Dietary salt intake and its relevance to ionic regulation in freshwater salmonids

SUMMARYOurs is the first study to demonstrate an influence of dietary sodium on waterborne copper uptake in fish. We examined possible interactions between dietary sodium and the response of freshwater rainbow trout (Oncorhynchus mykiss) to waterborne copper in light of recent evidence of interactions between sodium and copper metabolism in the gills. Trout were maintained for 6 days on one of four diets of increasing sodium concentration (0.25 mmol g-1, 0.51 mmol g-1, 0.76 mmol g-1 and 1.27 mmol g-1, which corresponds to 0.6%, 1.2%, 1.8% and 3% sodium by mass, respectively). At the end of 7 days, fish were exposed for 6 h to waterborne copper spiked with 64Cu to determine if the dietary sodium affected responses to a subsequent short-term waterborne copper exposure. The radiotracer allowed us to distinguish between Cu occurring in fish tissues before the experiment and `newly accumulated' Cu arising from the experimental exposure. Dietary sodium concentrations of 1.8% or 3% reduced newly accumulated copper concentrations in gill (from 93.9 ng g-1in control to 38.9 ng g-1 and 20.0 ng g-1 in fish fed 1.8% or 3% Na+-supplemented diets, respectively), liver (from 64.3 ng g-1 to 23.1 ng g-1 and 7.5 ng g-1,respectively), kidney (from 29.3 ng g-1 to 11.7 ng g-1and 7.8 ng g-1, respectively), plasma (from 64.7 ng g-1to 21.5 ng g-1 and 10.7 ng g-1, respectively) and gut(from 6.8 ng g-1 to 3.4 ng g-1 and 2.2 ng g-1, respectively) by 50.0-88.2%. The 3%Na+-supplemented diets also increased plasma and gut sodium concentrations by 38.1% (from 137.1 μmol g-1 to 189.3 μmol g-1) and 104.3% (from 56.5 μmol g-1 to 115.4 μmol g-1), respectively, relative to fish maintained on untreated diets. Whole body uptake rates of both sodium and copper were significantly reduced,and highly correlated (r=0.97) with one another, in fish fed high-sodium diets relative to controls. Moreover, sodium efflux was 12% and 38% higher in fish fed 1.8% and 3% sodium-enriched diets, respectively. Fish fed high-sodium diets also drank more water, but the contribution of drinking to waterborne copper uptake was negligible. From these results, we speculate that, at least in part, aqueous sodium and copper share a common branchial uptake route, probably through an apical sodium channel. According to this hypothesis, as the channel is downregulated with increasing internal sodium concentrations, both sodium and copper uptake from the water are inhibited.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Dietary sodium inhibits aqueous copper uptake in rainbow trout(Oncorhynchus mykiss)

Pyle

Kamunde

McDonald

et al. 2003

“…In freshwater salmonids, NaCl uptake from food may potentially exceed uptake from the water across the gills (Smith et al, 1989), a fact that was often overlooked until recently (reviewed by Wood and Bucking, 2011). Under the Amazonian soft water conditions, food might be an important source of ions, especially when the energy available for branchial uptake processes is limited under severe hypoxia.…”

Section: Introductionmentioning

confidence: 99%

Interactions between hypoxia tolerance and food deprivation in Amazonian oscars, Astronotus ocellatus (Agassiz)

Boeck

Wood

Iftikar

et al. 2013

SUMMARYOscars are often subjected to a combination of low levels of oxygen and fasting during nest-guarding on Amazonian floodplains. We questioned whether this anorexia would aggravate the osmo-respiratory compromise. We compared fed and fasted oscars (10-14 days) in both normoxia and hypoxia (10-20 Torr, 4 h). Routine oxygen consumption rates (Ṁ O2 ) were increased by 75% in fasted fish, reflecting behavioural differences, whereas fasting improved hypoxia resistance and critical oxygen tensions (P crit ) lowered from 54 Torr in fed fish to 34 Torr when fasting. In fed fish, hypoxia reduced liver lipid stores by approximately 50% and total liver energy content by 30%. Fasted fish had a 50% lower hepatosomatic index, resulting in lower total liver protein, glycogen and lipid energy stores under normoxia. Compared with hypoxic fed fish, hypoxic fasted fish only showed reduced liver protein levels and even gained glycogen (+50%) on a per gram basis. This confirms the hypothesis that hypoxia-tolerant fish protect their glycogen stores as much as possible as a safeguard for more prolonged hypoxic events. In general, fasted fish showed lower hydroxyacylCoA dehydrogenase activities compared with fed fish, although this effect was only significant in hypoxic fasted fish. Energy stores and activities of enzymes related to energy metabolism in muscle or gills were not affected. Branchial Na + uptake rates were more than two times lower in fed fish, whereas Na + efflux was similar. Fed and fasted fish quickly reduced Na + uptake and efflux during hypoxia, with fasting fish responding more rapidly. Ammonia excretion and K + efflux were reduced under hypoxia, indicating decreased transcellular permeability. Fasted fish had more mitochondria-rich cells (MRC), with larger crypts, indicating the increased importance of the branchial uptake route when feeding is limited. Gill MRC density and surface area were greatly reduced under hypoxia, possibly to reduce ion uptake and efflux rates. Density of mucous cells of normoxic fasted fish was approximately fourfold of that in fed fish. Overall, a 10-14 day fasting period had no negative effects on hypoxia tolerance in oscars, as fasted fish were able to respond more quickly to lower oxygen levels, and reduced branchial permeability effectively.

“…Additionally, parental mucus may act as a direct source of ions. Freshwater fish replace ions lost by passive efflux to the external environment through the active uptake of ions across the gills or through the diet (Smith et al, 1989). Experimental diets rich in ions help satisfy the osmoregulatory requirements of fish kept in freshwater, allowing energy normally used in osmoregulation to be used for growth (Gatlin et al, 1992).…”

Section: Introductionmentioning

confidence: 99%

Biparental mucus feeding: a unique example of parental care in an Amazonian cichlid

Buckley

Maunder

Foey

et al. 2010

SUMMARYVertebrates display a wide variety of parental care behaviours, including the guarding of offspring pre and post nutritional independence as well as the direct provision of nutrients during the early development period. The Amazonian cichlid Symphysodon spp. (discus fish) is unusual among fish species, in that both parents provide offspring with mucus secretions to feed from after hatching. This extensive provision of care, which can last up to a month, imposes a physiological demand on both parents and gives rise to conflict between the parent and offspring. Here, we investigated the relationship between parents and offspring during a breeding cycle, determining both mucus composition (total protein, cortisol, immunoglobulin, and Na + , K + and Ca 2+ concentrations) and the behavioural dynamics of the parent-offspring relationship. Over the course of a breeding cycle, a significant increase in offspring bite rate was recorded, with a concomitant increase in the frequency of turns the male and female parent took at caring for their young. A peak in mucus antibody provision was seen as offspring reached the free-swimming stage, suggesting a role analogous to colostrum provision in mammals. Mucus protein content was lowest during the second and third weeks of free swimming, and a weaning period, similar to that seen in mammalian parental care, occurred when the offspring had been free swimming for ~3weeks. In many ways, the parental behaviour of discus fish is more similar to mammalian and avian parental care than other fish species, and represents an exciting aquatic model for studying the parent-offspring conflict.