Effects of Restricted Water-Intake on Digestion, Urea Recycling and Renal-Function in Wombats (Marsupialia, Vombatidae) From Contrasting Habitats

Responses to limited water availability were studied in two species of wombats from mesic (Vombatus ursinus) or xeric (Lasiorhinus latifrons) habitats. Four Vombatus and three Lasiorhinus were fed a low-quality straw-based diet containing 0.6% nitrogen and 68% neutral detergent fibre (dry-matter basis). Restriction to 50% of ad libitum intakes of drinking water reduced dry-matter intakes by 30% but did not alter digestibilities of fibre or nitrogen. Nitrogen balances were negative and similar between species and water intakes. Urea pool size (C-14 urea) increased during water restriction but urea-entry rates and the proportion of urea recycled to the gut were similar between water intakes (78-89%). Tritiated water was given to wombats in single intramuscular or intraperitoneal doses. Times to equilibration of tritium in urinary water were large and variable (45 +/- 36 h). Urinary tritium concentrations often declined erratically after equilibration, and were 14 +/- 14% lower than the tritium concentration in the blood. These irregular kinetics for tritiated water suggest that the water-dilution method requires validation for the wombats. Urinary and faecal water losses were reduced by 60% during water restriction. Water was mainly lost in the faeces, which were drier in Lasiorhinus (41 % dry matter) than in Vombatus (31 %). As blood haematocrit and plasma osmolality were similar between water intakes, extracellular spaces were apparently maintained during water restriction. Glomerular filtration rates (creatinine clearance) were low (12 mL min-1) and similar between water intakes. Therefore, a more concentrated urine was produced by tubular resorption in water-restricted wombats. Lasiorhinus had greater urinary osmolalities and urine: plasma ratios of creatinine, which reflected a greater urine-concentrating ability than Vombatus. Apparent water intakes and the ability to reduce urinary and faecal water losses in the wombats are similar to those of kangaroos. The contrasting abilities of Vombatus and Lasiorhinus to minimise both these water losses are directly related to their separate distributions.

PARATHYROID HORMONE AND PHOSPHATE HOMEOSTASIS IN MAN

ABSTRACT To determine whether parathyroid hormone is required for renal response to phosphorus loading, the effects of ingestion of 3100 mg of phosphorus daily for 3 days were compared in 5 treated hypoparathyroid patients and 5 normal control subjects of comparable age and sex. In both normal and hypoparathyroid subjects the response to phosphorus loading was characterized by an increase in urinary excretion of phosphorus without a concomitant increase in the rate of glomerular filtration of phosphorus. Some mechanism other than changes in rates of parathyroid hormone secretion probably accounts for renal responses to changes in phosphorus intake. The timing of urine and serum collections with regard to food intake and the total amount of phosphorus in the diet were found to affect the assessment of renal handling of phosphorus. The finding that a 3-day period of oral phosphorus loading did not increase the serum phosphorus level in the hypoparathyroid patients suggests that phosphorus restriction is not always necessary in the treatment of hypoparathyroidism. The addition of one more condition to the list of those that may affect the percentage of renal tubular resorption of phosphorus in no way decreases the usefulness of this determination in the diagnosis of hyperparathyroidism.