AJP - Renal Physiology, Vol 254, Issue 1 45-F50, Copyright © 1988 by American Physiological Society
Calcifediol antagonizes PTH action on water and phosphate absorption in rabbit pars recta
R. A. Peraino, D. Rouse and W. N. Suki
Department of Medicine, Methodist Hospital, Houston, Texas 77030.
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 (Jv) or lumen-to-bath phosphate flux (JPil----b). However,
pharmacological concentrations did inhibit Jv by 20.9 +/- 5.6% (100 nM, P
less than 0.02) and 37.9 +/- 9.4% (1.0 microM, P less than 0.01) and
JPil----b by 18.5 +/- 6.6% (100 nM, P less than 0.05) and 40.5 +/- 8.6%
(1.0 microM, P less than 0.01). In the presence of 30 nM, 100 nM, or 1.0
microM calcifediol, neither 0.25 nor 1.0 U/ml PTH inhibited JPil----b. A
modest decline in Jv 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 Jv and JPil----b. To investigate the mechanism of
action of high calcifediol concentration, measurement of the collected
minus perfused tubule fluid chloride concentration difference was
performed.(ABSTRACT TRUNCATED AT 250 WORDS)
