An in vivo functional assay to characterize human STAT5B genetic variants during zebrafish development

Growth hormone (GH) binding to GH receptor activates janus kinase 2 (JAK2)-signal transducer and activator of transcription 5b (STAT5b) pathway, which stimulates transcription of insulin-like growth factor-1 (IGF1), insulin-like growth factor binding protein 3 (IGFBP3) and insulin-like growth factor acid-labile subunit (IGFALS). Although STAT5B deficiency was established as an autosomal recessive disorder, heterozygous dominant-negative STAT5B variants have been reported in patients with less severe growth deficit and milder immune dysfunction. We developed an in vivo functional assay in zebrafish to characterize the pathogenicity of three human STAT5B variants (p.Ala630Pro, p.Gln474Arg and p.Lys632Asn). Overexpression of human wild-type (WT) STAT5B mRNA and its variants led to a significant reduction of body length together with developmental malformations in zebrafish embryos. Overexpression of p.Ala630Pro, p.Gln474Arg or p.Lys632Asn led to an increased number of embryos with pericardial edema, cyclopia and bent spine compared with WT STAT5B. Although co-injection of WT and p.Gln474Arg and WT and p.Lys632Asn STAT5B mRNA in zebrafish embryos partially or fully rescues the length and the developmental malformations in zebrafish embryos, co-injection of WT and p.Ala630Pro STAT5B mRNA leads to a greater number of embryos with developmental malformations and a reduction in body length of these embryos. These results suggest that these variants could interfere with endogenous stat5.1 signaling through different mechanisms. In situ hybridization of zebrafish embryos overexpressing p.Gln474Arg and p.Lys632Asn STAT5B mRNA shows a reduction in igf1 expression. In conclusion, our study reveals the pathogenicity of the STAT5B variants studied.

Expression of acid-labile subunit (ALS) in developing and adult zebrafish and its role in dorso-ventral patterning during development

Mammalian acid-labile subunit (ALS) is a serum protein that binds binary complexes between Insulin-like growth factors (IGFs) and Insulin-like growth factor-binding proteins (IGFBPs) extending their half-life and keeping them in the vasculature. Human ALS deficiency (ACLSD), due to homozygous or compound heterozygous mutations in IGFALS, leads to moderate short stature with reduced levels of IGF-I and IGFBP-3. There is only one corresponding zebrafish ortholog gene and it has not yet been studied. In this study we elucidate the role of igfals during zebrafish development. In zebrafish embryos igfals mRNA is expressed throughout development, mainly in the brain and subsequently also in the gut and swimbladder. To determine its role during development, we knocked down igfals gene product using morpholinos (MOs). Igfals morphant embryos displayed dorsalization in different degrees of severity, including a shortened trunk and loss of tail. Furthermore, co-injection of human IGFALS (hIGFALS) mRNA was able to rescue the MO-induced phenotype. Finally, overexpression of either hIGFALS or zebrafish igfals (zigfals) mRNA leads to ventralization of embryos including a reduced head and enlarged tail. These findings suggest that als plays an important role in dorso-ventral patterning during zebrafish development.

MON-716 A Novel Human Heterozygous STAT5B Variant Leads to Impaired Growth and Developmental Defects in Zebrafish Embryos

