The integration of digestion and osmoregulation in the avian gut

Abstract

We review digestion and osmoregulation in the avian gut, with an emphasis on the ways these different functions might interact to support or constrain each other and the ways they support the functioning of the whole animal in its natural environment. Differences between birds and other vertebrates are highlighted because these differences may make birds excellent models for study and may suggest interesting directions for future research. At a given body size birds, compared with mammals, tend to eat more food but have less small intestine and retain food in their gastrointestinal tract (GIT) for shorter periods of time, despite generally higher mass-specific energy demands. On most foods, however, they are not less efficient at digestion, which begs the question how they compensate. Intestinal tissue-specific rates of enzymatic breakdown of substrates and rates of active transport do not appear higher in birds than in mammals, nor is there a demonstrated difference in the extent to which those rates can be modulated during acclimation to different feeding regimes (e.g. diet, relative intake level). One compensation appears to be more extensive reliance on passive nutrient absorption by the paracellular pathway, because the avian species studied so far exceed the mammalian species by a factor of at least two- to threefold in this regard. Undigested residues reach the hindgut, but there is little evidence that most wild birds recover microbial metabolites of nutritional significance (essential amino acids and vitamins) by re-ingestion of faeces, in contrast to many hindgut fermenting mammals and possibly poultry. In birds, there is some evidence for hindgut capacity to breakdown either microbial protein or protein that escapes the small intestine intact, freeing up essential amino acids, and there is considerable evidence for an amino acid absorptive capacity in the hindgut of both avian and mammalian hindgut fermenters. Birds, unlike mammals, do not excrete hyperosmotic urine (i.e. more than five times plasma osmotic concentration). Urine is mixed with digesta rather than directly eliminated, and so the avian gut plays a relatively more important role in water and salt regulation than in mammals. Responses to dehydration and high- and low-salt loads are reviewed. Intestinal absorption of ingested water is modulated to help achieve water balance in one species studied (a nectar-feeding sunbird), the first demonstration of this in any terrestrial vertebrate. In many wild avian species the size and digestive capacity of the GIT is increased or decreased by as much as 50% in response to nutritional challenges such as hyperphagia, food restriction or fasting. The coincident impacts of these changes on osmoregulatory or immune function of the gut are poorly understood.

Gut metagenomic analysis reveals prominent roles of Lactobacillus and cecal microbiota in chicken feed efficiency

“…In a recent study, numerous oligosaccharide-and polysaccharide-degrading enzyme-encoding genes and several pathways involved in the production of short-chain fatty acids (SCFAs) were observed in the cecal metagenome of the chicken 12 . The SCFAs were produced mainly by microbial fermentation in the hindgut and could be absorbed through the mucosa and catabolized for energy by the host 13 ; the SCFAs also inhibited acid-sensitive pathogens by lowering the pH 14 . Due to the rapid flow of the highly fluid, digested material and a higher acidity, the number of microbes in the duodenum was lower than that in the posterior intestine.…”

Chicken intestinal microbiome: Development and function

“…Są one absorbowane przez komórki nabłonka jelitowego i stanowią dodatkowe źródło energii dla gospodarza. LKT oddziałują również antagonistycznie na drobnoustroje chorobotwórcze oraz zwiększają absorpcję składników mineralnych (3,37,55,57,61). W metagenomie jelitowych mikrobiontów najliczniej reprezentowane są sekwencje odpowiedzialne za fermentację octanową, obejmujące między innymi ponad 30 kinaz octanowych/fosfotransferaz.…”

“…Mikrobiom jelitowy, zaliczany do najliczniejszych u zwierząt i człowieka, wydaje się decydować o wielu aspektach życia gospodarzy, w tym najważniejszych, jakimi są wykorzystanie i przetwarzanie pokarmu czy też zachowanie zdrowia. U ptaków, w odróżnieniu od ssaków, układ pokarmowy jest krótszy i następuje w nim szybsze trawienie i przesuwanie się treści pokarmowej (zwykle cały cykl od pobrania pokarmu do wydalenia nie przekracza 3,5 h), co oczywiście przekłada się na inną kompozycję zasiedlających jelita mikrobiontów i ich funkcję (3,35,55,58). W pierwszych dniach po wykluciu układ pokarmowy ptaków jest najintensywniej rozwijają-cym się narządem, stymulowanym również do tego przez zasiedlające go bakterie.…”

“…Te zaś ponownie stają się dla gospodarza źródłem energii i węgla. Są absorbowane przez komór-ki nabłonka jelitowego i włączane do wielu metabolicznych cykli gospodarza (55). Krótkołańcuchowe kwasy tłuszczowe nie tylko obniżają pH w środowisku jelit, regulując w ten sposób kompozycję mikrobiontów, ale również w formie niezdysocjowanej dyfundują do komórek wielu bakterii, w których podlegają dysocjacji i obniżają pH, doprowadzając do zahamowania funkcji wielu enzymów i w efekcie metabolizmu.…”

A Pharm-Ecological Perspective of Terrestrial and Aquatic Plant-Herbivore Interactions

“…Although enhanced paracellular absorption in birds relative to most mammals may compensate for shorter intestines of birds (Caviedes-Vidal et al, 2007), it may also expose birds to significantly greater amounts of water soluble PSMs (McWhorter et al, 2009), which can interfere with digestion and exert potentially toxic effects. Differences in the importance of paracellular absorption can translate to significantly different effects of PSMs on nutrient absorption (Skopec et al, 2010) and offers an opportunity for researchers to investigate mechanisms responsible for nutrient-PSM interactions.…”