On the variation of urinary excretion of creatinine and purine derivatives in pregnant and lactating ewes given diets with different protein contents

The effect of the physiological state and dietary protein level on urinary excretion of creatinine (C) and purine derivatives (PD) was studied in two experiments carried out with pregnant and lactating ewes to evaluate whether the PD/C ratio in urine can be confidently used as an index of PD excretion. In both experiments ewes were given ammonia-treated straw and concentrates including different levels of fish meal and the excretion in urine and milk and the plasma concentration of C, allantoin (AL), xanthine, hypoxanthine and uric acid was measured. Creatinine excretion (in urine and milk) was higher in pregnant ewes than in those lactating (492 and 420 (s.e. 10.0) mu mol/kg maternal live weight(0.75)) and no significant differences were found due to number of foetuses and dietary protein level. The coefficient of variation was 0.10 in both pregnancy and lactation and individual variation accounted for proportionately 0.78 and 0.93 of total variation. The AL/C ratio in urine was highly correlated with daily AL excretion (r = 0.90 and 0.78 in pregnant and lactating ewes, respectively). Changes in PD excretion with experimental treatments were mainly reflected in AL, as the main component (0.83) of total PD. Most of the variation in AL excretion was explained by differences in rumen fermentable organic matter intake (RFOMI) (R-2 = 0.79) and AL excretion did not differ between treatments when expressed per kg of RFOMI. In contrast to this the ratio AL/digestible organic matter intake decreased with increasing levels of fish meal in the diet. Urinary PD excretion was better related to estimated PD kidney tubular load (r = 0.76) than to PD plasma concentration (r = 0.64). The results suggest that creatinine excretion is scarcely affected by the number of foetuses in pregnancy and dietary protein level but if the AL/C purine is used instead of total collection as an index of purines absorbed in the duodenum, differences in urinary creatinine excretion due to physiological state must be accounted for.