Effect of the curvature of the O2 equilibrium curve on alveolar O2 uptake: theory

The effect of the curvature of the O2 equilibrium curve (OEC), in the range between mixed venous and alveolar PO2, on alveolar O2 uptake was quantitatively investigated in a simple homogeneous lung model. The O2 uptake achieved with a linear OEC (Mlin) was subtracted from the O2 uptake (M) attained with the physiologically curved, sigmoid OEC, and the relative difference was considered as the 'curvature effect', CE [= (M - Mlin)/M], indicating, if positive, the enhancement of O2 uptake by the non-linearity of the OEC. Calculations show CE to be close to nil (less than 1%) in normal lungs during rest both in normoxia and in hypoxia. CE is more important in heavy exercise both in normoxia (CE less than 19%) or slight hypoxia (CE less than 15%). In deep hypoxia, CE is negligible again even during exercise. Thus, the simplified approach to the analysis of alveolar O2 uptake using a linear OEC, in the mixed venous-to-alveolar PO2 range, constitutes in most cases a valid approximation.

Diffusion and perfusion limitation in alveolar O2 exchange: shape of the blood O2 equilibrium curve

The limitations imposed by diffusion (Ldiff) and perfusion (Lperf) on alveolar gas exchange can be estimated using a simple model of alveolar-capillary gas transfer (Piiper and Scheid (1981) Respir, Physiol. 46: 193-208). These limitations indicate the fractional increase of gas exchange that would occur by raising pulmonary conductances for diffusion or perfusion to functionally infinite values. The (simple) model assumes linear relations between concentration and partial pressure for the gases studied. We have investigated in this study the effects of this assumption for estimating Ldiff and Lperf for O2 whose blood equilibrium curve is particularly non-linear in normoxia. The calculations suggest that Lperf is only slightly overestimated by the assumption of linear blood O2 binding. For Ldiff, there is a significant overestimation in normoxia, but in hypoxia the linear equilibrium curve yields sufficiently accurate estimates. Calculations for data estimated for man on the summit of Mt. Everest suggest that alveolar O2 uptake in deep hypoxia at rest is mainly limited by perfusion and to a lesser degree by diffusion (Lperf greater than Ldiff). For the sustained exercise of climbing, on the other hand, diffusion limitation is more prominent than perfusion limitation (Ldiff greater than Lperf). Large values of Ldiff are estimated for normoxic O2 uptake across the skin of the gill-less and lung-less salamander, and here, the effects of the alinearity of the O2 equilibrium curve are pronounced. It is concluded that the simplified model of alveolar-capillary gas transfer, with linear O2 equilibrium curve, can be very useful to estimate diffusion and perfusion limitations from experimental data.

Time-variations of pretreatment peripheral blood S + G2/M-phase size determined by flow cytometry in adult acute myeloid leukaemia

The circadian and seasonal variations of pretreatment proliferative activity of peripheral blood (PB) as PB S + G2/M-phase size was determined by flow cytometry in 61 adult patients with acute myeloid leukaemia (AML). Pretreatment PB S-phase (p less than 0.002), G2 + M-phase (p less than 0.008) and S + G2/M-phase size are statistically correlated to the time of sampling, with the highest phase size at the end of the day. Time-variations of the blast cell count are slightly significant (p = 0.049). Cytological diagnosis-related differences in S + G2/M-phase (p less than 0.003) and white blood cell count (p less than 0.04) time-variations are observed. For all patients, no seasonal variations can be drawn, but in AML 1 (p less than 0.029) and AML 4-5 patients (p less than 0.003), the circadian variations of S + G2/M are affected by the seasons. The present results suggest that time may be taken into account in the monitoring of chemotherapy in acute leukaemia.

Effects of thromboxane on respiration and pulmonary circulation in the cat: role of vagus nerve

