Acute interactions between intestinal sugar and calcium transport in vitro
AJP Gastrointestinal and Liver Physiology
Published online on October 31, 2013
Abstract
Fructose consumption by Americans has increased markedly whereas Ca2+ intake has decreased below recommended levels. Since fructose metabolism decreases enterocyte ATP concentrations, we tested the hypothesis that luminal fructose acutely reduces active, diet-inducible Ca2+ transport in the small intestine. We confirmed that the decrease in ATP concentrations was indeed greater in fructose- compared to glucose-incubated mucosal homogenates from wildtype, and was prevented in fructose-incubated homogenates from ketohexokinase (KHK)-/-, mice. We then induced active Ca2+ transport by chronically feeding wildtype, fructose transporter GLUT5-/-, as well as KHK-/- mice, a low Ca2+ diet, and measured transepithelial Ca2+ transport in everted duodenal sacs incubated in solutions containing glucose, fructose or their nonmetabolizable analogs. The diet-induced increase in active Ca2+ transport was proportional to dramatic increases in expression of the Ca2+-selective channel TRPV6 as well as of the Ca2+ binding protein CaBP9k, but not that of the voltage-dependent L-type channel Ca(v)1.3. Crypt-villus distribution of CaBP9k seems heterogeneous, but low Ca2+ diets induce expression in more cells. In contrast, KHK distribution is homogeneous, suggesting that fructose metabolism can occur in all enterocytes. Diet-induced Ca2+ transport was not enhanced by addition of the enterocyte fuel glutamine, and was always greater in sacs of wildtype, GLUT5-/- and KHK-/- mice incubated with fructose or nonmetabolizable sugars than those incubated with glucose. Thus, duodenal Ca2+ transport is not affected by fructose and enterocyte ATP concentrations but instead may decrease with glucose metabolism, as Ca2+ transport remains high with 3-O-methylglucose that is also transported by the glucose transporter SGLT1, but cannot be metabolized.