Experimental determination of net protein charge, [A]_tot, and K_a of nonvolatile buffers in bird plasma

The quantitative mechanistic acid-base approach to clinical assessment of acid-base status requires species-specific values for [A]_tot (the total concentration of nonvolatile buffers in plasma) and K_a (the effective dissociation constant for weak acids in plasma). The aim of this study was to determine [A]_tot and K_a values for plasma in domestic pigeons. Plasma from 12 healthy commercial domestic pigeons was tonometered with 20% CO₂ at 37° C. Plasma pH, PCO₂, and plasma concentrations of strong cations (Na, K, Ca), strong anions (Cl, L-lactate), and nonvolatile buffer ions (total protein, albumin, phosphate) were measured over a pH range of 6.8-7.7. Strong ion difference (SID) (SID ₅ = Na + K + Ca - Cl - lactate) was used to calculate [A] _tot and K_a from the measured pH and PCO₂ and SID₅. Mean (±SD) values for bird plasma were as follows: [A]_tot = 7.76 ± 2.15 mmol/l (equivalent to 0.32 mmol/g of total protein, 0.51 mmol/g of albumin, 0.23 mmol/g of total solids); K _a = 2.15 ± 1.15 × 10^-7; and pK_a = 6.67. The net protein charge at normal pH (7.43) was estimated to be 6 meq/l; this value indicates that pigeon plasma has a much lower anion gap value than mammals after adjusting for high mean L-lactate concentrations induced by restraint during blood sampling. This finding indicates that plasma proteins in pigeons have a much lower net anion charge than mammalian plasma protein. An incidental finding was that total protein concentration measured by a multianalyzer system was consistently lower than the value for total solids measured by refractometer.