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 Vmax of 44.0 nmol/cm2 per h and a Km 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.
D. B. N. Lee, M. W. Walling, U. Gafter, V. Silis, J. W. Coburn