Mechanism of inhibition of Na⁺-glucose cotransport in the chronically inflamed rabbit ileum

Abstract

In a rabbit model of chronic ileal inflammation, we previously demonstrated that coupled NaCl absorption was reduced because of an inhibition of Cl−/[Formula: see text]but not Na+/H+exchange on the brush-border membrane (BBM) of villus cells. In this study we determined the alterations in Na+-stimulated glucose [Na+- O-methyl-d-glucose (Na+-OMG)] absorption during chronic ileitis. Na+-OMG uptake was reduced in villus cells from the chronically inflamed ileum. Na+-K+-adenosinetriphosphatase (ATPase), which provides the favorable Na+ gradient for this cotransporter in intact cells, was found to be reduced also. However, in villus cell BBM vesicles from the inflamed ileum Na+-OMG uptake was reduced as well, suggesting an effect at the level of the cotransporter itself. Kinetic studies demonstrated that Na+-OMG uptake in the inflamed ileum was inhibited by a decrease in the maximal rate of uptake for OMG without a change in the affinity. Analysis of steady-state mRNA and immunoreactive protein levels of this cotransporter demonstrates reduced expression. Thus Na+-glucose cotransport was inhibited in the chronically inflamed ileum, and the inhibition was secondary to a decrease in the number of cotransporters and not solely secondary to an inhibition of Na+-K+-ATPase or altered affinity for glucose.

T cell activation causes diarrhea by increasing intestinal permeability and inhibiting epithelial Na+/K+-ATPase

“…These data also indicated that tissue inflammation was associated with decreased Na + absorption. In animal models of colitis, both hyporesponsiveness to secretagogues (45) and decreased Na + -absorptive processes (Na + -dependent glucose absorption and non-nutrient-dependent Na + and Cl -transport) have been observed (46,47). Similar to the present findings, decreased Na + /K + -ATPase activity was measured in inflamed intestine (48).…”

Na-glucose and Na-neutral amino acid cotransport are uniquely regulated by constitutive nitric oxide in rabbit small intestinal villus cells

“…BBM vesicles (BBMV) from rabbit intestinal villus cells were prepared by CaCl 2 precipitation, and differential centrifugation as previously reported (23,24). BBMV were resuspended in a medium appropriate to each experiment.…”

“…For example, alanine is transported into villus cells by one of these Na-driven amino acid transporters specifically by the Na-neutral amino acid cotransporter system B 0 (Asct1)(3) and will be used to measure the effects of cNO on NAcT. Both cotransporters have been localized to the BBM of the absorptive villus but not the secretory crypt cells in the mammalian small intestine including rabbits (23,24). The favorable transcellular Na gradient for these secondary active transport processes is provided by Na-K-ATPase on the basolateral membrane (BLM) of villus cells (16).…”

“…SGLT-1 is unaffected in secretory diarrheal diseases such as cholera (12). However, it has been demonstrated the SGLT-1 is inhibited during chronic small intestinal inflammation (24). A variety of hormones has also been demonstrated to alter NAcT in the mammalian small intestine (8).…”

“…Although NAcT is inhibited in the chronically inflamed small intestine, it is of note that the mechanism of inhibition of this cotransport process is uniquely different from that of SGLT-1 during chronic ileitis (23). For example, during chronic ileitis, it has been demonstrated that whereas SGLT-1 inhibition is during chronic ileitis secondary to a decrease in cotransporter numbers, NAcT inhibition is secondary to a decrease in the affinity of the cotransporter for the amino acid without change in cotransporter numbers (23,24). These findings indicate that these cotransporters lend themselves to unique regulation.…”