Environmental Health PerspectivesVol. 26, pp. 207-215, 1978

Effects of Fluorocarbons, ChlorinatedSolvents, and Inosine on theCardiopulmonary Systemby Domingo M. Aviado*

The effects of fluorocarbons and chlorinated solvents on the cardiopulmonary system are reviewed. Thenew information, not hitherto reported, relates to the antagonistic action of inosine, a naturally occurringnucleoside formed in the body by deamination of adenosine. The effect of inosine on methylene chloridetoxicity was investigated in open chest dogs anesthetized with pentobarbital sodium. Methylene chloride(5% in air or 50,000 ppm) elicited a decrease of ventricular contractility represented by the dhninished leftventricular (dpldt)^a, and myocardial contractile force measured directly with a Walton-Brodie straingauge arch. Coronary blood flow decreased slightly after exposure to methylene chloride. Arterial bloodpressure and heart rate did not change. The negative inotropic effect of methylene chloride was reversedor prevented to a substantial extent by intravenous infusion of inosine (5 mg/kg-min). The effect of thelatter compound was also characterized by signiffcant coronary vasodilation. It was shown by the experi-ments that the cardiostimulatory action of inosine was associated with improved hypoxic adaptability ofthe coronary blood vessels. In contrast, the effect of catecholamines (epinephrine and isoproterenol) wasnot accompanied by such a beneffcial coronary vascular effect. On the basis of these results, the conclu-sion has been arrived at that inosine might be recommended as a useful antidote in methylene chloridepoisoning in particular, and of poisoning by chlorinated solvents and fluorocarbons in general.

IntroductionThe purpose of this article is to review the car-

diopulmonary effects of fluorocarbons and relatedcompounds which have been identified in man andexperimental animals. The experimental observa-tions were made at the author's laboratories at theUniversity of Pennsylvania, at the request of theFood and Drug Administration and the ConsumerProducts Safety Commission, to identify the effectsof intentional or accidental use of aerosol productscontaining fluorocarbon propellants and chlorinatedsolvents. This article starts with a discussion offluorocarbons, proceeds with chlorinated solvents,particularly methylene chloride, and then concludeswith a description of a nucleoside that can be usedto reverse or treat the cardiotoxic properties of thehalogenated propellants and solvents under consid-eration.

* Allied Chemical Corporation, P. 0. Box 1021R, Morristown,New Jersey 07960.

Cardiac Arrhythmia Induced byFluorocarbonsThe three most widely-used propellants in

aerosols containing bronchodilator drugs aretrichloromonofluoromethane (FC 11), dichlorotet-rafluoroethane (FC 114), and dichlorodi-fluoromethane (FC 12). The potential for these pro-pellants to produce cardiac arrhythmia has beendemonstrated in various animal species (1-8), andthe concentrations that induce cardiac arrhythmiasin the anesthetized dog are as follows: 0.25% for FC11, 2.5% for FC 114, and 10.0o for FC 12. Theseconcentrations are probably lower in animals with-out anesthesia and in animals with ischemic hearts(9, 10). In monkeys, ischemia increased the sen-sitivity of the heart to experimentally-induced car-diac arrhythmia (11).The mechanisms by which the fluorocarbons in-

duced cardiac arrhythmias are as follows: (a) sen-sitization of the heart to proarrhythmic effect ofephinephrine (5); (b) depression in myocardial con-

October 1978 207

tractility (12-14); (c) reduction in cardiac output andcoronary blood flow (13, 14); and (d) reflex increasein sympathetic and vagal impulses to the heart byirritation of mucosa in the upper and lower respira-tory tract (2). There is no evidence to implicatethree other mechanisms: release of epinephrinefrom the adrenal medulla and from the heart andblockade of conduction by a direct action of thefluorocarbon on the heart muscle. These possiblemechanisms can be readily investigated by analysisof catecholamines in the heart and adrenal glands inanimals exposed to the fluorocarbons. It should benoted that arrhythmia can be triggered without thefluorocarbon being absorbed from the respiratorytract. Irritation of the upper respiratory tract in-creases vagal tone, whereas irritation of the lowerrespiratory tract increases sympathetic activityto the heart (2, 15). It is this neurogenic mechanismwhich triggers cardiac arrest, especially if thefluorocarbons have been absorbed by the blood inconcentrations high enough to influence the heart.

