Enhancement of Coronary Vasodilator Actionof Adenosine Triphosphate by Dipyridamole

By Skoda Afomo, M.D., Ph.D., and George S. O'Brien, M.D., Ph.D.

ABSTRACTIt has been reported that previously administered dipyridamole intensifies the

coronary vasodilator action of adenosine compounds. In the present study, acombination of dipyridamole and ATP at doses that alone are ineffective wasexamined for its effect on the coronary sinus blood flow. In 10 anesthetizeddogs, heart rate, arterial blood pressure, cardiac output, and coronary sinusblood flow were measured before and during a 20-min constant-rate infusionof (a) ATP alone, 1 mg/min; (b) a combination of ATP, 1 mg/min, anddipyridamole, 0.005 mg/kg/min; and (c) dipyridamole alone, 0.005 mg/kg/min. Coronary sinus flow was measured by a newly developed thermodilutionflowmeter. Consistently during the infusion of ATP or dipyridamole alone nochanges in the measured parameters occurred, whereas during infusion of thecombination a very marked and sustained elevation (+493$) of coronary flowoccurred, associated with a moderate increase in cardiac rate (+37$) andoutput (+42$) and a decrease in arterial blood pressure (—17$). In 7 otherdogs, coronary vasodilator responses to ATP were determined before and 1 hourafter a single 10-mg dose of dipyridamole. After dipyridamole, the coronaryvasodilator action of ATP increased 5- to 100-fold. In a third group of 3 dogs,dipyridamole did not enhance the coronary vasodilator effects of nitroglycerin,bradykinin, or acetylcholine.

ADDITIONAL KEY WORDScoronary vasodilationanesthetized dogs

coronary sinus blood flowbradykinin

acetylcholinenitroglycerin

• Dipyridamole,1 2,6-bis (diethanolamino)-4, 8-dipiperidino-pyrimido-(5,4-d) pyrimi-dine, is a well known coronary vasodilator.Besides this immediate action, prior adminis-tration of dipyridamole greatly intensifies thecoronary vasodilator effect of adenosine (1);however, no specific data concerning thisinteresting action is available. In this studycoronary and systemic effects of the concur-rent administration of adenosine 5-triphos-phate (ATP) and dipyridamole are reported.In addition, the effect of prior administration

From the Cardiovascular Research Laboratory, De-partment of Medicine, University of Wisconsin Medi-cal School, Madison, Wisconsin 53706.

This work was supported in part by Grant HE05364 from the National Institutes of Health of theU. S. Public Health Service and by the WisconsinHeart Association.

Accepted for publication February 11, 1967.1PersanrJn, as the intravenous form, was supplied

through the courtesy of Dr. Paul A. Kennedy, Jr.,Geigy Pharmaceuticals, Ardsley, New York.

of dipyridamole on ATP-induced coronaryvasodilation was quantified. ATP was usedsince previous work (2) and preliminarytrials have shown no differences in the coro-nary vasodilator effects of ATP and adenosinewhen administered in equimolar doses; ATPis dephosphorylated to adenosine blood (3).Other types of coronary vasodilators were alsoexamined for possible potentiation by dipy-ridamole.

Methods

Three groups of dogs were studied. The ob-jective in the first group was to evaluate thecoronary vasodilator response to combinations ofATP and dipyridamole in doses which alone areineffective. Ten healthy fasting mongrel dogs(dogs 1-10) were used. Their weights, in the or-der of their assigned numbers, were 20.4, 22.2,18.0, 18.6, 23.6, 18.2, 18.0, 26.3, 17.2, and 23.7kg. Anesthesia was produced by 3 mg/kg of mor-phine sulfate subcutaneously followed in 1 hourby intravenous administration of allobarbital, 12.5

