Acidification of urine through dietary modification is considered to be the cornerstone for the management and prevention of struvite (magnesium ammonium phosphate) urolithiasis in cats (Taton et al. 1984). Urinary acidification, however, may be a risk factor for calcium oxalate urolithiaisis (Kirk et al. 1995, Osborne et al. 1995). The aim of this study was to compare the effect of commercial dry cat foods on urinary pH, and struvite and calcium oxalate relative supersaturation (RSS), the ultimate determinant of crystallization. Materials and methods. Nine different dry cat foods were fed to groups of six healthy, adult domestic short-haired cats (29 neutered females and 25 neutered males) for periods of between 14 and 21 d. Food allowances were calculated on the basis of an assumed maintenance requirement of 250 kJ/(kg body weight z d); the daily allowance was fed as two meals at 0800 and 1530 h. The cats were housed individually throughout each trial. All housing conditions and procedures fell within the UK Home Office regulations. During the last week of each trial, a 48-hour urine collection was made over dry ice to ensure rapid freezing of the samples. At the end of the collection period, the samples were thawed, and volume and pH were measured. The samples were acidified to pH 2 with hydrochloric acid and frozen for further analysis. Urine concentrations of sodium, potassium, magnesium, calcium, chloride, sulfate, phosphate, oxalate, citrate, pyrophosphate, ammonium and uric acid were analyzed by HPLC. A computer program, Equil 2 (Werness et al. 1985), was used to calculate urinary RSS for struvite and calcium oxalate from the concentrations of these analytes. This resulted in one struvite and one calcium oxalate RSS value for each cat during each trial. RSS is calculated from the activity product of the sample divided by the solubility product for the crystal in question; thus, values of ,1.0 correspond with undersaturation. Initially, an ANOVA was used to identify whether there were significant differences between mean trial urine pH and RSS values. Once significant differences within the data set were elucidated, it was further analyzed by a Newman-Keuls multiple range test . The relationship between mean urine pH and struvite and calcium oxalate RSS values was investigated using a simple regression procedure (Statgraphics plus version 2.1, Manugistics, Cambridge, MA.). Results. All diets resulted in the production of acidic urine, and all mean urine pH values were ,6.5. The mean urinary struvite RSS values varied as follows: two of the diets produced urine that was undersaturated, whereas the other seven products resulted in urine that was oversaturated with struvite to varying degrees. All of the diets produced urine that was oversaturated for calcium oxalate. There was no correlation between mean urine pH and struvite or calcium oxalate RSS values. Discussion. All diets tested resulted in a mean urine pH of ,6.5, a target suggested as appropriate for foods designed to prevent struvite urolithiasis. Nevertheless, diets A and H resulted in urine with a very high urinary struvite RSS, a value significantly greater than that for any of the other foods (P , 0.05). There were significant differences between the struvite RSS values for the other foods, and it is of interest that food F, which had the highest mean urine pH, actually resulted in one of the lowest struvite RSS values. Foods with the lowest urine pH values tended to have the highest calcium oxalate RSS values, although this relationship was not entirely consistent (Table 1). These data indicate that factors other than urine pH can be of importance in determining both struvite and calcium oxalate RSS.

Increasing evidence suggests involvement of free-radical species in the development of oxidative DNA damage, the consequences of which have been implicated in a number of degenerative disorders associated with the aging process. Here we report the application of a single-cell gel electrophoresis (comet) assay for assessing levels of DNA damage in canine and feline leukocytes. Leukocytes were collected from 24 healthy adult cats and dogs and subjected to DNA damage ex vivo by exposure to a range of hydrogen peroxide (H2O2) concentrations (0 -250 mol/L). The optimal concentration of H2O2 to induce a significant increase in DNA damage was 100 mol/L for both canine and feline leukocyte samples. Levels of DNA damage were assessed and quantified by visual and computer image analysis. The results obtained showed high correlations between visual scoring and computer image analysis for feline samples (percentage DNA in tail, R2 0.99; tail moment, R2 0.95; tail length, R2 0.90) and canine samples (percentage DNA in tail, R2 0.97; tail moment, R2 0.95; tail length, R2 0.91). In conclusion, this method provides a way of assessing levels of DNA damage utilizing visual and/or computer image analysis in the feline and canine systems. With the capacity of the comet assay to be able to measure end products of free-radical reactions, it is a useful tool for determining the optimal effects of dietary antioxidants on a reliable biomarker of oxidative stress such as cellular DNA status in cats and dogs. J. Nutr. 132: 1598S-1603S, 2002.