EFFECT OF PARATHYROID-HORMONE (HPTH[1-34]) INFUSION ON SERUM 1,25-DIHYDROXYVITAMIN-D AND PARATHYROID-HORMONE IN NORMAL WOMEN

Calcium absorption declines with age, Because 1,25-dihydroxyvitamin D (1,25(OH)(2)D) is the major hormone controlling calcium absorption, ch anges in vitamin D metabolism may account for the malabsorption of agi ng, Serum levels of 1,25(OH)(2)D have been reported to either decrease or remain unchanged with age, To assess the effect of aging on renal production of 1,25(OH)(2)D, we evaluated the response of renal 250HD 1 alpha hydroxylase to human parathyroid hormone (hPTH((1-34) stimulati on in 119 women ages 25-83 years, In this population, baseline serum 2 5OHD and 1,25(OH)(2)D values did not significantly change with age, bu t serum iPTH (r = 0.44; P < 0.001) and serum creatinine (r = 0.31; p < 0.01) increased with age, However, the stimulatory activity of hPTH(1 -34) on the renal production of 1.25(OH)(2)D declined with age (r(3) = -0.36; p < 0.001) and was most apparent after age 75, being 50% less than that of younger women. Besides age, the production of 1,25(OH)(2) D was found to be dependent on baseline serum iPTh (r = -0.31; p < 0.0 001). Administration of hPTH(1-34) led to suppression of endogenous PT H, and suppressibility of endogenous PTH declined with age (r = 0.53; p < 0.0001), The increase in serum PTH and decreased suppressibility o f PTR with age could be due to mild secondary hyperparathyroidism. The increase in PTR with age is probably responsible for maintaining norm al serum 1,25(OH)(2)D levels in elderly subjects; however, decreased m etabolism of 1,25(OH)(2)D in the elderly could also maintain normal se rum 1,25(OH)(2)D levels.