Experimental determination of net protein charge and A_tot and K_a of nonvolatile buffers in canine plasma

Acid-base abnormalities frequently are present in sick dogs. The mechanism for an acid-base disturbance can be determined with the simplified strong ion approach, which requires accurate values for the total concentration of plasma nonvolatile buffers (A_tot) and the effective dissociation constant for plasma weak acids (K_a). The aims of this study were to experimentally determine A_tot and K_a values for canine plasma. Plasma was harvested from 10 healthy dogs; the concentrations of quantitatively important strong ions (Na⁺, K⁺, Ca ²⁺, Mg²⁺, Cl^-, L-lactate) and nonvolatile buffer ions (total protein, albumin, phosphate) were determined; and the plasma was tonometered with CO₂ at 37°C. Strong ion difference (SID) was calculated from the measured strong ion concentrations, and nonlinear regression was used to estimate values for A_tot and K_a, which were validated with data from an in vitro and in vivo study. Mean (±SD) values for canine plasma were A_tot = (17.4 ± 8.6) mM (equivalent to 0.273 mmol/g of total protein or 0.469 mmol/g of albumin); K_a = (0.17 ± 0.11) × 10^-7; pK_a = 7.77. The calculated SID for normal canine plasma (pH = 7.40; PCO₂ = 37 mm Hg; [total protein] = 64 g/L) was 27 mEq/L. The net protein charge for normal canine plasma was 0.25 mEq/g of total protein or 0.42 mEq/g of albumin. Application of the experimentally determined values for A_tot, K _a, and net protein charge should improve understanding of the mechanism for complex acid-base disturbances in dogs.