Lack of influence of vitamin D deficiency on insulin release from the isolated pancreatic islets of rats

The effects of vitamin d on insulin release from isolated islets of rats

BACKGROUND

Vitamin D (vit D) affects glucose metabolism. Receptors of vitamin D have been identified in β cells and studies show that vitamin D deficiency reduces glucose-stimulated insulin secretion (GSIS).

OBJECTIVES

The aim of this study was to examine the effect of vitamin D on insulin release from isolated islets of rats.

MATERIALS AND METHODS

Islets were isolated from male Wistar rats, weighing 200-250 grams, using the collagenase digestion method. Insulin release was assessed following 24 and 48 hours coincubation of islets with vitamin D (0.1, 1 and 10 nM) and glucose (5.6, 11.1 and 16.7 mM). In addition, islets were preincubated with vitamin D for 24 and 48 hours and GSIS was assessed for one hour in the presence of 5.6 and 16.7 mM glucose.

RESULTS

Coincubation of islets with vitamin D (10 nM) and 11.1 mM glucose increased islet insulin release (37.27 ± 3.75 vs. 24.64 ± 2.83 ng/islet/24 hours; P < 0.05), while vitamin D (1 and 10 nM) decreased insulin release in the presence of 16.7 mM glucose (21.14 ± 3.58 and 18.65 ± 3.84 vs. 37.71 ± 4.63 ng/ islet/24 hours; P < 0.05). Islets preincubation with vitamin D (1 and 10 nM) increased GSIS in the presence of 16.7 mM glucose (4.39 ± 0.73 and 4.39 ± 0.63 vs. 2.07 ± 0.43 ng/islet/1 hour; P < 0.05).

CONCLUSIONS

Preincubation of islets with vitamin D increased GSIS but decreased insulin release in coincubation with high levels of glucose. Insulin secretion from β cells in the presence of glucose seems to be related to the dosage of vitamin D and duration of preincubation.

Hypocalcemia reduces rate of disappearance of glucose from plasma

The influence of hypocalcemia on glucose disposal from plasma was studied in 4.5 to 24 kg weaned, fasted, calcitriol deficient piglets, which suffered from type I inherited pseudo vitamin D deficiency rickets. The plasma glucose concentration of the piglets was clamped to 40% above its preinfusional level by a continuous intravenous infusion of glucose. This suppressed hepatic glucose production. Since the clamped plasma glucose concentration was below its renal threshold, the rate of steady state glucose infusion served as an estimate for glucose disposal from plasma. It was found that hypocalcemia, generated either by calcitriol deficiency or by continuous EDTA infusions, reduced glucose disposal from plasma by 56-76% compared to normocalcemic control piglets. It was suggested that the effect was not compounded by plasma phosphate, calcitriol or insulin. It was further suggested that hypocalcemia depressed the rate of glucose utilization.

The in vitro effect of 1 alpha,25-dihydroxyvitamin D3 on insulin production by neonatal rat islets

The in vitro effect of 1 alpha,25-dihydroxyvitamin D3 on the function of beta cells of the endocrine pancreas was investigated. Neonatal islets maintained in serum-free medium, or medium supplemented with 0.5% fetal bovine serum achieved a 2.5-fold increase in medium insulin levels in response to 10(8) M 1 alpha,25-dihydroxyvitamin D3 (P less than 0.001). The effect of 1,25-dihydroxyvitamin D3 required at least 96 h treatment to become evident and was similar at medium glucose concentrations of 10 and 20 mM. Cell-associated insulin was increased in 1 alpha,25-dihydroxyvitamin D3-treated cultures maintained in 0.5% serum. These data suggest that 1 alpha,25-dihydroxyvitamin D3 may have a direct effect in the beta cell.

Vitamin D and pancreatic islet function. I. Time course for changes in insulin secretion and content during vitamin D deprivation and repletion

After weaning, rats were given free access to a control or vitamin D-deprived diet for 2 to 5 weeks. In the vitamin D deficient rats, plasma concentrations of 25-(OH)D3, 1,25-(OH)2D3,24,25-(OH)2D3 calcium, glucose and insulin were all decreased. After an overnight fast, the plasma insulin concentration was also decreased even when the plasma glucose concentration was not significantly affected. The food intake and body growth was also impaired in vitamin D-deficient rats. Administration of vitamin D3 in oil for 3 to 6 days to vitamin D-deficient rats increased the plasma concentration of vitamin D metabolites, calcium and insulin, but not that of glucose, and stimulated food intake and body growth to a larger extent than in rats treated by oil alone. Vitamin D deprivation decreased and vitamin D treatment increased the insulin content of the whole pancreas or isolated islets and the secretory response of the islets to D-glucose. The changes in insulin release remained significant when the hormonal output was expressed relative to the insulin content of the islets. These findings confirm that vitamin D deficiency causes alterations of pancreatic B-cell function. Moreover, the time course for changes in biological variables during vitamin D deprivation and treatment suggests that such an alteration cannot be solely accounted for by concomitant abnormalities in either plasma calcium or glucose concentrations.