Local regulation of pulmonary blood flow and ventilation-perfusion ratios in the coatimundi.

Small catheters (ca. 3 mm diam at tip) were wedged in subsegmental bronchi in anesthetized coatimundi (Nasua nasua) during spontaneous breathing. Mixed expired gases of a group of lobules were sampled continuously without contamination from neighboring units, and local tidal volume, frequency, carbon dioxide production, and oxygen consumption were measured, as well as mixed venous PO2 and PCO2. Local ventilation-perfusion ratio, alveolar PO2, PCO2, and blood flow were calculated. There was a 22% reduction (range 15-38) in local perfusion (as percent of flow at PAO2 100 mmHg) per 10 mmHg fall in local alveolar oxygen tension over the PAO2 range 150-36 mmHg. Local hypercapnia had little effect on local flow. Local tidal volume (ca. 1% of total tidal volume) was unaffected by changes in alveolar gas tensions. The contribution of vasoconstriction or vasodilatation, as a negative feedback system, to the stability of local PAO2 was greatest close to the physiologic range (65-85 mmHg) falloderate efficiency.     

Insulin binding to cells of several tissues of the early chick embryo.

The binding of I¹²⁵-labeled insulin to isolated cells from several tissues of the 3- and 4-day chick embryo was determined over a concentration range of insulin from 2 × 10^?11 to 2 × 10^?7M. The cells were obtained from limb bud and nonlimb bud tissues of the 4-day chick, from the headless 3-day chick embryo, and from cartilage of the 12-day embryo. The amount of bound insulin was found to be similar for the cells from the different embryonic tissues. Some implications of these findings for the interpretation of the nature of the binding sites and the teratogenic effect of insulin are discussed.     

Monoclonal antibodies specific to quail embryo tissues: their epitopes in the developing quail embryo and their application to identification of quail cells in quail-chick chimeras.

Quail-chick chimeras have been used extensively in the field of developmental biology. To detect quail cells more easily and to detect cellular processes of quail cells in quail-chick chimeras, we generated four monoclonal antibodies (MAb) specific to some quail tissues. MAb QCR1 recognizes blood vessels, blood cells, and cartilage cells, MAb QB1 recognizes quail blood vessels and blood cells, and MAb QB2 recognizes quail blood vessels, blood cells, and mesenchymal tissues. These antibodies bound to those tissues in 3-9-day quail embryos and did not bind to any tissues of 3-9-day chick embryos. MAb QSC1 is specific to the ventral half of spinal cord and thymus in 9-day quail embryo. No tissue in 9-day chick embryo reacted with this MAb. This antibody binds transiently to a small number of brain vesicle cells in developing chick embryo as well as in quail embryo. A preliminary application of two of these MAb, QCR1 and QSC1, on quail-chick chimeras of neural tube and somites is reported here.     

Development of phenobarbital-sensitive control mechanisms for uridine diphosphate glucuronyltransferase activity in chick embryo liver

Uridine diphosphate (UDP) glucuronyltransferase activity in chick liver rises at hatching from near zero to adult levels. This rise will occur prematurely in embryo liver during organ culture. Increase in enzyme activity during organ culture differs with embryo age: in liver from 11-day old embryos it ceases at adult values; in liver from 5-day old embryos it continues to much higher-than-adult levels. Phenobarbital added to culture medium accelerates these rises in enzyme activity and elevates the plateau reached in 11-day embryo liver to that observed in 5-day embryo liver. Kinetic analysis of the changes in enzyme activity induced by phenobarbital during culture suggests that the regulatory mechanisms for enzyme activity are different in 5- and 11-day embryo liver and that these differences reflect developmental changes occurring in ovo.     

