Capsaicin-sensitive vagal afferents modulate posttranscriptional regulation of the rat Na⁺/glucose cotransporter SGLT1

Abstract

Stearns AT, Balakrishnan A, Rounds J, Rhoads DB, Ashley SW, Tavakkolizadeh A. Capsaicin-sensitive vagal afferents modulate posttranscriptional regulation of the rat Na ϩ /glucose cotransporter SGLT1. Am J Physiol Gastrointest Liver Physiol 294: G1078-G1083, 2008. First published February 28, 2008 doi:10.1152/ajpgi.00591.2007.-Introduction: the intestinal Na ϩ /glucose cotransporter (SGLT1) displays rapid anticipatory diurnal rhythms in mRNA and protein expression. The vagus nerve has been implicated in the entrainment of some transporters. We aimed to clarify the influence of the vagus nerve on the diurnal entrainment pathway for SGLT1 and examine the role of vagal afferent fibers. Methods: male Sprague-Dawley rats were randomized to three groups, total subdiaphragmatic vagotomy, selective deafferentation of the vagus with capsaicin, or sham laparotomy. Postoperatively, animals were maintained in a 12-h light-dark cycle with food access limited to night. On the ninth postoperative day, animals were euthanized to harvest jejunal mucosa at 6-h intervals starting at 10 AM. Whole cell SGLT1 protein was measured by semiquantitative densitometry of immunoblots. Sglt1 and regulatory subunit RS1 mRNA was assessed by quantitative PCR. Fluorogold tracer technique was used to confirm adequacy of the vagotomy. Results: the diurnal rhythm in intestinal SGLT1, with a 5.3-fold increase in Sglt1 mRNA at 4 PM, was preserved in both vagotomy and capsaicin groups. However, the rhythmicity in SGLT1 protein expression (2.3-fold peak at 10 PM; P ϭ 0.041) was abolished following either total vagotomy or deafferentation. Lack of change in RS1 mRNA suggests this is independent of the RS1 regulatory pathway. Conclusion: SGLT1 transcription is independent of the vagus. However, dissociation of the protein rhythm from the underlying mRNA signal by vagotomy suggests the vagus may be involved in posttranscriptional regulation of SGLT1 in an RS1 independent pathway. Disruption following afferent ablation by capsaicin suggests this limb is specifically necessary. glucose transport; vagus; vagotomy THE APICAL na ϩ /glucose cotransporter (SGLT1), localized on the brush-border membrane of small bowel enterocytes, is responsible for all secondary active absorption of glucose from the small bowel (14,19). Dysregulation of SGLT1 is increasingly recognized in type 2 diabetes mellitus, with expression increasing up to fourfold in both patients and animal models (4, 7-9). Similarly, knockout of the SGLT1 regulatory subunit protein RS1 in mice leads to both SGLT1 overexpression and severe obesity (27). Therapy aimed at modulating SGLT1 expression may therefore have clinical benefits in diabetes and obesity through slowing of glucose absorption in a manner similar to amylase inhibitors (5) and through increased ileal glucose delivery (38). SGLT1 expression is regulated by several inputs, including diet composition (11) and the timing of food intake (28). Despite significant progress in characterizing the physiological signaling mechanisms regulating SGL...

Impact of Roux-en-Y gastric bypass surgery on rat intestinal glucose transport

“…There is an appearance of a further, less mobile species, which may represent further posttranscriptional modifications such as ubiquitination. It is furthermore remarkable that gastric bypass and duodenal exclusion abolished the normal diurnal rhythms in intestinal glucose transport (but not transcription), a finding we have shown to be replicated by vagal afferent blockade (39). This supports our earlier conjecture that vagal afferents signaling nutrient delivery to the proximal intestine may regulate intestinal Sglt1.…”

“…Although care was taken during this procedure to avoid vagal injury, there is the possibility of inadvertent nerve injury. This may have contributed to the loss of diurnal rhythms in Sglt1, which have been shown to be regulated by vagal afferents (39).…”

“…Whole-cell protein lysates were extracted from frozen tissue aliquots harvested at HALO-15 and separated with Western blotting (39). Membranes were blotted for Sglt1 (Chemicon, Temecula, CA; 1:4,000) before being stripped and reblotted for actin (Neomarkers; 1:500).…”

Restricted Feeding Phase Shifts Clock Gene and Sodium Glucose Cotransporter 1 (SGLT1) Expression in Rats

“…Moreover, peripheral clocks adapt to restricted feeding at different rates; liver shifts much more quickly than lung (19). Although it is possible that the small intestine would achieve a complete phase shift following longer restricted feeding, the sufficiency of 4-d adaptation previously reported (27), the equal weights between rats in the 2 restricted groups, and the plateau in food intake in the LF rats [matching that expected for rats of that weight (25)] all suggest that the partial phase shift was due to factors other than incomplete adaptation. We note that the rapid adaptation observed in liver may result from more direct (i.e.…”

Ileal interposition improves glucose tolerance and insulin sensitivity in the obese Zucker rat

“…This was not further investigated because it is beyond the scope of this report. Post-RYGB alterations in intestinal glucose transport and metabolism have been recently described (28,31,36). However, because muscle represents the principal site of insulin-stimulated glucose transport in vivo, the observed increase in muscle glucose uptake presumably represents the predominant mechanism for improved glucose homeostasis after the IT procedure (14,27).…”