NITROGEN-BALANCE IN ADULT FEMALE MINK (MUSTELA-VISON) IN RESPONSE TO NORMAL FEEDING AND SHORT-TERM FASTING

Ten adult female mink (Mustela vison) were studied in a 7 d balance ex periment consisting of a 2 d pre-surgery feeding period, followed by s urgery, 1 d of recovery, 4 d of ad libitum feeding, and a 2 d fasting period. In this experiment (Expt A) the animals had osmotic pumps impl anted for continuous release of radioactively-labelled p-aminohippuric acid (p-aminobenzoyl-2-[H-3]glycine; [H-3]PAH; n 10) and C-14-labelle d inulin ([C-14]IN; n 5). Repeated 24 h collections of urine, correcte d to 100 % [H-3]PAH or [C-14]IN recovery, were used for accurate deter mination of N balances, 24 h urinary excretion of urea, creatinine, an d total N, and calculation of mean 24 h renal clearance rates for endo genous creatinine and inulin. N balances were slightly below zero, but not significantly different between feeding and fasting periods, indi cating that correction to 100 % [H-3]PAH recovery resulted in slight o verestimation of the final balances. During fasting, withdrawal of the dietary water and protein loads resulted in a dramatic decline in 24 h urinary volume, and urea and creatinine excretion. Large individual variations in 24 h urinary creatinine excretion (with relative variati on coefficients up to 30 %) confirmed that this is an unreliable index of the completeness of urine collection. In this respect, recovery ra tes of [H-3]PAH proved far more consistent. Renal clearance values obt ained in fed mink were in fair agreement with published data from cats , dogs and ferrets (Mustela putorius furo). Inulin clearance was about 30 % higher than endogenous creatinine clearance, although its declin e in response to fasting was not significant. In a separate study (Exp t B) another ten female mink were equipped with osmotic pumps containi ng [H-3]PAH for determination of 24 h excretion rates of purine deriva tives. During feeding, allantoin accounted for more than 97 % of the e xcretion of purine derivatives in urine, uric acid making up less than 2.5 %, xanthine and hypoxanthine less than 1 %. In fasted animals, ur inary excretion of each of these purine derivatives declined to less t han 50 % of the feeding value. In conclusion, an experimental techniqu e is presented for efficient and accurate measurements of daily urinar y excretion of nitrogenous constituents, which allows for correct dete rmination of N balances in adult mink and, presumably, in other mammal ian species.