Pulmonary gas exchange in Andean natives with excessive polycythemia--effect of hemodilution

Pulmonary gas exchange in Andean natives (n = 8) with excessive high-altitude (3,600-4,200 m) polycythemia (hematocrit 65.1 +/- 6.6%) and hypoxemia (arterial PO2 45.6 +/- 5.6 Torr) in the absence of pulmonary or cardiovascular disease was investigated both before and after isovolemic hemodilution by use of the inert gas elimination technique. The investigations were carried out in La Paz, Bolivia (3,650 m, 500 mmHg barometric pressure). Before hemodilution, a low ventilation-perfusion (VA/Q) mode (VA/Q less than 0.1) without true shunt accounted for 11.6 +/- 5.5% of the total blood flow and was mainly responsible for the hypoxemia. The hypoventilation with a low mixed venous PO2 value may have contributed to the observed hypoxemia in the absence of an impairment in alveolar capillary diffusion. After hemodilution, cardiac output and ventilation increased from 5.5 +/- 1.2 to 6.9 +/- 1.2 l/min and from 8.5 +/- 1.4 to 9.6 +/- 1.3 l/min, respectively, although arterial and venous PO2 remained constant. VA/Q mismatching fell slightly but significantly. The hypoxemia observed in subjects suffering from high-altitude excessive polycythemia was attributed to an increased in blood flow perfusing poorly ventilated areas, but without true intra- or extrapulmonary shunt. Hypoventilation as well as a low mixed venous PO2 value may also have contributed to the observed hypoxemia.     

Metabolism of 4-hydroxyaminoquinoline-1-oxide and its binding to DNA in chick embryos

The metabolic pathway of 4-hydroxyaminoquinoline-1-oxide (4HAQO) and its binding to DNA was studied in 2-day chick embryos administered [G-3H]4HAQO in a shell-less culture. The 4HAQO rapidly metabolized into non-carcinogenic compounds and 1 h after administration only very small amounts of free 4HAQO could be detected in the embryo cells. The amount of DNA-bound 4HAQO in the embryo cells reached a maximum 2 h after administration, then began to decrease. The maximum extent (mu mol/mol P of nucleotide) was 18.2, equivalent to 1 molecule of 4HAQO-purine adducts per 2.8 X 10(4) base pairs of DNA. It was possible to detect removal of 4HAQO-purine adducts from DNA in chick embryo cells in a shell-less culture. A dose-response relationship for the killing effect of 4HAQO on 2-day embryos was observed in the range of 0.24-24 nmol 4HAQO per embryo. The practicality of the present method of administration of 4HAQO for 'flash administration' of compounds to chick embryo and the advantages of the shell-less culture method which provides access for biochemical and developmental studies of chick embryos were also discussed.     

Operation Everest II: oxygen transport during exercise at extreme simulated altitude

A decrease in maximal O2 uptake has been demonstrated with increasing altitude. However, direct measurements of individual links in the O2 transport chain at extreme altitude have not been obtained previously. In this study we examined eight healthy males, aged 21-31 yr, at rest and during steady-state exercise at sea level and the following inspired O2 pressures (PIO2): 80, 63, 49, and 43 Torr, during a 40-day simulated ascent of Mt. Everest. The subjects exercised on a cycle ergometer, and heart rate was recorded by an electrocardiograph; ventilation, O2 uptake, and CO2 output were measured by open circuit. Arterial and mixed venous blood samples were collected from indwelling radial or brachial and pulmonary arterial catheters for analysis of blood gases, O2 saturation and content, and lactate. As PIO2 decreased, maximal O2 uptake decreased from 3.98 +/- 0.20 l/min at sea level to 1.17 +/- 0.08 l/min at PIO2 43 Torr. This was associated with profound hypoxemia and hypocapnia; at 60 W of exercise at PIO2 43 Torr, arterial PO2 = 28 +/- 1 Torr and PCO2 = 11 +/- 1 Torr, with a marked reduction in mixed venous PO2 [14.8 +/- 1 (SE) Torr]. Considering the major factors responsible for transfer of O2 from the atmosphere to the tissues, the most important adaptations occurred in ventilation where a fourfold increase in alveolar ventilation was observed. Diffusion from alveolus to end-capillary blood was unchanged with altitude. The mass circulatory transport of O2 to the tissue capillaries was also unaffected by altitude except at PIO2 43 Torr where cardiac output was increased for a given O2 uptake. Diffusion from the capillary to the tissue mitochondria, reflected by mixed venous PO2, was also increased with altitude. With increasing altitude, blood lactate was progressively reduced at maximal exercise, whereas at any absolute and relative submaximal work load, blood lactate was higher. These findings suggest that although glycogenolysis may be accentuated at low work loads, it may not be maximally activated at exhaustion.     