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404 AFONSO, O'BRIEN

mg/kg, urethane, 50 mg/kg, monoethylurea, 50mg/kg, and sodium pentobarbital 8 mg/kg. Car-diac output, femoral arterial blood pressure, car-diac rate, and coronary sinus blood flow weremeasured in these animals. Cardiac output wasdetermined by the dye dilution technique andcoronary sinus flow was measured by a thermo-dilution flowmeter intended for intravascular use(4). Briefly, its working principle is that heatproduced electrically by a resistance wire coilis uniformly distributed in the blood streamby the stirring action of a rotating blade, andthe temperature change is measured downstreamby a thermistor; blood flow is inversely propor-tional to the temperature change. Two cardiaccatheters were placed in the right atrium, one forthe infusion of drugs and the other for the in-jection of indocyanine green for dye dilutioncurves. The thermodilution flowmeter was in-serted through the jugular vein and placed inthe coronary sinus, under fluoroscopic control.Cardiac rate was measured from electrocardio-graphic records. Femoral arterial blood pressurewas recorded from a Statham strain gauge con-nected to a percutaneously inserted Coumandneedle. Mean arterial pressure was obtained byelectrical integration. Coronary sinus blood flow,arterial pressure, and electrocardiograms were re-corded on a direct writing Sanborn Polyviso. Dyedilution curves were obtained with a Gilson dyetracer model DTL and Macropolygraph. Left ven-tricular work was calculated from mean arterialblood pressure and cardiac output. To eliminatecoronary flow changes associated with changes inheart rate, in dogs 1, 2, 7 and 8 (Fig. 2), ratewas maintained constant by electrical stimulationof the right atrium via a catheter with a bipolarelectrode coiled against the atrial wall. Theexperiment was performed in three parts: (a)Femoral arterial blood pressure, cardiac rate, andcoronary sinus blood flow were measured con-tinuously before and during a 20-min constant-rate infusion of ATP in saline delivered throughthe right atrium catheter. Doses used were 1 mg/min in 8 dogs and 2 mg/min in 2 dogs. Cardiacoutput was determined before and at 10 minafter the start of the infusion, (b) The sameparameters were measured as mentioned above,before and during a continuous 20-min infusionof a mixture of dipyridamole and ATP. Thedoses were 0.005 mg/kg/min of dipyridamoleand 1 mg/min of ATP. (c) After this infusionand when coronary sinus flow had returned tothe control level, the same parameters measuredpreviously were redetermined before and during a20-min infusion of dipyridamole alone. Eightdogs received 0.005 mg/kg/min and the 2 dogswhich had previously received 2 mg/min of ATP

were given 0.01 mg/kg/min. Doses of dipyrida-mole and ATP were selected on the basis ofpreliminary experimentation which indicated thesedoses to be ineffective. Since dipyridamole wouldby expected to influence the action of ATP,infusion of ATP always preceded the infusionscontaining dipyridamole.

In the second group of seven dogs, the objectivewas to quantify the enhancement of the coronaryvasodilator action of ATP produced by dipyrida-mole. The animals were anesthetized in the samemanner as those in the first group. In the firstpart of the experiment the coronary vasodilatorresponses to effective doses of ATP given as con-tinuous infusions into the right atrium were de-termined. Then a single intravenous dose of 10 mgof dipyridamole was injected, and 1 hour latercoronary vasodilator responses to effective ATPinfusions were redetermined.

In the third group, three dogs were studiedto determine if the enhancing effect of dipyrida-mole extends to other types of coronary vaso-dilators such as nitroglycerin, bradykinin, andacetylcholine. These animals were anesthetizedas the previous ones. First, coronary vasodilatorresponses to single intravenous injections of nitro-glycerin, bradykinin, and acetylcholine were de-termined. Each agent was used in two doses: onechosen to produce a small increase in coronaryflow and the other twice this amount. Then, theresponses were redetermined for the same dosesas used earlier, 40 min after the intravenousadministration of 10 mg of dipyridamole. Thedogs weighed 20, 23.5 and 19 kg. Doses usedin the first dog were 75 and 37.5 fig of nitro-glycerin, 5 and 2.5 fig of bradykinin and 25 and12.5 fig of acetylcholine. The other two dogsreceived double these doses. The sequence of ad-ministration of these agents was at random.

