Influence of metabolic acidosis on serum 1,25(OH)2D3 levels in chronic renal failure.

Metabolic acidosis has been shown to alter vitamin D metabolism. There is also evidence that calcium may modulate 1,25(OH)2D3 by a parathyroid hormone (PTH)-independent mechanism. To investigate the effect of rapid correction of chronic metabolic acidosis on serum 1,25(OH)2D3 levels by free calcium clamp in chronic renal failure, 20 patients with mild to moderate metabolic acidosis (mean pH 7.31 +/- 0.04) and secondary hyperparathyroidism (mean intact PTH 156.47 +/- 84.20 ng/l) were enrolled in this study. None had yet received any dialysis therapy. Metabolic acidosis was corrected by continuous bicarbonate infusion for 3-4 h until plasma pH was around 7.4, while plasma ionized calcium was held at the preinfusion level by calcium solution infusion during the entire procedure. The plasma pH, bicarbonate, total CO2, sodium, and serum total calcium levels were significantly increased while serum concentrations of alkaline phosphatase and albumin were significantly decreased after bicarbonate infusion. The plasma ionized calcium, potassium, serum magnesium, inorganic phosphorus, and 25(OH)D levels showed no significant change before and after bicarbonate infusion. The serum 1,25(OH)2D3 levels were significantly increased (38.66 +/- 11.77 vs. 47.04 +/- 16.56 pmol/l, p < 0.05) after correction of metabolic acidosis. These results demonstrate that rapid correction of metabolic acidosis raises serum 1,25(OH)2D3 levels in vitamin D-deficient chronic renal failure patients, and may underline the importance of maintaining normal acid-base homeostasis in the presence of secondary hyperparathyroidism in chronic renal failure.