Creatine is involved in the energy delivery process in tissues and in the phosphorylated form (phosphocreatine) is directly involved in maintaining low adenosine diphosphate concentrations at locations where energy is utilized. Creatine is synthesized from the amino acids arginine and glycine in the kidney and the liver (see Fig. 4.8). Creatine degrades in vivo to creatinine which is not used by the animal and is rapidly excreted in the urine. The irreversible conversion of creatine into creatinine occurs spontaneously and at a relatively constant rate. As such, creatinine is often used to validate quantitative urine collection in animals and humans although the suitability of creatinine for the validation of quantitative urine collection in humans has been questioned (Knuiman et al., 1986). Quantitative data on endogenous creatine and creatinine excretion in the urine of adult cats and dogs are scarce as many studies used creatinine-containing diets. Sagawa et al. (1995) showed that creatinine is absorbed by the intestine and excreted in the urine of adult cats. In adult dogs, Hoppe et al. (1993) reported a daily endogenous creatinine excretions of 33.9 mg kg-1 body weight. In adult cats, Worden et al. (1960) reported 24 h urinary creatine and creatinine values of 10.0 and 52.5 mg kg-1 body weight, respectively. The accuracy of these values, however, should be questioned as problems with the analysis of creatine and creatinine were reported and the cats were fed diets which most likely would have contained creatine and creatinine. Hendriks et al. (1997a) reported 24 h endogenous creatinine excretion levels in adult cats fed a proteinfree diet of 30.9 mg kg-1, and a slightly higher but not significantly different value was found using the regression to zero protein intake approach (31.7 mg kg-1). Hendriks et al. (1999) reported a similar value of 30.5 mg kg-1 day-1 in adult cats fasted for 2 days.
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