Transmural Absorption of Pyridoxine HCI in Vitro in the Rat Jejunum 1

Abstract

Abstract

Transmural absorption of [³H]pyridoxine . HCl ([³HIPN) was studied in Sprague-Dawley rats in vitro utilizing everted sacs and perfused (serosal lumen) everted segments of jejunum. The predominant form of vitamin B₆ to appear in the serosal fluid was [³HIPN. Phosphorylated vitamin ([³H]B₆PO₄) was limited primarily to the jejunal wall (44.6 ± 3.4% of total tissue ³H) with only minimal amounts detectable in the serosal fluid (7.0 ± 1.5% of total serosal ³H). The appearance of absorbed vitamin in the serosal perfusate, expressed on a per micromolar [³H]PN basis, was significantly increased when the mucosal PN concentration was increased to 1 mM. Likewise, serosal appearance of vitamin was increased for 2 μM [³H]PN when 1 mM 4-deoxypyridoxine was added to the mucosal solution. When a high phosphate buffer (80 mM) was substituted for the Krebs' bicarbonate buffer (1.1 mM phosphate) in the serosal perfusing solution, tissue content of [³H]B₆PO₄ was increased a significant 49.7%. In contrast, substitution of phosphate buffer in the mucosal solution had no significant effect on tissue [³H]B₆PO₄. Conclusions are (i) PN is the principal form of absorbed vitamin B₆ to appear on the serosal side of everted rat jejunum in vitro; (ii) intracellular metabolism leads to a compartmentalization of [³H]PN after its uptake by mucosa; (iii) this metabolism is associated with a significant decrease in the serosal appearance of absorbed vitamin; and (iv) in vitro increases in tissue [³H]B₆PO₄ by phosphate inhibition of phosphatase hydrolysis are dependent upon the high phosphate concentrations being present on the serosal, not the mucosal, side of the jejunum.