The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2584S-2586S

Effects of Protein Supply on Plasma Urea and Creatinine Concentrations in Female Mink (Mustela vison)^1,2

Anne-Helene Tauson³ and Soren Wamberg^*

Department of Animal Science and Animal Health, The Royal Veterinary and Agricultural University, DK-1870 Frederiksberg C, Denmark and ^* Department of Physiology, Institute of Medical Biology, Odense University, DK-5000 Odense C, Denmark

KEY WORDS: carnivore · postprandial response · high protein · low protein · mink

	INTRODUCTION

Introduction References

In clinical veterinary medicine, single or serial measurements of plasma concentrations of urea and creatinine are widely used to evaluate the functional status of the kidneys. In carnivores such as dogs, cats and mink, however, the diagnostic value of these measures may be limited or uncertain, because they are markedly affected by nonrenal factors, and particularly by the amount and quality of dietary protein intake. Thus, Watson et al. (1981) showed that the plasma response in dogs was different when the animals were fed raw vs. heat-treated meat; the postprandial response in plasma urea was more affected by processing than that of creatinine. Changes in plasma concentrations of urea and creatinine, therefore, even in the postabsorptive state, must be interpreted with caution. The aim of this study was to evaluate the influence of dietary protein level on the postprandial changes in plasma concentrations of urea and creatinine in adult female mink given a single test meal.

Materials and methods.  Ten 2-y-old female mink (Mustela vison) of the pastel color type, weighing ~1060 g, were studied in two groups given either a high protein (HP) diet (n = 5) or a low protein (LP) diet (n = 5). The animals were confined in metabolism cages in the laboratory at natural (~8 h) daylight conditions for location (55°N, 12°E) and time of year (January). The experimental diets (Table 1) were based mainly on whole fish (cod) and whole chicken; the ingredients of animal origin were minced at a single occasion before the start of the experiment, deep frozen and stored until the day before feeding. They were taken out of the freezer at that time, thawed over night and mixed with dry ingredients on the day of feeding. The experiment was preceded by a 1-wk adaptation period in which the animals were given free access to the experimental diets; tap water was freely available throughout the study. The experiment started with a 12-h fasting period. The animals were then given free access to a weighed amount of the experimental diets for 15 min. The feed was then withdrawn, the amount consumed was recorded and a 48-h fasting period ensued. During fasting, blood samples were taken by venipuncture (Blixenkrone-Moller et al. 1987) 2, 4, 6, 8, 24 and 48 h after feeding. Results from the sampling at 48 h postfeeding were used as initial fasting values to ensure that the animals had reached the postabsorptive state. Plasma concentrations of urea and creatinine were determined by the urease (EC 3.5.1.5) method (Hallet and Cook 1971) and the alkaline picrate method (Chasson et al. 1961), respectively, using a Technicon RA-1000 AutoAnalyzer (Technicon Instruments, Tarrytown, NY) as previously described (Wamberg et al. 1992).

Results and discussion.  The voluntary feed intake after 12 h of restriction varied considerably among animals (Table 1, Fig. 1); moreover, it was affected by the dietary protein supply, and was 35.2 ± 12.0 g (mean ± SD) for the HP diet and 22.8 ± 8.0 g for the LP diet.

View larger version (25K): [in this window] [in a new window]   Fig 1. Individual changes in plasma urea concentrations of five adult female mink after ingestion of a high protein (HP) test meal. The closed symbols in the right-hand panel indicate the amount of feed ingested in a 15-min period after 12 h fasting. For details, see text.

View larger version (18K): [in this window] [in a new window]   Fig 2. Changes in plasma urea concentration (mean ± 2 SD) in adult female mink after ingestion of a high protein (, HP; n = 5) or a low protein (, LP; n = 5) test meal.

View larger version (18K): [in this window] [in a new window]   Fig 3. Changes in plasma creatinine concentration (mean ± 2 SD) in adult female mink after ingestion of a high protein (, HP, n = 5) or a low protein (, LP, n = 5) test meal.

The mean responses in plasma urea concentration (Fig. 2) were markedly influenced by the dietary protein supply, with peak values attained ~2 h after the test meal in mink fed the LP diet. In HP-fed mink, peak values were higher and were recorded after 4 h. In both groups, 24-h values were close to the initial fasting value, demonstrating that rapid feed passage rate occurs in mink, and that the 48-h fasting value was indeed representative of the "initial fasting value." Peak HP values and initial fasting values were in good agreement with Tauson et al. (1997). They reported data from animals with free access to a diet with a protein content similar to the HP diet in this investigation, and from the same animals during a 48-h fasting period. Individual response curves for plasma urea concentrations in HP-fed mink (Fig. 1) correlated well with the amount of feed ingested, and showed increases up to ~400% of the initial fasting value.

The plasma creatinine concentration curves (Fig. 3) showed a biphasic pattern, reflecting the combined effects of gastrointestinal absorption of the dietary load of preformed creatinine and its removal by renal glomerular filtration. The last-mentioned has been shown to increase remarkably in response to dietary protein loading in dogs (Premen 1988, Watson et al. 1981) and other carnivores (Hiatt and Hiatt 1942) and has also been clearly demonstrated in fed vs. fasted mink (Tauson et al. 1997).

Conclusions.  This experiment underscores the importance of feed-induced changes in plasma concentrations of urea and creatinine in carnivores. Interpretation should be made with caution and the effects of sampling time in relation to feeding and quantity of dietary protein intake taken into consideration.

	FOOTNOTES

	LITERATURE CITED

Introduction References

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• Hiatt E. P., Hiatt R. B. The effect of food on the glomerular filtration rate and renal blood flow in the harbor seal (Phoca vitulina L.) J. Cell. Comp. Physiol. 1942; 19:221-227
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• Tauson A.-H., Elnif J., Wamberg S. Nitrogen balance in adult female mink (Mustela vison) in response to normal feeding and short-term fasting. Br. J. Nutr. 1997; 77:83-96[Medline]
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0022-3166/98 $3.00 ©1998 American Society for Nutritional Sciences