Signal transducer and activator of transcription 5b (STAT5b) has been identified as a key downstream mediator of GH signaling in somatic growth. Autosomic recessive human mutations in STAT5B lead to severe growth retardation associated to immune dysregulation. On the other hand, some heterozygous STAT5B mutations have been associated to a milder form of the disease. We have identified a heterozygous novel STAT5B mutation by Whole Exome Sequencing (WES) in a 2.2-year-old boy who presented proportionate short stature (height -2.77 SDS) with mild immune dysregulation. He also had normal GH response to provocative tests, low IGF-I levels, and a limited response to IGF generation test. This variant is located within the highly conserved SH2 domain responsible for recognizing and interacting with tyrosine-phosphorylated target peptides. The aim of our study was to evaluate the functional consequences of this novel heterozygous human STAT5B variant (K632N), using the zebrafish as a biosensor system, to determine its pathogenicity. To do this, we performed overexpression experiments microinjecting construct-derived mRNA for the wildtype (WT) and mutant variant into zebrafish embryos at the 1-cell stage and assessed the consequences at 72 hours post fertilization (hpf). The missense variant was introduced into the full length STAT5B cDNA clone (Origene) by site-directed mutagenesis. To generate mRNA, WT and mutant forms of STAT5B cDNAs were linearized by digestion with XhoI, purified and subsequently transcribed with Mmessage Mmachine T7 Transcription Kit. Zebrafish embryos microinjected with 100 and 200 pg of mutant mRNA show a dose dependent significant reduction of body length at 72 hpf compared to those microinjected with the same dose of WT mRNA (p<0.001). Body length reduction with 100 pg of mutant mRNA was 4%, while with 200 pg was 12.7% (p<0.001). In addition, a significant number of embryos injected with mutant mRNA show developmental defects including pericardial edema, bent spine, and cyclopia compared to those injected with WT mRNA (p<0.001). In the case of pericardial edema, the number of affected embryos increased significantly with the mutant mRNA dose (p<0.005). In conclusion, our study was able to evidence the pathogenic nature of the STAT5B K632N variant since it leads to growth and developmental defects in zebrafish embryos. The zebrafish, and its conserved GH-IGF-I axis, constitutes an ideal in vivo model for characterizing the functional effect of genetic variants in ortholog human genes.

A novel heterozygous STAT5B variant in a patient with short stature and partial growth hormone insensitivity (GHI)

BACKGROUND

The most frequent monogenic causes of growth hormone insensitivity (GHI) include defects in genes encoding the GH receptor itself (GHR), the signal transducer and activator of transcription (STAT5B), the insulin like-growth factor type I (IGF1) and the acid-labile subunit (IGFALS). GHI is characterized by a continuum of mild to severe post-natal growth failure.

OBJECTIVE

To characterize the molecular defect in a patient with short stature and partial GHI.

PATIENT AND METHODS

The boy was born at term adequate for gestational age from non-consanguineous normal-stature parents. At 2.2 years, he presented proportionate short stature (height -2.77 SDS), wide forehead and normal mental development. Whole-exome analysis and functional characterization (site-directed mutagenesis, dual luciferase reporter assay, immunofluorescence and western immunoblot) were performed.

RESULTS

Biochemical and endocrinological evaluation revealed partial GH insensitivity with normal stimulated GH peak (7.8 ng/mL), undetectable IGF1 and low IGFBP3 levels. Two heterozygous variants in the GH-signaling pathway were found: a novel heterozygous STAT5B variant (c.1896G>T, p.K632N) and a hypomorphic IGFALS variant (c.1642C>T, p.R548W). Functional in vitro characterization demonstrated that p.K632N-STAT5b is an inactivating variant that impairs STAT5b activity through abolished phosphorylation. Remarkably, the patient's immunological evaluation displayed only a mild hypogammaglobulinemia, while a major characteristic of STAT5b deficient patients is severe immunodeficiency.

CONCLUSIONS

We reported a novel pathogenic inactivating STAT5b variant, which may be associated with partial GH insensitivity and can present without severe immunological complications in heterozygous state. Our results contribute to expand the spectrum of phenotypes associated to GHI.

Partial growth hormone insensitivity and dysregulatory immune disease associated with de novo germline activating STAT3 mutations

Germinal heterozygous activating STAT3 mutations represent a novel monogenic defect associated with multi-organ autoimmune disease and, in some cases, severe growth retardation. By using whole-exome sequencing, we identified two novel STAT3 mutations, p.E616del and p.C426R, in two unrelated pediatric patients with IGF-I deficiency and immune dysregulation. The functional analyses showed that both variants were gain-of-function (GOF), although they were not constitutively phosphorylated. They presented differences in their dephosphorylation kinetics and transcriptional activities under interleukin-6 stimulation. Both variants increased their transcriptional activities in response to growth hormone (GH) treatment. Nonetheless, STAT5b transcriptional activity was diminished in the presence of STAT3 GOF variants, suggesting a disruptive role of STAT3 GOF variants in the GH signaling pathway. This study highlights the broad clinical spectrum of patients presenting activating STAT3 mutations and explores the underlying molecular pathway responsible for this condition, suggesting that different mutations may drive increased activity by slightly different mechanisms.