Depression of Myocardial Contractility byChlorinated Solvents

In the anesthetized dog, the hemodynamic effectsof chlorinated solvents have been investigated. Thethreshold inspired concentrations are 0.05% fortrichloroethylene, 0.25% for methyl chloroform,and 2.5% for methylene chloride (16). The mosttoxic solvent is trichloroethylene and the least toxicis methylene chloride. The primary action of thechlorinated solvents is depression of myocardialcontractility, which reduces cardiac output andlowers the systemic arterial blood pressure. By in-activation of baroreceptors in the carotid sinusesand aortic arch, the low blood pressure increasessympathetic discharge to the heart, just ashypoxemia would stimulate chemoreceptors in thecarotid and aortic bodies. The end result would bean antagonism against the direct depressant effectof the solvent on the myocardium (16, 17).There are several mechanisms that have not been

elucidated, such as the role of the adrenal medullain influencing total systemic vascular resistance; theinfluence of the chlorinated solvent directly on thevenous return to the heart, and on the coronaryblood vessels. The experiments elaborating onthese mechanisms will necessitate the use of totalbody perfusion and total coronary perfusion (15,18).

In the subject who inhales the chlorinated sol-vent, there are primary events that would inducecirculatory shock and secondary compensatorymechanisms. It is not possible to assess the impor-tance of circulatory shock relative to cardiac arrest

and respiratory failure (19-21) as a cause of death.

Cardiotonic Properties of Inosineon Methylene ChlorideDepressed Heart

In the course of investigating the cardiac effectsof the inhalation of chlorinated solvents andfluorocarbon propellants (see above) we thought itworthwhile to test corrective drugs. Since depres-sion of contractility is the most important manifes-tation of toxic concentrations of methylenechloride, there was a need to find a cardiotonic drugthat did not provoke arrhythmias. That methylenechloride sensitizes the heart (17) to cardiac ar-thythmia has contraindicated the use of epinephrineand isoproterenol.

Inosine, a cardiotonic drug, does not sensitize theheart to arrhythmia (22, 23). Before inosine can beconsidered as a clinically useful cardiotonic agentfor a heart poisoned with methylene chloride, theantagonism must be established in the laboratory.The experiments reported below fulfill this pre-requisite, demonstrating that inosine counteractsthe cardiac effect of methylene chloride. It may alsobe effective with other structurally related chlori-nated solvents such as carbon tetrachloride,chloroform, methyl chloroform, and tetra-chloroethylene.

MethodsThe experiments were carried out on mongrel

dogs (12 to 20 kg) of either sex under pentobarbital(30 to 35 mg/kg IV) anesthesia. After insertion of atracheostomy tube, artificial ventilation was main-tained with a Starling Ideal respirator using roomair. The chest was opened in the fifth left inter-coastal space and the pericardium was slit to exposethe heart. A short segment of the left anterior de-scending coronary artery, close to its origin, wasdissected free and a Statham electromagnetic flowprobe of appropriate size (usually 2 mm) was placedaround the vessel. Phasic, as well as mean coronaryblood flow, was measured, the latter value beingobtained by means of electrical integration.Myocardial contractile force was recorded with aWalton-Brodie strain gauge sewn to the anteriorwall of the left ventricle supplied by the anteriordescending coronary branch. Usually a secondstrain gauge was applied to the base of the ventriclesupplied by the circumflex coronary artery. Apolyethylene catheter was inserted into the leftventricular cavity through the apex; ventricularpressure was measured with the aid of a Statham

Environmental Health Perspectives

transducer (P23AA) connected to the catheter. Themaximal rate of rise of the left ventricular pressure,(dpldt)max was obtained by using a derivative com-puter (8814 HP). Aortic pressure was recorded withanother Statham transducer by cannulating the leftcarotid artery. All recordings were made on a six-channel Sanborn 7700 recorder. No drugs were in-jected for at least 30 min after completing thesurgery to allow the animals to reach a steady state.Four experimental groups were studied.

