An appraisal of the in vivo role of phosphate as a modulator of urinary ammonium and titratable acid excretion in the acidotic rabbit

Infusion of an acidified ethanolic—dextrose solution enhances urinary ammonium excretion and increases acid resilience in non—mechanically ventilated acidotic rabbits

Hitherto, the rabbit has long been known to have a very poor tolerance to non—volatile acid. In this study, we tested the hypothesis that acid resilience in the acidotic rabbit can be increased by enhancing the plasma availability of a naturally occurring volatile fatty acid, namely acetate. To ascertain the relative merits of the respiratory and renal systems in contributing to that resilience, we conducted our studies in non—ventilated and mechanically ventilated acidotic animals. Using ethanol as a feeder of acetate, and to counteract the antidiuretic effects of surgical interventions, we induced acidosis in anaesthetised rabbits, by intravenously infusing an acidified ethanolic dextrose solution. We observed very potent respiratory regulation of arterial blood pH coupled with a notable renal response by way of a 25-fold increase in urinary ammonium excretion in the non—ventilated group. In contrast, arterial blood pH plummeted much more rapidly in the mechanically—ventilated animals, but the compensated renal response was enormous, in the form of an 85 -fold increase in urinary ammonium output. Despite this significant adaptive renal response, the non -mechanically ventilated group of rabbits showed the greater acid resilience. This was attributed to an acetate stimulated flux through a series of metabolic pathways, generating supplementary buffer in the form of bicarbonate and ammonia, complemented by a robust respiratory response.

The kinetics of inorganic phosphate excretion in the acidotic rabbit during intravenous phosphate loading: a pseudo-ruminant model

The rabbit is a much-used experimental animal in renal tubule physiology studies. Although a monogastric mammal, the rabbit is a known hindgut fermenter. That ruminant species excrete inorganic phosphate (Pi) mainly through the digestive system while non-ruminants eliminate surplus phosphate primarily through the renal system are acknowledged facts. To understand phosphate homeostasis in the acidotic rabbit, anaesthetized animals were infused with hydrochloric acid, after which they underwent intravenous phosphate loading. Biofluids were collected during the infusion process for analysis. Plasma Pi increased (7.9 ± 1.7 mmoles.Litre⁻¹ (N = 5) vs 2.2 ± 0.4 mmoles.Litre⁻¹ (N = 10) pre-infusion, (p < 0.001)), while urinary phosphate excretion was also enhanced (74.4 ± 15.3 from a control value of 4.7 ± 3 µmol.min⁻¹ (N = 9), pre-infusion, p < 0.001)) over an 82.5 minute Pi loading period. However, the fractional excretion of Pi (FePi) only increased from 14.2 ± 5.4% to a maximum of 61.7 ± 19% (N = 5) over the infusion period. Furthermore, the renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmPi/GFR) computed to 3.5 mmol.L⁻¹, while a reading of 23.2 µmol.min⁻¹.Kg.^0.75 was obtained for the transport maximum for Pi (TmPi). The high reabsorptivity of the rabbit nephrons coupled with possibly a high secretory capacity of the salivary glands for Pi, may constitute a unique physiological mechanism that ensures the rabbit hindgut receives adequate phosphate to regulate caecal pH in favour of the resident metabolically - active microbiota. The handling of Pi by the rabbit is in keeping with the description of this animal as a monogastric, pseudo-ruminant herbivore.