The apparent pK of carbonic acid in rainbow trout blood plasma between 5 and 15°C

Abstract

Values for carbon dioxide solubility (αCO 2) and the apparent first dislocation constant (pK app) of carbonic acid in rainbow trout plasma were measured at 5, 10 and 15°C so as to eliminate the uncertainties with continued used of mammalian values extrapolated from the much higher temperatures of their determination. Estimates of pK app were based on the in vivo measurement criteria most commonly used ( i.e. whole blood pH, P CO 2 and the C CO 2 of true plasma separated from red cells at room temperature). Apparent pK varied inversely with pH, the dpK app/dpH slopes at 10 and 15°C (−0.075 and −0.080, respectively) being significantly elevated with respect to that at 5°C (−0.004). At constant pH, dpK app/dTemp varied between −0.0160 (pH 7.4) and −0.0208 (pH 8.0), both of which are higher than theoretically and experimentally based literature data on separated plasma. When we repeated our pK app determinations (using identical methods) on rainbow trout separated plasma, we obtained dpK app/dT slopes ranging from −0.009 to 0.0110, similar to all previous determinations. In attempts to account for the discrepancies between our whole blood and plasma based pK app estimates, we found that the pH of whole blood was always lower than that of its isothermally separated true plasma (0.015 units lower at 15°C) and that this difference became magnified at lower temperatures (0.033 units lower at 5°C). Also, if cool blood was allowed to warm towards room temperature before and/or during anaerobic centrifugation for true plasma, CO 2 was found to leave the red cells and result in a higher plasma total CO 2 content relative to the amount contained in the original blood plasma (0.40 mM for a 15°C dT of separation). We conclude that use of pK app values from gasometric determinations on separated plasma is not appropriate for P CO 2 or [HCO 3 −] calculation by the Henderson-Hasselbach equation when the practice is to measure the whole blood pH and the total CO 2 content of true plasma separated at temperatures other than that of the original blood plasma.