Effects of medium pH on duodenal and ileal calcium active transport in the rat
Favus, M.J.; Bushinsky, D.A.; Coe, F.L.
American Journal of Physiology 251(5 Pt 1): G695-G700
1986
ISSN/ISBN: 0002-9513 PMID: 3096149 Document Number: 282956
To study the effect of pH on Ca active transport in vitro pH was varied from 7.1 to 7.7, and bidirectional transmural Ca fluxes were measured under short-circuited conditions across duodenum or ileum from rats fed either normal chow (NCD, 1.2% Ca) or low Ca (LCD, 0.002% Ca). Duodenum and ileum from rats fed LCD actively absorbed calcium at medium pH 7.4. Reduction in mucosal and serosal medium bicarbonate from 25 to 10 mM (pH 7.4 to 7.1) decreased duodenal net Ca absorption (Jnet) from 121 .+-. 32 to 39 .+-. 9 nmol .cntdot. cm-2 .cntdot. h-1 (P < 0.02) and ileal Jnet from 74 .+-. 13 to 22 .+-. 6 (P < 0.01). The decline in duodenal and ileal Ca Jnet was due to a decrease in the mucosal-to-serosal flux .**GRAPHIC**. Raising medium pH from 7.4 to 7.7 by increasing bicarbonate from 25 to 50 mM did not alter Ca .**GRAPHIC**. serosal-to-mucosal flux .**GRAPHIC**. or Jnet. Ca fluxes across ileum from rats fed NCD demonstrated net secretion, and increasing medium pH from 7.4 to 7.7 reduced net secretion. LCD ileal .**GRAPHIC**. was unaltered when pH was reduced from 7.4 to 7.1 by increasing the PCO2 content of the buffer from 30 to 81 mmHg while maintaining bicarbonate at 24 mM. The results indicate that in virtro conditions that simulate metabolic acidosis (low bicarbonate and pH, normal PCO2) inhibit 1,25D-mediate calcium .**GRAPHIC**. whereas conditions that simulate respiratory acidosis have no effect on Ca fluxes. The present studies suggest that decreases in calcium .**GRAPHIC**. is by a primary alteration in transport of rather ions other than direct effect on Ca transport.