CONTROL OF PROTEIN AND ENERGY METABOLISM BY EPINEPHRIN

拨款的最初目的是(a)使用氨基酸示踪剂确定肾上腺素降低氨基酸水平的生理机制，(b)确定肾上腺素是否是能量消耗慢性刺激和底物水平改变的重要影响因素，如在应激和创伤中所见，以及(c)确定由肾上腺素引起的初始急性发作后代谢率增加的底物提供能量。这些目标已在三项人体临床研究中得到解决，其中两项已发表，一项已提交发表。此外，我们已经扩展了我们的亮氨酸动力学研究，以解决使用不同的示踪剂对亮氨酸动力学测量的影响，这些测量对蛋白质合成氨基酸的处理有影响。目的1的目的是确定肾上腺素是如何降低人体血浆中氨基酸浓度的：通过增加系统中氨基酸的去除或减少蛋白质分解中氨基酸的外观。我们注射肾上腺素并测量亮氨酸和苯丙氨酸的动力学。我们确定肾上腺素严重影响氨基酸出现和消失的速度，净效应是氨基酸去除的净增加。然而，肾上腺素对氨基酸动力学的影响是短暂的，但在降低氨基酸浓度方面是慢性的。这些结果表明，肾上腺素输注对氨基酸代谢的影响是增加氨基酸代谢的代谢效率，使氨基酸通量率正常进行，但血浆氨基酸浓度较低。在后续研究中，我们使用相同的稳定同位素标记示踪剂，注入氨基酸来取代被肾上腺素抑制的浓度。这种情况被设计成类似于危重病人的肠外营养。再一次，我们发现肾上腺素对氨基酸代谢没有分解代谢作用，对静脉注射氨基酸作为肠外营养也没有负面影响。这些结果表明，肾上腺素升高对氨基酸和蛋白质代谢无不良影响。如果有的话，肾上腺素可能会促进蛋白质的保留，而不是在压力和创伤中浪费。在另一项关于肾上腺素对能量消耗影响的研究中，正常受试者输注生理盐水24小时，然后输注肾上腺素23小时，以使血浆肾上腺素浓度升高到高生理范围。肾上腺素输注短暂地增加心率和血压，但在肾上腺素输注期间显著增加能量消耗。肾上腺素的输注也产生尿流量和尿氮排泄的短暂增加。在肾上腺素输注停止后，尿量和氮排泄量的下降补偿了这种利尿。我们得出的结论是，肾上腺素会导致能量消耗的慢性增加，而这种增加的燃料来自碳水化合物氧化的增加，而不是脂肪或蛋白质。这些研究的结果表明，在应激和创伤中，肾上腺素的升高对维持蛋白质储存没有害处，只要以肠内或肠外营养的形式摄入能量，能量消耗的增加也不会有害。我们正在完成一项关于肾上腺素增加的氨基酸处理途径的后续研究。为了进行这项研究，我们需要能够直接测量蛋白质合成。我们已经开始开发这些技术，但与我们在上述研究中使用的[1- 13c]亮氨酸示踪剂相比，这些方法需要[1,2- 13c2]亮氨酸示踪剂。问题是，在酮异己酸脱羧过程中，位于2号碳上的13C不像1号碳那样被直接代谢。因此，使用不同的示踪剂测量亮氨酸氧化不一定相同。我们正在完成一项研究，以解决两种示踪剂之间的这些差异。当研究完成后，我们将能够确定两种示踪剂代谢氧化的差异，并通过其更复杂的代谢途径确定2号碳的途径。

The original aims of the grant were (a) to define using amino acid tracers, the physiologic mechanism by which epinephrine lowers amino acid levels, (b) to determine whether epinephrine is an important affector of chronic stimulation of energy expenditure and alteration of substrate levels as is seen in stress and trauma, and (c) to determine the substrates which supply the energy for the increased metabolic rate induced by epinephrine beyond the initial acute burst. These aims have been addressed in humans in three clinical studies, two of which have been published and one of which has been submitted for publication. In addition, we have extended our leucine kinetic studies to address the effect of using a different tracer upon leucine kinetic measurements that bear upon the disposal of amino acids for protein synthesis. The purpose of aim 1 was to define how epinephrine lowers amino acid plasma concentrations in humans: through increased removal from the system or decreased appearance of amino acids from protein breakdown. We infused epinephrine and measured leucine and phenylalanine kinetics. We determined that epinephrine acutely affects both rates of amino acid appearance and disappearance, with the net effect being a net increase in amino acid removal. However, the effect of epinephrine on amino acid kinetics was transient, but chronic in lowering amino acid concentrations. These results suggest that the effect of epinephrine infusion on amino acid metabolism is to increase the metabolic efficiency of amino acid metabolism allowing rates of amino acid flux to proceed normally, but at lower concentrations of plasma amino acids. In a follow-up study using the same stable isotopically labeled tracers, we infused amino acids to replace the concentrations which had been suppressed by infusion of epinephrine. This situation was designed to be similar to that used in parenteral nutrition of critically ill patients. Again, we found no catabolic effect of epinephrine upon amino acid metabolism and no negative effect upon delivery of intravenous amino acids as parenteral nutrition. These results indicate no deleterious effect of elevation of epinephrine upon amino acid and protein metabolism. If anything, epinephrine may promote retention of protein, rather than its wasting in stress and trauma. In another study addressing the question of epinephrine's effect on energy expenditure, normal subjects were infused for 24-h with saline, then for 23-h with epinephrine to increase plasma epinephrine concentrations into the high physiologic range. Infusion of epinephrine transiently increased heart rate and blood pressure, but significantly increased energy expenditure for the duration of the epinephrine infusion. Infusion of epinephrine also produced a transient increase in urine flow and in urinary nitrogen excretion. This diuresis was compensated by a drop in urine volume and nitrogen excretion after the epinephrine infusion was stopped. We concluded that epinephrine can produce a chronic increase in energy expenditure and that the fuel for this increase was from increased carbohydrate oxidation, not fat or protein. The results of these studies indicate that elevated epinephrine is not detrimental in stress and trauma with respect to maintaining protein stores, nor is the increase in energy expenditure produced harmful as long as energy intake is provided in the form of enteral or parenteral nutrition. We are in the process of completing a follow up study on the route of disposal of amino acids that is increased by epinephrine. To do this study, we need to be able to measure protein synthesis directly. We have begun developing these techniques, but the methods require a [1,2-13C2]leucine tracer compared to the [1-13C]leucine tracer we used in the above studies. The problem is that the 13C located in the 2nd carbon is not metabolized directly as is the 1st carbon during decarboxylation of ketoisocaproic acid. Thus, leucine oxidation will not necessarily be measured the same using the different tracers. We are completing a study addressing these differences between the two tracers. When the study is completed we will be able to define the difference between the metabolic oxidation of the two tracers and define the pathway of the 2nd carbon through its more convoluted route of metabolism.