Kinetics of Oxygen and Carbon Dioxide Exchange

A structure-function analysis of the left ventricle

This study presents a structure-function analysis of the mammalian left ventricle and examines the performance of the cardiac capillary network, mitochondria, and myofibrils at rest and during simulated heavy exercise. Left ventricular external mechanical work rate was calculated from cardiac output and systemic mean arterial blood pressure in resting sheep (Ovis aries; n = 4) and goats (Capra hircus; n = 4) under mild sedation, followed by perfusion-fixation of the left ventricle and quantification of the cardiac capillary-tissue geometry and cardiomyocyte ultrastructure. The investigation was then extended to heavy exercise by increasing cardiac work according to published hemodynamics of sheep and goats performing sustained treadmill exercise. Left ventricular work rate averaged 0.017 W/cm³ of tissue at rest and was estimated to increase to ∼0.060 W/cm³ during heavy exercise. According to an oxygen transport model we applied to the left ventricular tissue, we predicted that oxygen consumption increases from 195 nmol O₂·s^-1·cm^-3 of tissue at rest to ∼600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 90% of the oxygen demand rate and consistent with work remaining predominantly aerobic. Mitochondria represent 21-22% of cardiomyocyte volume and consume oxygen at a rate of 1,150 nmol O₂·s^-1·cm^-3 of mitochondria at rest and ∼3,600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 80% of maximum in vitro rates and consistent with mitochondria operating near their functional limits. Myofibrils represent 65-66% of cardiomyocyte volume, and according to a Laplacian model of the left ventricular chamber, generate peak fiber tensions in the range of 50 to 70 kPa at rest and during heavy exercise, which is less than maximum tension of isolated cardiac tissue (120-140 kPa) and is explained by an apparent reserve capacity for tension development built into the left ventricle.

Apparent diffusion limitations for CO(2) excretion in rainbow trout are relieved by injections of carbonic anhydrase

Experiments were performed in vivo to elucidate the underlying mechanism(s) of apparent diffusion limitations for CO(2) excretion in rainbow trout (Oncorhynchus mykiss). Ligation of two gill arches and the associated expected reduction in gill surface area of 30% caused pronounced respiratory acidosis as indicated by elevated arterial blood P(CO(2)) (Pa(CO(2))) and reduced arterial blood pH. Under conditions of normoxia, arterial blood P(O(2)) (Pa(O(2))) was not significantly (statistically) reduced. However, during hypoxia (water P(O(2))=70-80 mmHg), the apparent trend for reduced Pa(O(2)) values became statistically significant in fish with 15% surface area reduction. To determine whether the elevated Pa(CO(2)) in fish with reduced surface area (30%) reflected true diffusion limitations or chemical equilibrium limitations imposed by the relatively slow rate of red blood cell Cl(-)/HCO(3)(-) exchange, fish were injected with carbonic anhydrase (CA) to permit catalysis of HCO(3)(-) dehydration within the plasma. Injection of CA caused a lowering of Pa(CO(2)) by 0.87+/-0.32 mmHg within 120 min and thus essentially eliminated the increase in Pa(CO(2)) (1.04+/-0.33 mmHg) that was caused by the reduction in surface area. These results clearly demonstrate that the elevation in Pa(CO(2)) evoked by gill surface area reduction is a consequence of chemical equilibrium limitations rather than true diffusion limitations, per se.

Different red blood cell characteristics in a primitive agnathan (M. glutinosa) and a more recent teleost (O. mykiss) influence their strategies for blood CO2 transport

This study examines how the different red blood cell (rbc) characteristics in two lower vertebrates, the phylogenetically primitive hagfish and a more recent teleost, the rainbow trout, influence their strategies for blood CO2 transport. Deoxygenation of the blood resulted in a significant increase in rbc CO2 content in hagfish, but there were no significant changes in the CO2 content of plasma or whole blood under these conditions. In contrast, deoxygenation increased the CO2 content of the rbc, plasma and whole blood in the trout. These results demonstrate that the Haldane effect is much less important for CO2 transport in the hagfish as compared to the trout. The relative importance of the rbc and plasma in blood CO2 transport were roughly similar in hagfish and trout and were very different from that previously documented in another primitive vertebrate, the lamprey. In trout, however, the role of the rbc in CO2 carriage was increased upon the addition of the beta-adrenergic agonist isoproterenol (10(-5) M) to the blood. Taken together, these results and those recently collected for lampreys demonstrate that changes in rbc characteristics during vertebrate evolution have probably resulted in several important transitions in the strategy for blood CO2 transport.

