Calcium and Inorganic Phosphate Transport in Rat Colon

Abstract

In the small intestine, 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] stimulates both calcium (Ca) and inorganic phosphate (Pi) absorption. This is mediated through an increase in mucosal-to-serosal flux (Jms) whereas the serosal-to-mucosal flux (Jsm) remains unchanged. We now report that in rat proximal colon, 1,25(OH)₂D₃ produces active Ca absorption without affecting Pi transport, and that this induced active Ca absorption is associated with alterations in kinetics of both Jms and Jsm so that both processes demonstrate saturable components. Vitamin D-deficient rats were given daily injections of solvent (−D) or 270 ng 1,25(OH)₂D₃ (+D) for 3 d. ⁴⁵Ca and [³²P]phosphate fluxes were measured employing the Ussing technique using a modified Krebs-Ringer-HCO₃ buffer ([Ca] 1.25, [Pi] 1.18, [glucose] 11 mM). In −D rats there was no net flux (Jnet) of either Ca or Pi. In +D rats net active Ca absorption was observed (−D = 3.3 nmol/cm² per h ±3.4 (SEM); +D = 27.3 ±3.8, n = 11, P < 0.001) whereas Pi transport was unchanged, i.e., still no Jnet. Pi Jms was not different from Pi Jsm measured at the following buffer [Pi]: 0.0118, 0.118, 1.18, and 2.36 mM. Ca saturation kinetics were estimated using buffer [Ca] from 0.0125 to 5.0 mM. Saturable processes were demonstrated for both Jms and Jsm. Jnet for Ca across colon from +D rats exhibited saturation at [Ca] > 3 mM, with an estimated V_max of 44.0 nmol/cm² per h and a K_m of 0.9 mM. This colonic model may provide a useful system for studying 1,25(OH)₂D₃-induced molecular events related to Ca but not Pi transport. The apparent action of 1,25(OH)₂D₃ on Ca secretory process may furnish new insights into the mechanism of action of vitamin D.