Renal responses to calcium deprivation in young rabbits

To evaluate the renal adaptations to dietary Ca deprivation, young growing female albino rabbits were fed a Ca-deficient diet for 8 consecutive days while they were housed in metabolism cages. Urinary Ca excretion rates decreased markedly within 24 h of Ca deprivation, reached a nadir by day 5, and remained low thereafter. Concomitantly, urinary P excretion increased. The hypocalciuria during the 8-day study was accounted for by an increased tubular reabsorption of Ca. Urinary cyclic AMP excretion was increased on the first day of Ca deprivation. Plasma immunoreactive parathyroid hormone and calcitriol concentrations were elevated after 8 days of Ca deprivation. Conversely, plasma calcifediol concentrations were decreased. We conclude that there are rapidly induced, appropriate renal homeostatic adaptations to dietary Ca deprivation in the rabbit and suggest that increases in endogenous plasma parathyroid hormone concentration, in part, play a role in mediating these changes.