Fig. 3.

Parameters of calcium metabolism. Graphs show serum calcium, 25-hydroxy- vitamin D (25OHD), alkaline phosphatase (bone turnover marker) and osteocalcin (bone formation marker) values in normonatremic (open bars) and hyponatremic (black bars) aged F344BN rats at the end of Phase I and Phase II of the study. Results indicate that chronic hyponatremia reduced serum calcium and 25OHD levels, and that a 10-fold increased vitamin D supplementation during Phase II was needed to normalize calcium and 25OHD levels. Serum alkaline phosphatase concentrations were increased regardless of vitamin D status in the hyponatremic rats compared to the normonatremic rats, consistent with ersistently increased bone resorption to liberate sodium from the bone matrix. Reduced osteocalcin in Phase I indicated decreased bone formation, which returned to the levels of the normonatremic rats following vitamin D supplementation in Phase II. Asterisks indicate statistically significant differences from normonatremic controls (p < 0.05). All data are means ± SEM