Renal responses to phosphorus deprivation in young rabbits

To evaluate the renal adaptations to dietary P deprivation, young growing female rabbits were fed a P-deficient diet for 10 consecutive days while they were housed in metabolism cages. Urinary Ca excretion rates increased markedly within 24 h of P deprivation, remained high for each of the 10 days that dietary P was low, and returned to control values within 24 h of consuming a normal-P feed. The hypercalciuria was attributable to both an increased filtered load and decreased tubular reabsorption of Ca. Urinary P excretion rates decreased gradually in response to a low P diet and reached a nadir only after 9 days of deprivation. Urinary P excretion rates recovered to control values within 24 h of feeding a normal-P diet. Increased tubular reabsorption of P alone accounted for the hypophosphaturia. Plasma P concentration was reduced significantly after 10 days of dietary P deprivation, and this was associated with a significant increase in plasma calcitriol concentration. We conclude that dietary P deprivation in the rabbit effects the hypophosphatemia, hypophosphaturia, and hypercalciuria that characterize this condition in rats, dogs, and humans. Furthermore, the elevation in plasma calcitriol concentration that has been observed with dietary P deprivation in healthy rats and humans also occurs in the laboratory rabbit.