All dogs were heparinized with 500 units ofheparin/kg given intravenously at the beginningof the experiment.

Results

A typical coronary sinus flow tracing in adog of the first group, whose heart rate waspaced, is shown in Figure 1. During infusionsof ATP or dipyridamole alone, there wasno appreciable change in coronary flow orpressure, but during infusion of the mixtureof the two, there was a striking and sustainedincrease in flow accompanied by a decrease inarterial pressure. Since the coronary flowchanges were of particular interest in thisstudy, values of coronary sinus flow for in-dividual dogs were plotted before and during

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ENHANCEMENT OF ATP ACTION BY DIPYRIDAMOLE 405

FIGURE 1

Coronary stows blood flow (CF) and blood pressure (BP) in a 20.4-kg dog, whose heart waspaced, before (A), during the first 10 min (B), and at the end of 20 min of continuous infusion(C) of 1 m%/min of ATP (I); 1 mg/min ATP with 0.1 mg/min of dipyridamole (II) and 0.1mg/min of dipyridamole atone (III). Arrows show time infusions started. There were no appre-ciable changes in coronary flow in I and III, hut a great increase in II.

the infusion of ATP alone, ATP with dipy-ridamole, and dipyridamole alone. Theseresults are presented in Figure 2. No signifi-cant or consistent changes in flow were ob-served during the infusions of either substancealone, except in the 2 dogs (9 and 10)that received double doses of dipyridamole;in these dogs a small increase in flow occurredat the end of the infusion. In marked contrast,during infusion of ATP with dipyridamole,every dog responded with a dramatic in-crease in flow which was maintained through-out the infusion. In all dogs, values of coro-nary sinus flow, arterial pressure, cardiacoutput and heart rate (unpaced dogs only)measured at 10 min after the start of eachinfusion were compared to those obtained be-fore the start of the infusion and analyzed

CircuUtum Ktiurcb, Vol. XX, April 1967

statistically. Results shown in Table 1 indicatethat no significant changes occurred in coro-nary flow, pressure, output, and rate duringthe infusion of ATP or dipyridamole alone.During infusion of the two combined, a verylarge and statistically significant increase(+493$) in coronary sinus flow occurred,associated with a much smaller but signifi-cant increase in cardiac output (+42%), amoderate increase in heart rate (+378S), anda decrease in mean femoral arterial bloodpressure (—Yl%). Ventricular work increased26%; however, this change did not reachstatistical significance (P<0.1).

A representative recording of coronarysinus blood flow in a dog from the secondgroup is illustrated in Figure 3. Before ad-ministration of dipyridamole, infusions of

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406 AFONSO, O'BRIEN

7.6 and 15.3 mg/min of ATP raised the coro-nary blood flow from 27 ml/min to 39 and134 ml/min respectively. One hour after the

• too3

\

ATP + DIPYRIOAMOU 0IPYRI0AMOLE

MIMUTC*

FIGURE 2

Coronary sinus blood flow for each dog of the firstgroup during continuous infusions. Flow increasedstrikingly in all animals during infusion of ATP anddipyridamole. Hearts of dogs 1, 2, 7 and 8 were paced.

single dose of 10 mg of dipyridamole, aninfusion of 0.76 mg/min of ATP raised coro-nary flow to 92 ml/min, which is intermediatein magnitude between the levels obtained be-fore dipyridamole with doses of ATP tenand twenty times larger. Thus, in this dog,dipyridamole enhanced the coronary vaso-dilator action of ATP by ten to twenty times.In Table 2 coronary vasodilator responsesof ATP before and 1 hour after dipyridamoleare given for each dog. In all seven dogssensitivity to ATP increased after dipyrida-mole, the increase varying from 5 to over100 times. Ten to 15 min after dipyridamolealone, coronary flow was significantly ele-vated (average +135%) and returned to con-trol level within 1 hour.

In the three dogs which received nitro-glycerin, bradykinin, and acetylcholine, in-creases in coronary blood flow produced byeach dose of each agent were measured. In-creases in flow obtained before dipyridamolewere comparable to the respective ones afterdipyridamole. Coronary vasodilation was notenhanced.

