Effects of Chronic High Sucrose With Dietary or Drinking Inclusion on the Electrogenic Glucose Absorption in the Intestinal Tract of Mice (Mus musculus)

Abstract

The long-term impact of high sugar diets, a factor in the development of obesity and type-2 diabetes, on the intestinal electrogenic sodium dependent glucose transport, the first portal of entry for glucose, is unknown. Here female C57bl/6 mice fed a normal standard chow diet, and 20% sucrose in the drinking water for 8 months, or 35% sucrose inclusion in the diet for 12 months were assessed. The drinking water sucrose treated mice developed obesity, whereas the solid dietary sucrose treated mice did not. Jejunal, ileal and colonic segment differences for electrogenic sodium dependent glucose transport kinetics, mRNA expression of sodium dependent glucose transporters, inflammatory mediators, and insulin signaling genes were assessed in all groups, as novel differences between segments were found in normal mice. Ex vivo intestinal Ussing chamber studies in normal mice characterizing the electrogenic sodium dependent glucose transport followed Hill Equation sigmoidal kinetics demonstrating low affinity, high capacity transport (Vmax of 100.8 ± 24.2 uA/cm2, K0.5 17.6 ± 0.9 mM) in the jejunum, high affinity, high capacity transport in the ileum (Vmax of 111.4 ± 17.5 uA /cm2, K0.5 7.4 ± 1.0 mM) and the absence of transport in the colon. The preferential fit of the kinetics to the Hill Equation sigmoidal kinetics in each of the tissues, suggest mouse SGLT are working allosterically, or that there are multiple transporters working together to create the currents observed, more than in other mammals previously reported. Although segmental differences in inhibition by dapagliflozin, an SGLT2 inhibitor or phloridzin dihydrate, an SGLT1 inhibitor were evident, gene expression analysis of the SGLT 1- 6 could not fully explain these regional differences in kinetics. Non-the-less, the kinetics were highly modified by sucrose treatments, with a significant shift in the segmental transport, with a decrease in transport in the proximal segments of the intestine and increase distally in both groups. Most notable was, a significant increase in the Vmax and K0.5 in the ileum of drinking water sucrose treated mice and the appearance of colonic glucose induced Hill equation kinetic transport in the solid dietary sucrose treated mice. Interestingly, the novel currents induced in mice treated with both drinking water sucrose and solid dietary sucrose diet were insensitive to inhibition by dapagliflozin or phloridzin dihydrate. This indicates that neither SGLT1 or SGLT2 were responsible for the changes in transport induced by the treatments. Paradoxically, the mRNA expression of SGLT1 was significantly increased in the jejunum, ileum and colon of the drinking water sucrose treated mice and, SGLT2 was significantly increased in the jejunum and colon of the solid dietary sucrose treated mice. Additionally, none of the other SGLT family members known for glucose transport assessed by qRT-PCR could account for the observed kinetic changes. This is suggestive of an orphan sodium dependent glucose transporter or posttranslational modification of the identified transporters. Finally, these kinetic changes do not seem to be caused by inflammation or dysfunctions in the insulin signaling pathway, as the genes for both inflammatory mediators and insulin signaling were generally unchanged from control mice in both groups. The exceptions, not consistent in all segments, was a significant increase in TGF-b1 in the drinking water sucrose treated mouse jejunum and ileum, and IRS-2 in the drinking water sucrose treated mouse jejunum. Identifying these novel segmental kinetic differences in electrogenic glucose absorption in the mouse intestine and the changes induced by chronical sucrose provided in the drinking water, including the appearance of a putative orphan transporter, not only adds to the understanding of the pathophysiology of the obesity, type-2 diabetes but could direct future therapy.