Acute interactions between intestinal sugar and calcium transport in vitro

Fructose consumption by Americans has increased markedly, whereas Ca(2+) intake has decreased below recommended levels. Because fructose metabolism decreases enterocyte ATP concentrations, we tested the hypothesis that luminal fructose acutely reduces active, diet-inducible Ca(2+) transport in the small intestine. We confirmed that the decrease in ATP concentrations was indeed greater in fructose- compared with glucose-incubated mucosal homogenates from wild-type and was prevented in fructose-incubated homogenates from ketohexokinase (KHK)(-/-) mice. We then induced active Ca(2+) transport by chronically feeding wild-type, fructose transporter glucose transporter 5 (GLUT5)(-/-), as well as KHK(-/-) mice a low Ca(2+) diet and measured transepithelial Ca(2+) transport in everted duodenal sacs incubated in solutions containing glucose, fructose, or their nonmetabolizable analogs. The diet-induced increase in active Ca(2+) transport was proportional to dramatic increases in expression of the Ca(2+)-selective channel transient receptor potential vanilloid family calcium channel 6 as well as of the Ca(2+)-binding protein 9k (CaBP9k) but not that of the voltage-dependent L-type channel Ca(v)1.3. Crypt-villus distribution of CaBP9k seems heterogeneous, but low Ca(2+) diets induce expression in more cells. In contrast, KHK distribution is homogeneous, suggesting that fructose metabolism can occur in all enterocytes. Diet-induced Ca(2+) transport was not enhanced by addition of the enterocyte fuel glutamine and was always greater in sacs of wild-type, GLUT5(-/-), and KHK(-/-) mice incubated with fructose or nonmetabolizable sugars than those incubated with glucose. Thus duodenal Ca(2+) transport is not affected by fructose and enterocyte ATP concentrations but instead may decrease with glucose metabolism, as Ca(2+) transport remains high with 3-O-methylglucose that is also transported by sodium-glucose cotransporter 1 but cannot be metabolized.