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Am J Physiol Regul Integr Comp Physiol
2023 May 01;3245:R645-R655. doi: 10.1152/ajpregu.00249.2021.
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Na+-dependent intestinal glucose absorption mechanisms and its luminal Na+ homeostasis across metamorphosis from tadpoles to frogs.
Ishizuka N
,
Nagahashi M
,
Mochida Y
,
Hempstock W
,
Nagata N
,
Hayashi H
.
Abstract
The abrupt morphological changes of the intestine during metamorphosis have been detailed in frogs. The features of intestinal metamorphosis are shortening of the intestine and remodeling of the intestinal epithelium. It is believed that the purpose of the morphological changes of the intestine is adaptation from aquatic herbivorous to carnivorous life. However, little is known about the physiological importance of these morphological changes. To elucidate the functional changes during metamorphosis, we measured luminal Na+ concentrations and Na+-dependent glucose uptake in tadpoles and adult African clawed frogs Xenopus laevis. The small intestine was isolated and divided into four segments in length, the luminal contents collected for analysis of ion concentration by ion chromatography. Phlorizin-sensitive glucose-induced short-circuit current (ΔIsc) was measured in intestinal preparations mounted in Ussing chambers. Although dietary sodium intake was extremely low in tadpoles, luminal Na+ concentration gradually increased along the proximal to the middle part of the intestine (>70 mM), and this Na+ concentration was comparable with that of carnivorous adult frogs. The increment of glucose-induced ΔIsc was observed in tadpole intestine. We also measured the ΔIsc induced by acetic acid, which is the major short-chain fatty acid produced by fermentation. The expression levels of mRNA for Na+-dependent glucose transporter 1 and tight junction protein claudin-15 in each intestinal segment was measured. These results suggest that luminal Na+ homeostasis is important and luminal Na+ is kept at a high concentration for Na+-dependent nutrient absorption mechanisms.
Figure. 1.
Effect of methimazole, a thyroid hormone synthesis inhibitor, on development of the tadpoles. Representative image of giant tadpoles [Nieuwkoop and Faber (NF) stage 57] and frogs used in these studies (A). Tadpoles were reared in the presence of 0.5 mM methimazole for about 50 days (B). The intestine of the tadpole and frogs was cut through the mesentery and the length was measured by stretching the intestine out straight. The arrowhead indicates the switch back point. Hematoxylin and Eosin-stained cross sections of upper small intestine (I1 segment) in control tadpole (C) and giant tadpole (D). Enlarged view of intestinal epithelial cells in a control tadpole (E) and a giant tadpole (F).
Figure 2.
Effect of glucose on short-circuit current (Isc). Representative Isc trace of glucose-induced Isc changes in tadpoles (A) and frogs (C). Where indicated by the arrows, glucose was added to the mucosal side. The final concentration of glucose is shown in mM. The concentration dependence of glucose-induced Isc in tadpoles (B, n = 10) and frogs (D, n = 6). The curves were fit to Michaelis–Menten kinetics. Km, Michaelis-Menten constant, Vmax, maximal velocity.
Figure 3.
Luminal Na+ and K+ concentration along the intestinal axis. The luminal contents of each intestinal segment were collected from tadpoles (A, n = 6, 7, 7, 7 in I1, I2, I3, and I4, respectively) and frogs (B, n = 5, 4, 5, 5 in stomach, I1, I2, and I3, respectively) and Na+ and K+ concentrations were measured in each intestinal segment. Data were analyzed by one-way ANOVA, and Tukey’s multiple comparison test was used for the post hoc test. Each ion concentration was compared between segments, with significant differences noted by different letters.
Figure 4.
Effect of luminal NaCl dilution on transepithelial potential difference in tadpoles. Representative trace of luminal NaCl dilution (A). Where indicated by arrows, the mucosal bathing solution was changed. Relative permeability of Na+ (PNa/PCl) was calculated by Goldman–Hodgkin–Katz equation (B, n = 3, 4, 3 in I1, I2, and I3, respectively). Data were analyzed by one-way ANOVA, and Tukey’s multiple comparison test was used for the post hoc test. PNa/PCl is compared between segments, with significant differences noted by different letters. PNa/PCl > 0.66 indicates cation selectivity.
Figure 5.
Effect of luminal NaCl dilution on transepithelial potential difference in frogs. Representative trace of luminal NaCl dilution (A). Where indicated by arrows, the mucosal bathing solution was changed. Relative permeability of Na+ (PNa/PCl) was calculated by Goldman–Hodgkin–Katz equation (B, n = 3). Data were analyzed by one-way ANOVA, and Tukey’s multiple comparison test was used for the post hoc test. PNa/PCl is compared between segments, with significant differences noted by different letters. PNa/PCl > 0.66 indicates cation selectivity.
Figure 6.
The expression of mRNA for SGLT1 and claudin-15 along the intestinal axis in tadpoles and frogs. The relative mRNA expression level of SGLT1 for tadpoles (A) and frogs (C). Claudin-15 mRNA expression in the intestine of tadpoles (B) and frogs (D). Data are normalized to smn2. Data are presented as the means ± SE from tadpoles (n = 3) and frogs (n = 3). S: stomach, the intestines were divided into four equal parts (I1–I4). Data were analyzed by one-way ANOVA, and Tukey’s multiple comparison test was used for the post hoc test. Relative mRNA expression is compared between segments, with significant differences noted by different letters.
Figure 7.
Acetate-induced current in tadpoles and frogs. Representative short-circuit current (Isc) trace of acetate-induced Isc changes in tadpoles (A) and frogs (B). Where indicated by the arrows, acetate was added to the mucosal side. The final concentration of acetate is shown in mM.