Experimental GroupsGroup 1. Methylene Chloride after Inosine

(Five Dogs). Methylene chloride (5% in air) wasadministered via the inlet of the respirator. The gasmixture was prepared by mixing the appropriateamount of solvent into a known amount of air, asdescribed previously (16). After complete recoveryfrom the control administration of methylenechloride for 5 min, the same procedure was re-peated while the dogs had been continuously in-fused intravenously with a moderate dose (5 mg/kg-min) of inosine (Trophicardyl). The inhalation ofmethylene chloride started in the fifth minute of in-osine infusion which was continued till the end ofthe inhalation period (5 min duration).Group 11. Inosine after Methylene Chloride

(Five Dogs). In this series, after a methylenechloride (5% in air) inhalation period of 5 min dura-tion, the uninterrupted inhalation of the solvent wascontinued for a further 5 min, but during this secondphase, inosine (5 mg/kg-min) was infused simul-taneously.Group III. Inosine, Catecholamines, and

General Hypoxia (Six Dogs). In this series, gen-eral hypoxia was induced by ventilating the lungswith 5% oxygen in nitrogen for 2 min; a new steadystate of all circulatory variables was obtained inevery dog at the end of the hypoxic period. Afterrecording the control response to hypoxia, the samemaneuver was repeated during the last 120 sec ofexperimental periods of 6 min duration, when theanimals were infused with inosine (10 mg/kg-min),epinephrine (0.5 ,ug/kg-min) and isoproterenol (0.5,g/kg-min), respectively.Group IV. Inosine and Adenosine (Three

Dogs). In these dogs, the influence of inosine (2.5mg/kg-min) on the circulatory effect of adenosinewas tested. Adenosine was given intravenously indoses of 0.02, 0.10, and 1.0 mg/kg.

Statistical AnalysisCirculatory variables during the last minute of

infusion and/or inhalation periods were chosen for

data analysis. All values quoted in the text are meanvalues + S.E. The results were expressed both asabsolute and percentage values, 100% being thecontrol level before the administration of drugs orinhalation of methylene chloride. The results wereexamined statistically by using the Student t-test forpaired and/or unpaired data. Changes are consid-ered significant ifp < 0.05.

Measured Experimental VariablesMeasured experimental variables were as fol-

lows: Mean arterial pressure, in mm Hg, was mea-sured from a catheter inserted through the leftcarotid.

Heart rate, in beats/min, was computed from theaortic pressure curves, taken at a paper speed of 25mm/sec.

(dp/dt)max, the maximum rate of rise of left ven-tricular pressure taken in mm Hg/sec was derivedfrom the left ventricular pressure; the latter variablewas measured from a catheter in the left ventricularcavity inserted through the apex.

Myocardial contractile force, in percent of con-trol, was measured with a Walton-Brodie straingauge sutured to the exposed surface of the leftventricle.Coronary blood flow, mean in ml/min, was mea-

sured with a flow probe around the left anterior de-scending coronary artery and obtained by electricalintegration of the phasic flow signal.Coronary vascular resistance, mean, in dyne-

sec/cm5, was the ratio of mean arterial pressure indyne/cm2 and the coronary blood flow in cm3/sec.Coronary blood flow, late diastolic, in ml/min,

was measured in the late diastolic phase of the car-diac cycle, when the coronaries are free from ex-travascular compression.Coronary vascular resistance, late diastolic, in

dyne-sec/cm5 is the ratio of late diastolic arterialpressure in dyne/cm2 and the late diastolic coronaryblood flow in cm3/sec.