Enhanced cloning efficiency of mouse bone marrow macrophage progenitors correlates with increased content of CSF-1 receptor of their progeny at low oxygen tension

Mononuclear phagocytes are located in every tissue of metazoan organisms. In this extravascular space, they are designated as macrophages and are known to sense and process many signals including the local oxygen tension (PO2), which ranges from 150 mmHg at the lung apices to around 40 mmHg in mixed venous blood and most organs, and to less than 10 mmHg in tissues where long-term and dynamic remodeling processes occur. Most tissue macrophages survive and maintain their differentiated status within an environment bathed by colony-stimulating factor (CSF)-1 through the CSF-1 receptor, encoded by the Csf1r gene. In order to investigate the mRNA expression profile of macrophages as a function of PO2, we developed an in vitro model in which monocyte-derived macrophages were generated from mouse bone marrow progenitor cells grown and maintained under low (36 mmHg) or atmospheric (142 mmHg) PO2, in the presence of L929-conditioned medium (L-CM) as a source of CSF-1. We show that CSF-1-reactive C57BL/6 bone marrow cells displayed an increased cloning efficiency under a PO2 of 36, compared with 142 mmHg. Furthermore, we provide evidence of the overexpression of both CSF-1 receptor protein and mRNA by mouse monocyte-derived macrophages generated from bone marrow under low PO2.     

Studies on avian erythrocyte metabolism. XVI. Accumulation of 2,3-bisphosphoglycerate with shifts in oxygen affinity of chicken erythrocytes

The ability of the chicken erythrocyte to accumulate 2,3-bisphosphoglycerate (2,3-P2-glycerate) and its effect upon the oxygen affinity (P50) of the cell suspensions have been determined. Erythrocytes from chick embryos, which contain 4-6 mM 2,3-P2-glycerate, and from chickens at various ages, which contain 3-4 mM inositol pentakisphosphate but no 2,3-P2-glycerate, were incubated with inosine, pyruvate, and inorganic phosphate. Red blood cells from 20-day chick embryos incubated in Krebs-Ringer, pH 7.45, containing 20 mM inosine and 20 mM pyruvate had an increase in 2,3-P2-glycerate from 4.3 to 11.9 mM after 4 h. Importantly, as 2,3-P2-glycerate concentration increased there was a corresponding increase in P50 of the cell suspension. Further, erythrocytes from 9- and 11-week, and 7-, 14-, 24-, and 28-month-old chickens when incubated similarly with inosine and pyruvate accumulated 2,3-P2-glycerate with corresponding increases in P50 of the cell suspensions. The ability of the red cell to accumulate this compound under the incubation conditions used apparently decreases with age of the bird (e.g., 11.9 mM in the 20-day embryo to 1.1 mM in the 28-month-old chicken after 4 h incubation). Despite the presence of significant amounts of inositol-P5, the accumulation of 2,3-P2-glycerate markedly decreases oxygen affinity of the cell suspensions. The delta P50/mumol increase in 2,3-P2-glycerate in the red cells of the 20-day chick embryo after 4 h incubation is 1.5 Torr; conversely, the delta P50/mumol decrease in 2,3-P2-glycerate in the red cells of the 17-day embryo after 6 h incubation in the presence of sodium bisulfite is 2.8 Torr. The demonstrated ability of the chicken erythrocyte to accumulate 2,3-P2-glycerate in response to certain substrates suggests that regulation of concentration of this compound could contribute significantly to regulation of blood oxygen affinity in birds.     

