The effect of urine pH on plasma disposition of ampicillin sodium was evaluated. A single dose of 10 mg/kg of body weight was administered IV to Thoroughbreds with alkaline (pH > 8.0) or acidic (pH < 4.5) urine. Urine alkalinity was achieved and maintained by oral administration of up to 400 mg of sodium bicarbonate/kg/d, and acidity was achieved and maintained by oral administration of up to 400 mg of ammonium chloride/kg/d. Ampicillin sodium was measured in the plasma of horses by use of an agar diffusion microbiological assay with Bacillus subtilis as the test organism. The plasma disposition kinetics of ampicillin sodium best fitted a 2-exponential decay pattern, and statistically significant differences were not evident in elimination half-life, area under the plasma concentration time curve, volume of distribution, or body clearance rate between horses with alkaline or acidic urine. Results indicate that changes in urine pH over a range encountered in clinically normal horses are unlikely to affect plasma pharmacokinetic variables of ampicillin sodium after IV administration of the drug.

Hyperxanthinuria and xanthine uroliths have been recognized with increased frequency in dogs with ammonium urate uroliths that had been given allopurinol. We hypothesized that dietary modification might reduce the magnitude of uric acid and xanthine excretion in urine of dogs given allopurinol. To test this hypothesis, excretion of metabolites, volume, and pH were determined in 24-hour urine samples produced by 6 healthy Beagles during periods of allopurinol administration (15 mg/kg of body weight, PO, q 12 h) and consumption of 2 special purpose diets: a 10.4% protein (dry matter), casein-based diet and a 31.4% protein (dry matter), meat-based diet. Significantly lower values of uric acid (P = 0.004), xanthine (P = 0.003), ammonia (P = 0.0002), net acid (P = 0.0001), titratable acid (P 0.0002), and creatinine (P = 0.01) excreted during a 24-hour period were detected when dogs consumed the casein-based diet and were given allopurinol, compared with the 24-hour period when the same dogs consumed the meat-based diet and were given allopurinol. For the same 24-hour period, urine pH values, urine volumes, and urine bicarbonate values were significantly (P = 0.0004, P 0.04, and P = 0.002, respectively) higher during the period when the dogs were fed the casein-based diet and given allopurinol than when they were fed the meat-based diet and given allopurinol. Endogenous creatinine clearance was significantly (P = 0.006) lower when dogs were fed the casein-based diet and given allopurinol than when they were fed the meat-based diet and given allopurinol. Significantly lower concentrations of plasma uric acid (P 0.0001), plasma xanthine (P = 0.01), and serum urea nitrogen (P = 0.0001) were detected when dogs consumed the casein-based diet and were given allopurinol than when they consumed the meat-based diet and were given allopurinol.

Renal clearance procedures were performed on adult mixed-breed dogs with a wide range of renal function. Endogenous creatinine clearance was computed after analyzing plasma and urine for creatinine by use of 2 methods, PAP and kinetic Jaffe. For 20-minute clearance procedures, [14C]inulin clearance was measured simultaneously with endogenous creatinine clearance. For 111 twenty-minute clearance procedures performed on 24 dogs, [14C]inulin clearance was highly correlated with creatinine clearance for both methods of creatinine analysis (R2 = 0.979 for [14C]inulin-PAP; R2 = 0.943 for [14C]inulin-Jaffe). The absolute values for PAP and [14C]inulin clearance were nearly the same (PAP-to-[14C]inulin clearance ratio = 1.03 +/- 0.08), but those for Jaffe clearance were substantially less than those for [14C] inulin clearance Jaffe-to-[14C]inulin clearance ratio = 0.88 +/- 0.10). The Jaffe-to-[14C] inulin clearance ratio was inversely correlated with degree of renal function (R2 = 0.464), whereas the PAP-to-[14C]inulin clearance ratio was not correlated with degree of renal function (R2 = 0.060). Thus, Jaffe-determined creatinine clearance varied, in relation to [14C] inulin clearance, depending on degree of renal function. In 4 clinically normal dogs, 20-minute and 24-hour sample collections analyzed by use of the PAP method gave clearance values significantly greater, for both periods, than did Jaffe analyses. The PAP-determined creatinine clearance values were less than, but not significantly different from 20-minute exogenous creatinine clearance values determined 10 days after 24-hour collections. For 20-minute and 24-hour collections, the difference in clearance values between the PAP and Jaffe methods was attributable mostly to lower plasma creatinine values for the PAP method (mean +/- SEM, plasma PAP-to-Jaffe ratio = 0.798 +/- 0.053). However, urine creatinine values also were less by use of the PAP method.

Samples of serum and urine were obtained simultaneously from 56 healthy lactating cows to determine ranges of fractional excretion (FE) of calcium (Ca), phosphate (PO4), magnesium (Mg), sodium (Na), potassium (K), and chloride (Cl). Samples were obtained at 3 stages of lactation: period 1 = 1 to 7 days, 2 = 83 to 112 days, and 3 = 175 to 197 days. The FE of electrolytes were significantly different among periods 1, 2, and 3 for Ca (P < 0.001), PO4 (P < 0.025) and Mg (P < 0.025), but were not significantly different for Na, K, and Cl. Least squares mean FE of Ca was lowest in period 1 and not significantly different for periods 2 and 3, whereas mean FE values for PO4 and Mg were highest in period 2 and not significantly different for periods 1 and 3. The mean FE values of Na, K, and Cl did not change with stage of lactation. Age and category of milk production (high, medium, and low) did not influence the FE values of the electrolytes.

To evaluate underlying causes of calcium oxalate urolithiasis, 24-hour excretion of urine metabolites was measured in 6 Miniature Schnauzers that formed calcium oxalate (CaOx) uroliths during periods when they were fed a standard diet and during periods when food was withheld. Serum concentrations of parathyroid hormone and 1,25-dihydroxyvitamin D also were evaluated. Serum calcium concentrations were normal in all 6 affected Miniature Schnauzers; however, during diet consumption, mean 24-hour urinary excretion of calcium was significantly (P = 0.025) higher than calcium excretion when food was withheld. In 1 dog, urinary calcium excretion was lower during the period of food consumption, compared with the period when food was withheld. Compared with clinically normal Beagles, Miniature Schnauzers that formed CaOx uroliths excreted significantly greater quantities of calcium when food was consumed (P = 0.0004) and when food was withheld (P = 0.001). Miniature Schnauzers that formed CaOx uroliths excreted significantly less oxalate than clinically normal Beagles during fed (P = 0.028) and nonfed (P = 0.004) conditions. Affected Miniature Schnauzers also excreted abnormally high quantities of uric acid. Excretion of citrate was not different between Miniature Schnauzers with CaOx urolithiasis and clinically normal Beagles. In 5 of 6 Miniature Schnauzers with CaOx urolithiasis, concentrations of serum parathyroid hormone were similar to values from age- and gender-matched Miniature Schnauzers without uroliths. The concentration of serum parathyroid hormone in 1 dog was > 4 times the mean concentration of clinically normal Miniature Schnauzers. Mean serum concentrations of 1,25-dihydroxyvitamin D in Miniature Schnauzers with calcium oxalate urolithiasis were similar to concentrations of clinically normal Miniature Schnauzers.