Effect of physical activity on calciotropic hormones and calcium balance in rats

The response of calciotropic hormones and bone turnover to exercise and immobilization was examined by the measurement of calcium balance, bone turnover indexes, levels of parathyroid hormone, nephrogenous adenosine 3',5'-cyclic monophosphate (cAMP), and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] weekly for 6 wk in three groups of rats: control, exercise trained, and immobilized. Early in the experiment, increases were observed in excretion of urinary calcium, hydroxyproline, and in serum alkaline phosphatase after both exercise and immobilization. It was not until the latter part of the experimental period that changes were observed in nephrogenous cAMP and intestinal absorption efficiency of calcium. In the fasting state, the exercise group had a drop in serum calcium and phosphate and a rise in nephrogenous cAMP and serum 1,25(OH)2D3 compared with the control group. The exercised animals experienced an increase in bone mass, whereas the immobilized animals had a decline in bone mass. Thus exercise stimulates bone growth, resulting in an increased demand for minerals that is satisfied by an increase in serum 1,25(OH)2D3 levels and increased intestinal absorption of calcium. The increase in calcium absorption suppresses parathyroid hormone production (nephrogenous cAMP) in the exercised animal. Immobilization resulted in increased bone resorption that suppressed parathyroid hormone, nephrogenous cAMP, and the intestinal absorption of calcium.