Pulmonary vasoconstrictor response to acute decrease in blood P50

The O2 sensor that triggers hypoxic pulmonary vasoconstriction may be sensitive not only to alveolar hypoxia but also to hypoxia in mixed venous blood. A specific test of the blood contribution would be to lower mixed venous PO2 (PvO2), which can be accomplished by increasing hemoglobin-O2 affinity. When we exchanged transfused rats with cyanate-treated erythrocytes [PO2 at 50% hemoglobin saturation (P50) = 21 Torr] or with Créteil erythrocytes (P50 = 13.1 Torr), we lowered PvO2 from 39 +/- 5 to 25 +/- 4 and to 14 +/- 4 Torr, respectively, without altering arterial blood gases or hemoglobin concentration. Right ventricular systolic pressure increased from 32 +/- 2 to 36 +/- 3 Torr with cyanate erythrocytes and to 44 +/- 5 Torr with Créteil erythrocytes. Cardiac output was unchanged. Control exchange transfusions with normal rat or 2,3-diphosphoglycerate-enriched human erythrocytes had no effect on PvO2 or right ventricular pressure. Alveolar hypoxia plus high O2 affinity blood caused a greater increase in right ventricular systolic pressure than either stimulus alone. We concluded that PvO2 is an important determinant of pulmonary vascular tone in the rat.     

Changes in heart rate during progressive hyperoxia in the dogfish Scyliorhinus canicula L.: evidence for a venous oxygen receptor

Progressive hyperoxia caused a gradual increase in arterial blood oxygen tension (PaO2). Initially there was no change in venous O2 tension (PvO2) but in extreme hyperoxia (PO2 650 mmHg) it increased to 2.5 times the normoxic (PO2 150 mmHg) level (Table 1). Ventilation frequency gradually decreased down to 73% of the normoxic value as PO2 rose towards a maximum at 700 mmHg (Fig. 1). In moderately hyperoxic water (mean PO2 233 mmHg) heart rate (fH) increased significantly above the normoxic level. Further increases in ambient PO2 caused a progressive reduction in fH to a level significantly below the normoxic rate in extreme hyperoxia (Fig. 2). Injection of atropine abolished these changes, and the atropinized fH was similar to that measured during moderate hyperoxia. The initial increase in fH during progressive hyperoxia is attributed to release of vagal tone, due to removal of normoxic stimulation of peripheral oxygen receptors; whereas, the secondary bradycardia is attributed to the stimulation of oxygen receptors located in the venous system. Injection of 5 ml of hyperoxaemic blood into the venous system of normoxic fish caused a transient bradycardia (Fig. 3), lasting a mean of 73 sec, which is the approximate time for passage of the blood volume of the venous system through the heart. This bradycardia was neither pH dependent nor a pressor response and provides supporting evidence for the existence of a venous oxygen receptor.     

Myocardial oxygenation and high-energy phosphate levels during graded coronary hypoperfusion

This study was performed to determine the myocyte PO(2) required to sustain normal high-energy phosphate (HEP) levels in the in vivo heart. In 10 normal dogs, myocyte PO(2) values were calculated from the myocardial deoxymyoglobin resonance (Mb-delta) intensity determined with (1)H-NMR spectroscopy during sequential flow reductions produced by a hydraulic occluder that decreased coronary perfusion pressure to approximately 60, 50, and 40 mmHg and, finally, during total occlusion. Myocardial blood flow was measured with microspheres, and HEP levels were determined with (31)P magnetic resonance spectroscopy. During control conditions, Mb-delta was undetectable. Myocardial blood flow was 1.11 +/- 0.06 ml. min(-1). g(-1) during basal conditions and decreased with sequential graded occlusions to 0.78 +/- 0.05, 0.58 +/- 0.03, and 0.38 +/- 0.04 ml. min(-1). g(-1), respectively; blood flow during total occlusion was 0.07 +/- 0.02 ml. min(-1). g(-1). Reductions of blood flow caused progressive increases of Mb-delta, which were associated with decreases of phosphocreatine (PCr), ATP, and the PCr-to-ATP ratio, as well as progressive increases of the P(i)-to-PCr ratio. There was a strong linear correlation between normalized blood flow and Mb-delta (R(2) = 0.89, P < 0.01). Reductions of HEP and PO(2) were also highly correlated (although nonlinearly); with the assumption that myoglobin was 90% saturated with O(2) during basal conditions and 5% saturated during total coronary occlusion, the intracellular PO(2) values for 20% reductions of PCr and ATP were approximately 4. 4 and approximately 0.9 mmHg, respectively. The data indicate that O(2) availability plays an increasing role in regulation of oxidative phosphorylation when mean intracellular PO(2) values fall below 5 mmHg in the in vivo heart.     

