Intestinal phosphate absorption: influence of vitamin D and non-vitamin D factors.
The transport of inorganic phosphate (P) was measured in the absence of electrochemical gradients across rat jejunum in vitro. Active P absorption was demonstrated in young, vitamin D-deficient (-D) rats, whereas active P secretion was found in normal, non-vitamin D-deficient adult rats, suggesting regulation of intestinal P transport by age-dependent but vitamin D-independent mechanisms. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] stimulated mucosal-to-serosal P flux (Jm----s) without affecting serosal-to-mucosal P flux (Js----m), causing further increases in net P absorption (Jnet) in -D rats and reduction in net P secretion in normal adult rats, confirming a previously described role of this hormone on P absorption. We then examined the effect of increasing extracellular (buffer) P concentration [P] on this 1,25(OH)2D3-stimulated active P absorption in jejunum from -D rats. At [P] of 0.024, 0.24, and 2.4 mM, 1,25(OH)2D3 consistently stimulated Jm----s without affecting Js----m, causing an increment in Jnet. At 7.5 mM [P], however, this Jm----s stimulatory effect of 1,25(OH)2D3 was no longer observed. Moreover, at this [P] the Js----m exceeded Jm----s in both -D and 1,25(OH)2D3-repleted rats, converting the active P absorption into active P secretion in both treatment groups. These observations suggest the participation of local mechanisms, such as the ambient [P] of the transporting enterocytes, in the regulation of intestinal P absorption. Finally, the influence of dietary P deprivation on P absorption was studied in -D rats, with or without either vitamin D3 or 1,25(OH)2D3 repletion.(ABSTRACT TRUNCATED AT 250 WORDS)
Lee
D B
DB
Walling
M W
MW
Brautbar
N
N
D000818:Animals; D001693:Biological Transport, Active; D002117:Calcitriol / Q000494:pharmacology; D007408:Intestinal Absorption; D007473:Ion Channels / Q000378:metabolism; D007583:Jejunum / Q000378:metabolism; D007700:Kinetics; D008297:Male; D010710:Phosphates / Q000378:metabolism*; D051381:Rats; D014807:Vitamin D / Q000494:pharmacology*; D014808:Vitamin D Deficiency / Q000378:metabolism
D007473:Ion Channels; D010710:Phosphates; D014807:Vitamin D; D002117:Calcitriol