Infusion of stoichiometrically equal quantities of acid and base (neutral acid-base infusion) in the cat elicits pulmonary hypertension and rapid shallow breathing (J. Appl. Physiol. 62: 2362-2370, 1987), and thromboxane A2 (TxA2), released from platelets, is responsible for these effects (Respir. Physiol. 71: 169-183, 1988). To investigate the involvement of vagal afferent fibers in these responses, we reversibly blocked signal conduction in the vagus of the cat by bilaterally cooling the vagus nerves to 1 degree C and measured the cardiorespiratory parameters in response to neutral acid-base infusion and infusion of the TxA2 mimetic U-46619. Vagal cooling before infusion caused tidal volume (VT) to increase and respiratory frequency (fresp) to decrease, whereby total ventilation (VE) was slightly enhanced, but did not affect right ventricular blood pressure (Prv). Infusion of neutral acid-base after vagal cooling prompted Prv to rise, on average from 35 Torr to a peak of 60 Torr, and a similar rise was elicited by infusion of U-46619. However, vagal cooling abolished any effect on VT or fresp of both acid-base and U-46619 infusion. After rewarming the vagus nerves, infusion of U-46619 caused fresp to increase and VT to decrease (rapid shallow breathing) with a concomitant rise in Prv, similar to what had been observed in the earlier studies. Our data suggest that the effects of TxA2 and of its mimetic U-46619 on respiration are mediated by the stimulation of vagal afferent fibers, whereas pulmonary hypertension is unrelated to vagal activity.

Thrombin-induced cytosolic alkalinization in human platelets occurs without an apparent involvement of HCO3-/Cl- exchange

We have estimated the changes in cytosolic pH (pHi) that occur when human platelets are stimulated by thrombin. Changes in pHi were estimated (i) from the H+ efflux across the plasma membrane using an extracellular pH electrode and (ii) using an intracellular pH-sensitive fluorescent dye (BCECF). Stimulation of platelets with thrombin (0.5 unit/ml) resulted in an H+ efflux that averaged 7.7 +/- 1.6 mumol/10(11) platelets (means +/- SD) leading to an increase in pHi, from 7.05 +/- 0.04 to 7.45 +/- 0.05. Both H+ efflux and pHi changes were unaffected by 0.1 mM 4,4-diisothiocyanostilbene-2,2 disulphonate (DIDS), 0.1 mM 4'-acetamido 4'-isothiostilbene-2,2'-disulphonic acid (SITS), or 0.5 mM bumetanide, suggesting no involvement of anion transport systems, e.g. an HCO3-/Cl- exchange. Removal of HCO3- or Cl- from the suspending buffer had no effect on the extent of the rise in pHi. After blockade of Na+/H+ exchange by 100 microM ethylisopropylamiloride (EIPA), thrombin induced a decrease in pHi the rate of which averaged 0.39 unit/min in HCO3(-)-containing medium, and 0.57 unit/min in HCO3(-)-free medium. The cytosolic buffer capacity for H+ was determined by the nigericin/NH4Cl technique in BCECF-loaded platelets and averaged 25.3 mmol/(1xpH) in buffer containing 8 mM HCO3-, but only 17.2 mmol/(1xpH) in HCO3(-)-free buffer. The total amount of H+ transferred by Na+/H+ exchange can be estimated from our measurements at 10 mmol/l platelet cytosol in the absence of HCO3- and to 14 mmol/l platelet cytosol in the presence of HCO3-, and is in good agreement with the estimated amount of Na+ uptake by ADP-stimulated platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Buffering characteristics of eel blood at the extreme conditions in the swimbladder

CO2 binding in whole blood and true plasma of the eel was estimated by measuring CO2 content and pH in blood aliquots equilibrated with various PCO2 values over a wide range expected to occur in the swimbladder. Bicarbonate concentration, [HCO3-], was calculated using the CO2 solubility coefficient, which was measured to average 50 mumols.L-1.Torr-1 (20 degrees C). Buffer lines of non-bicarbonate buffers were obtained in plots of [HCO3-] against pH, and non-bicarbonate buffer values, beta NB, were obtained by curve fitting. In the pH range 6.6-8.2, the buffer line for oxygenated whole blood was sigmoid, while that for deoxygenated blood increased its slope monotonously with increasing pH. The beta NB for oxygenated blood displayed a maximum of about 8.6 mmol.L-1.pH-1 at pH = 7.4 and dropped down to below 1 mmol.L-1.pH-1 at higher and lower pH. Similar shapes of the buffer lines were obtained in true plasma; the [HCO3-] levels and beta NB values were, however, somewhat higher than in whole blood. These data are useful to assess the back-diffusion of CO2 and HCO3- in the rete mirabile of the fish swimbladder and to estimate the effects CO2 back-diffusion exerts on the counter-current enhancement of O2 in the rete.

