Effect of dietary magnesium level on urinary and faecal excretion of calcium, magnesium and phosphorus in adult, ovariectomized cats.

Dietary Mg restriction is generally considered to contribute to the prevention of struvite urolithiasis in cats, but its effects on faecal and urinary excretion of Ca and P have not been systematically investigated. The present study seeks to fill the gap. In a 4 x 4-week crossover study, ovariectomized cats were fed on purified diets containing either 0.40, 0.79, 1.59 or 3.17 mmol Mg/MJ (0.19, 0.38, 0.76 and 1.52 g Mg/kg diet). Increasing the dietary Mg level from 0.40 to 3.17 mmol Mg/MJ by the addition of extra MgCO3 raised urinary and faecal excretion of Mg from 0.14 to 0.68 mmol/MJ and from 0.28 to 1.66 mmol/MJ respectively. The 8-fold increase in Mg intake significantly raised urinary excretion of Ca from 0.06 to 0.09 mmol/MJ. Apparent absorption and urinary excretion of P were depressed by 13 and 25% respectively when the dietary Mg level was raised from 0.40 to 3.17 mmol/MJ. A dietary level of 0.40 mmol Mg/MJ (0.19 g Mg/kg diet) was found to be sufficient to maintain Mg balance in the adult ovariectomized cats fed on the purified diet.

Dietary phosphorus restriction to half the minimum required amount slightly reduces weight gain and length of tibia, but sustains femur mineralization and prevents nephrocalcinosis in female kittens.

The effects of dietary P restriction to half the recommended minimum level on growth, bone and renal mineralization and urinary composition were studied in female kittens. In two separate experiments, 8-week-old weanling kittens were fed on purified diets containing either 4.6 or 9.2 mmol P/MJ (2.8 or 5.6 g P/kg diet). In the second experiment there was an additional low-P diet in which the Ca concentration was reduced from 9.5 to 4.8 mmol/MJ (7.5 v. 3.8 g Ca/kg diet). P restriction slightly but systematically reduced weight gain (to a maximum of 16%) and growth of the tibia (by 1-4%); the former effect was statistically significant (P < 0.05) between the ages of 15 and 20 weeks in Expt 1 only, and the latter did not reach statistical significance at any time point (P > or = 0.13). No adverse effect of P restriction was found on mineralization of femur at the age of 39 weeks. Kidney Ca concentrations were significantly lowered (Expt 1, 6 v. 20 mumol/g dry weight, P < 0.001; Expt 2, 7 v. 16 mumol/g dry weight, P < 0.01) in cats fed on the low-P diets, this effect not being affected by the dietary Ca:P ratio. Urinary P concentration was significantly depressed (by 50-96%) after feeding the low-P diets (P < 0.001). P intake did not influence P, Ca and Mg retention during the period of 15 to 39 weeks of age.

Increasing phosphorus intake reduces urinary concentrations of magnesium and calcium in adult ovariectomized cats fed purified diets.

We assessed the phosphorus requirement of adult cats and the relationship between phosphorus intake and the fecal and urinary excretion of calcium, magnesium and phosphorus. Female cats (ovariectomized at the onset of sexual maturity) were fed purified diets containing 4.6, 9.2, 18.4 or 27.7 mmol phosphorus/MJ in a 4 x 4-wk crossover study. During the experiment, balance studies were performed and blood samples were taken. A dietary level of 4.6 mmol phosphorus/MJ was found to be sufficient to maintain phosphorus balance and normal plasma concentrations of phosphorus in the adult ovariectomized cats. Increasing levels of dietary phosphorus in the form of NaH2PO4.2H2O caused lower urinary pH values, lower urinary concentrations of calcium and magnesium, and higher urinary concentrations of phosphorus. When dietary levels of phosphorus were raised, the percentage of apparent absorption of magnesium was lower, whereas that of phosphorus was higher. Although it could be predicted that dietary phosphorus levels higher than the National Research Council recommendation of 9.2 mmol/MJ markedly reduced urinary struvite saturation, these higher levels are discouraged because they are associated with lower plasma phosphorus concentrations and creatinine clearance.

Dietary calcium chloride vs. calcium carbonate reduces urinary pH and phosphorus concentration, improves bone mineralization and depresses kidney calcium level in cats.

The effect of dietary calcium chloride vs. calcium carbonate on mineral metabolism was studied in cats. Ovariectomized cats and female kittens were fed purified diets with a normal calcium level (9.5 mmol Ca/MJ) but containing either calcium carbonate or calcium chloride, or were fed diets with a high calcium level (17.7 mmol Ca/MJ) containing either calcium carbonate alone or equimolar amounts of both calcium carbonate and calcium chloride. A 4 x 4-wk cross-over study using adult cats and a 31-wk parallel study using kittens were conducted. Calcium, phosphorus and magnesium balances were established regularly. In the course of the experiment with the kittens, blood samples were taken and X-ray photographs of the tibiae made. At the age of 39 wk, the kittens were killed, and organs and bones were collected. In both adult cats and kittens fed the high calcium diets, urinary concentrations of magnesium and phosphorus and apparent absorption of these minerals were lower than after feeding the normal calcium diets. Urinary pH and phosphorus concentration were lower in cats and kittens fed diets with calcium chloride instead of calcium carbonate. Body weight gain and tibia growth in the kittens tended to be greater after feeding the diets with calcium chloride. Calcium chloride vs. calcium carbonate and also supplemental calcium chloride in the high calcium diet significantly stimulated femur density and reduced renal calcium concentration.

