The effect of serotonin on in vitro insulin secretion and biosynthesis in mice

Tesz

Journal of Biological Chemistry

2001

The release and oxidation of 5-hydroxytryptamine from 5-hydroxytryptamine-preloaded ␤-cells has been used as a surrogate marker for insulin secretion. Findings made using this methodology have been used to support the concept that insulin stimulates its own release. In the present studies, the effects of 5-hydroxytryptamine on stimulated insulin secretion from isolated perifused rat islets was determined. When added together with stimulatory glucose, 5-hydroxytryptamine (0.5 mM) significantly reduced both phases of 8 mM glucose-induced secretion and reduced the first phase of 15 mM glucoseinduced release by 60% without any effect on sustained insulin release rates. Preloading of ␤-cells with 0.5 mM 5-hydroxytryptamine for 3 h resulted in a more severe impairment of 15 mM glucose-induced secretion. First and second phase release rates were reduced by 70 and 55%, respectively. In addition, this pretreatment protocol also abolished 200 M tolbutamide-induced insulin secretion from perifused islets. These findings confirm that 5-hydroxytryptamine is a powerful inhibitor of stimulated insulin secretion. The responses of 5-hydroxytryptaminepreloaded ␤-cells may not accurately reflect the biochemical events occurring during the physiologic regulation of insulin secretion. The suggestion that insulin stimulates its own secretion based exclusively on amperometric measurements should be reconsidered.The concept that insulin exerts biochemical and physiologic effects on the cell that secretes it, the pancreatic ␤-cell, has been suggested based on a number of observations. Over 30 years ago (1, 2), a negative feedback effect of insulin on its own release was proposed. A large number of subsequent studies (3-9) in which insulin or connecting peptide levels were measured arrived at a similar conclusion. We recently reported (10), in agreement with several prior studies (11,12), that the phosphatidylinositol 3-kinase (PI3K) 1 inhibitor wortmannin amplified glucose-induced insulin secretion. This conclusion was based upon insulin secretion measurements from isolated perifused rat islets that retain physiologic insulin secretory responses similar to those observed using the perfused rat pancreas preparation (13-15). We also suggested that an impairment in PI3K signaling in ␤-cells might be responsible for the hyperinsulinemia noted in a variety of insulin-resistant states including obesity and type 2 diabetes. Thus, whereas disruption of PI3K signaling results in insulin resistance in peripheral insulin-dependent tissues (16, 17), the same biochemical alteration in the ␤-cell results in compensatory hyperinsulinemia. This elegant communications network allows the ␤-cell to match the degree of insulin resistance and the secretion of insulin thus maintaining glucose tolerance. This concept presupposes that insulin exerts a negative feedback effect on its own secretion and utilizes the same biochemical signaling systems described in liver, muscle, and adipose tissues (18 -20).In contrast to the numerous studies demonstrating that in...

supporting

confidence: 93%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Are 5-Hydroxytryptamine-preloaded β-Cells an Appropriate Physiologic Model System for Establishing That Insulin Stimulates Insulin Secretion?

Tesz

Journal of Biological Chemistry

2001

Journal of Endocrinology

“…4), previously gassed with CO 2 /O 2 (5/95%), containing 1% B MAIZTEGUI and others . Islet glucokinase in insulin resistance (w/v) BSA and different glucose concentrations (2, 4, 8, 12, 16, and 20 mM;Gagliardino et al 1974). At the end of the incubation period, aliquots from the medium were taken to measure insulin by RIA (Herbert et al 1965).…”

Section: Insulin Secretion In Vitromentioning

confidence: 99%

Islet adaptive changes to fructose-induced insulin resistance: β-cell mass, glucokinase, glucose metabolism, and insulin secretion

Maiztegui¹,

Borelli²,

Raschia³

et al. 2008

b-Cell mass, hexokinase/glucokinase (HK/GK) activity, glucose metabolism and insulin secretion were studied in the islets of rats with fructose-induced insulin resistance (IR). Normal male Wistar rats were fed a standard commercial diet and water without (control, C) or with 10% fructose-rich diet (FRD) for 3 weeks. Blood glucose (strips), triglyceride (commercial kit), and insulin (RIA) levels were measured at the time of death. Glucose-induced insulin release, glucose metabolism ( 14 CO 2 andH]-glucose) and HK/GK activity (G-6-P production), transcription (RT-PCR), protein expression (Western blot), and cellular compartmentalization were measured in isolated islets (collagenase digestion). FRD rats presented normoglycemia but impaired glucose tolerance, hypertriglyceridemia, hyperinsulinemia, and increased HOMA-IR index. In these rats, b-cell mass decreased significantly by 33%, with a 44% increase in the percentage of apoptotic cells. Glucoseinduced insulin release and islet glucose metabolism were higher in FRD rats. While GK activity (total and cytosolic fraction) and protein expression were significantly higher in FRD islets, HK showed no change in any of these parameters. Our results demonstrate that the changes induced by dietary-induced IR upon b-cell function and mass are strongly conditional on the nutrient model used. In our model (intact animals with impaired glucose tolerance), GK activity increases through mechanisms previously shown only in vitro or under highly hyperglycemic conditions. Such an increase plays a pivotal role in the adaptive increased release of insulin in response to IR, even in the presence of marked b-cell mass reduction.

Journal of Endocrinology

“…Groups of five islets were incubated for 60 min at 37 C in 0·6 ml of Krebs-Ringer bicarbonate buffer, pH 7·4, previously gassed with CO 2 /O 2 (5%/95%), containing 1% (w/v) BSA and 3·3 or 16·7 mM glucose (Gagliardino et al 1974). At the end of the incubation period, aliquots were taken from the medium to measure insulin by RIA (Herbert et al 1965, Linde et al 1980.…”

Section: Insulin Secretion In Vitromentioning

confidence: 99%

Enhanced expression of hexokinase I in pancreatic islets induced by sucrose administration

Maiztegui

Borelli²,

Massa³

et al. 2006

Administration of a sucrose-rich diet (SRD) to normal hamsters induces an insulin-resistant state and a significant increase of insulin secretion and -cell mass. Islets isolated from these animals had a marked increase in glucose metabolism and glucose-induced insulin secretion, at both low and high glucose concentrations. They also presented increased hexokinase (HK) activity, without measurable changes in glucokinase (GK) activity. In this study we measured HK and GK activity in homogenates of islets isolated from normal control and SRD-fed hamsters, as well as in their particulate and cytosolic fractions. We also measured transcription rate (mRNA by reverse transcriptase PCR) and expression levels (Western blotting) of both enzymes in these islets. We found an increase in HK activity and expression levels, without measurable changes in HK mRNA level in SRD-fed animals. Whereas a similar GK activity was measured in homogenates of islets isolated from both groups, such activity was significantly higher in the cytosolic fraction of SRD islets. On the other hand, GK transcription rate and expression level were similar in both experimental groups. Our results suggest that the increased -cell secretory response to low glucose can be partly ascribed to an increased activity of islet HK consecutive to an enhanced expression of the enzyme, while the enhanced response to high glucose could be due to changes in GK compartmentalization.