Endogenous excretion of purine derivatives, creatinine and nitrogen in crossbred (Bos indicus x Bos taurus) cattle

A study was carried out to elucidate the effect of fasting on endogenous urinary excretion of purine derivatives (PD), creatinine and nitrogen in crossbred (Bos indicus x Bos taurus) cattle. Daily excretion rates (μmol/kgW^0.75) of allantoin, uric acid and creatinine during fasting averaged 247.5, 49.1 and 746.3, respectively. The proportional contribution of allantoin (0.82–0.83) and uric acid (0.17–0.18) to total PD excretion (296.7) during pre-fasting and fasting period of the trial remained statistically (P>0.05) similar. The excretion rates of PD decreased significantly during fasting relative to pre-fasting period, however average daily urinary creatinine excretion (0.719–0.746 mmol/kg W^0.75/d) remained insignificant irrespective of level of feed offered to animals during various phases of fasting trial. Endogenous urinary nitrogen excretion (g/d or mg/kg W^0.75/d) observed during fasting was significantly (P<0.01) lower to the corresponding total N excretion recorded during pre-fasting period. The proportion of allantoin nitrogen to total nitrogen excretion decreased from 14.63 in pre-fasting to 6.57 per cent during fasting period. The creatinine concentration in plasma increased significantly during fasting (P<0.05). Similarly, the glomerular filtration rate (GFR, L/d or L/kg W^0.75/d) as proportion of plasma concentration of creatinine (mmol/L) to urinary creatinine excretion (mmol/d) was higher during pre-fasting compared to fasting period. Renal clearance expressed in terms of tubular load of allantoin, uric acid and total PD (mmol/d) showed a significantly (P<0.05) higher rate of tubular load during pre-fasting relative to fasting. Re-absorption of PD (mmol/d or % tubular load) during pre-fasting and fasting followed similar trend as observed for tubular load of its various constituents. The endogenous PD value (297μmol/kg W^0.75) obtained during the trial has been used for developing a model for predicting rumen microbial protein supply, however, further studies are warranted to improve the sensitivity and precision for estimating the contribution of endogenous PD in urinary PD excretion by Indian cattle.