The disequilibrium pH: a tool for the localization of carbonic anhydrase

The disequilibrium pH is defined as any discrepancy between the measured pH and the pH which would exist if CO2-HCO3-H+ reactions were at equilibrium. Measurement of the disequilibrium pH can be used to assess the status of CO2-HCO3--H+ reactions and, in combination with carbonic anhydrase (CA) or CA inhibitor treatments, may also be used to localize CA. Renal physiologists have used disequilibrium experiments to determine that HCO3- reabsorption in the kidney tubule occurs via proton secretion, and that CA activity is available to ultrafiltrate CO2-HCO3-H+ reactions in the proximal convoluted tubule, but not the distal tubule. Disequilibrium experiments were also used in investigating the availability of CA to CO2-HCO3--H+ reactions in water at the fish gill; the opposing results obtained in two studies have not yet been resolved. Respiratory physiologists have used the disequilibrium technique in vivo and with saline-perfused preparations to assess the availability of CA to plasma CO2-HCO3--H+ reactions following gas exchange. Saline-perfused preparations enable direct localization of CA activity, while in vivo measurements encompass the numerous factors affecting CO2-HCO3--H+ equilibration in a multi-phase solution. Given the many organs in which membrane-bound CA activity has now been identified, the usefulness of the disequilibrium pH technique has increased beyond its original applications in renal and pulmonary physiology.

An analysis of carbon dioxide transport in arterial and venous blood of the rainbow trout, Oncorhynchus mykiss, following exhaustive exercise

Arterial and venous cannulations were used to examine the characteristics of CO2 transport in pre and post branchial blood both at rest and during recovery from exercise. As in previous studies, exercise caused a marked decrease in the extracellular pH (pHe) in both arterial and venous blood. Except for a transient increase in venous blood immediately following exercise, plasma total CO2 ([CCO 2]pl) and whole blood total CO2 ([CCO 2]wb) decreased in both arterial and venous blood during recovery. Exercise also resulted in an increase in red blood cell total CO2 concentration ([CCO 2]i) and in the partial pressure of CO2 (PCO 2) in both arterial and venous blood. Activation of the adrenergic mechanism at the level of the red blood cell likely contributed to the increases observed in ([CCO 2]i) following exercise. At rest, the majority of the total [CCO 2] carried in arterial and venous blood could be attributed to the plasma, with 2 and 9% carried in the red blood cells, respectively. However, exercise resulted in an increase in the percentage of C02 carried within the red blood cell to 13.5 and 20% in arterial and venous blood, respectively. The total CO2 difference between pre and post branchial blood also increased following exercise suggesting an increase in CO2 excretion.

Kinetics of the Root effect and of O2 exchange in whole blood of the eel

Oxygen transfer kinetics in blood of the eel (Anguilla rostrata, A. anguilla) were measured spectrophotometrically in thin blood layers covered by Gore-Tex membranes, which allowed fast changes of the gas phase at the blood surface (Heidelberger and Reeves, 1990 J. Appl. Physiol. 68: 1854-1864). The following main results were obtained for A. rostrata (similar values were measured for A. anguilla): (1) step change in PO2 of the gas phase between 0 and 37 kPa at low PCO2 (0.19 kPa, blood pH, 8.1; 20 degrees C) yielded mean half times (t(on)) for O2 uptake of 7.1 msec, and for O2 release (t(off)), of 42.8 msec. Similar values were obtained at high PCO2 (19 kPa; blood pH, 6.9), indicating O2 kinetics to be independent of pH and PCO2; (2) decreasing the high PO2 from 37 to 14 kPa significantly prolonged oxygen uptake kinetics, but release kinetics were unaltered; (3) changing PCO2 from 0.19 to 19 kPa at constant high PO2 (37 kPa) resulted in a reduction of hemoglobin oxygen saturation (SO2) (Root-off reaction), with t(off) averaging 44.8 msec; likewise, changing PCO2 from 19 to 0.19 kPa increased SO2 with t(on) averaging 64.8 msec (Root-on reaction). As these half times comprise reactions at the hemoglobin molecule and conversion between CO2 and H(+)/HCO3-, the Root effect kinetics of the hemoglobin molecule appear to be even faster. It is concluded that the O2 exchange kinetics of eel blood are comparable with those of human blood.(ABSTRACT TRUNCATED AT 250 WORDS)