Tomoko Kagawa

Estimation of the Transfer Coefficients of Oxygen and Carbon Monoxide in the Boundary of Human and Chicken Red Blood Cells by a Microphotometric Method

The reaction rates of O2 and CO with the human and chicken red blood cell (RBC) were measured by using a microphotometric apparatus. In the experiments on the human RBC, a small amount of RBCs were put in an air-tight reaction cuvette. Gas mixtures containing various concentrations of O2 and CO were sequentially injected into the cuvette and the change in O2 and CO saturation of hemoglobin was measured from the change in transmission of the RBCs at 402 and 416.5 nm. The reaction rate of CO with RBCs was significantly influenced by photodissociation of carboxyhemoglobin (COHb). To eliminate this, a short-pass filter (400 to 435 nm) and a sector (100 Hz) were used. By comparing the measured reaction rates of O2 and CO with the theoretical rates obtained from the numerical solutions of the partial differential equations of the diffusions of O2 and CO, the transfer coefficients of O2 and CO (eta O2 and eta CO) in the RBC boundary, including the RBC membrane and water layer around the RBC, were estimated. Both the values showed good agreement, ranging from 0.3 to 2.5 x 10(-6) cm.sec-1.Torr-1. Furthermore, the chorioallantoic capillary of chicken embryo was used for the measurements of the reaction rates of O2 and CO with RBC through the capillary membrane. The reaction rates of O2 and CO in the chorioallantoic capillary were slower than those obtained in the human RBC. By comparing the measured reaction rates and the numerical solutions, the eta O2 and eta CO in the boundary, including the capillary membrane, plasma, and RBC membrane, were estimated. These two values ranged from 0.1 to 0.4 x 10(-6) cm.sec-1.Torr-1 and showed good agreement. These results suggest that the diffusion rates for O2 and CO across the capillary and RBC membrane are similar.

Relationship between Alveolar-Arterial Po2 and Pco2 Differences and the Contact Time in the Lung Capillary

From the numerical solutions of simultaneous O2 and CO2 diffusions in the RBC, we calculated the P(A-a)CO2 by varying the tc at various PvO2 and PvCO2 levels, whereas the PAO2 and PACO2 were kept constant at 90 and 40 Torr, respectively. From the results we could clarify the relationship between the P(A-a)CO2 and the tc and its dependency on PvO2 and PvCO2. Furthermore, the influence of the tc on the C(a-v)O2, C(v-a)CO2 and R was clarified quantitatively.

Relation between the contact time and venous and alveolar PCO2 at rest.

The simultaneous partial differential equations for diffusions of O2, CO2, and HCO 3 − ions in the red blood cell (RBC) were solved numerically, taking chemical reactions of Bohr- and Haldane-effects into account. The diffusion equations and the chemical reactions were computed alternatively in an increment time of 2 msec. After solving each of the three diffusion equations, the Po2, O2 saturation (S), Pco2, pH and HCO 3 − content were corrected by using the equations of Bohr- and Haldane-effects, and a modified Henderson-Hasselbalch equation (Kagawa and Mochizuki, 1984). The Bohr-shift was calculated from Hill’s equation by assuming its K value to be a function of the intracellular pH. The change in intracellular Pco2 due to the Haldane effect was also evaluated by means of the modified Henderson-Hasselbalch equation, in which the buffer value was taken as 44 mmol · 1(RBC)−1 · pH c −1 . The computed Pco2 profiles during the Haldane effect in a closed vessel was compatible wit the experimental data of Klocke (1973). The extracellular Po2 profile computed during the Bohr-off-shift in a closed system coincided well with the experimental data of Nakamura and Staub (1964) and Forster and Steen (1968).

EFFECT OF DIFFUSION HETEROGENEITY ON OXYGEN TENSION IN TISSUE

Publisher Summary This chapter discusses the effect of diffusion heterogeneity on oxygen tension in tissue. It describes a theoretical study on the diffusion heterogeneity that was performed to elucidate the extent to which the diffusion rate and the PO 2 profile were influenced by the facilitated diffusion because of such an enzyme as cytochrome P-450. In the study, a simulation was made in a three-dimensional tissue model, assuming that a flat diffusion layer with an appropriate thickness was lying side by side at a constant distance. The PO 2 profile was calculated from the differential equation of the diffusion with a zero-order O 2 consumption rate in the steady state by using a point iterative method. The influence of the heterogeneity was apparently observed in a large cell of about 20 μm, suggesting a possibility to estimate the diffusivity of the facilitated diffusion layer quantitatively through the simulation technique. The PO 2 pattern in the semi-infinite tissue model was first calculated by changing the density of the facilitated diffusion layer, its thickness, and the PO 2 at the boundary. The diffusion quantity across the unit surface area increased with the PO 2 in the medium. The former was proportional to the square root of the latter; however, the ratio of the increase in diffusion quantity through the facilitated layer to the total diffusion quantity was almost independent of the PO 2 in the outer medium.

