'Pulse' oral calcitriol in uraemic patients: rapid modification of parathyroid response to calcium.

We performed dynamic parathyroid function tests, with generation of parathyroid hormone/ionized calcium (PTH/iCa) response curves, obtained during sequential hypercalcaemic and hypocalcaemic dialysis, on six haemodialysis patients before and after 4-weekly 'pulse' doses of oral calcitriol. There were no significant changes in the pretreatment, 2-week and 4-week values of blood ionized calcium, serum total calcium, phosphate, and alkaline phosphatase. PTH decreased significantly after 4 weeks of treatment with calcitriol (P less than 0.03), and the post-treatment PTH/iCa curves were all displaced leftwards and downwards, indicating a marked inhibitory effect of calcitriol on parathyroid responsiveness across and beyond the physiological range of calcaemia. The mean shift of the PTH/iCa relationship was equivalent to a reduction of iCa of 0.2 mM for any given concentration of PTH, and conversely to a 70% reduction of PTH for any given blood iCa. The results suggest that wide spacing of oral 'pulse' doses of calcitriol can achieve favourable modification of the PTH/iCa relationship, and that dosing interval and dose size, rather than route of administration, may be the major determinants of the previously reported superiority of intravenous over daily oral calcitriol regimens.