Abstract
The kinetics of bicarbonate-chloride exchange across the human red cell membrane was studied by following the time course of extracellular pH in a stopped-flow rapid-reaction apparatus during transfer of H+ into the cell by the CO2 hydration-dehydration cycle, under conditions where the rate of the process was determined by HCO3--Cl- exchange flux across the membrane. The flux of bicarbonate increased linearly with [HCO3-] gradient from 0.6 to 20 mM across the red cell membrane at both 37 degrees C and 2 degrees C, and decreased as transmembrane potential was increased by decreasing extracellular [Cl-]. An Arrhenius plot of the rate constants for the exchange indicates that the Q10 is strongly dependent on temperature, being about 1.7 between 24 degrees C and 42 degrees C and about 7 between 2 degrees C and 12 degrees C. These data agree well with the published values for Q10 of 1.2 between 24 degrees C and 40 degrees C and of 8 between 0 degrees C and 10 degrees C. The results suggest that different processes may determine the rate of HCO3--Cl- exchange at low vs. physiological temperatures, and that the functional (and/or structural) properties of the red cell membrane vary markedly with temperature.
Highlights
The movement of anions across the red blood cell membrane, in particular the exchange of bicarbonate for chloride during and after COz outflow in the lungs or uptake in the tissues, is an important if not r a t e - determining step in the net transport of metabolic waste products from cells to environment
From the classical experimental data of Roughton (1964) one can calculate that approximately half of the total CO2 exchanged in the lungs in a resting human has to come from HCO3- that moves into the red cell from plasma during passage of blood through the lungs
Since the experimental half-time for C1--C1- exchange across the red cell membrane is about 0.2 s (Tosteson, 1959) and H + equilibration is on the order of tens of seconds (Forster and Crandall, 1975), while the average time spent by red cells in the pulmonary capillaries has been estimated to range from 0.1 to 2 s (Roughton, 1945; Roughton and Forster, 1957; see review by Piiper, 1969), it appears that red cell transmembrane exchanges limit the amount o f CO~ which can be eliminated from blood in the lung capillary
Summary
The movement of anions across the red blood cell membrane, in particular the exchange of bicarbonate for chloride during and after COz outflow in the lungs or uptake in the tissues, is an important if not r a t e - determining step in the net transport of metabolic waste products from cells to environment. From the classical experimental data of Roughton (1964) one can calculate that approximately half of the total CO2 exchanged in the lungs in a resting human has to come from HCO3- that moves into the red cell from plasma during passage of blood through the lungs. Since the experimental half-time for C1--C1- (or HCO3--C1-) exchange across the red cell membrane is about 0.2 s (Tosteson, 1959) and H + equilibration is on the order of tens of seconds (Forster and Crandall, 1975), while the average time spent by red cells in the pulmonary capillaries (pulmonary transit time) has been estimated to range from 0.1 to 2 s (Roughton, 1945; Roughton and Forster, 1957; see review by Piiper, 1969), it appears that red cell transmembrane exchanges limit the amount o f CO~ which can be eliminated from blood in the lung capillary. Dirken and Mook (1931), Piiper (1964), and Hemingway et al (1970), using continuous-flow rapidmixing filtration techniques, reported half-times for HCO~--CL- exchange that ranged from 0.04 to 0.2 s
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