CO2 back-diffusion in the rete aids O2 secretion in the swimbladder of the eel

In order to estimate the relative importance of back-diffusion of O2 and CO2 for their partial pressure enhancement in the swimbladder, we have determined O2 and CO2 partial pressure and content and pH in microsamples collected non-obstructively from the afferent and efferent rete vessels in the European eel. 1. The PO2 increased significantly along the arterial vessels of the rete (from 33 to 303 Torr), with no change in O2 content, suggesting O2 not to be exchanged in the rete counter-current system. 2. A corresponding increase of PCO2 (from 4 to 35 Torr) was accompanied by a significant rise in CO2 content (from 8 to 15 mmol.L-1), suggesting significant CO2 back-diffusion in the rete. 3. Changes in PO2 during passage of blood through the swimbladder epithelium were variable and small, and the PO2 in rete venous blood was similar to that in rete arterial blood, explaining the lack of O2 back-diffusion. 4. Using blood CO2 dissociation data, about 70% of the rise in arterial CO2 content was estimated to derive from diffusion of CO2, the remaining 30% from diffusion of HCO3-, from venous to arterial rete capillaries, or from H+ transport in the opposite direction. The data indicate that CO2 back-diffusion in the rete does not only raise the rete arterial PCO2; it also reduces the O2 capacity (Root effect) and thus enhances the arterial PO2.

Na+/H+ exchange modulates Ca2+ mobilization in human platelets stimulated by ADP and the thromboxane mimetic U 46619

According to recent observations ADP stimulates platelets via activation of Na+/H+ exchange which increases cytosolic pH (pHi). This event initiates formation of thromboxane A2 (via phospholipase A2) and, thereafter, inositol 1,4,5-trisphosphate (via phospholipase C) which is known to mobilize Ca2+ from intracellular storage sites. We investigated changes in pHi and cytosolic free Ca2+, [Ca2+]i, activating platelets with ADP and the thromboxane mimetic U 46619. We found that ADP (5 microM) increased pHi from 7.15 +/- 0.08 to 7.35 +/- 0.04 (n = 8) in 2'-7'-bis-(carboxyethyl)-5,6-carboxyfluorescein-loaded platelets, whereas thromboxane A2 formation was inhibited by indomethacin. ADP also induced a dose-dependent Ca2+ mobilization in fura2-loaded platelets which again was not affected by indomethacin. [Ca2+]i increased by 54 +/- 10 nM (n = 8) at 1 microM and by 170 +/- 40 nM (n = 7) at 10 microM ADP above the resting value of 76 +/- 12 nM (n = 47). Inhibition of Na+/H+ exchange by ethylisopropylamiloride (EIPA) reduced ADP-induced Ca2+ mobilization by more than 65% in indomethacin-treated platelets. This inhibition could be completely overcome by artificially raising pHi using either NH4Cl or the Na+/H+ ionophore monensin. We found that U 46619 increased pHi by 0.18 +/- 0.05 at 0.1 microM and by 0.29 +/- 0.07 (n = 7) at 1.0 microM above the resting value via an EIPA-sensitive mechanism. In conflict with the proposed role of the Na+/H+ exchange we found that U 46619 raised [Ca2+]i via a mechanism that for more than 50% depended on intact Na+/H+ exchange. Again, artificially elevating pHi restored U 46619-induced Ca2+ mobilization despite the presence of EIPA. Thus, our data show that Na+/H+ exchange is a common step in platelet activation by prostaglandin endoperoxides/thromboxane A2 and ADP and enhances Ca2+ mobilization independently of phospholipase A2 activity.

Counter-current blood flow in tissues: protection against adverse effects

In hypoxia, the tissue counter-current can thus, by virtue of the Bohr effect, increase tissue Po2 and thus tissue oxygenation. In hyperoxia, on the other hand, the counter-current system, acting as a diffusion shunt, can protect the tissue against adverse O2-toxic effects. It thus appears, that the counter-current system is advantageous for O2 supply to tissues.

Gas exchange in the fish swimbladder

The fish swimbladder acts as a device to adjust for neutral buoyancy at various depths. High gas pressures, corresponding to the ambient hydrostatic pressure, are encountered, most of which is made up by O2 and N2. To prevent gas loss, the swimbladder wall is made impermeable by guanine crystals in its wall. Gas deposition is made possible by lactic acid production in the swimbladder epithelium, which increases blood gas partial pressures of inert gases (salting-out effect), O2 (Bohr and Root effects) and CO2 (conversion from HCO3-). The hairpin counter-current blood flow in the rete mirabile enhances this partial pressure increase to the tremendous values, up to several 100 atm, encountered in deep sea dwellers. Flow balance in the rete capillaries is found to be crucial, and salt back-diffusion to be advantageous, for the concentrating efficiency in the rete mirabile.

