Schröder B, Schöneberger M, Rodehutscord M, Pfeffer E, Breves G
Department of Physiology, School of Veterinary Medicine, Bischofsholer Damm 15, 30173 Hannover, Germany.
J Comp Physiol B. 2003 Aug;173(6):511-8. doi: 10.1007/s00360-003-0359-3. Epub 2003 Jun 14.
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 Km 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 Vmax 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 Km 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 Vmax 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.
本研究旨在探讨长期低蛋白饮食供应对空肠刷状缘膜氨基酸转运能力的潜在调节作用。为此，我们使用中性氨基酸L-丙氨酸（代表非必需氨基酸）和L-亮氨酸（代表必需氨基酸）作为模型物质。将10只8周龄、体重19 - 27 kg的绵羊羔羊分配到两种饮食方案中，分别给予充足或减少的蛋白质供应，这通过在浓缩饲料中分别提供17.9%和9.7%的粗蛋白来实现。饲养期为4 - 6周。同样，将8只5 - 7周龄、体重8 - 14 kg的山羊幼崽分配到充足蛋白质供应组（粗蛋白20.1%，饲养期9 - 12周）或减少蛋白质供应组（10.1%，饲养期17 - 18周）。羔羊饮食蛋白质减少导致体重显著下降0.6±0.7 kg，而对照动物体重增加了1.9±0.7 kg（P<0.05）。血浆尿素浓度显著降低了60%（低蛋白组2.3±0.1对比对照组5.7±0.2 mmol·l⁻¹，P<0.001）。在山羊幼崽中，饮食蛋白质摄入量减少导致日增重从181±8 g显著下降48%至94±3 g（P<0.001），日干物质摄入量从568±13 g显著下降27%至417±6 g（P<0.01）。血浆中相应的尿素浓度从5.2±0.4降至1.2±0.2 mmol·l⁻¹，降低了77%（P<0.01）。对低蛋白饮食供应影响下绵羊和山羊分离的空肠刷状缘膜囊泡中丙氨酸摄取初始速率的动力学分析表明，表观Km在不同物种间无显著差异，且不受饲养方案的显著影响，范围在0.12至0.16 mmol·l⁻¹之间。然而，饮食蛋白质减少导致转运系统的Vmax值显著降低25 - 30%，与物种无关。对绵羊和山羊空肠刷状缘膜囊泡中亮氨酸摄取初始速率的动力学分析表明，亮氨酸摄取由Na⁺依赖性和Na⁺非依赖性过程介导。与丙氨酸类似，亮氨酸摄取的表观Km值在不同物种间无差异，且不受低蛋白饮食影响，范围在0.08至0.15 mmol·l⁻¹之间。与丙氨酸转运机制不同，饮食蛋白质减少导致绵羊中Na⁺依赖性亮氨酸转运的Vmax值增加53%，山羊中增加230%。同样，绵羊和山羊中Na⁺非依赖性亮氨酸摄取分别被刺激85%和200%。本研究表明，由于饮食蛋白质减少，小反刍动物空肠肠细胞刷状缘膜水平的氨基酸吸收发生了适应性变化。虽然低蛋白饮食导致非必需氨基酸丙氨酸的转运能力降低，但必需氨基酸亮氨酸的转运能力却受到显著刺激。由此，必须讨论在肠道氨基酸转运机制适应性变化中相当不同的反馈机制的参与情况。