ResultsThe first two groups of experiments consist of

administering inosine and methylene chloride in thissequence and in the reversed order. The results arediscussed as a group because they indicate an-tagonism in both cases.

Inosine Versus Methylene Chloride. Inosineadministration consistently increased myocardialcontractility, whether administered before or afterthe inhalation of methylene chloride (Groups I andII). The responses characterizing the changes of

October 1978 209

Table 1. Effect of inosine (5 mg/kg-min) on cardiac depression induced by methylene chloride (5% v/v) inhalation.

Group I Group IIMethylene Cl Inosine after

Control after inosine Control methylene Cl

Time of methylene Cl 0 5 0 5 0 5 10inhalation, min

Time of inosine infusion, min 5 10 0 5 10Mean arterial blood pressure, 123 4.4 127 3.4 123 ± 4.3 127 ± 6.2 126 9.5 130 6.9 144b 11.2 135 7.9mm Hg

Heart rate, min-' 172 9.6 171 9.4 182b± 11.6 181b+ 11.2 154 4.3 151 ±3.8 147 4.5 153 3.6(dpldt)max, mm Hg/sec 3850 ± 528 3225b ± 465 5385b ± 714 4175 + 691 4200 ± 463 3025b ± 498 3975 ± 655 5925b ± 998Myocardial contractile 100 0 70b 6.7 136b 9.5 87 + 11.9 100 0 60 7.1 77 + 9.1 122b 7.5

tone,%Mean left AD coronary 18.6 ± 2.4 17.6 ± 2.8 22.9b ± 3.3 21.1 ± 3.0 28.5 ± 5.5 28.9 ± 7.2 43.0b ± 10.5 43.5b ± 5.8

blood flow, mlminMeancoronaryvascular 60.0±8.3 64.8"± 11.6 47.8b±8.6 51.5± 10.3 42.2±9.1 44.0_9.2 33.4b 8.2 27.6"±3.4

resistance x 10w, dyne-sec/cmeLate diastolic left AD 25.9 ± 2.7 25.4 ± 3.6 41.4b 7.7 32.8b±_ 4.6 47.6 7.9 46.9 ± 10.6 67.7b_ 14.4 67.2b 7.5

coronary flow, mlminLate diastolic vascular 40.0 ± 3.4 40.1 ± 6.9 25.0" ± 5.0 29.3b ± 5.0 21.3 ± 3.1 24.6 4.5 18.3 ± 3.7 16.7b ± 2.1

resistance x 104, dyne-sec/cm5a Mean values ± SEM.b Significant difference of change (p < 0.05) from control values.

MAP

A. NOSI BEFORMETHYLENE Cl

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HR dp/dt max MF MCBF MCVR DCGF DCVR

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B N0S1 AFTER + + +

METHYLENE

-40-20.

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METHYLENE Cl (5%) lone INOSINE (5mf/kg/ngn) alne INOSINE + METHYLENE a

FIGURE 1. Protective effect of inosine against inhalation of methylene chloride; (MAP), mean arterial pressure; (HR) heart rate; (MF)myocardial contractile force; (MCBF) mean coronary blood flow; (MCVR) mean coronary vascular resistance; (DCBF) late-diastolic coronary blood flow; (DCVR) late-diastolic coronary vascular resistance. Mean values SE; + denotes significant changefrom control.

Environmental Health Perspectives

60-

40-

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210

cardiovascular variables are tabulated in Table 1,while the percentage changes are graphically dis-played in Figure 1. The predominant feature of themethylene chloride toxicity is the depression ofmyocardial contractility; this effect is clearly re-vealed by data in Table 1 (under the "control"headings) and by the black columns of Figure 1. Thelevels of arterial blood pressure and heart rate didnot decrease after the inhalation of the solvent; theformer variable even exhibited a statistically non-significant increase. At the same time, the values ofthe contractility indices [(dp/dt)max and myocardialcontractile force] substantially and significantly di-minished after the inhalation of methylene chloride.

