Figure 4
Intestinal barrier function after long term hypoxia at high temperature (Experiment 2). This experiment aimed at mimicking DO levels measured in sea cages in fjords sheltered from waves, wind and strong currents. In these situations, cyclic drops in DO levels are frequently observed during slack water at tidal reverse and further decreased DO levels are observed during high temperatures. Four oxygen treatment regimes were initiated: fixed 50% or 85% DO levels, or 50% or 85% DO levels in two different 6 hour cycles (4:2 h at 85:50%; 1:5 h at 85:50%) at 16°C. Four fish per tank from fixed 85% and 50% DO groups were sampled. The intestine was removed and opened longitudinally, divided into a proximal and a distal part, washed in ice-cold salmon Ringer solution and mounted in Ussing chambers. The electrical parameters; transepithelial resistance (TER), short-circuit current (SCC) and transepithelial potential (TEP) were measured. TER is mainly a measure of the paracellular permeability, and SCC describes the sum of active transports. This, together with the passive leakage of charged molecules across the epithelium is reflected in the TEP. Ussing chamber data was analysed in a genral linear model with intestinal region and treatment (with tank nested within treatment) as factors. The paracellular permeability of the intestinal epithelium was also assessed as the apparent permeability (Papp) of 14C-mannitol. TER was lower in the 50% DO group in both intestinal regions (p < 0.001) (A). An interaction between intestinal region and treatment was found for Papp (p < 0.001), revealing a decrease in Papp of the proximal intestine whereas there was an increase in the distal region in the 50% DO group compared to the 85% DO group (B). In both proximal and distal intestine (p < 0.05) the capability to maintain a potential difference between mucosa and serosa was reduced as shown by decreased TEP (C). No differences in SCC could be observed between treatments in neither proximal nor distal intestine (D). All data are expressed as means ± SEM and p < 0.05 was regarded as significant and indicated as *, p < 0.01 as ** and p < 0.001 indicated as ***.