2063 INTRODUCTION Prior to the last decade, there was relatively little research on the physiological events associated with feeding and fasting in elasmobranchs (reviewed by Holmgren and Nilsson, 1999). However, since 2005, we have initiated a research program in this area (reviewed by Wood et al., 2010) on the spiny dogfish shark (Squalus acanthias), chosen because it is an opportunistic predator that is relatively abundant, and because its feeding ecology has been well studied (Bonham, 1954; Holden, 1966; Jones and Geen, 1977; Tanasichuk et al., 1991; Hanchet, 1991; Laptikhovsky et al., 2001; Alonso et al., 2002). In nature, the dogfish consumes large meals at irregular intervals; many dogfish are caught with nothing in their stomach, but a single meal may be very large (1–10% of body mass). Digestive processing is slow, taking more than 5 days at 11°C (Wood et al., 2007b). During this period there are profound changes in acid- base balance (Wood et al., 2005; Wood et al., 2007a; Wood et al., 2009), metabolism (Walsh et al., 2006; Kajimura et al., 2006; Wood et al., 2010) and ionoregulation (Kajimura et al., 2006; Matey et al., 2009). However, studies on the gastrointestinal tract itself in relation to feeding are limited to one descriptive investigation (Wood et al., 2007b) of changes in chyme volume and composition in various segments over this 5+ day digestive period. In that study (Wood et al., 2007b), the animals were fed with dead hake (Merluccius productus), a teleost fish that is one of their most common natural prey items (Jones and Geen, 1977), with an internal osmolality only ~40% that of S. acanthias. It was concluded that the seawater drinking rate, which is very low, probably remained unchanged after feeding, but that nevertheless, the osmolality of the chyme was quickly raised to that of the blood plasma of the shark. This appeared to occur as a consequence of large additions of urea, Na + , Cl – and unmeasured osmolytes to the chyme, though removal of water by osmosis may have also played a role. HCl in particular appeared to be secreted into the chyme by the two sections of the stomach, creating the systemic ‘alkaline tide’ (Wood et al., 2005). Na + and Cl – ions appeared to be secreted mainly in the two sections of the stomach, and urea mainly in the intestine. However, it was unclear whether the urea secretion originated from the bile, the pancreas or intestinal wall secretions. Later in the digestive process, Na + , Cl – , K + and urea appeared to be reabsorbed by the intestine and perhaps also the colon, together with a 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 Ca 2+ and Mg 2+ , 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 CO 2 /HCO 3 – 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 CO 2 /HCO 3 – secretion; if interpreted as HCO 3 – 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 CO 2 /HCO 3 – secretion caused by feeding, but had no effect on Na + , Cl – or water absorption. Key words: cardiac stomach, pyloric stomach, intestine, spiral valve, colon, gut sac. Received 29 October 2012; Accepted 29 January 2013 The Journal of Experimental Biology 216, 2063-2072 © 2013. Published by The Company of Biologists Ltd doi:10.1242/jeb.082313 RESEARCH ARTICLE An in vitro study of urea, water, ion and CO 2 /HCO 3 – transport in the gastrointestinal tract of the dogfish shark (Squalus acanthias): the influence of feeding Hon Jung Liew 1,2,3 , Gudrun De Boeck 1,2 and Chris M. Wood 1,2,4, * 1 Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada, V0R 1B0, 2 Systemic Physiological and Ecotoxicological Research, Department of Biology, University of Antwerp, Groenenborgerlaan 171, BE-2020 Antwerp, Belgium, 3 Institute of Tropical Aquaculture, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia and 4 Department of Biology, McMaster University, 1280 Main St West, Hamilton, ON, Canada, L8S 4K1 *Author for correspondence (woodcm@mcmaster.ca) THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY