Stimulus For Insulin Release Research Articles

Nitric oxide is involved in the insulin release in rats by l-arginine

The insulinogenic effect ofl-arginine has been demonstrated bothin vivo andin vitro, but the mechanism by which this amino acid stimulates the pancreatic B-cells to release insulin is not entirely clear. Recently it was shown thatl-arginine-derived nitric oxide may mediatel-arginine-induced insulin release, and data were also provided to suggest that nitric oxide (NO) has no part in this process. To further investigate whether NO is involved in the release of insulin induced byl-arginine, we infused different doses ofl- andd-arginine in rats. L-Arginine (25 and 100 mg/kg/minute) elicited dose-dependent increases of the plasma insulin levels by up to 18.65±2.13 and 48.6±6.6 U/L, respectively, and increased the plasma glucose levels by up to 1.18±0.13 and 1.43±0.1 mmol/L, respectively. D-Arginine (25 and 100 mg/kg/minute) also elicited dose-dependent increases of the plasma insulin levels by up to 9.08±1.23 and 23.33±2.33 U/L, and increased the plasma glucose levels by up to 0.32±0.09 and 0.46±0.08 mmol/L. Thus, the increases in plasma insulin and glucose levels were significantly smaller during infusion ofd-arginine. We conclude that the plasma insulin response to i.v. infusion ofl-arginine is at least partly mediated by augmented NO synthesis by the pancreatic islets, althoughl-arginine-derived NO is not an obligatory stimulus for insulin release.

Enzyme-Linked Immunosorbent Assay of Autoantibodies against Mitochondrial Glycerophosphate Dehydrogenase in Insulin-Dependent and Non-Insulin-Dependent Diabetic Subjects

The mitochondrial enzyme FAD-linked glycerophosphate dehydrogenase (mGDH) plays a key role in the recognition of glucose as a stimulus for insulin release from the pancreatic islet B-cell. In the present study, an ELISA procedure was used for the measurement of mGDH antibodies in both insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetic patients. Positive readings, exceeding the upper limit of the normal range, were recorded in 7 out of 12 IDDM patients, as distinct (P< 0.01) from 2 out of 12 nondiabetic subjects of comparable age. The study conducted in 41 NIDDM patients and 15 control subjects of similar age indicated that the incidence of mGDH-positive cases was not significantly different in the diabetic (4/41) and control (1/15) groups, the measurement of optical density in the positive cases barely exceeding the upper limit of the normal range. These findings indicate that the mitochondrial enzyme mGDH often acts as an antigenic determinant in IDDM, but not in NIDDM, patients.

Autoantibodies against mitochondrial glycerophosphate dehydrogenase in patients with IDDM

The mitochondrial enzyme FAD-linked glycerophosphate dehydrogenase (mGDH) plays a key role in the recognition of d-glucose as a stimulus for insulin release from the pancreatic islet B-cell. This study reveals that autoantibodies against this enzyme are not uncommonly found in patients with insulin-dependent diabetes mellitus (IDDM) examined at the onset of the disease. Antibodies reacting with a recombinant mGDH fragment product were observed in the serum of four out of 15 type-1 diabetics, but in none of 15 control subjects. The serum of patients positive for the recombinant mGDH fragment also recognized native mGDH in a rat testis extract, provided that the enzymatic protein was first exposed to an anti-mGDH rabbit serum. Antibodies against mGDH were also found in four out 12 patients with autoimmune thyroiditis. These findings reveal that a mitochondrial enzyme, that represents an essential component of the islet B-cell glucose-sensing device, may act as an antigenic determinant in patients with IDDM or other autoimmune diseases.

Glyceraldehyde and the pancreatic beta-cell.

D-glyceraldehyde is commonly used as a stimulus of insulin release from the pancreatic β-cell1–3. It is generally accepted that the triose can be metabolized in the β-cell via the glycolytic pathway, thus stimulating insulin release in a manner analogous to glucose1–3. In numerous studies of tumoural β-cell lines which, owing to abnormalities in glucose transport or metabolism, fail to respond to the sugar, glyceraldehyde has been employed as a ‘nutrient’ type stimulus4–6. The results of our studies on glyceraldehyde actions on the HIT-T15 cell line suggest, however, that the triose is poorly metabolized in these cells and that, unlike glucose, it stimulates the β-cell by a mechanism which does not involve its glycolytic metabolism.