Solute back-diffusion raises the gas concentrating efficiency in counter-current flow

We have extended the counter-current model of the rete mirabile of the fish swimbladder to include the effects of inert gas secretion into the swimbladder as well as solute back-diffusion in the rete capillaries. (1) Gas secretion attenuates the inert gas concentrating efficiency of the rete, i.e. its ability to produce high inert gas partial pressures in blood at the swimbladder pole. The maximum attainable gas secretion rate depends on the salting-out effect, i.e. on the ratio of solubility in venous and arterial blood of the rete. (2) Solute back-diffusion leads to a significant increase in the concentrating efficiency, and this is due to the salting-out effect produced by the solute when it diffuses back into the arterial capillary blood. This enhancement is particularly prominent when gas and solute permeabilities of the rete vessels are high. (3) Estimates for physiologic parameters in the European eel suggest that lactate back-diffusion may contribute significantly to the gas concentrating efficiency of the rete mirabile.

Water and lactate movement in the swimbladder of the eel, Anguilla anguilla

Hemoglobin (Hb) and lactate (La) concentrations were measured in small (150 microliters) blood samples collected with micropipettes from the inflow and outflow vessels of the rete mirabile of the eel swimbladder. Hemoglobin was used as a marker of the intravascular space. 1. Hemoglobin concentrations suggest that there was no significant water movement between the arterial and venous capillaries in the rete, but a significant 6% water efflux from the vascular space into the swimbladder epithelium. No significant differences in osmolality were observed between the sites of measurement. 2. Of the lactate present in the blood entering the venous capillaries of the rete, 30% derived from release by the swimbladder epithelium; 38% of the lactate entering the venous capillaries diffused back in the rete tissue into its arterial capillaries. 3. Theoretical models suggest that any water movement in the swimbladder, leading to blood flow mismatch in the rete counter-current system, reduces its efficiency to concentrate inert gases, whereas lactate back-diffusion enhances this efficiency.

Cardiopulmonary function in exercising bar-headed geese during normoxia and hypoxia

To investigate possible physiologic mechanisms that allow the bar-headed goose to perform strenuous physical activity when flying at high altitude (e.g., above 9,000 m), we measured cardiopulmonary variables during running exercise (treadmill; 0.6 m.sec-1; 2 degrees incline) while the bird breathed either normoxic (21% O2) or hypoxic (7% O2) gases via a face mask. 1. During normoxic exercise, O2 uptake rate doubled and both ventilation and cardiac output increased. Blood gases and pH in arterial, mixed venous and blood from the leg, however, remained virtually unaltered. 2. Hypoxia at rest stimulated ventilation to rise but not cardiac output. The birds reached a steady state with virtually unaltered O2 uptake. 3. Exercise during hypoxia further stimulated ventilation, resulting in elevated arterial PO2 and O2 content compared to hypoxia at rest. However, O2 uptake increased only slightly, and cardiac output did not rise over the resting hypoxic value. The hyperventilation resulted in respiratory alkalosis and increased CO2 output, with R values being as high as 2.0. 4. It is concluded that neither ventilation nor pulmonary gas transfer were the limiting step in supplying O2 to the working muscles during hypoxic exercise in our experiments. It is more likely that muscle blood flow or diffusion from muscle capillaries to mitochondria, or both, determined the aerobic capacity under these conditions.

Efficiency of parabronchial gas exchange in deep hypoxia: measurements in the resting duck

