Low extracellular calcium enhances beta cell sensitivity to the stimulatory influence of 1,25-dihydroxyvitamin D3 on insulin release by islets from vitamin D3-deficient rats

The beneficial effect of 1,25-dihydroxyvitamin D3 [1,25 (OH)2 D3] on insulin secretion from beta cells in hypocalcemic vitamin D3-deficient rats is now well established. Moreover, few data concerning the mechanism of 1,25 (OH) 2D3 efficiency as a function of the severity of hypocalcemia. In the present experiment, we submitted islets from vitamin D3-deficient rats to in vitro exposure to a range of decreasing extracellular Ca2+ concentrations ([Ca2+]ex), from 0.5 mM to 0.6 mM, during a 6-h 10-8 M 1,25 (OH) 2D3 induction. Thereafter, we compared the effect of this pretreatment on the islets' insulin response to a given stimulus. Various stimuli were used, and we measured in parallel the variations of 86Rb+ and 45Ca2+ efflux and insulin release into the perifusion medium. In the presence of 1,25 (OH) 2D3, we observed an inverse correlation between the [Ca2+]ex pre-exposure and the amplitude of the insulin response to certain stimuli studied, suggesting that beta cells that were pre-exposed to low [Ca2+]ex became more sensitive to the beneficial effect of 1,25 (OH) 2D3 on insulin release. This effect was observed when beta cells were activated by acetylcholine but only during its second phase of stimulation, and more particularly with the barium plus theophylline stimulus. In contrast, insulin release was not affected by [Ca2+]ex pre-exposure during 1,25 (OH) 2D3 induction in response to acetylcholine during its first phase of stimulation, thus excluding any mechanism mediated via nutrient pathways, membrane depolarization, or inositol triphosphate (IP3)-dependent events. Moreover, the islets that were pre-exposed to a 10-fold [Ca2+]ex exhibited only a 50% lower 45Ca2+ content after 45Ca2+ loading, suggesting a different or relatively more efficient storage capacity in the presence of low extracellular calcium. Studies of 45Ca2+ efflux showed that the mobilization of Ca2+ stores induced by a barium plus theophylline stimulus, in the absence of calcium in the perifusion medium, was more efficient in islets pre-exposed to low [Ca2+]ex, whereas the acetylcholine-IP 3-induced mobilization of Ca2+ from reticular stores was not affected. These results generated the hypothesis that 1,25 (OH)2D3 may prepare the beta cells during their pre-exposure to low [Ca2+]ex to become more efficient as concerns insulin release via a more efficient mobilization of 45Ca2+ stores (mitochondrial?) and by an activation of release potentiating systems via protein kinase C protein kinase A pathways.

Regulatory effect of 1,25-dihydroxyvitamin D3 on insulin release and calcium handling via the phospholipid pathway in islets from vitamin D-deficient rats

The effect of 10(-8) M 1,25-dihydroxyvitamin D3 [1,25 (OH)2D3] on the phosphoinositide pathway, was studied on [3H] inositol and 45Ca2+ efflux and on insulin release of islets from vitamin D-deficient rats, during an acetylcholine (Ach) stimulus in perifusion. The insulin release, which was low in vitamin D-deficient rats, was enhanced by this treatment. The 3H flux, reflecting phosphoinositide breakdown, was also increased. The 45Ca2+ flux was stimulated both during the first 14 min peak (mobilization of IP3-sensitive reticular Ca2+ stores) and during the following sustained small elevation of 45Ca2+ flux, reflecting protein kinase C (PKC) activation and consequently increased phosphorylation of Ca2+ channel proteins. These effects were larger during perifusions performed in the presence of glucose which is known to open Ca2+ channels, suggesting a synergistic influence of glucose and 1,25(OH)2D3. This positive influence of 1,25(OH)2D3 in Ca2+ entry by Ca2+ channels was confirmed by the use of nifedipine-a Ca2+ channel blocker-which suppressed the 45Ca2+ flux and lowered insulin secretion. Moreover, the sustained 45Ca2+ flux also disappeared in islets from vitamin D-deficient rats supplemented by 1,25(OH)2D3 but perifused without extracellular Ca2+ supporting the hypothesis of 1,25(OH)2D3-induced activation of PKC. Thus, 1,25(OH)2D3 may provide supplementary calcium to the B cell by regulating the intracellular signalling processes involving phospholipid metabolism, PKC induction, Ca2+ mobilization and Ca2+ entry by Ca2+ channels.

Stimulatory effect of 1,25-dihydroxyvitamin D3 on calcium handling and insulin secretion by islets from vitamin D3-deficient rats

Among the various vitamin D3 metabolites, 1,25-(OH)2D3 is the specific secosteroid hormone that can enhance, in vitro, the weak insulin response to glucose of islets from vitamin D3-deficient rats. Because this potentiating effect is preceded by an increase in Ca2+ handling, several putative sites of action were studied by measuring 45Ca2+ and 86Rb+ (as K+ tracer) efflux during perifusions in the presence of various stimuli known to affect Ca2+ movements in different ways: high glucose without calcium, high calcium without glucose, high potassium, or barium-theophylline without calcium or glucose. The present results show that 1,25-(OH)2D3 may activate Ca2+ handling by at least two mechanisms: (1) an increase of Ca2+ entry via voltage-dependent Ca2+ channels in the experiments in which extracellular Ca2+ was present and where Ca2+ channels were opened; this 1,25-(OH)2D3 influence on Ca2+ channels was not mediated by a possible indirect influence on K+ channels because 86Rb+ fluxes were never observed to be affected by the steroid; (2) an enhancement of 45Ca2+ mobilization from intracellular stores as suggested by barium-theophylline stimulation and probably also via the Ca2+ stimulus. Both of these 1,25-(OH)2D3 influences tended to provide more calcium to the B cell of vitamin D3-deficient rats. But this prerequisite was not sufficient in itself to potentiate the insulin response; indeed, experiments with barium-theophylline suggested that 1,25-(OH)2D3 may also activate a cAMP-dependent exocytosis process.

Effect of 1,25 dihydroxyvitamin D3 on isolated islets from vitamin D3-deprived rats

Insulin release is impaired by vitamin D3 deficiency but can be restored by in vivo administration of 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]. A direct influence of 1,25(OH)2D3 on the B-cell was studied in vitro with islets from 5-wk vitamin D3-deprived rats. This hormone (10(-12) to 10(-6) mol/l) added to the incubation medium had a stimulatory dose-dependent effect on insulin response to 8.3 mmol/l glucose 6 h later. Moreover, perifusion experiments performed after different times of incubation demonstrated that after 6 h 1,25(OH)2D3 increased in particular the first phase of insulin response to 16.7 mmol/l glucose. The 45Ca fluxes, followed in parallel, were never modified by 1,25(OH)2D3 in the absence of glucose but were enhanced during the glucose stimulus, whereas 86Rb fluxes were never affected by 1,25(OH)2D3. These results demonstrated that 1,25(OH)2D3 acts in vitro on B-cells, but with a 6-h delay to potentiate the glucose-induced insulin release, concomitant with intracellular calcium redistribution.