimulated (pro)insulin biosynthesis was significantly inhibited, with only a slight inhibition of total protein biosynthesis. Inhibition could be abolished in the additional presence of an anti-interferon antibody. Interferon did not affect insulin release, total insulin content, or glucose oxidation of the islets. The stimulation of (pro)insulin biosynthesis by adenosine, D-glyceraldehyde, mannose, N-acetylglucosamine and leucine was also inhibited by interferon, with no effect on insulin release. At concentrations of dsRNA (double-stranded RNA) said to induce interferon (1-100 micrograms/ml), glucose-stimulated (pro)insulin biosynthesis was inhibited without significantly affecting insulin release. The dsRNA may itself inhibit stimulated (pro)insulin biosynthesis or may function indirectly by the induction of interferon.     

Plasma levels of total and active ghrelin in acromegaly and growth hormone deficiency

Ghrelin is an endogenous growth hormone (GH) secretagogue recently isolated from the stomach. Although it possesses a strong GH releasing activity in vitro and in vivo, its physiological significance in endogenous GH secretion remains unclear. The aim of this study was to characterize plasma ghrelin levels in acromegaly and growth hormone deficiency (GHD). We investigated plasma total and active ghrelin in 21 patients with acromegaly, 9 patients with GHD and 24 age-, sex- and BMI-matched controls. In all subjects, we further assessed the concentrations of leptin, soluble leptin receptor, insulin, IGF-I, free IGF-I and IGFBP-1, 2, 3 and 6. Patients with acromegaly and GHD as well as control subjects showed similar levels of total ghrelin (controls 2.004+/-0.18 ng/ml, acromegalics 1.755+/-0.16 ng/ml, p=0.31, GHD patients 1.704+/-0.17 ng/ml, p=0.35) and active ghrelin (controls 0.057+/-0.01 ng/ml, acromegalics 0.047+/-0.01 ng/ml, p=0.29, GHD patients 0.062+/-0.01 ng/ml, p=0.73). In acromegalic patients plasma total ghrelin values correlated negatively with IGF-I (p<0.05), in GHD patients active ghrelin correlated with IGF-I positively (p<0.05). In the control group, total ghrelin correlated positively with IGFBP-2 (p<0.05) and negatively with active ghrelin (p=0.05), BMI (p<0.05), WHR (p<0.05), insulin (p=0.01) and IGF-I (p=0.05). Plasma active ghrelin correlated positively with IGFBP-3 (p=0.005) but negatively with total ghrelin and free IGF-I (p=0.01). In conclusion, all groups of the tested subjects showed similar plasma levels of total and active ghrelin. In acromegaly and growth hormone deficiency plasma ghrelin does not seem to be significantly affected by changes in GH secretion.     

Relative hypoglycemia and hyperinsulinemia in mice with heterozygous lipoprotein lipase (LPL) deficiency. Islet LPL regulates insulin secretion.

Lipoprotein lipase (LPL) provides tissues with fatty acids, which have complex effects on glucose utilization and insulin secretion. To determine if LPL has direct effects on glucose metabolism, we studied mice with heterozygous LPL deficiency (LPL+/-). LPL+/- mice had mean fasting glucose values that were up to 39 mg/dl lower than LPL+/+ littermates. Despite having lower glucose levels, LPL+/- mice had fasting insulin levels that were twice those of +/+ mice. Hyperinsulinemic clamp experiments showed no effect of genotype on basal or insulin-stimulated glucose utilization. LPL message was detected in mouse islets, INS-1 cells (a rat insulinoma cell line), and human islets. LPL enzyme activity was detected in the media from both mouse and human islets incubated in vitro. In mice, +/- islets expressed half the enzyme activity of +/+ islets. Islets isolated from +/+ mice secreted less insulin in vitro than +/- and -/- islets, suggesting that LPL suppresses insulin secretion. To test this notion directly, LPL enzyme activity was manipulated in INS-1 cells. INS-1 cells treated with an adeno-associated virus expressing human LPL had more LPL enzyme activity and secreted less insulin than adeno-associated virus-beta-galactosidase-treated cells. INS-1 cells transfected with an antisense LPL oligonucleotide had less LPL enzyme activity and secreted more insulin than cells transfected with a control oligonucleotide. These data suggest that islet LPL is a novel regulator of insulin secretion. They further suggest that genetically determined levels of LPL play a role in establishing glucose levels in mice.