Selective stimulation of insulin secretion by CCK-4 analogues having N-terminal modifications

Synthetic analogues of CCK-4 in which Trp was replaced by D-Pro (peptide I), Thz (peptide II) and delta Pro (peptide III) have been studied for their insulin and glucagon releasing activities from the islets of Langerhans in vitro. Peptide I has been found to be the most potent insulin releaser among the three analogues and its activity is comparable to that of CCK-4. Unlike CCK-4, its three analogues (peptides I-III) do not stimulate the release of glucagon with basal concentration of glucose in the medium. However, with increasing glucose concentration, all the three analogues potentiate the glucose stimulus for insulin release.

Mechanism of Action of Growth-Hormone-Releasing Hormone in Stimulating Insulin Secretion in vitro from Isolated Rat Islets and Dispersed Islet Cells

Human growth-hormone-releasing hormone [(1-44)NH2] (hGHRH) was a potent stimulus for insulin release from rat islets of Langerhans in vitro; the optimum concentration used was 10(-11) M. The dose response curves for hGHRH effects on insulin secretion were notably different in intact islets of Langerhans compared to cultured dispersed islet cells. Pancreatic islets responded to a very low hGHRH concentration (10(-12) M), but at a higher hGHRH concentration (10(-9) M) no stimulation of insulin release was observed. When somatostatin antiserum was included in the incubation medium, hGHRH (10(-9) M) stimulated insulin release from intact islets. In cultured dispersed islet cells, which are principally insulin-secreting B cells, hGHRH directly and potently stimulated insulin release even at a concentration of 10(-9) M. Addition of somatostatin (10(-7), 10(-8) M) significantly reduced the hGHRH-induced insulin-secretory responses of dispersed islet cells. hGHRH (10(-11)-10(-9) M) raised islet cAMP levels; individually, hGHRH and theophylline exerted positive effects on insulin release, their combined effect was greater than that caused by either one. We conclude that hGHRH directly affects insulin secretion in vitro by a cAMP-dependent mechanism, and that the difference in responses of intact islets versus islet cells to increasing concentrations of hGHRH may be related to hGHRH-induced release of somatostatin in intact rat islets.

?-Cell recognition of stereoisomers of D-glucose

The ability of all eight D-aldohexose steroisomers to stimulate insulin release and biosynthesis was compared with their ability to serve as a metabolic substrate for isolated islets of Langerhans as judged by formation of lactate. Insulin release and synthesis were stimulated by glucose or mannose but not by allose, altrose, gulose, idose, galactose or talose. No potentiary effects of allose, altrose, gulose, idose, or talose were found on insulin release in the presence of 4 mmol/l glucose nor did these sugars inhibit insulin release in the presence of 20 mmol/l glucose. Lactate formation was increased above values found in the absence of added substrate by 20 mmol/l D-glucose or mannose, but not by allose, altrose, gulose, galactose or talose. The results support the substrate-site hypothesis for the recognition of sugars as stimuli of insulin release and synthesis.

Cellular Mechanisms of Insulin Release: Effects of Retinol on Insulin Release and Islet Ultrastructure

In previous studies using perifused collagenase-isolated rat islets, retinol (vit A) inhibited glucose-induced Diphasic insulin release and glucose oxidation. To further evaluate vit A action and elucidate cellular mechanisms of insulin release, we tested the effects of vit A on glucose uptake by islets, on other stimuli of insulin release, and oh islet ultrastructure and the effects of calcium on vit A inhibition of insulin release. Vit A did hot inhibit 2-deoxy-D-glucose uptake into islets. Vit A inhibited second phase 9.7 and 13.9 mM glucoseinduced insulin release to 50%, 10 mM glyceraldehydeinduced release to 64%, and 20 mM leucine-induced release to 66% of control. First phase, release was unaffected. Kinetic analysis of the effects of vit A on glucose-induced release indicated a change in the Vmax but not in the Km, suggesting inhibition of a potentiator of insulin release by vit A. Calcium (4 mM) opposed vit A inhibition of glucose-induced release. Ultrastructurally, vit A decreased the volume fraction of secretory granules to 90% of control, increased the volume fraction of material within the rough endoplasmic reticulum to 147% of control, and increased the outer and decreased the inner mitochondrial compartment volume fractions by 14%. Our functional and ultrastructural findings, in all, suggest that vit A inhibitioh of second phase insulin release is not specific for glucose and may be mediated in part through impairment of mitochondrial function and intracellular glucose oxidation.

