The Critical Importance of Urinary Concentrating Ability in the Generation of Urinary Carbon Dioxide Tension

Jose A. L. Arruda; Luiz Nascimento; Pradeep K. Mehta; Donald R. Rademacher; John T. Sehy; Christof Westenfelder; Neil A. Kurtzman

doi:10.1172/JCI108847

The Critical Importance of Urinary Concentrating Ability in the Generation of Urinary Carbon Dioxide Tension

Jose A. L. Arruda, Luiz Nascimento, Pradeep K. Mehta, Donald R. Rademacher, John T. Sehy, Christof Westenfelder, and Neil A. Kurtzman

Published October 1, 1977 - More info

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Abstract

Measurement of urine to blood (U-B) carbon dioxide tension (P_CO₂) gradient during alkalinization of the urine has been suggested to assess distal H⁺ secretion. A fact that has not been considered in previous studies dealing with urinary P_CO₂ is that dissolution of HCO₃ in water results in elevation of P_CO₂ which is directly proportional to the HCO₃ concentration. To investigate the interrelationship of urinary HCO₃ and urinary acidification, we measured U-B P_CO₂ in (a) the presence of enhanced H⁺ secretion and decreased concentrating ability i.e., chronic renal failure (CRF), (b) animals with normal H⁺ secretion and decreased concentrating ability, Brattleboro (BB) rats, and (c) the presence of both impaired H⁺ secretion and concentrating ability (LiCl treatment and after release of unilateral ureteral obstruction). At moderately elevated plasma HCO₃ levels (30-40 meq/liter), normal rats achieved a highly alkaline urine (urine pH > 7.8) and raised urine HCO₃ concentration and U-B P_CO₂. At similar plasma HCO₃ levels, BB rats had a much higher fractional water excretion and failed to raise urine pH, urine HCO₃ concentration, and U-B P_CO₂ normally. At a very high plasma HCO₃ (>50 meq/liter), BB rats raised urine pH, urine HCO₃ concentration, and U-B P_CO₂ to the same levels seen in normals. CRF rats failed to raise urine pH, urine HCO₃, and U-B P_CO₂ normally at moderately elevated plasma HCO₃ levels; at very high plasma HCO₃ levels, CRF rats achieved a highly alkaline urine but failed to raise U-B P_CO₂. Dogs and patients with CRF were also unable to raise urine pH, urine HCO₃ concentration, and U-B P_CO₂ normally at moderately elevated plasma HCO₃ levels. In rats, dogs, and man, U-B P_CO₂ was directly related to urine HCO₃ concentration and inversely related to fractional water excretion. At moderately elevated plasma HCO₃ levels, animals with a distal acidification defect failed to raise U-B P_CO₂; increasing the plasma HCO₃ to very high levels resulted in a significant increase in urine HCO₃ concentration and U-B P_CO₂. The observed urinary P_CO₂ was very close to the P_CO₂ which would be expected by simple dissolution of a comparable amount of HCO₃ in water. These data demonstrate that, in highly alkaline urine, urinary P_CO₂ is largely determined by concentration of urinary HCO₃ and cannot be used as solely indicating distal H⁺ secretion.