Together with the induced cardiac depression, thecoronary blood flow decreased slightly (both themean and the late diastolic values), and the vascularresistance increased slightly. These latter changes,however, were not found to be statistically signifi-cant, except in one set of measurements (mean re-sistance values in Group I).The action of inosine on the circulatory system is

best envisaged by the second (white) columns inFigure 1 and the third row of Table 1, respectively.These effects, as they have been fully describedpreviously (23), consist of cardiostimulating andcoronary vasodilator components: the values of(dpldt)max and myocardial contractile force signifi-

w I

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FIGURE 2. Behavior of the hypoxic coronary adaptation expressed in percent vascular conductance: range of coronary adaptation toinhalation of 5% 02 + 95% N2. There is a significant increase during inosine infusion.

Table 2. Effect of inhalation of 5% 02 on the circulation."

Inosine Epinephrine IsoproterenolControl (10 mg/kg-min) (0.5 Ag/kg-min) (0.5 ,ug/kg-min)

21%02 5%O2 21%02 5% 02 21% 02 5% 02 21%902 5% 02Mean arterial blood 129 ± 6.0 136 ± 1.7 125 ± 5.2 123 ± 9.4 156 ± 12 147 ± 10.0 104b ± 17.2 104b 16.0

pressure, mm HgHeartrate,min-' 153±7.2 159b±6.3 170a±5.7 173b±6.9 159±7.1 159±7.2 193b±7.9 194b"8.5(dp/dt)ma, mm Hg/sec 3395 ± 457 3938b ± 563 5292b ± 594 6167b ± 805 5875b ± 551 6042b ± 476 5563b ± 819 5564b ± 991Myocardial 100 0 109 2 140a 8 148b 8 161b 10 156b 9 179b 18 157b ± 12

contractile force, %Mean left AD blood 17.9 2.1 22.8b 2.8 32.5b± 5.0 47.5b 8.2 31.6 6.1 31.3b 5.1 52.0b 6.5 58.8b ± 9.1

flow, ml/minMean coronary vascular 65.5 ± 6.1 51.5b ± 5.2 36.1" ± 7.6 26.0b ± 6.8 47.6 ± 9.4 43.7b ± 7.9 16.2b ± 2.5 14.9" ± 2.8

resistance x 10, dyne-sec/cme

Late diastolic left AD 28.6 ± 3.9 36.3b 5.0 60.7b" 10.8 90.8b" 19.0 46.5 9.1 47.1b 7.9 106.6b" 13.6116.2b± 21.2flow, ml/min

Late diastolic vascular 35.4 ± 4.0 29.5b ± 4.2 17.9" 4.4 13.6" 4.6 27.5 ± 6.2 25.2b ± 5.2 6.7b ± 1.1 6.2b ± 1.0resistance x 10, dyne-sec/cmea Mean values + SEM.b Significant changes (p < 0.05) from the normoxic control value.

October 1978 211

cantly increased together with the enhancement ofthe coronary flow parameters and the diminution ofthe vascular resistance in the coronary bed. Theseeffects were accompanied by a moderate, but sig-nificant tachycardia.When the methylene chloride inhalation was re-

peated during the uninterrupted infusion of inosine,the cardiovascular action of the solvent was foundto be substantially different from the controlmethylene chloride effect, (see Fig. 1 and Table 1).The action of methylene chloride was no longerpowerful enough to elicit a significant depression ofcardiac contractility, although the positive inotropiceffect of inosine became diminished. Similarly, al-though the inosine-induced coronary vasodilationdecreased as a consequence of methylene chlorideinhalation, the vasodilatory effect of inosine wasstill prevalent enough to be statistically significantin the late diastolic phase of the cardiac cycle.An essentially similar picture emerges from the

experiments where the inosine infusion was startedduring the uninterrupted administration ofmethylene chloride by inhalation (Group II). Figure

