Renal reabsorption of bicarbonate in puppies: effect of extracellular volume contraction on the renal threshold for bicarbonate.

Abstract

Extract: It has been recognized for many years that infants have lower concentrations of bicarbonate in plasma than older children and adults, due neither to a limited renal capacity to excrete hydrogen ion nor to a reduced capacity for reabsorption of bicarbonate, but to a low renal threshold for bicarbonate. The purpose of the present investigation was to determine whether the neonate and young of another species also could be shown to have a “physiologic acidosis” and, if so, to use this model for exploration of factors that might influence the threshold for bicarbonate. Mean pH of blood and concentration of bicarbonate in plasma in 11 young suckling puppies were 7.24 ± 0.018 and 18.2 ± 0.55 mmoles/liter, respectively, levels considerably lower than those found in adult dogs. In all but one animal pH of urine was below 6.0. Inasmuch as puppies thus appear to be in a state of “physiologic acidosis” and to have no limitation in acidification of the urine, we feel that this animal provides a suitable model for further examination of the characteristics of renal reabsorption and excretion of bicarbonate during immaturity. Titrations of bicarbonate performed in the usual manner demonstrated a threshold of 17–20 mmoles/liter, with a mean of 18.3. In the absence of significant volume expansion, maximal rates of reabsorption of bicarbonate were not observed despite filtered loads that were double those at threshold. Fractional excretion of sodium during control periods in animals which were not dehydrated averaged 2.9 ± 1.8% compared with 0.16 ± 0.08% in animals which were slightly dehydrated by their previous treatment with ammonium chloride. In this group, by replacement of gastric fluid (primarily sodium and potassium chloride) with a solution of sodium and potassium bicarbonate, renal handling of bicarbonate at varying filtered loads could be studied in the absence of volume expansion and with the continued stimulus for reabsorption of sodium. Under these circumstances, the renal threshold for bicarbonate was found to be markedly increased (25.2 ± 1.12). Saline was administered to two animals after the threshold had been passed. At any given level of filtered load the rate of excretion of bicarbonate in urine after administration of saline greatly exceeded the rate of excretion during gastric suction. The data in the present study thus indicate that the decreased threshold for bicarbonate in the young animal is not due to a limitation in intrinsic capacity for reabsorption of bicarbonate, inasmuch as capacity could be increased markedly under the stimulus