Abstract
The periodic oral administration of high doses of 1, 25-dihydroxyvitamin D (1, 25-(OH)2D pulse therapy) has been reported to be effective in reducing the serum levels of parathyroid hormone (PTH) in severe cases of secondary hyperparathyroidism (Nephron 51: 130-131, 1989). To clarify the mechanisms of this therapy, the effects of short-term oral 1, 25-(OH)2D on the response of PTH to blood calcium level were evaluated in 6 hemodialysis patients with secondary hyperparathyroidism. Blood ionized calcium and serum intact PTH were measured during high calcium dialysis (Ca=5.5mEq/l) and low calcium dialysis (Ca=2.5mEq/l) before and after two 4μg doses of oral 1, 25-(OH)2D.
The basal PTH fell from 333.5±185.2pg/ml to 181.2±95.1pg/ml after 1, 25-(OH)2D without a change of blood ionized calcium. The linear regression line between the blood ionized calcium and the serum PTH (Y=AX+B, Y; PTH, X; free calcium) shifted downward after 1, 25-(OH)2D. The maximum PTH induced by conditions of low calcium fell from 394.7±132.1pg/ml to 223.3±154.0pg/ml, while the minimum PTH induced by conditions of high calcium did not change significantly.
We conclude that short term oral 1, 25-(OH)2D pulse therapy suppresses PTH secretion without changing blood calcium levels and that it changes the response of PTH to blood calcium. These results suggest that the suppressive effect of 1, 25-(OH)2D on PTH is partly attributable to the change of sensitivity of the parathyroid gland to blood calcium.
