Biomedical subjects

F C Schuit

Publications and source records attributed to F C Schuit.

29 records · Page 2Linked to original sources

PubMed1992

Complementation of HLA-DQA and -DQB genes confers susceptibility and protection to insulin-dependent diabetes mellitus.

Lack of an aspartic acid 57 in the HLA-DQ beta chain was introduced as a genetic marker of insulin-dependent diabetes mellitus (IDDM). Because 25% of the control population carries the same marker, we analyzed the DQ locus for the presence of more specific disease susceptibility markers, taking into account a possible role for the polymorphic DQA gene. We thereby identified the DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) genotype which was detected in 30% of the 268 typed IDDM patients and only in 1% of the 331 typed healthy controls, resulting in a relative risk of 35. This genetic marker was more frequent in patients with clinical onset before age 18 years (36%) than in patients diagnosed between age 18 and 40 years (22%) and was not observed in patients with non-IDDM. The new susceptibility genotype DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) may explain the well-known excess of DR3/DR4 heterozygous IDDM patients and is expected to help identify individuals at risk for developing the disease.

Adolescent↗

PubMed1991

Measuring the balance between insulin synthesis and insulin release.

The absolute rates of hormone synthesis and release were determined in purified pancreatic B cells. Newly synthesized proteins were labeled with L-[3,5-3H]tyrosine or L-[2,5-3H]histidine. When medium glucose was less than or equal to 10 mM, the production of insulin exceeded or equaled its release. Raising the glucose levels above 10 mM did not further increase the rate of insulin synthesis (67 +/- 10 fmol/10(3) cells/2 hour) but elevated that of insulin release up to 3-fold the production rates (181 +/- 10 fmol/10(3) cells/2 hour). In the presence of glucagon or of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate the cells also released 3-fold more hormone that they synthesized; release was however reduced to 25% of the rate of production in the presence of epinephrine. It is concluded that glucose as well as hormonal regulators of islet B cells can influence, bi-directionally, the balance between the rates of insulin synthesis and release.

Animals↗

PubMed1991

Preferential stimulation by glucose of its oxidation relative to glycolysis in purified insulin-producing cells.

A rise in extracellular D-glucose concentration increases to a greater relative extent the conversion of both D-[5-3H]glucose to 3HOH and D-[6-14C]glucose to 14CO2 in rat purified insulin-producing cells than previously observed in pancreatic islets. In the pure B-cells, the ratio between D-[6-14C]glucose oxidation and D-[5-3H]glucose utilization increases, in a sigmoidal manner, as a function of the hexose concentration. The preferential stimulation by D-glucose of mitochondrial oxidative events is proposed to represent an unusual but essential feature of the metabolic and, hence, functional response of these fuel-sensor cells.

Acetyl Coenzyme A↗

PubMed1989

Sensitivity of rat pancreatic A and B cells to somatostatin.

Islet A and B cells were purified from the rat pancreas and examined for their respective sensitivity to somatostatin. Both somatostatin-14 (S14) and -28 (S28) inhibited glucagon and insulin release through direct interactions with the corresponding cell types. A dose-dependent suppression of the secretory activities was paralleled by a reduction in cellular cyclic AMP formation with similar ED50 values for both actions. The somatostatin effects on pancreatic hormone release may thus be mediated via an inhibition of adenylate cyclase activity. In pancreatic A cells, S14 and S28 were equally potent inhibitors with ED50 values ranging from 2 x 10(-12) to 2 x 10(-11) mol/l. Pancreatic B cells exhibited a similar sensitivity to S28 as the A cells (ED50 of 2 to 5 x 10(-11) mol/l), but not to S14 (ED50 of 2 x 10(-9) mol/l). Extrapolation of these in vitro sensitivities of islet A and B cells to the in vivo situation suggests that both cell types can respond to circulating S28 levels and that A cells are sensitive to both locally and distally released S14. Islet B cells appear insensitive to the normal peripheral S14 levels but could respond to locally released somatostatin. The marked difference in the sensitivities of islet A and B cells to S14 suggest that these cell types are equipped with different somatostatin receptors. This notion was further supported by the cell-selective actions of the synthetic S14 analogues [D-Trp8, D-Cys14]S14 and desAsn5[D-Trp8, D-Ser13]S14.

Animals↗

PubMed1988

Glucose stimulates proinsulin biosynthesis by a dose-dependent recruitment of pancreatic beta cells.