Serum IGF-1 is insufficient to restore skeletal size in the total absence of the growth hormone receptor

States of growth hormone (GH) resistance, such those observed in Laron dwarf patients, are characterized by mutations in the GH receptor (GHR), decreased serum and tissue IGF-1 levels, impaired glucose tolerance, and impaired skeletal acquisition. IGF-1 replacement therapy in such patients increases growth velocity but does not normalize growth. Herein we combined the GH-resistant (GHR knockout [GHRKO]) mouse model with mice expressing the hepatic Igf-1 transgene (HIT) to generate the GHRKO-HIT mouse model. In GHRKO-HIT mice, serum IGF-1 levels were restored via transgenic expression of Igf-1, allowing us to study how endocrine IGF-1 affects growth, metabolic homeostasis, and skeletal integrity. We show that in a GH-resistant state, normalization of serum IGF-1 improved body adiposity and restored glucose tolerance but was insufficient to support normal skeletal growth, resulting in an osteopenic skeletal phenotype. The inability of serum IGF-1 to restore skeletal integrity in the total absence of GHR likely resulted from reduced skeletal Igf-1 gene expression, blunted GH-mediated effects on the skeleton that are independent of serum or tissue IGF-1, and poor delivery of IGF-1 to the tissues. These findings are consistent with clinical data showing that IGF-I replacement therapy in patients with Laron syndrome does not achieve full skeletal growth.

Unbound (bioavailable) IGF1 enhances somatic growth

Understanding insulin-like growth factor-1 (IGF1) biology is of particular importance because, apart from its role in mediating growth, it plays key roles in cellular transformation, organ regeneration, immune function, development of the musculoskeletal system and aging. IGF1 bioactivity is modulated by its binding to IGF-binding proteins (IGFBPs) and the acid labile subunit (ALS), which are present in serum and tissues. To determine whether IGF1 binding to IGFBPs is necessary to facilitate normal growth and development, we used a gene-targeting approach and generated two novel knock-in mouse models of mutated IGF1, in which the native Igf1 gene was replaced by Des-Igf1 (KID mice) or R3-Igf1 (KIR mice). The KID and KIR mutant proteins have reduced affinity for the IGFBPs, and therefore present as unbound IGF1, or 'free IGF1'. We found that both KID and KIR mice have reduced serum IGF1 levels and a concomitant increase in serum growth hormone levels. Ternary complex formation of IGF1 with the IGFBPs and the ALS was markedly reduced in sera from KID and KIR mice compared with wild type. Both mutant mice showed increased body weight, body and bone lengths, and relative lean mass. We found selective organomegaly of the spleen, kidneys and uterus, enhanced mammary gland complexity, and increased skeletal acquisition. The KID and KIR models show unequivocally that IGF1-complex formation with the IGFBPs is fundamental for establishing normal body and organ size, and that uncontrolled IGF bioactivity could lead to pathological conditions.

Is Lipotoxicity presents in the early stages of an experimental model of autoimmune diabetes? Further studies in the multiple low dose of streptozotocin model

An increased availability of plasma free fatty acids (FFA) seems to play a role in the early stages of experimental type 1 diabetes mellitus induced in C57BL/6J mice by multiple low doses of streptozotoxin (mld-STZ). We analyzed the temporal changes of: (1) plasma and skeletal muscle lipids and their relationship with glucose metabolism; (2) triglyceride (Tg) concentration in isolated islets; (3) intraperitoneal glucose tolerance test; and (4) insulin secretion patterns when the three mutually interactive glucose signaling pathways were activated. Animals were killed by cervical dislocation at days 4, 6, 7, 8, 9 and 12 post first injection of mld-STZ. Compared with control mice, we observed: (1) at day 6, a significant increase of plasma FFA and both muscle and islet Tg content and a significant decrease of muscle pyruvate dehydrogenase activity. These parameters further deteriorated with time. (2) plasma Tg, glucose and insulin levels and glucose tolerance test were significantly different only after day 8. (3) an increase in both phases of the glucose plus palmitate-stimulated insulin secretion was observed at day 4. This effect progressively decreased since day 7 up to day 9. Moreover, an inhibitory action of cerulenin over glucose plus palmitate-stimulated insulin secretion was observed between days 6 and 9. Taken together these results suggest that early alteration in carbohydrate and lipid metabolism could represent a "metabolic window" which would develop between days 6 and 8. Afterwards, subsequent immunological alterations, apoptosis and necrosis induced the destruction of β cells and would mask the results mentioned above.