Nitric oxide and endothelin in oxygen-dependent regulation of vascular tone of human umbilical vein

We investigated the possible contribution of nitric oxide (NO) and endothelin (ET) to oxygen-dependent regulation of human umbilical vein vascular tone by simultaneous registration of intracellular membrane potential and isometric tension of vessel strips with and without NO synthase inhibition [10-4 M N omega-nitro-L-arginine methyl ester (L-NAME)], ETA receptor blockade (10(-5) M BQ-123), or ETB receptor blockade (10(-7) M BQ-788) at Po2 values in the bath solution between 5 and 104 mmHg. Increasing PO2 above the physiological intrauterine range resulted in depolarization and an increase of isometric tension, whereas lowering PO2 resulted in hyperpolarization and a decrease in isometric tension. Removal of the endothelium reversed these effects. At PO2 values below 39 mmHg, intact preparations treated with either L-NAME, BQ-788, or BQ-123 were more depolarized than controls. In the case of treatment with L-NAME or BQ-123, this was accompanied by an increase in isometric tension. We conclude that it is NO that mediates the hypoxic hyperpolarization and vasodilatation of the human umbilical vein and that ET, via activation of ETB1 receptors on endothelial cells, contributes to this effect.     

Influence of G-suit abdominal bladder inflation on gas exchange during +GZ stress

Available data relating duration of +GZ stress to blood gas exchange status is limited. Furthermore, studies focusing on pulmonary gas exchange during +GZ stress when abdominal restriction is imposed have yielded conflicting results. To examine the time course of blood gas changes occurring during exposure to +GZ stress in dogs and the influence of G-suit abdominal bladder inflation on this time course, seven spontaneously breathing pentobarbital-anesthetized adult mongrel dogs were exposed to 60 s of up to +5 GZ stress with and without G-suit abdominal bladder inflation. Arterial and mixed venous blood were sampled for blood gas analysis during the first and last 20 s of the exposure and at 3 min postexposure. Little change in blood gas status was seen at +3 GZ regardless of G-suit status. However, with G-suit inflation, arterial PO2 fell by a mean of 14.7 Torr during the first 20 s at +4 Gz (P less than 0.01, t test) and 20.6 Torr at +5 GZ (P less than 0.01). It continued to fall an additional 10 Torr during the next 40 s at both +4 and +5 GZ. Arterial PO2 was still 5-10 Torr below control values (P less than 0.05) 3 min postexposure. A second series of experiments paralleling the first focused on blood gas status during repeated exposure to acceleration. Blood gas status was assessed in five dogs during the late 20 s of two 60-s exposures separated by 3 min at 0 GZ. No significant differences between the initial and repeated exposures were detected. The data indicate that G-suit abdominal bladder inflation promotes increased venous admixture.     

Origin of the iris sphincter muscle in chick embryo

The origin of the iridial sphincter muscle in chick embryo was investigated by means of immunohistochemistry. Desmin immunoreactive cells are shown in the mesenchymal stroma overlying the anterior epithelial layer of the iris in 4 1/2-day chick embryos. In 9-11-day chick embryos also some cells of the posterior epithelium near the pupillary margin, and of the iridial lamella show a slighter desmin-immunoreactivity. This finding agrees with a double origin of the iridial sphincter muscle: an early mesenchymal one and a later epithelial other.     

Pulmonary O2 diffusing capacity at exercise by a modified rebreathing method.