Substitution of dietary calcium chloride for calcium carbonate reduces urinary ph and urinary phosphorus excretion in adult cats.

In a 4x4-wk cross-over study, eight adult cats were given four moist diets containing identical amounts of calcium (13.9 mmol/MJ) but with different ratios of calcium carbonate to calcium chloride, the calcium salts providing half of the total dietary calcium. Increasing amounts of calcium chloride were substituted for equimolar amounts of calcium carbonate. Higher intakes of calcium chloride caused significantly lower pH values in postprandial and 24-h urine samples. The urinary excretion of ammonium and titratable acid rose with increasing calcium chloride intake. The urinary concentrations of calcium and magnesium were not affected by the type of calcium salt, but the urinary excretion and concentration of phosphorus were significantly depressed when the amount of calcium chloride in the diet was increased. The results are discussed in the context of dietary prevention of and therapy for struvite urolithiasis in cats.

Increasing calcium intakes lower urinary concentrations of phosphorus and magnesium in adult ovariectomized cats.

The effect of dietary calcium level on the fecal and urinary excretion of calcium, phosphorus and magnesium was studied in adult cats. Ovariectomized cats were fed purified diets containing 3.2, 4.8, 9.5 or 19.3 mmol calcium/MJ in a 4 x 4 wk crossover study. During the experiment, balance studies were performed and blood samples were taken. The adult ovariectomized cats maintained calcium balance at all four levels of calcium tested. Extra dietary calcium, in the form of CaCO3, caused a slight increase in urinary pH. Urinary concentrations of phosphorus and magnesium dropped, but that of calcium remained unchanged, when the cats were fed diets with increasing calcium levels. The percentages of apparent absorption of phosphorus and magnesium fell when calcium intake was raised. The dietary calcium level did not affect plasma concentrations of calcium, magnesium and phosphorus or plasma activity of alkaline phosphatase.

Vitamin D, within its range of fluctuation in commercial rat diets, does not influence nephrocalcinogenesis in female rats.

Massive, toxic doses of vitamin D have been shown to cause nephrocalcinosis in rats, but the effect of this vitamin within its range of fluctuation in commercial rat diets was unknown. Therefore, in two experiments with young female rats, the effect on nephrocalcinosis of a moderately increased level of vitamin D in the diet was studied, that is 5000 IU/kg versus the recommended concentration of 1000 IU/kg. This was done using purified diets with 0.5% (w/w) calcium and 0.04% magnesium containing either 0.2 or 0.6% phosphorus (P). Rats fed the diets containing 0.6% P showed severe kidney calcification compared to those fed the 0.2%-P diets. The level of vitamin D in the 0.2 and 0.6%-P diets did not affect kidney calcification. Bone density was increased after feeding diets containing 5000 instead of 1000 IU of vitamin D/kg. This study suggests that, within 28 days, a moderate increase of the amount of vitamin D in the diet has no influence on the development of kidney calcification. This in turn suggests that the variation in nephrocalcinosis severity and incidence seen in practice in rats fed different commercial diets is unlikely to be related to the different vitamin D concentrations in these diets. However, in rats fed such diets bone metabolism may be influenced differently.

An alternative method for the quantitative collection of faeces and urine of cats as validated by the determination of mineral balance.

Generally, in order to collect faeces and urine of cats, the animals are housed in metabolism cages with wire-mesh bases. We have developed an alternative method to collect faeces and urine quantitatively and separately, using cages with solid floors. The use of such cages allows long-term experiments to be carried out without increased risk for injury to feet or legs. Modified litter boxes were constructed so that faeces and urine were separated and the cats could bury their faeces. With the use of this method the excretion of calcium, magnesium and phosphorus by mature cats was determined. Mineral retention over a period of ten days did not differ significantly from zero, suggesting that the method developed is valid for the measurement of mineral excretion. Evidence is presented that under our experimental conditions faeces should be collected for at least five days so as to cancel out largely the impact of day-to-day variation in defecation on the variance of mineral retention.

The use of porcine hepatocytes for biotransformation studies of veterinary drugs.

1. Cultures of porcine hepatocytes with high viability were isolated from a liver sample by a simple procedure. In ageing monolayer cultures the cytochrome P-450 content and 7-ethoxycoumarin-O-deethylase activity decreased gradually while glutathione levels increased. 2. The nitrofuran, furazolidone, was rapidly metabolized, partly resulting in the formation of 3-(4-cyano-2-oxobutylidene amino)-2-oxazolidone. 3. Acetylating and deacetylating activities towards sulphadimidine and its N4-acetyl metabolite were present in porcine hepatocytes. Relative and absolute levels of these activities varied in different batches of hepatocytes. 4. No differences were seen in a number of enzyme activities measured in cytosolic and microsomal fractions isolated from different lobes of one liver. Differences between livers from different animals were marked.