An in vitro study of urea, water, ion and CO2/HCO3− transport in the gastrointestinal tract of the dogfish shark (Squalus acanthias): the influence of feeding

Abstract

SUMMARY In vitro gut sac preparations made from the cardiac stomach (stomach 1), pyloric stomach (stomach 2), intestine (spiral valve) and colon were used to examine the impact of feeding on transport processes in the gastrointestinal tract of the dogfish shark. Preparations were made from animals that were euthanized after 1–2 weeks of fasting, or at 24–48 h after voluntary feeding on a 3% ration of teleost fish (hake). Sacs were incubated under initially symmetrical conditions with dogfish saline on both surfaces. In comparison to an earlier in vivo study, the results confirmed that feeding caused increases in H+ secretion in both stomach sections, but an increase in Cl− secretion only in stomach 2. Na+ absorption, rather than Na+ secretion, occurred in both stomach sections after feeding. All sections of the tract absorbed water and the intestine strongly absorbed Na+ and Cl−, regardless of feeding condition. The results also confirmed that feeding increased water absorption in the intestine (but not in the colon), and had little influence on the handling of Ca2+ and Mg2+, which exhibited negligible absorption across the tract. However, K+ was secreted in the intestine in both fasted and fed preparations. Increased intestinal water absorption occurred despite net osmolyte secretion into the mucosal saline. The largest changes occurred in urea and CO2/HCO3− fluxes. In fasted preparations, urea was absorbed at a low rate in all sections except the intestine, where it was secreted. Instead of an increase in intestinal urea secretion predicted from in vivo data, feeding caused a marked switch to net urea absorption. This intestinal urea transport occurred at a rate comparable to urea reabsorption rates reported at gills and kidney, and was apparently active, establishing a large serosal-to-mucosal concentration gradient. Feeding also greatly increased intestinal CO2/HCO3− secretion; if interpreted as HCO3− transport, the rates were in the upper range of those reported in marine teleosts. Phloretin (0.25 mmol l−1, applied mucosally) completely blocked the increases in intestinal urea absorption and CO2/HCO3− secretion caused by feeding, but had no effect on Na+, Cl− or water absorption.