The rebreathing technique for the measurement of the pulmonary O2 diffusing capacity, DO2, previously developed for resting conditions [Cerretelli et al., J. appl. Physiol. 37, 526-532 (1974)] has been modified for application to exercise and simplified to one rebreathing maneuver only. The changes consist: 1) in administering in the course of a normoxic exercise a priming breath of an O2 free mixture just before the onset of rebreathing in order to achieve rapidly the appropriate starting PO2 values on the linear part of the O2 dissociation curve as required by the method; 2) in calculating mixed venous blood O2 tension by extrapolation of the alveolar to mixed venous blood PO2 equilibration curve, instead of determining it separately. While the mean DO2 value of 21 measurements on 5 subjects at rest was 30 ml-min-1 - Torr-1 +/- 3 (S.E.), in 2 subjects exercising on a bicycle ergometer, DO2 was found to increase from a resting value of about 32 ml- min-1 - Torr-1 to 107 ml - min-1 - Torr-1 for an eightfold increase of O2 uptake. The validity and the applicability of the method are critically discussed.     

Evidence for tissue diffusion limitation of VO2max in normal humans

We recently found [at approximately 90% maximal O2 consumption (VO2max)] that as inspiratory PO2 (PIO2) was reduced, VO2 and mixed venous PO2 (PVO2) fell together along a straight line through the origin, suggesting tissue diffusion limitation of VO2max. To extend these observations to VO2max and directly examine effluent venous blood from muscle, six normal men cycled at VO2max while breathing air, 15% O2 and 12% O2 in random order on a single day. From femoral venous, mixed venous, and radial arterial samples, we measured PO2, PCO2, pH, and lactate and computed mean muscle capillary PO2 by Bohr integration between arterial (PaO2) and femoral venous PO2 (PfvO2). VO2 and CO2 production (VCO2) were measured by expired gas analysis, VO2max averaged 61.5 +/- 6.2 (air), 48.6 +/- 4.8 (15% O2), and 38.1 +/- 4.1 (12% O2) ml.kg-1.min-1. Corresponding values were 16.8 +/- 5.6, 14.4 +/- 5.0, and 12.0 +/- 5.0 Torr for PfVO2; 23.6 +/- 3.2, 19.1 +/- 4.2, and 16.2 +/- 3.5 Torr for PVO2; and 38.5 +/- 5.4, 30.3 +/- 4.1, and 24.5 +/- 3.6 Torr for muscle capillary PO2 (PmCO2). Each of the PO2 variables was linearly related to VO2max (r = 0.99 each), with an intercept not different from the origin. Similar results were obtained when the subjects were pushed to a work load 30 W higher to ensure that VO2max had been achieved. By extending our prior observations 1) to maximum VO2 and 2) by direct sampling of femoral venous blood, we conclude that tissue diffusion limitation of VO2max may be present in normal humans. In addition, since PVO2, PfVO2, and PmCO2 all linearly relate to VO2max, we suggest that whichever of these is most readily obtained is acceptable for further evaluation of the hypothesis.     

Oxygen transport in the rat brain cortex at normobaric hyperoxia

The distribution of oxygen tension (PO(2)) in microvessels and in the tissues of the rat brain cortex on inhaling air (normoxia) and pure oxygen at atmospheric pressure (normobaric hyperoxia) was studied with the aid of oxygen microelectrodes (diameter = 3-6 microm), under visual control using a contact optic system. At normoxia, the PO(2) of arterial blood was shown to decrease from [mean (SE)] 84.1 (1.3) mmHg in the aorta to about 60.9 (3.3) mmHg in the smallest arterioles, due to the permeability of the arteriole walls to oxygen. At normobaric hyperoxia, the PO(2) of the arterial blood decreased from 345 (6) mmHg in the aorta to 154 (11) mmHg in the smallest arterioles. In the blood of the smallest venules at normoxia and at normobaric hyperoxia, the differences between PO(2) values were smoothed out. Considerable differences between PO(2) values at normoxia and at normobaric hyperoxia were found in tissues at a distance of 10-50 microm from the arteriole walls (diameter = 10-30 microm). At hyperbaric hyperoxia these values were greater than at normoxia, by 100-150 mmHg. In the long-run, thorough measurements of PO(2) in the blood of the brain microvessels and in the tissues near to the microvessels allowed the elucidation of quantitative changes in the process of oxygen transport from the blood to the tissues after changing over from the inhalation of air to inhaling oxygen. The physiological, and possibly pathological significance of these changes requires further analysis.     

