Dietary protein reduction in sheep and goats: different effects on l-alanine and l-leucine transport across the brush-border membrane of jejunal enterocytes

Abstract

It was the aim of this study to examine the potential regulatory effects of a long-term low dietary protein supply on the transport capacity of the jejunal brush-border membrane for amino acids. For this purpose, we used the neutral amino acids l-alanine (representative for nonessential amino acids) and l-leucine (representative for essential amino acids) as model substances. Ten sheep lambs, 8 weeks of age and 19–27 kg body weight, were allotted to two dietary regimes with either adequate or reduced protein supply which was achieved by 17.9% and 9.7% of crude protein in the concentrated feed, respectively. The feeding periods were 4–6 weeks in length. Similarly, eight goat kids of 5–7 weeks of age and 8–14 kg body weight were allotted to either adequate (crude protein 20.1%, feeding period 9–12 weeks) or reduced protein supply (10.1%, feeding period 17–18 weeks). Dietary protein reduction in lambs caused a significant body weight loss of 0.6±0.7 kg, whereas the body weight in control animals increased by 1.9±0.7 kg (P<0.05). Plasma urea concentrations decreased significantly by 60% (low protein 2.3±0.1 versus control 5.7±0.2 mmol l⁻¹, P<0.001). In kids, reduction of dietary protein intake led to significant decreases of the daily weight gain by 48% from 181±8 g to 94±3 g (P<0.001) and daily dry matter intake by 27% from 568±13 g to 417±6 g (P<0.01). Respective urea concentrations in plasma were reduced by 77% from 5.2±0.4 to 1.2±0.2 mmol l⁻¹ (P<0.01). Kinetic analyses of the initial rates of alanine uptake into isolated jejunal brush-border membrane vesicles from sheep and goats as affected by low dietary protein supply yielded that the apparent K_m was neither significantly different between the species nor significantly affected by the feeding regime thus ranging between 0.12 and 0.16 mmol^.l⁻¹. Reduction of dietary protein, however, resulted in significantly decreased V_max values of the transport system by 25–30%, irrespective of the species. Kinetic analyses of the initial rates of leucine uptake into jejunal brush-border membrane vesicles from sheep and goats yielded that leucine uptake was mediated by Na⁺-dependent as well as Na⁺-independent processes. Similar to alanine, apparent K _m values of leucine uptake were neither different between the species nor affected due to low dietary protein and ranged between 0.08 and 0.15 mmol l⁻¹. In contrast to the alanine transport mechanism, dietary protein reduction resulted in increased V _max values of Na⁺-dependent leucine transport by 53% in sheep and 230% in goats. Similarly, Na⁺-independent leucine uptake was stimulated by 85% and 200% in sheep and in goats, respectively. This study shows adaptation of amino acid absorption at the brush-border membrane level of jejunal enterocytes of small ruminants due to dietary protein reduction. Whereas the transport capacity for the nonessential amino acid alanine was reduced due to low dietary protein, the transport capacity for the essential amino acid leucine was markedly stimulated. From this, the involvement of rather different feedback mechanisms in adaptation of intestinal amino acid transport mechanisms has to be discussed.