Introduction
After the latest discoveries on how the gut health influences the general wellbeing in humans, the fish intestine is getting more and more attention. The gut acts as a selective barrier allowing nutrients in and blocking the entry to pathogens.
The inclusion of new ingredients in fish feed formulation represents a challenge for the correct functionality of the gut. Not only the formula has to be nutritionally balanced but it should also promote the health of the intestine.
Feed additives are designed to provide a benefit for the fish, helping in the transport, absorption and/or incorporation of nutrients. In order to evaluate such a benefit, the Ussing chamber has been used as diagnostic tool to assess the tissue electrical properties of the gut (electrophysiology). This technique allows the analysis of the intestine ex-vivo if the tissue is kept alive, and reflects its in vivo functionality.
In this work, electrophysiology and histology have been used to evaluate the impact of a feed additive on the gut health of a marine fish, gilthead seabream (S. aurata) and a freshwater fish, tilapia (Oreochromis sp.).
Material and methods
Gilthead seabream: fish of 98,07±1,02 g average individual body weight were distributed in 6 tanks of a marine (32 psu) recirculating aquaculture system (RAS) held at 23ºC. A group of 25 animals were stocked per tank. Fish were fed either a control diet (diet A) or the same diet to which a gut health promoting additive (Sanacore GM®, Adisseo) was added (diet B). Feeds contained 10% FM and the additive was included at 0,5%. Feeds were isoproteic and isolipidic (CP/CF = 45/18). Fish were fed manually twice per day 6 days per week. Trial lasted 8 weeks.
Tilapia: fingerlings of 9.95±1,02 g average individual body weight were stocked in 12 tanks of a freshwater RAS. Fish were fed a positive control diet (diet T+), a negative control diet (diet T-) and a diet S, containing the additive (Sanacore GM®, Adisseo). Diet T+ contains 10 % FM and diets T- and S had 0% FM inclusion. Diets were isoproteic and isolipidic (CP/CF = 33/5). Fish were fed automatically distributing the daily ration during 12 hours per day. Trial lasted 8 weeks.
Epithelial electrophysiology in Ussing chambers
The anterior intestine, was isolated and mounted as previously described (Gregorio et al., 2013) with apical (luminal) and basolateral (blood side) sides of the tissue identified on a tissue holder of 0.25-0.30 cm2 and positioned between two half-chambers containing 2 ml of physiological saline. Bioelectrical parameters for each tissue were recorded continuously during the in vitro period onto Labscribe3 running in a MacIntosh computer using IWorx188 and Lab-Trax-4 data acquisition systems, from the time of mounting for 90 min. Epithelial resistance (Rt, Ω.cm2) was manually calculated (using Ohm’s law) from the current deflections induced by a bilateral +2 mV pulse of 3 s every minute.
Results & conclusions
Gilthead seabream: diet A and B had a similar performance in terms of SGR and FCR as well as in somatic indexes. Significant difference was found in the transepithelial resistance of fish fed the two diets (Fig.1).Tissue resistance in the intestine of healthy seabream juveniles of this size is expected to be higher than 150 Ω cm2. The time-response in vitro in the Ussing chamber experiment shows a typical progression to plateau values in groups A and B. Fish fed diet B showed a significantly higher average values of tissue electrical resistance than fish fed diet A.
Tilapia: there are very few reports on epithelial tissue resistance in the intestine of tilapia. Values obtained in previous experiments with tilapia species points to values between 45-75 Ω. cm2. Values barely changed during the time in vitro, and no significant differences between groups were observed after statistical analysis. However, fish fed with the diet S showed an increase of the epithelium resistance in tilapia although no significant difference was found between diet S and diet T+. Diet T- without additive inclusion showed the lowest Rt values (Fig.2).
References
Gregoìrio SF, ESM Carvalho, S EncarnaçaÞo, JM Wilson, DM Power, AVM Canaìrio, J Fuentes. (2013). Adaptation to different salinities exposes functional specialization in the intestine of the sea bream (Sparus auratus L.). Journal of Experimental Biology 216 (3): 470- 9 http://dx.doi.org/10.1242/jeb.073742