Expression of apical Na⁺–l-glutamine co-transport activity, B⁰-system neutral amino acid co-transporter (B⁰AT1) and angiotensin-converting enzyme 2 along the jejunal crypt–villus axis in young pigs fed a liquid formula

Gut apical amino acid (AA) transport activity is high at birth and during suckling, thus being essential to maintain luminal nutrient-dependent mucosal growth through providing AA as essential metabolic fuel, substrates and nutrient stimuli for cellular growth. Because system-B⁰ Na⁺–neutral AA co-transporter (B⁰AT1, encoded by the SLC6A19 gene) plays a dominant role for apical uptake of large neutral AA including l-Gln, we hypothesized that high apical Na⁺–Gln co-transport activity, and B⁰AT1 (SLC6A19) in co-expression with angiotensin-converting enzyme 2 (ACE2) were expressed along the entire small intestinal crypt–villus axis in young animals via unique control mechanisms. Kinetics of Na⁺–Gln co-transport activity in the apical membrane vesicles, prepared from epithelial cells sequentially isolated along the jejunal crypt–villus axis from liquid formula-fed young pigs, were measured with the membrane potential being clamped to zero using thiocyanate. Apical maximal Na⁺–Gln co-transport activity was much higher (p < 0.05) in the upper villus cells than in the middle villus (by 29 %) and the crypt (by 30 %) cells, whereas Na⁺–Gln co-transport affinity was lower (p < 0.05) in the upper villus cells than in the middle villus and the crypt cells. The B⁰AT1 (SLC6A19) mRNA abundance was lower (p < 0.05) in the crypt (by 40–47 %) than in the villus cells. There were no significant differences in B⁰AT1 and ACE2 protein abundances on the apical membrane among the upper villus, the middle villus and the crypt cells. Our study suggests that piglet fast growth is associated with very high intestinal apical Na⁺–neutral AA uptake activities via abundantly co-expressing B⁰AT1 and ACE2 proteins in the apical membrane and by transcribing the B⁰AT1 (SLC6A19) gene in the epithelia along the entire crypt–villus axis.