Tobacco Documents Online

R IFELL6 -_ = RONALD F. COBURN oxygen tension has been questioned in recent years. This field was stimulated by the discovery of 2,3-diphosphoglycerate but extensive research has failed to dem- onstrate a beneficial effect on tissue oxygenation. The reason for this appears to be due to the major controi of tissue pO, (at-least in heart and skeletal muscle) residing in the microcirculation, with control of capillary density and diffusion distance between capillary and mitochondria: On the basis of data seen in hypoxic hypoxemia experiments (experiments where effects of decreased arterial blood pO, were studied) performed on the intact heart or skeletal muscle (10, 11), it is expected that a small decrease in capillary pO, due to shift of the oxyhemoglobin dissociation curve to the left would result in microcirculatory compensation and maintenance of tissue pO, nearly at the normal level. It is well known that anemia is better tolerated by man or by experimental animals than an equivalent loss of functioning hemoglobin resulting from CO binding. The usual explanation is that this shows deleterious effects of shifts of the-oxyhemoglobin dissociation curve which occur as a result of CO binding to hemoglobin. But another explanation could very well be a deleterious effect on tissue due to binding of CO to intracellu- lar hemoproteins. The evidence that CO toxicity to mammalian tissues is a result of binding to hemoglobin is somewhat indirect and includes the following: (a) The known ef- fects of CO binding on oxygen carrying capacity and oxyhemoglobin dissociation curve; (b) demonstration of small decreases in tissue pO, (measured with micro- oxygen electrodes). resulting from increases in blood HbCO (32). A decrease in oxygen uptake (VO.) of tissues resulting from increases in HbCO does not prove that the mechanism is a decrease in capillary pO„ since a decrease in VOs could be due to inhibition of diffusion of O, within the cell, or inhibition of O. metabolism by the cell: A common approach to the study of the mechanism of CO toxicity is to compare "toxicity" with "toxicity due to hypoxic hypoxemia." In such studies attempts are usually made tadecrease mean capillarypO, to a similar extent and if adverse effects are similar with CO and with hypoxic hypoxemia it is concluded that the CO effects are all due to CO binding to hemoglobin. Roth and Rubin (26) have pointed out the hazards of this approach since the systemic effects on the circulation can be markedly different probably due to tack of, or smaller, effects of elevated HbCO on chemoreceptors. There-also may be a difference in the local response of the microcirculation to decreases- in oxygen delivery induced by the two different methods. - ~------ -- -- . .. . - - BINDING OF CARBON MONOXIDE TO INTRACELLULAR ENZYMES IN _ PERIPHERAL TISSUES In considering the possibility that CO may bind to intracellular hemoprotein enzymes in sufficient quantities to inhibit function, I first want to emphasize the relationship of Oz tension and CO tension to CO binding. Although CO will bind to copper-containing enzymes at a relatively high pCO, the primary candidates for intracellular enzymes that have high enough CO affinities to bind CO at physiolog- ical tissue pCO are iron-containing hemoproteins. Figure I illustrates O: and CO competition in two intracellular hemoproteins, myoglobin and cytochrome oxri _dase!_ - E.` Acn^" I

t RONALD F. COBURN j rEX "%oa.sq co21 95%nz•s%cnz -, 02 195 a0, •_% Co2 - t 0 0 Fiea.2. Titration of the two intracellular respiratory pigments in the perfused working rat heart 177. htyoglobin and cytochrome oxidase spectra were measured and effects of removal of O, from the perfusion solution were determined. This was folloued by stepwise increases_in the pOr of the perfusing soluGon. Note the changes in spectra were parallel in the two different compounds. Reprinted with permission of the authors and the publisher, from Ref. (7). I and cytochrome oxidase during fall in perfusing fluid pOs in an isolated heart. Figure 2 shows data demonstrating parallel reduction of the two compounds dur- ing changes in extracellularpOz. The oxygen affinities of the two compounds are markedly different, cytochrome oxidase having a much higher affinity than myo- globin. The most likely explanation for this finding is that there.are steep oxygen tension gradients inside-myocardial cells. Thus. intracellularpO, gradients may be so steep that the pOz in proximity to the mitochondrial terminal oxidase may be low enough so that CO binding at normal or elevated tissue pCO becomes signifi- cant. EXPERIMENTAL MEASUREMENTS OF CO TOXICITY IN ISOLATED SMOOTH MUSCLE STRIPS Proving that CO binding to cytochrome oxidase is a mechanism of CO toxicity in vivo will require development of techniques to measure CO binding, which-is not possible at present. Another approach is to remove tissue and directly study effects of CO on oxidative metabolism under conditions where hemoglobin is absent. We have developed such a preparation for this type of study. Isolated strips of rabbit aorta were mounted in an organ bath perfused with Krebs solution. The organ bath pOi could be determined with an oxygen electrode. Oxygen up- take was measured by measuring the rate of decrease in Fluid pOe .vhen the top of the organ bath was closed, isolating the system from atmospheric oxygen. We could study effects of increasing organ bathpCO and COIO_ on oxygen uptake and on isometric mechanical tension of (his smooth muscle. To our knowledee. there are no previous measurements of CO effects on isolated tissues. Some of the data reported here has been published previously (12). Oxygen uptake (VO_) of rabbit aorta is completely inhibited by I nh4 NaCN and inhibited 97ib by antimycin A, evidence that VO, reflects mitochondrial electron chain transport. The-pOe gra- dients between the outside and the core of the strip are not defined in this prepara- tion; factors which determine the pO: gradient in isolated tissue are illustrated in Fig. 3. Another complicating factor in interpreting data obtained in this prepara- - r ----- -------- I - -- -- ------ -- ---- -- a~ri3hr -- / ~ ~ ~ E ~ N N ~ O

