Effect of age on renal responsiveness to parathyroid hormone and calcitonin in rats

The purpose of these studies was to determine whether the responsiveness of the kidney to parathyroid hormone (PTH) and calcitonin changed with age. Experiments were performed in young (3 months old), adult (12-14 months old) and old (22-24 months old) male Fischer 344 rats fed normal diets and thyroparathyroidectomized. Parathyroid hormone was administered i.p. at 24, 12 and 2 h before death and calcitonin was given i.p. at 12 and 2 h before death. Parathyroid hormone significantly increased the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) by renal slices from young but not adult or old animals. A similar age-related decline in the capacity of PTH to raise serum 1,25-dihydroxyvitamin D (1,25-(OH)2D) levels was also seen. Parathyroid hormone significantly decreased tubular reabsorption of phosphorus, increased concentrations of urinary cyclic AMP (cAMP) and increased serum concentrations of calcium in all age groups. In contrast, calcitonin significantly increased 1,25-(OH)2D3 production by renal slices from both young and adult animals. Calcitonin decreased serum concentrations of calcium in young but not in adult rats. These results suggest that there are maturational changes in the PTH- and cAMP-dependent pathways in the kidney but not in the calcitonin- and cAMP-independent pathways. The changes in the PTH- and cAMP-dependent pathways affect the stimulation of 1,25-(OH)2D production but not the inhibition of phosphate transport.