Expression of Mineral and Urea Transport-Related Genes in the Salivary Glands and Ruminal Epithelium of Young Goats Fed Nitrogen- and/or Calcium-Reduced Diets.

An adequate supply of protein and minerals, particularly nitrogen (N) and calcium (Ca), is essential for ruminant growth and metabolic function. The rumen and salivary glands are crucial for nutrient and electrolyte homoeostasis via specialised transport proteins. Dietary reductions in N and Ca may elicit adaptive molecular and physiological responses, but their effects on epithelial transport are still not entirely understood. This study investigated the effects of dietary N and Ca reductions on ruminal and salivary fluid composition and the mRNA expression of Ca-, phosphate (Pi)-, and urea-transporting proteins in the ruminal epithelium and salivary glands of young goats. Goats received diets with reduced N, reduced Ca, or combined N and Ca reduction. Ruminal and abomasal fluids, saliva, and tissue samples were analysed for mineral concentrations, urea, and short-chain fatty acids (SCFAs). mRNA expression of transporters, including CASR, TRPC3, TRPV3, PMCA, NCX1, UTB, SLC4A4, ATP1B1 (Na+/K+-ATPase β-subunit), AQP5, NaPi IIb, and PiT1 was quantified using qPCR. Dietary N reduction decreased ruminal and salivary urea concentrations, reduced mRNA expression of SLC4A4, AQP5, and NaPi IIb in salivary glands, and lowered ruminal SCFA concentrations, indicating impaired microbial fermentation. The dietary Ca reduction increased salivary Pi and induced upregulation of AQP5, ATP1B1, and PiT1. In the ruminal epithelium, N reduction increased PMCA and UTB expression, whereas Ca reduction had only minor effects on Ca transporters. Ruminal Pi and Ca concentrations remained stable, reflecting strong stabilisation of mineral concentrations in the ruminal environment, whereas abomasal fluid showed greater sensitivity to dietary changes. These results indicate tissue-specific transcriptional and compositional responses of ruminant epithelia to dietary N and Ca reduction. N reduction primarily affects the expression of genes related to nutrient and solute transport, whereas Ca reduction is associated with selective transcriptional changes in mineral-related transporters. These results provide evidence for adaptive epithelial responses to nutritional variation, although functional consequences at the protein or transport level require further investigation.