RONALD F. COBURN REFERENCES I. Abboud. R., Andersson. G.. and Coburn. R. F. Evaluation of stagnant pulmonary capillary blood during breath holding in dogs-J. Appf. Phrsiol. 37, 397-409 (1974)- 2. Aronow, W. S.. and Isbell, M-. W. Carbon monoxide effect on exercise-induced angina pecioris. Ana. Irnern. Sled. 79, 392-395 (1973). 3. Ball. E. G.. Striumatter. C- F., and Cooper, O. The reaction of cytochrome oxidase with carbon monoxide. l. Biol. Chenr. 193, 635-647 (1951)_ 4. Chance. B.. Ericinska, M.. and Wagner. M. Mitochondrial responses to carbon monoxide toxic ity. Ann. N.Y. Arad. Sri. 174, 193-'_04 I1970). 5. Chance. B.. and Ericinska. M. Flow flash kinetics of the cytochrome at-oxygen reaction in coupled and uncoupled mitochondria using the liquid dye laser. Arch. Binchear. Biophic. 143, 675-687 (1971). - 6. Chance, B. Discussion of L. H. Opids effects of regional ischemia on metabolism of glucosc and fatty acids. Circ. Res- 38(Suppl. 1). 69-74 (1976). 7. Clark. B. 1., and Coburn. R. F. Mean myoglobin oxygen 4ension. during exercise at maximal oxygen uptake. J. AppL Phcsinf. 39, 135-144 (1979. 8. Coburn. R. F., Forster, R. E-. and Kane, P. B. Considerations of the physiological variables that deiermine the blood carboxyhemoglobin concentration in man. J. Clin. Inrest. 44. 1899-1910 (1965). - - 9. Coburn. R. F. The carbon monoxide body stores. Ann- N.Y. Acnd. Sri. 174, 11 -22 (19701, 10. Coburn, R- F.. and Maycrs. L- B. Myoglobin O:-tension determined from measurements of car- boxymyoglobin in skeletal muscle. Anrer. J. PMaio1. 220, 66-74 (1971). 11. Coburn, R. F.. Ploegmakers. F.. Gondrie. P-, and Abboud. R. Myocardial myoglobin oxygen tension. Amer. I. Phraiol. 224, 870-876 (1973). . 12. Coburn. R. F., Grubb. B., and Aronson, R. Effect of cyanide on oxygen tension dependent mechanical tension in rabbit aorta. Circ. Rer., in press. 13. Degn, A., and Wohlrab. H. Measurement of sleady-state values of respiration rate and osidaiion levels of respiratory pigments at low oxygen tensions. A new technique. Binrhini. Binphye. Aria 245, 347-355 (1971). 14. Forman, H. J.. and Feigelson, P. Kinetic evidence indicating the absence during catalysis of an unbound ferroprotoporpAqren form of tryptophan oxidase. Biurhemivin10, 760-763(1971). 15. Goldbaurn. L. R.. Ramircz, R. G.. and Absalon. K. B. W hat is the mechanism of carbon monoxide loxiciry.' Arins- Space Erniron. ,Lled. 46, 1289-1'_91 (1975). 16. Goldbaum, L. R., Orellano. T., and Dergal. E. Studies on the relation between carboxytiemoglo- bin concentration and toxicity. AtiaL Spnce Enriron. .tfed. 48, 969-970 (1977)- 17. Gdther, M-, Lutz. F-, and Malorny. G. Carbon monoxide panial pressure in tissue of d1&rcnt animals. Em-iron. Rrs. 3. 303-309 (1970). 18. Haldane. 1., and Smith, 1. L. The oxygen tension of arterial blood. J. Phrsiol. (Lundon) 20, 497-520 (1896)- - 19. Holland. R. A. B. Rate of O. dissociation from O,Hb and relative combination rate of CO and 0, in mammals at 37°C. Rnp._Phctiul. 7, 30-42 (1969). 20. Horvath, S. M.. Raven. P- B.. Dahms. T. E., and Gray, D- J. Maximal aerobic capacity at differ- ent Ievds of carboxyhemoglobin. J. ADph Physinf. 38, 300-303 (1975). 21_. _JObsis. F. F., Keizer. L H-, IaManna, J. C., and Rosenthal. M. R<Bectane_e spectropholometry of cytochromc an, in riru. l. Appf. Phyriol. 43. 858-87? (1977). - - 22. Jones. R. H., Ellicoll. M. F.. Codigan, 1. B., and Gaensler, E. A. The relationship bcween als'eo- lar and blood carbon monoxide concentrations during breath holding. J. Lab- Cfn. Sfed 51, 553-557 41958). 23. Longo, L..D., Power, G-G.. and Forster. R. E. Respiratory function of the placenta as deter- mined with carbon monocide in sheep and dogs. l- Clin. Incert. 46, 812-826 (1967). 24. Luomanmaki,K..andCoburn.R.F.Effectofinetabolismanddislributionofcarbonmono:ideon blood and body slores. Anrer. J- Physiul. 217, 354--363 (1969). 25. Rich. T. L., and Williamson. 1. R- Optical evidence for steep oxygen gradients and sharp border zones in hypoxic tissue. Fed. Proc. 37, 780 (1978)- 26. Roth. R. A-, and Rubin, R. 1. Comparison of the effect of carbon monoxide and of hypu.ic ~ -- -.-_- . I . ~ .;.~ CopyriyT k