A study on strategies for improving growth and body composition after renal transplantation

Allograft function and metabolic effects of four treatment regimens, namely, methylprednisone (MP) standard dose (MP-STD), deflazacort (DFZ), MP-late steroid withdrawal (MP-LSW), and MP-very low dose (MP-VLD), were evaluated in prepubertal patients. MP was decreased by month 4 post-transplantation to 0.2 mg/kg/day in MP-STD and DFZ patients and to <0.1 mg/kg/day in MP-LSW and MP-VLD patients. Starting in month 16 post-transplant, MP was switched to DFZ in the DFZ group and totally withdrawn in the MP-LSW group. Creatinine clearance diminished in the MP-STD and MP-LSW groups from 77 +/- 6 to 63 +/- 6 ml/min/1.73 m(2)and from 103 +/- 5 to 78 +/- 3 ml/min/1.73 m(2), respectively (p < 0.01 and p < 0.001, respectively). Height increased >0.5 SDS only in the MP-LSW and MP-VLD groups. The body mass index and fat body mass for height-age increased only in the MP-STD patients (p < 0.05 and p < 0.01, respectively). Fat body mass decreased in the DFZ group (p < 0.05), total cholesterol and LDL-cholesterol increased in the MP-STD group, while LDL-cholesterol and total cholesterol/HDL-cholesterol ratio decreased in the DFZ group (p < 0.01). Lumbar spine bone mineral density (BMD) for height-age showed an increase in the MP-LSW and MP-VLD groups (p < 0.01). Our data suggest that MP-LSW and MP-VLD strategies improve linear growth, BMD, the peripheral distribution of fat, and preservation of the bone-muscle unit and maintain the normal lipid profile. The MP-LSW patients had a concerning rate of acute rejections and graft function deterioration in prepubertal patients.

Role of lipids in the early developmental stages of experimental immune diabetes induced by multiple low-dose streptozotocin

The present work examines the role of lipids in the development of the Type 1 diabetes induced by the administration of multiple low doses of streptozotocin (STZ) in C57BL/6J mice. The study was performed before and after the onset of clear hyperglycemia, and the results were as follows. First, 6 days after the first dose of STZ, while plasma glucose and insulin levels remained similar to those observed in the control mice, plasma free fatty acid (FFA) levels were significantly increased ( P < 0.05). At that time, a marked increase of triglyceride content in gastronemius muscle was accompanied by a diminished activity of pyruvate dehydrogenase complex, suggesting an impaired glucose oxidation. Furthermore, a decrease of both triglyceride content and lipoprotein lipase activity was observed in the epididymal fat tissue. Second, 12 days after the first injection of STZ, hyperglycemia was accompanied by hypertriglyceridemia, a more pronounced increase of plasma FFA, and a significant ( P < 0.05) reduction of insulinemia. At this time, both the adipose tissue and the gastrocnemius muscle showed a further deterioration of all parameters mentioned after 6 days. Moreover, in the gastrocnemius muscle, an impaired nonoxidative pathway of glucose metabolism was observed [significant reduction ( P < 0.05) of glycogen mass, glucose-6-phosphate content, and glycogen synthase activities] at this time point. Finally, the data suggest for the first time that, in mice, Type 1 diabetes induced by multiple low doses of STZ and enhanced lipolysis of fat pads leads to an increase in the availability of plasma FFA, which seems to play a role in the early steps of diabetes evolution.