Cardio-respiratory parameters and air sac and blood gases were measured in the unrestrained, unanesthetized duck during exposure to varied levels of inspired hypoxia. As ventilation increased with hypoxia, the gas partial pressures in the air sacs and in arterial blood approached the inspired values, the differences being less than 3 Torr for air sacs and less than 5 Torr for arterial blood. To analyze the relative significance of ventilation, diffusion and perfusion in limiting parabronchial gas exchange, the conductances for ventilation (Gvent), diffusion (Gdiff = O2 diffusing capacity) and perfusion (Gperf) were calculated at all hypoxic levels. With increasing hypoxia, down to PIO2 = 50 Torr, all three conductances increased. Whereas Gvent continued to increase beyond this level, Gdiff remained constant, at about 0.27 mmol.min-1.Torr-1, while Gperf decreased, from a value of 0.22 mmol.min-1.Torr-1 at PIO2 = 50 Torr to 0.12 mmol.min-1.Torr-1 at PIO2 = 30 Torr. This reduction in Gperf may result from a direct hypoxic effect on the heart. Whereas ventilation is the main limiting factor for parabronchial gas exchange at rest down to hypoxia levels of PIO2 = 50 Torr, perfusion becomes the main limiting factor at deeper hypoxia. It is suggested that the higher tolerance of hypoxia exhibited by birds compared to mammals is not due to the higher efficiency of parabronchial compared with alveolar gas exchange, but reflects the ability of birds to tolerate lower arterial PCO2 levels than mammals.

Significance of the Bohr effect for tissue oxygenation in a model with counter-current blood flow

Counter-current arrangement of afferent and efferent blood flow in tissues is commonly considered to be detrimental to tissue oxygenation, since O2 diffusion would shunt O2 away from the tissue. We have investigated the combined effects of counter-current CO2 and O2 exchange in a simple model, paying particular attention to the Bohr effect. We have obtained the following main results. (1) Back-diffusion of CO2 leads to increasing CO2 partial pressure (PCO2) and CO2 content along the afferent vessel. This is enhanced when fixed acid is released by the tissue into the venous blood, e.g. during hypoxia, which leads to a further PCO2 increase therein. (2) The increasing PCO2, with concomitant decrease in pH, in the afferent blood leads to a decrease in blood O2 affinity (Bohr effect) and thus results in increased PO2. (3) The resulting O2 diffusion shunt diminishes the O2 content in afferent blood, but for most conditions its PO2 remains higher than without the Bohr effect. (4) During hypoxia, both the PO2 in blood reaching the tissue (Pta) as well as in that leaving it (Ptv) are significantly elevated above the level without the Bohr effect. Moreover, with fixed acid release both Pta and Ptv for O2 can be higher than the arterial PO2 value. (5) During hyperoxia, O2 diffusion shunt prevents the tissue PO2 levels from increasing to levels that might be regarded as toxic. It is concluded that a diffusion shunt in tissues stabilizes the O2 partial pressure at the tissue when it varies in arterial blood (hypoxia or hyperoxia).

Activation of Na+/H+ exchange and Ca2+ mobilization start simultaneously in thrombin-stimulated platelets. Evidence that platelet shape change disturbs early rises of BCECF fluorescence which causes an underestimation of actual cytosolic alkalinization

Although an increase in cytosolic pH (pHi) caused by Na+/H+ exchange enhances Ca2+ mobilization in platelets stimulated by low concentrations of thrombin [Siffert & Akkerman (1987) Nature (London) 325, 456-458], studies using fluorescent indicators for pHi (BCECF) and [Ca2+]i (fura2) suggest that Ca2+ is mobilized while the cytosolic pH decreases. Several lines of evidence indicate that the initial fall in BCECF fluorescence is not due to cytosolic acidification but is caused by a platelet shape change. (1) Pulse stimulation of platelets by successive addition of hirudin (4 unit/ml) and thrombin (0.2 unit/ml) induced a shape change of 43 +/- 8% and a fall in BCECF fluorescence, which both remained unchanged when Na+/H+ exchange was inhibited by ethylisopropylamiloride (EIPA, 100 microM). (2) Increasing the thrombin concentration to 0.4 unit/ml doubled the shape change and the fall in BCECF fluorescence, but again EIPA had no effect on these responses. (3) Treating platelets with 2 microM-ADP induced shape change and a decline in BCECF fluorescence that was unaffected by EIPA. (4) A second addition of thrombin to platelets that had already undergone shape change induced an immediate increase in BCECF fluorescence without a prior decrease. (5) Activation of protein kinase C by 1,2-dioctanoyl-sn-glycerol (DiC8) neither induced shape change nor a decline in BCECF fluorescence; in contrast BCECF fluorescence rapidly increased indicating an immediate cytosolic alkalinization. Concurrent analysis of [Ca2+]i under conditions in which shape change did not interfere with BCECF fluorescence showed that cytosolic alkalinization and Ca2+ mobilization started almost simultaneously. These observations suggest that cytosolic alkalinization is not preceded by a fall in pHi and can support Ca2+ mobilization induced by weak agonists.

