Quantification of Mg(2+), Ca(2+) and H(+) transport by the gastrointestinal tract of the goldfish, Carassius auratus, using the Scanning Ion-selective Electrode Technique (SIET)

An in vitro gut-sac technique and the scanning ion-selective electrode technique (SIET) were used to characterize Mg(2+), Ca(2+), and H(+) transport at both the mucosal and serosal surfaces of non-everted and everted gastrointestinal tissues obtained from Carassius auratus. As part of the study, two magnesium ionophores were compared (II vs. VI). Unfed animals displayed uniform transport of all ions along the intestine. Feeding resulted in elevated Mg(2+) and Ca(2+) transport when the gut lumen contained chyme however, under symmetrical conditions this increased transport rate was absent. Furthermore, zonation of divalent cation transport was present for both Ca(2+) and Mg(2+) under non-symmetrical conditions while the zonation remained for Ca(2+) alone under symmetrical conditions. High dietary Mg(2+) decreased absorption and induced secretion of Mg(2+) in the posterior intestine. Uptake kinetics in the esophagus suggest large diffusive and/or convective components based on a linear relationship between Mg(2+) transport and concentration and lack of inhibition by ouabain, an inhibitor of Na(+)-K(+)-ATPase. In contrast, kinetics in the rectum were suggestive of a low affinity, saturable carrier-mediated pathway. A decrease in Mg(2+) and Ca(2+) transport was observed in the posterior intestine (both at the mucosal and serosal surfaces) in response to ouabain. This impact was greatest for Ca(2+) transport and when applied to the mucosal fluid and measured in everted preparations. In contrast a putative Mg(2+) transport inhibitor, cobalt(III)hexamine-chloride, did not affect Mg(2+) transport. This is the first study to use SIET approaches to study ion transport in the gut of teleost fish. This is also the first study to provide characterization of Mg(2+) transport in the gut of C. auratus. Due to the limited selectivity of Magnesium ionophore II, subsequent studies of tissues bathed in physiological saline should be made using Magnesium Ionophore VI.