Fish Oil Affects Pancreatic Fat Storage, Pyruvate Dehydrogenase Complex Activity and Insulin Secretion in Rats Fed a Sucrose-Rich Diet

Rats fed a sucrose-rich diet (SRD) develop hypertriglyceridemia and a marked decline in beta cell function. The purpose of this study was to determine whether changes in triglyceride concentration and/or altered pyruvate dehydrogenase complex (PDHc) activity contribute to the beta cell dysfunction, and to analyze the effect of dietary fish oil on the altered patterns of insulin secretion and peripheral insulin resistance. Rats were fed an SRD for 210 d. One-half of the rats continued consuming the SRD until d 270. The other half received an SRD in which fish oil (FO) was partially substituted for corn oil until d 270. A group of rats was fed a control diet (CD) throughout the experiment. The islets of rats fed the SRD had a greater triglyceride concentration and lower PDHc activity than those fed the CD. Insulin secretion patterns under the stimulus of glucose, palmitate or L-arginine were impaired in SRD-fed compared with CD-fed rats. This was accompanied by peripheral insulin resistance, mild hyperglycemia, a sharp increase of plasma triglyceride and free fatty acid levels and greater epididymal and retroperitoneal fat weights. FO normalized and/or improved these variables. Our results indicate that the increased fat storage and decreased PDHc activity in the beta cells play a key role in the abnormal insulin secretion of rats chronically fed an SRD. This is consistent with the reversion of these alterations by dietary FO.

Muscle lipid metabolism and insulin secretion are altered in insulin-resistant rats fed a high sucrose diet

Feeding rats a sucrose rich diet (SRD) induces hypertriglyceridemia and insulin resistance. The purposes of this study were to determine the time course of changes in lipid and glucose metabolism in the gastrocnemius muscle, both in the basal state and after the euglycemic hyperinsulinemic clamp, in rats fed a SRD for 3, 15 or 30 wk, and to analyze the changes in glucose-stimulated insulin secretion from perifused isolated islets from SRD-fed rats and their relationships to peripheral insulin insensitivity. A control group of rats was fed a control diet (CD) for the same period of time. After 3 wk of consuming the SRD, long-chain acyl CoA (LCACoA) levels in muscle were greater than in rats fed the CD, an early indication of the disturbance of lipid metabolism. Neither glycogen storage nor glucose oxidation were impaired at this time. Moreover, the biphasic patterns of glucose-stimulated insulin secretion showed a marked increase in the first peak, which helped maintain normoglycemia in SRD-fed rats. After 15 or 30 wk of consuming the SRD, triglyceride and LCACoA levels in muscles were greater than in rats fed the CD. Glucose oxidation as well as insulin-stimulated glycogen synthase activity and glycogen storage were lower than in rats fed the CD. Moreover, the altered pattern of insulin secretion further deteriorated. This was accompanied by peripheral insulin resistance and moderate hyperglycemia. Our results indicate that the dyslipemia present in rats chronically fed a SRD may play an important role in the progressive deterioration of insulin secretion and sensitivity in this animal model.

Decreased tubular uptake of L-3,4-dihydroxyphenylalanine in streptozotocin-induced diabetic rats

BACKGROUND/AIMS

This study determined alterations in renal dopamine production in streptozotocin-treated rats and explored the mechanisms underlying this alteration.

METHODS

Streptozotocin (65 mg/kg) or vehicle was administered to 3-month-old male Wistar rats. Treated animals had hyperglycemia, glycosuria and increased diuresis, natriuresis and excretion of L-dopa. Urinary dopamine and dihydroxyphenylacetic acid were similar to those in control animals. The production of dopamine by renal cortex slices from treated rats was significantly less than that from control animals. The addition of glucose (8.4-18.4 mM) to the incubation medium decreased about 40% the uptake of L-dopa by isolated proximal tubular cells. Scatchard analysis of the saturation curves obtained in this condition showed a decrease in the V(max) without changes in the K(m).

RESULTS

Our results confirm previous studies suggesting a renal dopaminergic deficiency in insulin-dependent diabetes and provide evidence strongly suggesting that a decrease in the number of tubular L-dopa transport sites is the underlying defect of this deficiency.

CONCLUSION

These results highlight the role of the uptake of dopa as an important modulator of renal dopamine synthesis.

Troglitazone (CS-045) normalizes hypertriglyceridemia and restores the altered patterns of glucose-stimulated insulin secretion in dyslipidemic rats

Rats fed a sucrose-rich diet ([SRD] 63% wt/wt) up to 270 days develop stable hypertriglyceridemia, impaired glucose tolerance, and insulin insensitivity. The aim of the present study is to investigate whether the hypoglycemic agent troglitazone introduced as a pharmacologic intervention could improve and/or reverse the whole-body insulin insensitivity and related abnormalities present after feeding normal rats with a SRD long-term. For this purpose, male Wistar rats were fed a SRD for 210 days. While half of the animals continued with this diet for up to 270 days, troglitazone (0.2 g/dL wt/wt) was added to the SRD of the other half for up to 270 days. Troglitazone markedly reduced in vivo the hepatic triglyceride secretion rate (TGSR) and enhanced its removal from the circulation, leading to a normalization of plasma triglyceride levels. It also normalized the whole-body peripheral insulin resistance, the glucose homeostasis, and the elevated free fatty acids (FFAs) without detectable changes in plasma insulin levels. The clear alteration of the biphasic pattern of glucose-stimulated insulin secretion in the in vitro perfused beta-cell islets of rats fed the SRD long-term (270 days) was also completely normalized when the SRD was supplemented with troglitazone for 2 months. The normalization of the altered patterns of glucose-stimulated insulin secretion, as well as the enhancement of peripheral insulin sensitivity without detectable changes in plasma insulin, might be largely a result of the significant action of troglitazone in the decrease of circulating lipids and enhancement of whole-body glucose metabolism.

Role of skeletal muscle on impaired insulin sensitivity in rats fed a sucrose-rich diet: effect of moderate levels of dietary fish oil

In the present study we investigated: (1) the contribution of the skeletal muscle to the mechanisms underlying the impaired glucose homeostasis and insulin sensitivity present in dyslipemic rats fed a sucrose-rich diet (SRD) over a long period of time and (2) the effect of fish oil on these parameters when there was a stable hypertriglyceridemia before the source of fat (corn oil) in the diet was replaced by isocaloric amounts of cod liver oil. Our results show an increased triglyceride content in the gastrocnemius muscle with an impaired capacity for glucose oxidation in the basal state and during euglycemic clamp. This was mainly due to a decrease of the active form of pyruvate dehydrogenase complex (PDHa) and an increase of PDH kinase activities. Hyperglycemia, normoinsulinemia, and diminished peripheral insulin sensitivity also were found. Even though there were no changes in the insulin levels, the former metabolic abnormalities were completely reversed when the source of fat was changed from corn oil to cod liver oil. The data also suggest that in the gastrocnemius muscle of rats fed a SRD over an extended period, an increased availability and oxidation of the lipid fuel, which in turn impairs the glucose oxidation, contributes to the abnormal glucose homeostasis and to the peripheral insulin insensitivity. Moreover, the parallel effect on insulin sensitivity, glucose, and lipid homeostasis attained through the manipulation of dietary fat (n-3) in the SRD suggests a role of n-3 fatty acid in the management of dyslipidemia and insulin resistance.

Effect of moderate levels of dietary fish oil on insulin secretion and sensitivity, and pancreas insulin content in normal rats