Stimulus-secretion coupling of glucose-induced insulin release. Metabolism of alpha- and beta-D-glucose in isolated islets.

Alpha-D-Glucose is known to exert more marked insulinotropic action than B-D-glucose. Both anomers are phosphorylated at the same rate by rat islet homogenates. The islet glucose-6-phosphate dehydrogenase displays a preferential affinity towards beta-D-glucose-6-phosphate, and this coincides with a higher sorbitol content in the islets exposed to beta-D-glucose. On the contrary, the islet phosphoglucose isomerase is stereospecific for alpha-D-glucose 6-phosphate and, hence, the concentration of glucose 6-phosphate is lower and that of the alpha-anomer to lactate and CO2 is also higher than that of beta-D-glucose. This increased glycolytic flux is associated with a more marked inhibitory action on 14Ca efflux, a more pronounced stimulation of 45Ca net uptake and a higher rate of insulin release in the islets exposed to alpha-D-glucose. The more marked insulinotropic action of alpha- as a distinct from beta-D-glucose is thus compatible with the view that glycolysis represents the key component of the sensor device through which glucose is identified in the pancreatic B-cell as a stimulus for insulin release.

Regulation of in Vitro Insulin Release from a Transplantable Syrian Hamster Insulinoma1

An insulin-producing islet cell tumor of the Syrian hamster has been studied in vitro for its capacity to respond to known stimuli of insulin release. Insulin secretion during short term incubation and perifusion of fragments of tumor was detected by radioimmunoassay. Insulin release was increased 2-4 fold by 40 mM potassium in the presence of calcium, glucose (22 mM), glucagon (0.3-3.0 muM), N6,02'-dibutyryl adenosine 3',5'-monophosphate (cAMP; 6mM), and theophylline (10 mM). Concentrations of glucagon that induced insulin release were also effective in activating adenylate cyclase in the membranes of tumor cells. Thus, this tumor appears to possess a cAMP-mediated mechanism for insulin release. Somatostatin (0.8-25 mum) inhibited glucagon-induced insulin release without altering basal or glucagon stimulated adenylate cyclase activity. It would appear that inhibition of glucagon induced insulin release by somatostatin is not mediated by adenylate cyclase. We propose that insulin release by this tumor is sufficiently similar to that found in normal islets so as to make it a suitable model for biochemical studies that require large quantities of homogeneous tissue.

Human foetal pancreatic insulin secretion in response to ionic and other stimuli.

SUMMARY Pieces of human foetal pancreas were incubated under control conditions and in media containing different stimuli of insulin release. Insulin secretion was stimulated from the pancreases of foetuses (83–625 g body weight) which were of 16–24 weeks gestational age. Potassium (60 mmol/l), barium (2·54 mmol/l) and ouabain (10−5 mol/l) were effective stimuli in all experiments. Glucagon (5 μg/ml), theophylline (1 mmol/l) and dibutyryl 3′,5′-cyclic adenosine monophosphate (1 mmol/l) stimulated insulin secretion in media containing 0, 0·6 or 3·0 mg glucose/ml. Theophylline and dibutyryl 3′,5′-cyclic adenosine monophosphate were effective in all experients and glucagon stimulated insulin release in four out of six experiments. At all ages studied, histological examination of the pancreas after each experiment revealed islets of Langerhans containing β cells. In most cases the islets were of the mantle type but occasionally bipolar islets were seen. Cellular normality, as judged by light microscopy, was preserved after periods of incubation for up to 5½ h. Glycogen was demonstrable in the pancreatic acinar tissue but not in the islets. The results of these experiments indicate that, between the 16th and 24th week of foetal life, the human β cell is capable of releasing insulin in vitro when stimulated appropriately.