The nature of the electron spin resonance signal during aerobic uptake of Mn2+ in mitochondria from rat liver

Rat liver mitochondria take up aerobically large amounts of divalent cations in the absence of exogenous phosphate. The electron spin resonance (ESR) spectrum of matrix Mn2+ reveals the presence of two components: one, a sextet signal, corresponding to hydrated Mn2+; another, a spin exchange signal, attributed either to Mn2+ binding to specific high-energy membrane sites or to complexes of Mn2+ with inorganic phosphate. Identification of the spin exchange signal with a Mn-Pi complex is favoured by the evidence that the spin exchange signal is observed at pH 7.5 but not at pH 6.5 in the absence of exogenous Pi, but at both pH 7.5 and 6.5 in the presence of exogenous Pi. On the other hand both in the absence or presence of exogenous Pi inhibition by N-ethylmaleimide of Pi transport, abolishes the spin exchange signal. This signal is again observed when Pi is generated in the matrix, in the presence of N-ethylmaleimide, by ATP hydrolysis, and again abolished by oligomycin. Finally, addition of uncouplers results in a very slow disappearance of the signal. The amount of Mn2+ participating in the spin exchange signal has been calculated to be in the range of 50-60 nmol X mg protein-1. This amount is compatible with the amount of endogenous Pi present or generated in average mitochondrial preparations. The ESR spectrum obtained by superimposing the spectra of Mn3(PO4)2 precipitate and hydrated Mn2+, in appropriate concentrations and ratios, resembles closely the ESR spectrum during aerobic Mn2+ uptake in mitochondria. The band width of the spin exchange signal of Mn3(PO4)2 is not constant and varies between 40 and 22 mT depending on the state of aggregation of the complex. The kinetics of aggregation can be followed in solution as a function of the concentration of Mn2+, Pi and of pH. Similar kinetics can also be followed during aerobic Mn2+ uptake by controlling the rate of Mn2+ influx. The present data support the previous proposal [Pozzan et al. (1976) Eur. J. Biochem. 71, 93-99] that the spin exchange signal is essentially due to a Mn3(PO4)2 precipitate in the mitochondrial matrix.     

HbO2 dissociation in man during prolonged work in chronic hypoxia.

In healthy human sojourners to 3,100 m we studied exercise-induced shifts in HbO2 dissociation: their regulation in femoral venous blood and their net effect on estimated capillary PO2 (PC-O2) in working skeletal muscle. Prolonged heavy work effected an increase of 10.3 plus or minus 0.9 mmHg in in vivo P50 (7.30 PH-v, 41 degrees C-v, and 45 Pv-CO2); due entirely to the additive effects of increased venous temperature and [H+]. The rightward curve shift during work at 3,000 m, compared to that at 250 m, produced a similar increase in in vivo P50 but a reduced net effect on PC-O2, because Cv-02 at 3,100 m was reduced similar to 2 ml/100 ml to the lower converging portions of the curve. The lower Cv-O2 (and Pv-O2) at 3,100 M was attributable to a small decrease in total systemic blood flow. The net effect of the rightward curve shift during exercise on mean to end-capillary PO2 was positive in most cases (+1 to +8 mmHg PCO2). However, it was shown that the levels of mean to end-capillary PO2 (28-13 mmHg), which would have been obtained during exercise in the absence of any rightward curve shift, were more than adequate to sustain a steady state of aerobic energy production in working skeletal muscle. These data do not support the concept of a significant contribution to oxygen delivery to working skeletal muscle from in vivo shifts in HbO2 dissociation, during either acclimatization to high altitude or during prolonged muscular work.