A Method for Estimating Contact Time of Red Cells in Lung Capillaries from O2 and CO2 Concentrations in Rebreathing Air in Man

In a previous paper (Mochizuki et al., 1986) we described the relation between alveolar- and venous-Pco2 and the contact time (tc). However, at that time, Pco2-dependency of the arterio-venous difference in O2 content ((a-v)Co2), the contact-time-dependency of the Haldane effect, and linearity of the relation between the experimental gas exchange ratio and Pco2 (R-Pco2 line) in rebreathing air were not taken into account. Recently, we have precisely analysed the above correlations from the numerical solutions of the simultaneous O2 and CO2 diffusions in the red blood cell (RBC). Based upon the results we have derived a corrected contact time equation. When the time constant of the reaction rate of the extracellular dehydration reaction was less than 0.2 sec, good agreement was observed between the contact time obtained from the pulmonary diffusing capacity for CO (Uchida, Shibuya and Mochizuki, 1986) and that from the present method.

Secondary CO2 Diffusion Following HCO3- Shift Across the Red Cell Membrane

In order to elucidate the effect of CO2 diffusion in the red blood cell (RBC) on the Bohr shift, it is absolutely necessary to evaluate the diffusivity of CO2 and HCO3 within the RBC as well as across the RBC membrane. Uchida et al.(1) measured the diffusion coefficients of CO2 and HCO 3 - ions in hemoglobin solution (Hb) in a Hb layer with a constant thickness by varying Hb concentration. Extrapolating the relation between the diffusion coefficient and Hb concentration, the coefficients of CO2 and HCO 3 - within the RBC were estimated as follows: D(CO2) = 3.4 × 10-6, and D(HCO 3 - ) = 1.4 × 10-6 cm2/sec, respectively. Furthermore, Niizeki et al. (2) measured the diffusion rate of CO2 into the RBC by using a stopped flow method, where the HCO 3 - shift was suppressed by contrplling intracellular Cl- concentration preliminarily. The half-time, being about 77 msec, was longer than the values measured by a rapid flow method by Pilper (3), and Constantine et al.(4). From Niizeki’s data the diffusivity across the boundary layer, or the transfer coefficient of CO2, η(CO2) was estimated to be; η(CO2) = 2 × 10-6 cm/sec • Torr, on an average. From our microphotometric observation it was confirmed that the Bohr shift does not occur in a buffer solution.

Estimation of the Rate of CO2 Diffusion Into the Red Cell Through the Bohr Shift

When the reactions of the red blood cell with O2 and CO2 are observed in a reaction chamber of a micro-photometric apparatus, the plasma layer around the cell tends to be superfluously thick and usually becomes highly resistive to the O2 and CO2 diffusions. Nevertheless, since the diffusivity of CO2 through aqueous solution, in general, is about 20 times as high as that of O2, the rates of the Bohr-on- and off-shifts have been thought to be almost the same as those of the oxygenation and deoxygenation. In our observation, however, the Bohr-on-shift was constantly slower than the oxygenation, while the Bohr-off-shift was comparable with the deoxygenation rate. The half-time of the Bohr-on-shift was about 60% longer than that of the oxygenation. This fact suggested that the diffusion rates of CO2 and/or HCO3 ions were comparable with that of O2 and they could even be evaluated quantitatively by comparing the Bohr-shifts with the oxygenation and deoxygenation rates. Thus, in the present study the estimation of the CO2 diffusion rate into and out of the red cell was attempted from the measured rates of the simple O2 reactions and the Bohr shift.

The Effect of Deoxygenation Rate of the Erythrocyte on Oxygen Transport to the Cardiac Muscle

Up to the present time O2 delivery to the cardiac muscle has been studied theoretically by many authors (1, 2, 11, 12) using a Krogh’s cylinder model. However, relatively little attention has been paid to the influence of deoxygenation of red blood cells (RBC) on the O2 delivery. Mochizuki (8) measured the O2 dissociation rate of oxygenated hemoglobin and RBC by using a rapid flow method, and found that the deoxygenation rate of RBC was proportional to the PO2 difference between RBC and surrounding medium, suggesting that the diffusion inside RBC and across the cell membrane was a rate limiting factor. The rate factor, Fc’, which is given by dividing the O2 quantity taken up by 1 ml RBC by the PO2 difference between RBC and surrounding medium, was 0.02 – 0.03 sec-1•mmHg-1. Recently, Tazawa et al (10) reported that the deoxygenation rate of RBC in the chorioallantoic capillary of chick embryos ranged from 0.008 to 0.009 sec-1•mmHg-1. In contrast to our observations, Lawson and Forster (5) previously described that the PO2 difference between RBC and plasma in tissue was negligibly small, when the rate factor measured by them in a RBC suspension mixed with hydrosulfite (4) was used.