MEAN ARTERILRSSffE

lB as well. as Table 1 show that the depressivemethylene chloride effect was partially coun-teracted or even fully reversed by inosine: the val-ues of myocardial contractility were no longer re-duced in this period of experimental observationsand the slight increase of coronary vascular tone,which was elicited by the inhalation of solvent, hadbeen replaced by a significant coronary vasodilatoraction, i.e., the effects related to the myocardialblood supply were completely reversed. After dis-continuing the methylene chloride inhalation theuninhibited inosine action became obvious.Hypoxia. In the next series of experiments,

performed in six dogs (group III), a mixture of 5%oxygen and 95% nitrogen was administered for 2min in order to gauge the effect of inosine on thehypoxic dilator capacity of the coronary vessels.The modulator effect of inosine has been comparedto that of epinephrine and isoproterenol. Figure 2depicts a typical response, while the results ob-tained in six dogs are summarized in Table 2 andFigure 3. Briefly, the vasodilator and inotropic ac-tions of inosine were found to be associated with the

dp/G mot VENTRCULAR MEAN IOW MEAN DORCNW LATE OISRMUC LA1E =FORCE FLOW Ta5NCE RWM FLO W

280-

240-

200-

160-

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420-0-

-40-1_0"

21%.0 5% 217.02 57.020-------0CONTROL

21702 502 21%02 5%02 2c02 5%02 21%02 5%02 21702 5%02x a ............A

INbJOSINE EPINEPHRIE IS0p0tEIENOL(IOmg/kg/nin) (5 pgi/kg/min) (5 pg/kg/min)

FIGURE 3. Changes of cardiovascular responses to hypoxia during intravenous infusion of inosine, epinephrine and isoporterenol.Abbreviations as in Fig. 1.

Environmental Health Perspectives212

increase of hypoxic coronary adaptation (an aug-mented vascular capacity for further vasodilation),while those of epinephrine or isoproterenol, them-selves equally potent augmentors of coronary flowand cardiac activity, were not. Although, in this re-spect, the large differences between inosine on onehand, and the catecholamines on the other, are evi-dent by the behavior of coronary flow itself, com-monly the true relationships of vascular reactivityare somewhat obscured by the fact that during theinfusion of the three drugs, the arterial blood pres-sure was also differently affected (Fig. 3). The in-spection of the calculated vascular resistance valuesis not very helpful in this particular case, since thelatter parameter, being the product of the pressureand the reciprocal value of the flow, tends tominimize any vasodilator effect superimposed on analready established one. Therefore, in situationslike this, it is more helpful to use the parameter offlow conductance C instead of the flow resistanceR; (where C = lIR). The relations are clarified byFigure 4, where the range of hypoxic coronary re-sponses is expressed in percent of the control(nonhypoxic, preinfusion) vascular conductance,i.e., in the same relative units. The conductancechanges indicate a significant modulator effect ofinosine on hypoxic coronary adaptability.Modulating Action of Inosine on the Aden-

osine Coronary Effect. The results describedabove revealed the unexpected modulation ofhypoxic coronary adaptability during inosine infu-sion. Since adenosine, a nucleoside chemically re-

MEAN E LATE DIASTOLIC

g150

125

100

75

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CONTROLn (6)

I

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(6)l0 mg/kg/mn

>0.1 >0.1 >0.1 >0.8EPI1XEPHRINE IS0 0EfNOL

(6) (4)05 Ig/hg/rin 0.5 pg/kg/rin

FIGURE 4. Percent changes of mean and late diastolic conduc-tance in six dogs subjected to hypoxia. Mean valus _ SE.

