Abstract
Diets fed to ruminants should contain nitrogen (N) as low as possible to reduce feed costs and environmental pollution. Though possessing effective N-recycling mechanisms to maintain the N supply for rumen microbial protein synthesis and hence protein supply for the host, an N reduction caused substantial changes in calcium (Ca) and phosphate homeostasis in young goats including decreased intestinal transepithelial Ca absorption as reported for monogastric species. In contrast to the transcellular component of transepithelial Ca transport, the paracellular route has not been investigated in young goats. Therefore, the aim of the present study was to characterise the effects of dietary N and/or Ca reduction on paracellular transport mechanisms in young goats. Electrophysiological properties of intestinal epithelia were investigated by Ussing chamber experiments. The expression of tight junction (TJ) and adherens junction (AJ) proteins in intestinal epithelia were examined on mRNA level by qPCR and on protein level by western blot analysis. Dietary N reduction led to a segment specific increase in tissue conductances in the proximal jejunum which might be linked to concomitantly decreased expression of cadherin 17 mRNA. Expression of occludin (OCLN) and zonula occludens protein 1 was increased in mid jejunal epithelia of N reduced fed goats on mRNA and partly on protein level. Reduced dietary Ca supply resulted in a segment specific increase in claudin 2 and claudin 12 expression and decreased the expression of OCLN which might have been mediated at least in part by calcitriol. These data show that dietary N as well as Ca reduction affected expression of TJ and AJ proteins in a segment specific manner in young goats and may thus be involved in modulation of paracellular Ca permeability.
Highlights
IntroductionLow dietary nitrogen (N) contents are desirable for economic and environmental reasons to reduce feed costs and N output
In ruminant feeding systems, low dietary nitrogen (N) contents are desirable for economic and environmental reasons to reduce feed costs and N output
Expression of CDH17 mRNA was significantly decreased in the proximal jejunum and ileum in N reduced fed goats (P = 0.02 and P = 0.05), while dietary Ca reduction led to increased expression levels in the proximal jejunum (P = 0.05)
Summary
Low dietary nitrogen (N) contents are desirable for economic and environmental reasons to reduce feed costs and N output. For a sufficient synthesis of microbial protein by ruminal microbes, besides a certain amount of N sources, an adequate supply of dietary energy and macrominerals must be guaranteed. If these conditions are met, microbial protein accounts for the main protein supply of the host and ruminants were thought to cope with a reduced dietary N content, in contrast to monogastric species which showed severe changes in mineral, especially calcium (Ca) homeostasis when consuming a protein reduced diet [1,2,3,4]. Diminished transcellular Ca absorption in goats fed an N reduction or an N and Ca reduction was associated with reduced plasma calcitriol concentrations as the main regulator of transcellular Ca transport in the small intestine [5]
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