Evidence that factors other than 1,25 dihydroxyvitamin D may play a role in augmenting intestinal calcium absorption in patients with primary hyperparathyroidism

Could there be a role of serum zonulin increase in the development of hypercalcemia in primary hyperparathyroidism

PURPOSE

To evaluate the serum level of zonulin, which is an intestinal permeability (IP) biomarker, in primary hyperparathyroidism (PHPT) and to investigate the relationship between zonulin, calcium, and parathormone (PTH) levels.

METHODS

The study included 34 healthy control (HC) and 39 patients with PHPT. Serum calcium, phosphorus, magnesium, creatinine, albumin, and 24 h urine calcium levels were measured in all groups. Serum levels of zonulin were measured quantitatively by enzyme-linked immunosorbent assay (ELISA). Urinary ultrasonography (to assess the presence of nephrolithiasis) and dual energy X-ray absorptiometry (to assess the presence of osteoporosis) were used to evaluate complications related to PTHP.

RESULTS

Serum zonulin levels were significantly higher in the PHPT group than the HC group (p < 0.001). Zonulin levels were significantly positively correlated with plasma PTH and serum calcium levels (r = 0.600, p < 0.001 and r = 0.610, p < 0.001; respectively). There was no correlation between serum zonulin levels and adenoma volume.

CONCLUSION

Serum zonulin level increases in patients with PHPT. Serum zonulin levels show a moderate/strong positive correlation with serum calcium and plasma PTH levels. This suggests that IP increase may play a role in the development of hypercalcemia in patients with PHPT.

Duodenal active calcium transport in female rats increases with serum calcitriol concentrations, but reaches a plateau far below maximal calcitriol levels

To determine the relationship between serum calcitriol concentration and duodenal active calcium (Ca) transport, a wide range of circulating calcitriol concentrations (18-950 pg/ml) was obtained by feeding nonmated, lactating, and weaned rats vitamin D-sufficient diets containing 0.04, 0.06, 0.1, or 0.4% Ca. Ca transport was measured in vitro with the everted gut sac technique using both the proximal (D-1) and distal (D-2) duodenal halves. The ratio of serosal [Ca]/mucosal [Ca] (S/M) as well as the amount of Ca transported was calculated. The S/M ratio correlated with the serum calcitriol concentration over the range 18-90 pg/ml with slopes for the regression lines of 0.066 +/- 0.010 (R2 = 0.64, n = 27) for D-1 and of 0.036 +/- 0.005 (R2 = 0.73, n = 24) for D-2. The regression lines are significantly different from zero (p < 0.001) and from each other (p < 0.01). For D-1, a plateau of the S/M ratio of 7-9 appeared to be reached at a calcitriol concentration of approximately 90 pg/ml, and the plateau was maintained over the range 90-900 pg/ml calcitriol. For D-2, a plateau of the S/M ratio of 4-6 appeared to be reached at 200-300 pg/ml calcitriol. Calculating the amount of Ca transported per 10 cm per hour revealed a pattern similar to that of the S/M ratio. When net Ca absorption was determined from balance studies over 4 days for rats on a 0.04% Ca diet, maximal absorption (mg/day) was already observed at a serum calcitriol concentration of 60-70 pg/ml (n = 14). We conclude that active Ca transport correlates with serum calcitriol concentrations, but that the transport capacity quickly reaches a maximal value, which is maintained over a 10-fold higher range of serum calcitriol concentrations.

Verapamil induces PTH resistance but increases duodenal calcium absorption in rats

In vitro, verapamil inhibits duodenal Ca absorption, parathyroid hormone (PTH) secretion, and PTH-stimulated bone resorption. This study was designed to determine if any effects of chronic oral verapamil treatment on PTH secretion-action are reflected by changes in vitamin D metabolism, duodenal Ca absorption, and bone Ca content. Rats (100 g) received verapamil in the drinking water at doses of 4, 20, or 100 mumol.kg-1.day-1 for 2 wk. Verapamil administration did not significantly affect growth, plasma Ca or phosphate, or bone Ca content. However, verapamil treatment was associated with a dose-dependent 90% increase in plasma PTH levels. The elevated PTH was accompanied by a 22% decrease in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels, such that there was a significant negative correlation (r = -0.52; P less than 0.01) between PTH and 1,25(OH)2D3 levels. Despite the decreased plasma 1,25(OH)2D3 levels, verapamil treatment was associated with a dose-dependent increase in duodenal Ca absorption. The increased Ca absorption did not seem to be caused by a verapamil-induced increased intestinal sensitivity to 1,25(OH)2D3, since verapamil-treated vitamin D-deficient rats showed the same absorptive response to administered 1,25(OH)2D3 as untreated rats. Thus chronic oral verapamil treatment induces an apparent PTH resistance but does not appear to have major effects on overall Ca homeostasis in young male rats.