Oxygen transfer kinetics of red blood cells of the turtle Pseudemys scripta elegans

The kinetics of O2 uptake into, and release from, the red blood cells (RBC) of the turtle Pseudemys scripta elegans were determined with a stopped-flow technique at varied temperature (10-30 degrees C) and pH (7.5-7.9). The results were compared to those obtained for RBC of other vertebrates and related to morphometric and physiological data on gas/blood diffusion in turtle lungs. The O2 transfer conductance of RBC, G, for O2 release into high concentrations of dithionite, considered to represent the best estimate of true RBC transfer conductance for O2 uptake and release, averaged 0.17 +/- 0.01 at 30 degrees C, 0.13 +/- 0.01 at 20 degrees C, 0.09 +/- 0.01 at 10 degrees C (mean +/- SD, in mmol.min-1.Torr-1.(mlRBC)-1). These values are about one half the corresponding value for human RBC, and this difference may be due to the larger size of turtle RBC (volume, 327 microns 3) compared to human RBC (90 microns 3). The temperature dependence of G, Q10 = 1.3 indicates that, as in human RBC, diffusion through aqueous media is the main limiting factor for O2 exchange. Morphometric data on the lungs of Pseudemys scripta suggest that the resistance to O2 transfer by RBC is lower than that offered by the gas-blood barrier. The total apparent transfer resistance to CO, obtained from previous measurements of pulmonary diffusing capacity for CO in the same species, is much higher than that predicted from the combination of RBC O2 kinetics and morphometric data on gas-blood barrier.

Afferent vagal activity during hyperthermic polypnea in the pigeon

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.

Cyclooxygenase inhibition and effects of hypoxia on pulmonary circulation and gas exchange in anesthetized dogs

To investigate whether endogenously produced prostanoids are involved in hypoxic pulmonary vasoconstriction, pulmonary hemodynamic and gas exchange parameters and eicosanoid metabolites were measured in 5 anesthetized, artificially ventilated dogs (mean body weight 27 kg). Hypoxia elicited pulmonary vasoconstriction, but blood plasma levels of thromboxane B2 (TXB2) and 6-keto-prostaglandin F 1 alpha (6kPGF1 alpha) (stable metabolites of TXA2 and prostaglandin I2, respectively) remained unchanged. Administration of the cyclooxygenase inhibitor indomethacin blocked the synthesis of prostanoids, so that 6kPGF1 alpha and TXB2 levels decreased to values below the detection level (10 pg.ml-1) both during normoxia or hypoxia, but did not affect pulmonary vascular resistance or the alveolar-arterial PO2 difference (PAi-Pa)O2. The pulmonary vascular bed remained, however, responsive to TXA2 as evidenced by infusion of the TXA2 mimetic, U 46619, which significantly increased the pulmonary vascular resistance and (PAi-Pa)O2. Our data suggest that prostanoids are not involved in eliciting the effects of hypoxia on pulmonary hemodynamics and gas exchange efficiency.

Absence of CO2-sensitive venous chemoreceptors in the cat

We tested for the presence of CO2-sensitive venous chemoreceptors in anesthetized, paralyzed, artificially ventilated cats (N = 8). Systemic venous PCO2 was elevated (venous CO2 loading) by continuously passing blood withdrawn from the femoral artery (20 ml/min) through an extracorporeal gas exchanger, ventilated with 50% CO2 and 50% O2, and returning this hypercapnic blood to the femoral vein. Respiratory output was assessed by means of the amplitude of the integrated phrenic neurogram. Results of venous CO2 loading were compared to those of airway CO2 loading in which inspired CO2 levels were adjusted to give the same arterial PCO2 (PaCO2) as in venous loading. Despite large differences in mixed venous PCO2 (PvCO2) during venous CO2 loading (PvCO2 = 55 Torr, PaCO2 = 37 Torr) compared to airway CO2 loading (PvCO2 = 45 Torr, PaCO2 = 37 Torr), phrenic output was unchanged. However, phrenic output was elevated 33% when PaCO2 was increased 6-7 Torr by raising inspired CO2 and reduced 50% when PaCO2 was lowered 6-7 Torr by lowering inspired CO2, thereby substantiating the responsiveness of the respiratory control system to changes in PaCO2. The respiratory output response to changes in venous CO2 was also tested at a higher PaCO2 (40 Torr, created by adding 1% CO2 to the inspired air) and, as before, no change in phrenic output occurred when PvCO2 was elevated at a constant PaCO2. These experiments provide direct evidence for the absence of chemoreceptors in the central veins, right heart, and pulmonary arterial system of the cat that would respond to changes in PvCO2.