Glucose is a well-known stimulus of proinsulin biosynthesis. In purified beta cells, the sugar induces a 25-fold increase in the synthesis of insulin immunoreactive material over 60-min incubation. Autoradiographic analysis of the individual cells shows that this effect is achieved via dose-dependent recruitment of pancreatic beta cells to biosynthetic activity. Recruitment of beta cells is also seen in isolated islets exposed to glucose. The sigmoidal dose-response curve for glucose-induced proinsulin biosynthesis thus reflects a heterogeneous responsiveness of pancreatic beta cells rather than a progressively increasing activity of functionally homogeneous cells. Dose-dependent recruitment of functionally diverse cells may be a ubiquitous mechanism in tissue function.

Animals↗

PubMed1988

Interaction of sulfonylureas with pancreatic beta-cells. A study with glyburide.

In this study on purified rat pancreatic beta-cells, we show that the second-generation sulfonylurea glyburide stimulates insulin release through a direct interaction with the beta-cells. During static incubations, 2 microM glyburide releases 0.16 pg insulin per beta-cell, which corresponds to a half-maximal glucose stimulation. This effect occurs independently from the glucose-recognition unit, being detectable at both nonstimulatory and stimulatory glucose concentrations and proceeding without alterations in the rate of glucose oxidation. The secretagogue action of glyburide appears not to be mediated through cAMP but is potentiated by cAMP-generating substances such as glucagon (10(-8) M; 0.31 pg insulin released per beta-cell). Its 10-fold higher potency in isolated islets is attributed to the markedly higher cAMP levels that are maintained in islet beta-cells under the influence of locally released glucagon. Perifused pancreatic beta-cells respond to glyburide with a biphasic insulin release. After removal of the drug, the cells continue to secrete insulin at the same rate for greater than or equal to 30 min. This prolonged secretory activity coincides with a cellular accumulation of the drug, primarily in association with membranes of secretory vesicles and mitochondria. Tolbutamide also stimulates insulin release from pure beta-cells, but it is less powerful on a molar basis and does not lead to a sustained hormone release after its removal from the extracellular medium. We conclude that the hypoglycemic action of glyburide is at least partly the result of a direct interaction with pancreatic beta-cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1986

Differences in adrenergic recognition by pancreatic A and B cells.

The adrenergic control of glucose homeostasis is mediated in part through variations in the release of pancreatic hormones. In this study, purified pancreatic A and B cells were used to identify the recognition and messenger units involved in the adrenergic regulation of glucagon and insulin release. Catecholamines induced beta-adrenergic receptor activity in A cells and alpha 2-adrenergic receptor activity in B cells. The two recognition units provoked opposite variations in the production of cellular cyclic adenosine monophosphate, the beta-adrenergic unit enhancing the nucleotide's permissive effect on amino acid-induced glucagon release and the alpha 2-adrenergic unit inhibiting that upon glucose-induced insulin release. In both cell types, catecholamines interact powerfully with the synergistic control of hormone release by nutrient- and (neuro)hormone-driven messenger systems.

Adrenergic beta-Agonists↗

PubMed1985

A new in vitro model for the study of pancreatic A and B cells.

A method is developed for the preparation of single, pure, and viable rat pancreatic A and B cells in numbers sufficient for in vitro analysis. Islet isolation and dissociation techniques have been modified to increase the yield in islet cells per pancreas and per experiment. Islet cells are separated on the basis of their light scatter activity and flavin adenine dinucleotide autofluorescence into single non-B cells, single B cells, and structurally coupled B cells. Islet non-B cells are further purified into single A cells by autofluorescence-activated sorting according to the cellular nicotinamide adenine dinucleotide phosphate content at 20 mM glucose. Apart from offering the advantage of separating cells according to their functional characteristics, this procedure succeeds in the simultaneous isolation of 95-100% pure A and B cells. More than 50% of the cells in the initial islet preparation are recovered as single purified cells which can be maintained in culture. The isolated pancreatic A and B cells have been defined in terms of their cell volume, DNA and hormone content, and ultrastructural characteristics. The availability of pure pancreatic A and B cells is expected to contribute to our understanding of the regulation of glucagon and insulin release.

Animals↗

PubMed1985

Interplay of nutrients and hormones in the regulation of glucagon release.

