An ATPase dependent, radiosensitive acidic microclimate essential for intestinal folate absorption

Methyltetrahydrofolic acid transport was studied across everted sacs of rat jejunal segments from control and whole-body X-irradiated (700 rad) rats at 10-5 M concentrations (at which optimum transport occurs) at various pHs. The folate transport from mucosal to serosal compartment was inhibited by about 55% in irradiated rats at the pH of the intestinal chyme (6.5). Extraneous ATP in the incubation system could restore the defective transport of the irradiated intestine. The maximum folate transport which occurred at pH 4.0 was not adversely affected by whole body irradiation. An acidic, pH dependent, passive uptake of 5-methyltetrahydrofolic acid was observed. The normal absorption barrier of the small bowel was not disrupted by the acidification process as practically no uptake was observed with irradiated segments pretreated at pH 4.0 except in the presence of ATP. Leucine and serine transport at a zero concentration gradient indicated active transport mechanisms which were not affected by acidification. Their uptake was additively increased in the presence of glucose and ATP, further indicating that the normal physiology of the intestines was not affected by the acidification process. An intestinal mucosal cell surface ATPase was observed which was Mg2+ dependent. It could hydrolyse solution phase ATP and thus generate the protons necessary for the acidification of a microenvironment where passive uptake of the neutral folate species could occur. The ATPase activity was inhibited about 90% by 50 mM-Na azide at pH 6.5. Below this concentration folate transport was also inhibited. Na azide did not inhibit folate transport at pH 4.0, suggesting that its inhibition of folate uptake at pH 6.5 is related to its inhibitory effect on ATPase, rather than on folate transport per se. ATPase activity was therefore essential for folate transport at the pH of the intestinal chyme. (author)