Calcifediol antagonizes PTH action on water and phosphate absorption in rabbit pars recta

Reports of the effects of calcifediol (25-hydroxycholecalciferol) on phosphate excretion, alone or with parathyroid hormone (PTH), or with adenosine 3',5'-cyclic monophosphate (cAMP), have been conflicting. The purpose of this investigation was to examine the effects on fluid and phosphate transport by the rabbit pars recta of calcifediol alone or with PTH. In tubule segments obtained from rabbits fed a normal diet, the addition of 10 or 30 nM calcifediol to the bath did not alter fluid absorption (J(V)) or lumen-to-bath phosphate flux (J(l→b)(Pi)). However, pharmacological concentrations did inhibit J(v) by 20.9 ± 5.6% (100 nM, P < 0.02) and 37.9 ± 9.4% (1.0 μM, P < 0.01) and J(l→b)(Pi) by 18.5 ± 6.6% (100 nM, P < 0.05) and 40.5 ± 8.6% (1.0 μM, P < 0.01). In the presence of 30 nM, 100 nM, or 1.0 μM calcifediol, neither 0.25 nor 1.0 U/ml PTH inhibited J(l→b)(Pi). A modest decline in J(v) did occur with 1.0 U/ml PTH in the presence of 30 nM and 100 nM calcifediol. Unlike calcifediol, when 10 nM calcitriol (1,25-dihydroxycholecalciferol) was present in the bath, 0.2 U/ml PTH significantly depressed J(v) and J(l→b)(Pi). To investigate the mechanism of action of high calcifediol concentration, measurement of the collected minus perfused tubule fluid chloride concentration difference was performed. When 1.0 μM calcifediol was added, the chloride concentration difference fell from 14.2 ± 2.5 meq/liter, with vehicle alone, to 8.2 ± 2.3 meq/liter (P = 0.02). These results indicate that calcifediol, in superphysiological concentrations, directly inhibits fluid and phosphate absorption by the pars recta and, in more physiological concentrations, antagonizes the activity of PTH. We further suggest that calcifediol, in high concentrations, and PTH both act by inhibiting tubule bicarbonate absorption.