Oxygen transfer of red blood cells: experimental data and model analysis

Kinetics of O2 uptake and release by human red blood cells (RBC) as measured by stopped-flow techniques were simulated using an RBC model shaped as a spheric shell. The O2 transfer mechanisms in this model include diffusion and reaction within the RBC and diffusion and convection in the medium surrounding the RBC. Unknown model parameters were determined by comparing simulations with experimental data. The following conclusions were drawn. (1) Both diffusion and convection contribute to O2 transport in the medium surrounding the RBC, and this transport importantly limits the overall O2 transfer kinetics in stopped-flow experiments. (2) Intraerythrocyte transport mechanisms become predominant in limiting O2 transfer, and can thus be investigated by stopped-flow techniques, only when the perierythrocyte O2 transport resistance is minimized, e.g. by high levels of dithionite in measurements of O2 release from RBC. (3) Intraerythrocyte O2 transfer is shown to be mainly limited by diffusion of O2 and, to a lesser extent, by diffusion of oxyhemoglobin ('facilitated O2 diffusion') and by O2/hemoglobin reaction. The results suggest that diffusion is the main process limiting O2 uptake and release by RBC, the finite reaction kinetics of O2 with hemoglobin exerting a smaller limiting effect.

Solubility of nitrogen and argon in eel whole blood and its relationship to pH

The solubility coefficients (alpha) for the inert gases, nitrogen (N2) and argon (Ar), were measured by mass spectrometry in whole blood of the freshwater-adapted European eel, Anguilla anguilla, at varied pH and at two temperatures, 5 and 15 degrees C. The pH was altered either by varying PCO2 (0.75-75 mmHg; 1 mmHg = 133.3 Pa) or by adding fixed acid (HCl or lactic acid). No dependence of alpha on pH (range 5.5-8.4) or on lactate concentration (range 0.2-25 mmol l-1) was detectable. Average values (+/− S.D.) for alpha (mumol l-1 mmHg-1) were: alpha N2 = 1.25 +/− 0.01, alpha Ar = 2.60 +/− 0.05 at 5 degrees C and alpha N2 = 1.09 +/− 0.03, alpha Ar = 2.12 +/− 0.07 at 15 degrees C. These data yield values for Q10 of 0.87 for nitrogen and 0.82 for argon, and for activation energy, Ea (kJ mol-1 K-1), of −9.2 for nitrogen and −13.4 for argon. The results do not support earlier reports on significant pH dependence of alpha in eel blood and suggest, in contrast, that no fundamental differences exist in respect of inert gas solubility between whole blood of the eel and of other vertebrates.

Acid infusion elicits thromboxane A2-mediated effects on respiration and pulmonary hemodynamics in the cat

We have recently reported that infusion of stoichiometrically equal quantities of acid and base (neutral acid-base infusion) in the cat resulted in rapid, shallow breathing and in pulmonary hypertension (Orr et al., 1987). To investigate the mechanisms involved in these effects, we have measured in the anesthetized cat thromboxane (TX) B2 and 6-keto-prostaglandin (PG) F1 alpha, the stable metabolites of TXA2 and PGI2, in blood as well as cardiorespiratory parameters in response to neutral acid-base infusion. The first acid-base infusion prompted right ventricular blood pressure (Prv) to rise from 30 to a peak of about 55 mm Hg, with a concomitant rise in the right ventricular TXB2 level from below detection level to over 500 pg/ml. The second or third infusion evoked no (or small) rises in Prv and TXB2, individual values of Prv and TXB2 being tightly correlated. After blockade of TX synthesis by Dazmegrel, no changes were observed even at the first acid-base infusion in either Prv or TXB2. The TXA2 mimetic, U 46,619, caused Prv to rise with no change in TXB2, and this effect was repeatable. Increases were also observed in ventilation, particularly in respiratory rate. We conclude that acid exposure of blood stimulates TX synthesis and release from platelets, which in turn leads to pulmonary hypertension and to hyperventilation. The fact that these effects cannot be repeated within the same animal is due to a lack in TX release but not to a loss of responsiveness of the TX receptors in the lung.