Table 3. Potentiation of adenosine effect on thecoronaries by inosine.a

Dose of Mean coronary blood flow, ml/minaadenosine, Inosinemg/kg Control (2.5 mg/kg-min)

0.0 17.3 ± 1.4 20.6 ± 0.90.02 20.0 ± 2.1 40.4 ± 4.7b0.10 26.8 ± 2.7 44.6 ± 5. lb1.0 44.6 ± 5.5 50.6 ± 5.9a Mean values ± SEM.b Significant differences between inosine and control animals.

lated to inosine, is the suspected key substance inthe physiologic mechanism of hypoxic coronaryvasodilation, the interaction of these two com-pounds was tested in the last series of three dogs(Group IV). A significant potentiation of the coro-nary vasodilator effect of adenosine was observedby inosine infused in a threshold dose which, initself, scarcely affected the basic level of coronaryblood flow (Table 3).

DiscussionIn these studies, as in former experiments con-

ducted by Aviado et al. (17) the acute cardiovascu-lar toxicity of methylene chloride was found to becharacterized by an almost selective myocardialdepressant action. This was indicated by the un-equivocal decrease in left ventricular (dpldt)max andthe myocardial contractility measured directly bystrain gauges sutured to the ventricular wall. De-spite the weakened cardiac inotropism, the arterialblood pressure remained unchanged or even in-creased slightly. A similar pattern was observed informer experiments (17) with the additional obser-vation of a significantly diminished cardiac output.These observations led Aviado et al. (17) to con-clude that methylene chloride action is associatedwith a general vasoconstrictor effect in the systemicarterial bed. The present studies revealed a similartendency in the coronary circulation. The averagelevel of the coronary vasomotor tone as reflected bythe calculated value of vascular resistance did notexhibit dramatic changes after exposure tomethylene chloride. However, it was quite clearthat the depressant action of methylene chloride onthe cardiac muscle is not accompanied by a paralleldepressant effect on the smooth muscle cells ofcoronary blood vessels and the coronary blood flowwas usually slightly decreased during the inhalationof the solvent. Since the level of coronary flow is setprimarily by the rate of myocardial metabolic re-quirement, which, in turn, is considerably influ-enced by ventricular contractility, the observed

October 1978

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213

changes in coronary tone could probably be as-cribed to a secondary vascular adjustment to thediminished cardiac performance. Although the lat-ter mechanism can be regarded as a natural(physiologic) adaptive phenomenon, its conse-quences might be detrimental in special cases ofhuman toxicity, when the blood supply to the heartis already in jeopardy due to organic disorders ofthe coronary vessels. Sudden cardiac depressionassociated with a relative reduction of the ven-tricular blood supply might be a major factor in ini-tiation of serious cardiac arrhythmias, such as ven-tricular fibrillation, infrequently seen in methylenechloride poisoning of human subjects with heartdisease. Such possibilities raise the challengingproblems of effective prevention and appropriatetreatment of the cardiac depression brought aboutby methylene chloride.As a first approximation, every drug is a poten-

tially beneficial one if it can prevent the ar-rhythmogenic action of methylene chloride. How-ever, the efficacy of drugs should be described on amuch narrower basis; if the potential hazards in-volved in the administration of the agent came closeto (or even match) those involved in methylenechloride inhalation, the attempted therapy is self-defeating.

Inosine, which was selected in this study tocounteract the harmful cardiac effect of methylenechloride, is a drug of apparently low toxicity. Thefundamental cardiac stimulatory and coronary vas-odilator actions elicited by inosine resemble thoseinduced by catecholamine administration, as it wasreported previously by other workers (24, 25) andby Juhasz-Nagy and Aviado (23). At the same time,inosine was reported to be an agent with virtually noacute toxicity, the LD50 of the compound being sev-eral grams per kilogram body weight. Contrary tothe well-known sensitizing effect of catecholaminestoward arrhythmogenic tendencies, no such effecthas been reported in connection with the inosineadministration. In this study, particular attentionwas given to the inosine actions exerted on thephysiologic adaptive capacity of the coronary ves-sels. The reason for doing so was that no relevantexperiments had been reported until now concern-ing this aspect of inosine treatment. At the sametime, it seemed of the utmost importance to eluci-date this aspect considering the critical importanceof maintenance of adequate blood supply andadequate coronary reactivity in the diseased humanheart.

