Summary In the present study, a proline‐deficient diet was fed to rats in order to reduce the plasma proline concentration to that observed during fasting and then the carotid‐artery‐renal‐vein differences in various amino acids and intrarenal amino acid concentrations were measured in order to investigate the kinetics of amino acid metabolism in the kidney. The plasma proline concentration in the carotid arteries of the proline‐deficient diet group 8 h after the beginning of the last feed was significantly lower than that in the control group, and the concentration of proline was as low as that of the fasted rats in the control group. The results of the carotid artery‐renal vein difference in proline showed that, at 8 and 16 h after the beginning of the last feed, proline was absorbed by the kidney in the control group and was released from the kidney in the proline‐deficient diet group. The results of the carotid artery‐renal difference in arginine 8 h after the beginning of the last feed showed that the amount of arginine released from the kidney was significantly lower in the proline‐deficient diet group than in the control diet group. The results of the carotid artery‐renal vein difference in intrarenal free amino acids 8 h after the beginning of the last feed showed that the concentrations of amino acids that are metabolically related to proline, such as, arginine, ornithine and alanine, were significantly lower in the proline‐deficient diet group than in the control diet group. There was significant positive correlation between the intrarenal proline concentration and the arteriovenous difference in proline 8 h after the beginning of the last feed (r = 0.653, p < 0.05). At 8 h after the beginning of the last feed, the release of proline from the kidney was observed in the proline‐deficient diet group, and there was significant positive correlation between the carotid‐artery‐vein difference in proline and intrarenal arginine concentration (r = 0.887, p < 0.05). The above findings suggest that when the intrarenal proline concentration decreases, proline is released from the kidney, and when the intrarenal proline concentration increases, proline is absorbed by the kidney. In addition, when the amount of proline released from the kidney increases, the intrarenal arginine concentration decreases. In combination, these results indicate that a portion of the proline released from the kidney is synthesized from arginine in the kidney. Zusammenfassung Beziehung zwischen der Prolinbilanz der Rattenniere und der intrarenalen Aminosäurekonzentration. Nach Fütterung von Ratten mit prolinfreiem Futter ergaben sich so niedrige Serumprolinkonzentrationen, wie sie bei fastenden Ratten beobachtet wurden. In der vorliegenden Studie untersuchten wir die Dynamik des Aminosäuremetabolismus in der Niere, indem wir in diesem Zustand die Prolinkon‐zentrationsdifferenz swischen der A. carotis und der V. renalis und die Konzentration der freien Aminosäuren in der Niere bestimmten. Die Messung der Plasmaprolinkonzentration in der A....

Introduction  Proline is widely found in all types of mammalian tissue, and accounts for about 20% of the amino acids that constitute collagen (A dams 1970). Proline is nutritionally nonessential but biologically it is an important amino acid and consequently mammalian organisms synthesize the required amounts of proline even in the absence of sufficient proline consumption via food. The metabolism of higher animals is unique, and amino acid metabolism differs from one tissue to the next. Some organs are capable of synthesizing nonessential amino acids for use throughout the body. For example, arginine is primarily synthesized in the kidney and then released and distributed throughout the body. Pyrroline‐5‐carboxylate reductase is the enzyme responsible for the final stage of proline synthesis, and its activity has been confirmed in many important organs and tissue such as the cartilage, liver, small intestine, kidney and thymus gland (H erzfeld et al. 1977; S mith and P hang 1978). However, the different levels of pyrroline‐5‐carboxylate reductase activity among these organs has led to the belief that different amounts of proline are synthesized in these organs. In order to ascertain biological responses to dietary proline deficiency, it is important to identify the organs that release and distribute proline throughout the body when insufficient proline is consumed through the diet, thus reducing the blood proline concentration. Few studies have investigated this issue, but when ascertaining biological responses to dietary proline deficiency, it is more important to elucidate the effect of dietary proline deficiency on the metabolism of proline and other amino acids that are closely related metabolically to proline, in proline synthesizing organs. One of the most effective ways to assess amino acid metabolism in a target tissue of higher animals is to measure the difference between the arterial and venous concentrations of amino acids. I shikawa (1974) measured arteriovenous differences in order to examine the release of proline from the kidney and small intestine of fasted rats and the uptake of proline by the liver. In a previous study, it was found that when the plasma proline concentration was reduced to the fasting level by the consumption of a proline‐deficient diet, proline was released from the kidney (W atanabe et al. 1995, 1997). In the present study, to ensure the induction of dietary proline deficiency, a completely purified diet containing all amino acids except for proline was prepared and fed to rats under experimental conditions. To investigate the role of the small intestine and liver in supplying and ingesting proline when the uptake of proline through food is restricted, the release and uptake of amino acids in the small intestine and liver were assessed by measuring carotid artery–portal vein and portal–hepatic vein differences in proline in rats.

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