The role of nutrients and hormones in the regulation of glucagon release is investigated in pancreatic A cells purified by autofluorescence-activated cell sorting. Purified A cells lack secretory activity in 1-h incubation at 1.4 mM glucose. Their release mechanism can be activated by arginine, alanine, and glutamine, alone or in combination. Glucose inhibits amino acid-induced glucagon release through a direct insulin-independent action upon pancreatic A cells. Nutrient-induced glucagon release is suppressed by somatostatin and amplified by (Bu)2cAMP or epinephrine. The epinephrine stimulus is inhibited by 10(-11) M somatostatin and abolished by 10(-10) M of this peptide. The effects of somatostatin and epinephrine are associated with parallel changes in cellular cAMP levels, which is not the case for the variations induced by amino acids or glucose. It is confirmed that calcium is an essential requirement for glucagon release. In contrast to its exquisite sensitivity for somatostatin, the glucagon release process is relatively insensitive to insulin during a 1-h exposure. The hormone affects solely epinephrine-induced glucagon release and its inhibitory action is partial and only observed at 10(-7) M. This suppressive effect of insulin is not attributable to variations in glucose handling but appears associated with the stimulatory effect of epinephrine. It is concluded that a nutrient-induced signal interacts with a hormone-inducible cAMP signal to activate the secretory process in pancreatic A cells.

Alanine↗

PubMed1985

Interplay of nutrients and hormones in the regulation of insulin release.

Single pancreatic B cells are purified by autofluorescence-activated cell sorting, and their secretory activity is measured after overnight culture. Compared to intact islets, the isolated cells release 2-fold more insulin under basal conditions and 5-fold less during nutrient stimulation. Their secretory activity can be induced by glucose, leucine, or arginine, but only 0.3-1.7% of their hormone content is liberated at 20 mM nutrient concentrations. This poor nutrient-induced insulin release from purified B cells is attributed to their low cAMP levels and is markedly increased after addition of (Bu)2cAMP, of glucagon, or of pancreatic A cells. These results strongly support the concept that the potent in vivo insulin-releasing action of glucose and leucine is not only dependent on their fuel capacity in pancreatic B cells but also on the concurrent cAMP levels in these cells. In isolated islets, endogenously released glucagon apparently determines the cAMP production in B cells and thus participates in the nutrient-induced secretory process. Somatostatin and epinephrine were shown to exert their suppressive effects via the glucagon-dependent messenger system. It is concluded that nutrients and hormones interact with two different messenger systems which amplify each others' stimulatory effect upon insulin release. cAMP might represent the hormone-induced messenger which sets the B cell's sensitivity and secretory capacity for nutrient stimuli such as glucose. The higher insulin secretory response observed after reaggregation of single B cells could not be attributed to an altered activity in the nutrient or hormonal regulatory units, raising the possibility that the aggregated state of the cells is rather responsible for a better organization or cooperation of the secretory effector unit.

Amino Acids↗

PubMed1985

Regulation of adenosine 3',5'-monophosphate levels in the pancreatic B cell.

The poor glucose-induced insulin release from single purified B cells has been attributed, in part, to the low cellular cAMP levels. The present study demonstrates that isolated B cells exhibit a markedly lower cAMP formation than B cells lodged in intact islets and suggest that this deficiency is caused by their separation from glucagon-containing A cells. cAMP levels in purified B cells are rapidly and potently elevated by glucagon from 10(-10) M on, reaching the values of islet B cells at 10(-9) M. In contrast, exogenous glucagon stimulates cAMP formation in isolated islets only from 3.10(-9) M on, suggesting that endogenously released glucagon is mainly responsible for the higher cAMP levels in islet B cells. Somatostatin counteracts glucagon-induced cAMP production in purified B cells and, therefore, has also the potential to mediate an intra-islet regulation of B cell functions. Neither insulin nor pancreatic polypeptide affect cAMP formation in pancreatic B cells. Glucose alone does not influence cAMP levels in purified B cells, but enhances glucagon-induced cAMP formation in these cells. The glucose-dependent increase in islet cAMP is therefore not considered as the nutrient-induced mediator for hormone release but as a minor amplification of the glucagon-dependent signal. Experiments on reaggregated islet cells permit the reconstruction of the events which regulate cAMP levels in isolated islets. Further support is hereby given to the hypothesis that a normal glucose-induced insulin release from intact islets requires the simultaneous synarchic participation of a nutrient-dependent and a hormone-dependent messenger system.

1-Methyl-3-isobutylxanthine↗