This problem directed our attention to the possi-bility of an influence of inosine on hypoxic coronaryadaptation. We observed a significant improvementof the adaptive vascular capacity during inosine

administration. The effect was found to be superim-posed on the significant, but not excessive, coro-nary vasodilation elicited by the drug. In contrast,no similar augmentation occurred during infusion ofcatecholamines (sometimes referred to as "malig-nant" coronary vasodilators) which elicited a com-parable or even greater increase of myocardial flowand contractility. The explanation may be that, un-like most pharmacologic agents, exogenous inosinecreates, possibly by its virtue of being a "natural"modulator substance of coronary tone, conditionsfor better metabolic adaptation in the coronary bed,and thus, for better oxygen supply of cardiac mus-cle working under hypoxic stress. The unique fea-tures of inosine, being both a therapeutic agent andone of the constituents that can be extracted fromthe heart muscle, seem promising from a clinicalstandpoint.

It is the opinion of the author that in evaluatingthe coronary actions of drugs, the effects exerted onphysiologic vascular adaptability is of far greaterimportance than the mere vasodilator effects, thelatter being frequently associated with the exhaus-tion of the physiologic coronary reserve, and in thissense, potentially undesirable. At the same time,since the exact mechanism of physiologic coronaryadaptation to cardiac hypoxia and increase of heartmetabolism is not known at present, it is not easy toclassify cardiovascular therapeutic agents accord-ing to the latter point of view. There is every reasonto suppose,, however, that adenosine release fromthe myocardial muscle plays an important role inthe physiologic mechanism of coronary adaptation(26). Moreover, a close relationship is known toexist between the actions and metabolism ofadenosine and inosine, respectively. It was as-sumed that if an improvement in physiologic coro-nary adaptation can be achieved by inosine admin-istration, the coronary vasodilator action ofadenosine should also be increased by the com-pound. This supposition was fully confirmed inthese experiments. Similar observations that relatedto the potentiation of adenosine effects by inosinehave been reported previously (25, 27). Hence, theclose parallel between the potentiation of hypoxiccoronary vasodilation and the adenosine-inducedcoronary vasodilation as elicited by the administra-tion of inosine makes the latter compound a prom-ising candidate for treatment aimed at the improve-ment of cardiac activity weakened by methylenechloride in particular, and by chlorinated solventsand fluorocarbons in general.

Concluding RemarksThe effect of inosine on methylene chloride tox-

Environmental Health Perspectives214

icity was investigated in open chest dogs anes-thetized with pentobarbital sodium. Methylenechloride (5% in air or 50,000 ppm) elicited a de-crease of ventricular contractility represented bythe diminished left ventricular (dpldt)max andmyocardial contractile force measured directly witha Walton-Brodie strain gauge arch. Coronary bloodflow decreased slightly after exposure to methylenechloride. Arterial blood pressure and heart rate didnot change. The negative inotropic effect ofmethylene chloride was reversed or prevented to asubstantial extent by intravenous infusion of inosine(5 mg/kg-min). The effect of the latter compoundwas also characterized by significant coronary vas-odilation. It was shown by the experiments that thecardiostimulatory action of inosine was associatedwith improved hypoxic adaptability of the coronaryblood vessels. In contrast, the effect of cate-cholamines (epinephrine and isoproterenol) was notaccompanied by such a beneficial coronary vasculareffect. On the basis of these results, it is concludedthat inosine might be a useful antidote for poisoningfrom methylene chloride or other chlorinated sol-vents or fluorocarbons.

REFERENCES

1. Aviado, D. M. Cardiopulmonary effects of fluorocarboncompounds. In: Proceedings of the 2nd Annual Conferenceon Environmental Toxicology. Aerospace Medical ResearchLaboratory, Wright-Patterson Air Force Base, Ohio, 1971,p.31.

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