The effect of omega-3 fatty acids derived from fish and marine mammals on subjects with normal glucose tolerance is still unclear. The aim of the present study was to test whether the hypolipidemia that follows the chronic administration of cod liver oil, rich in polyunsaturated fatty acids (omega-3), to normal rats is accompanied by changes in glucose metabolism, insulin secretion and sensitivity, and pancreatic insulin content. To achieve this goal, male Wistar rats were fed with a semisynthetic diet (w/w): 62.5% cornstarch, 7% cod liver oil plus 1% corn oil, and 17% protein (CD + CLO). Control rats were fed with the same semisynthetic diet with the only exception that the source of fat was 8% (w/w) corn oil (CD). Both diets were administered ad libitum for 1 month. At the end of the experimental period, the results obtained were as follows (mean +/- SEM): serum triacylglycerol (mM): CD + CLO 0.21 +/- 0.04 vs. CD 0.58 +/- 0.05 (p < 0.05); free fatty acids (microM): CD + CLO 257 +/- 20 vs. CD 288 +/- 22 (p = NS); total cholesterol (mM): CD + CLO 1.13 +/- 0.09 vs. CD 1.82 +/- 0.06 (p < 0.05); high-density lipoprotein cholesterol (mM): CD + CLO 0.58 +/- 0.08 vs. CD 1.07 +/- 0.04 (p < 0.05); plasma glucose (mM): CD + CLO 6.30 +/- 0.29 vs. CD 6.28 +/- 0.10 (p = NS); liver triacylglycerol (mumol/liver): CD + CLO 104.1 +/- 11.4 vs. CD 136.8 +/- 4.3 (p < 0.05); glycogen (mumol/g wet weight): CD + CLO 298.3 +/- 21.0 vs. CD 297.0 +/- 19.0 (p = NS); glucose-6-phosphate dehydrogenase (U/liver): CD + CLO 37.9 +/- 2.2 vs. CD 58.8 +/- 5.0 (p < 0.05); triacylglycerol secretion (nmol/min/100 g body weight): CD + CLO 101.0 +/- 2.0 vs. CD 166.0 +/- 9.7 (p < 0.01); removal of fat emulsion (K2% min-1): CD + CLO 15.0 x 10(-2) +/- 0.8 x 10(-2) vs. CD 8.2 x 10(-2) +/- 0.2 x 10(-2) (p < 0.01); intravenous glucose tolerance (kg 10(-2): CD + CLO 2.68 +/- 0.37 vs. CD 2.70 +/- 0.14 (p = NS); immunoreactive insulin (microU/ml/ min): with the area under the curve between 0 and 30 min CD + CLO 544 +/- 60 vs. CD 1,050 +/- 38 (p < 0.05), with the area under the curve between 0 and 60 min CD + CLO 1,188 +/- 150 vs. CD 2,160 +/- 137 (p < 0.05), and pancreas insulin content (microU/mg pancreas): CD + CLO 1.85 +/- 0.29 vs. CD 2.04 +/- 0.12 (p = NS). In conclusion, the present study shows that the strong hypolipidemic effect produced by the administration of low doses of fish oil to normal rats is accompanied by a significant reduction of plasma insulin levels without changes in glucose tolerance. Since no changes in pancreatic insulin content were observed, lower plasma insulin levels, both basal and after an intravenous glucose challenge, may be the result of an increased peripheral insulin sensitivity in normoglycemic animals.

Effect of salicylate poisoning on insulin secretion. Studies on its mechanism of action

Sodium salicylate poisoning increased glucose-induced insulin secretion by slices of rat pancreas. The alpha adrenergic blocker phentolamine, but not salicylate poisoning, overcame the inhibitory effect of epinephrine on insulin secretion. Theophylline (5, 10 and 15 mM) significantly increased the insulin secretion induced by 11 mM glucose. The highest insulin response was obtained when theophylline was used at a 10 mM concentration. However, when pancreas slices from salicylate poisoned rats were used, a 5 mM concentration was sufficient to achieve maximal insulin response. Salicylate poisoning diminished the free tubulin pool, an action that was impaired by imidazole; however, imidazole did not modify the effect of the ionophore A23187. The results suggest that: a) sodium salicylate poisoning increases B-cell response to glucose; b) changes in alpha adrenergic activity are not related to the mechanism of action of salicylate; c) an increment in the cAMP concentration may mediate the stimulatory effect of salicylate poisoning on insulin secretion; d) the effect of salicylate on the microtubular system is indirect and probably mediated through an increment in pancreatic cAMP.

A comparative study of two insulin secretion inhibitors: somatostatin and diazoxide

We studied the effect of the ionophere A 23187 and of phosphodiesterase inhibitors and activators (Theophylline, Pentoxiphylline and Imidazol) on insulin secretion and on the pool of free tubulin in rat pancreas in the presence of somatostatin and diazoxide. The results suggest that: 1. The inhibitory effect of somatostatin on insulin secretion does not seem to be related mainly to an inhibition of cAMP production. The decrease in calcium translocation induced by somatostatin could inhibit the cAMP participation in the mechanism of hormonal secretion. 2. Somatostatin seems to inhibit the movement of calcium towards the cytoplasm from outside and from within the cell. Diazoxide seems to inhibit only the entrance of calcium from outside the cell but does not seem to inhibit the entrance determined by theophylline and pentoxiphylline from intracellular compartments. 3. Arginine glucose stimulation in the presence of A 23187-induced calcium translocation is able to determine insulin secretion although cAMP degradation is increased by imidazol. 4. Somatostatin and diazoxide inhibit pancreatic tubulin polimerization; however, the effect seem to be indirect and related to the inhibition of calcium translocation determined by both substances.