[Methods for isotopic exploration of protein metabolism. Effects of anesthetics and surgical stress]

Abstract

Techniques using stable isotopes, the only ones authorized in France, allow a new insight into protein metabolism. These methods use an amino acid labelled with nitrogen 15, carbon 13 or deuterium, making it possible to measure the synthesis and overall breakdown of the body, as well as the speed of tissue synthesis and specific protein synthesis. Analytically, the techniques using nitrogen 15 are based on measurements of the enrichment in terminal urinary products: total nitrogen, urea or ammonia. Methods using carbon 13 have become preferred, even though they are more invasive, as they require sampling of blood and expired air. In hypercatabolic states, particularly in intensive care, reaching a steady state while measuring the flux of an amino acid entails the use of a short perfusion preceded by priming, to reduce the risk of recycling. The flooding dose method, consisting of the simultaneous injection of a labelled amino acid and a large quantity of unlabelled amino acid, seems to be of particular interest for measuring specific protein synthesis. Whole body breakdown is easily calculated from flux of labelled amino acid. The combination of isotopic infusion with the net forearm balance for amino acid allows to measure changes in muscle protein synthesis and breakdown. In contrast, measurement of breakdown is more difficult. For example, fractional breakdown rate and tissue sites of catabolism of plasma protein cannot be determined in humans. These isotopic techniques have contributed to improving our understanding of protein metabolism as far as synthetic and catabolic processes are concerned. The effect of food intake on protein turnover is different in muscle and liver. The immediate responses of food intake is an increase of muscular protein synthesis with a decrease of it's breakdown, while liver protein synthesis does not change. In contrast, liver breakdown increases during post absorptive state. Insulin plays a major role in regulation of protein synthesis and inhibits proteolysis. Surgical stress is characterized by an increase of whole body protein synthesis and breakdown and a net protein catabolism. Uncomplicated surgery, but not general anaesthesia, induces change in protein turnover. The protein synthesis rate is unaffected by general anaesthesia while a decrease is seen after surgery. The effect of anaesthetic agents is not well characterized. In rats, lung protein synthesis is reduced by halothane, pentobarbitone and midazolam combined with fentanyl while liver protein synthesis is depressed by all these agents, excepted by midazolam/fentanyl. The effects of anaesthetic agents on skeletal muscle and heart are minor.