AJP - Renal Physiology, Vol 251, Issue 5 911-F918, Copyright © 1986 by American Physiological Society
Ca and P regulation of 1,25(OH)2D3 synthesis by vitamin D-replete rat tubules during acidosis
C. B. Langman, D. A. Bushinsky, M. J. Favus and F. L. Coe
Elevated blood ionized Ca concentration appears to be necessary for
suppression by chronic metabolic acidosis (CMA) of serum
1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels in vitamin D-replete rats
eating a low-Ca diet (LCD). The present study asks whether in vitro
1,25(OH)2D3 production by proximal tubule cells from such rats is
suppressed and whether suppression can be reversed in vitro by an altered
ionic milieu. Young rats were fed LCD and given 1.5% NH4Cl in their
drinking water for 10 days to produce CMA. Compared with controls, CMA rats
had low serum 1,25(OH)2D3 levels. Tubules prepared from CMA rats produced
1,25(OH)2D3 at a low rate compared with control; variation of medium Ca and
pH from 7.2 to 7.4 did not increase 1,25(OH)2D3 production. Reducing medium
phosphorus concentration ([P]) increased 1,25(OH)2D3 production by tubules
from control but not CMA rats. Increasing medium [P] increased 1,25(OH)2D3
production by tubules from CMA but not control rats. CMA appears to alter
proximal tubule cell response to medium [P] so that 1,25(OH)2D3 production
rises when medium [P] is increased but not when it is reduced. Medium pH
and Ca concentration do not appear to be important regulators of renal
1,25(OH)2D3 production in short-term incubations.
