Plasticity of osmoregulatory function in the killifish intestine:drinking rates, salt and water transport, and gene expression after freshwater transfer

Abstract

SUMMARY We have explored intestinal function in the euryhaline killifish Fundulus heteroclitus after transfer from brackish water (10%seawater) to fresh water. Plasma Na+ and Cl–concentrations fell at 12 h post-transfer, but recovered by 7 days. Drinking rate decreased substantially at 12 h (32% of control value) and remained suppressed after 3 and 7 days in fresh water (34 and 43%). By contrast, there was a transient increase in the capacity for water absorption measured across isolated intestines in vitro (3.3- and 2.6-fold at 12 h and 3 days),which returned to baseline after 7 days. These changes in water absorption could be entirely accounted for by changes in net ion flux: there was an extremely strong correlation (R2=0.960) between water absorption and the sum of net Na+ and net Cl–fluxes (3.42±0.10 μl water μmol–1 ion). However,enhanced ion transport across the intestine in fresh water would probably not increase water uptake in vivo, because the drinking rate was far less than the capacity for water absorption across the intestine. The increased intestinal ion absorption after freshwater transfer may instead serve to facilitate ion absorption from food when it is present in the gut. Modulation of net ion flux occurred without changes in mRNA levels of many ion transporters (Na+/K+-ATPase α1a,carbonic anhydrase 2, CFTR Cl– channel,Na+/K+/2Cl– cotransporter 2, and the signalling protein 14-3-3a), and before a measured increase in Na+/K+-ATPase activity at 3 days, suggesting that there is some other mechanism responsible for increasing ion transport. Interestingly, net Cl– flux always exceeded net Na+ flux, possibly to help maintain Cl– balance and/or facilitate bicarbonate excretion. Our results suggest that intestinal NaCl absorption from food is important during the period of greatest ionic disturbance after transfer to fresh water, and provide further insight into the mechanisms of euryhalinity in killifish.

The influence of dissolved organic matter (DOM) on sodium regulation and nitrogenous waste excretion in the zebrafish (Danio rerio)

“…We are aware of no previous studies of DOM or pH effects on drinking. In freshwater fish, drinking rates are traditionally thought to be very low, but the higher values in the present study (approximately 2.0 and 3.0 ml kg −1 h −1 at circumneutral and acidic pH, respectively) are similar to those measured in freshwater trout (Pyle et al, 2003;Robertson and Wood, 2014) and killifish acclimated to 10% seawater (Scott et al, 2006). The much lower drinking rates reported by for zebrafish acclimated to ion-poor water may reflect the use of a much smaller marker molecule ( 3 [H]PEG-400) that can be absorbed across the intestinal tract (Wood and Grosell, 2012), thereby underestimating the true rate.…”

Independence of net water flux from paracellular permeability in the intestine ofFundulus heteroclitus, a euryhaline teleost

“…In each gut sac preparation, one of the radiolabelled PEG compounds was placed in the mucosal saline at a concentration of 0.5-1.0Ciml -1 , representing a maximum total PEG concentration of 0.5-1.0gml -1 . Methods generally followed those developed earlier (Scott et al, 2006). Fish were lightly anaesthetized in ), killed by cephalic concussion, and the intestinal tract dissected out while the fish was kept cold on ice.…”

“…Probably for this reason, intestinal Cl -(as well as Na + and H 2 O) uptake rates increase during acclimation to freshwater (Scott et al, 2006), and persist at higher rates than in seawater-acclimated animals (Scott et al, 2008;Wood et al, 2010). The difference becomes more marked at 1-3 and 12-24h after a meal when Cl -uptake rate, HCO 3 -secretion rate (to which Cl -uptake is partially coupled) and water absorption are all significantly greater in freshwater killifish (Wood et al, 2010).…”

Exploring visual plasticity: dietary carotenoids can change color vision in guppies (Poecilia reticulata)

“…These assays used primers designed for bluefin killifish (Lucania goodei) (Fuller et al 2005). The primer sequence for the reporter gene (elongation factor 1-α) was taken from the work on Fundulus heteroclitus (Scott and Schulte 2005;Scott et al , 2006. These fishes all occur in the order Cyprinodontiformes (Nelson 2006).…”