Discussion

Results of experiments in this study dem-onstrate that the coronary vasodilator action

TABLE 1

Systemic and Coronary Hemodynamic Changes before and during Infusion

Measurements ATP

ATP

Dipyridamole Dipyridamole

Cardiacoutput(liter/min)

Mean arterialblood pres-sure (mm Hg)

Coronary sinusblood flow(ml/min)

Heart rate(beats/min)

ControlStudy% ChangeP<

ControlStudy% ChangeP<

ControlStudy% ChangeP<

ControlStudy% ChangeP<

1.7041.603

-5 .90.2

112114+ 1.2

0.6

64640.0

8289

+9.00.1

1.8692.650

+41.70.01

11596

-16.60.001

64366

+492.90.001

88121

+37.40.02

1.9922.008

+0.80.9

107102—4.1

0.1

6571

+8.80.2

8496

+ 14.20.5

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ENHANCEMENT OF ATP ACTION BY DIPYRIDAMOLE

IA B

CF 35ml/min

407

BPmmHg

CFml/min

R P

mmHg

Ji A0 3 , > • '

1407 0

35

1 7 R

2 0 0

1OO

0

:!-

r

B• M M M J

fiiij

FIGURE 3

I: A, control recordings; B, response to 7.6 mg/min of ATP; C, response to 15.3 mg/min ATP;D, response to intravenous injection of dipyridamole (arrow shows time of injection). II: A,3 min after injection of dipyridamole; B, 20 min after injection, and C, 1 hour after injection.Infusion of 0.76 mg/min ATP began at arrow.

Coronary Sinus

Dog

1234567

Weight(kg)

19.024.516.019.119.023.717.2

Blood Flow

Control(ml/min)

30.052.717.527.069.239.233.2

TABLE

during ATP Infusions

Before dipyridamoleATP mg/min

1 91 3.8 7.6 15.3

42.5 166.0148.0 280.0

17.539.0 134.0

100.0 156.080.0 253.0

50.0 268.0

2

1 Hour after dipyridamoleATP mg/min

0.076 0.19 0.38 0.76

100.0154.0

54.092.0

187.0258.0

275.0

Incr<

<10x<10x

<20x

lased sensitivityto ATP

>100X

>40X40X40 X

>5X>5X

>10x

of ATP is greatly enhanced by prior or con-current administration of dipyridamole. Thesefindings are in accordance with the observa-tion of Bretschneider et al. (1) of intensifica-tion of the effect of intravenous adenosineafter dipyridamole. On the basis of recent

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biochemical studies (3, 5, 6), it is likely thatthe intensification of adenosine action is dueto an adenosine-sparing effect of dipyrida-mole in blood and possibly at cellular levels.It was shown in their studies that dipyrida-mole effectively prevents the disappearance

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408 AFONSO, O'BRIEN

of adenosine added to whole blood. Themechanism of this protection was attributedto a reduction in the permeability of the redblood cell membrane to adenosine, thus avoid-ing its destruction by the intracellular adeno-sine deaminase. The combined effects of ATPand dipyridamole can be classified as supra-additive, since in the first group of dogs noappreciable changes in flow were observedduring the infusions of ATP or dipyridamolealone.

It should be emphasized that coronarysinus blood flow measured in this study re-fers to the flow at the cross section of thecoronary sinus where the thermistor is locatedand includes only the coronary venous drain-age up to this location. In this study it isimportant to note that the changes in cardiacoutput during the infusion of the combinationof ATP and dipyridamole are relatively small(+41.7%) when compared to the exceedinglygreat increase in coronary blood flow. Suchan enormous increase in coronary flow can-not be attributed to the small changes in leftventricular work and cardiac rate. Consid-ering that about half of the increase in car-diac output can be accounted for by the in-crease in the measured coronary flow, whichis only a part of the total coronary bloodflow, it is reasonable to believe that the com-bination of ATP and dipyridamole has aselective action on the coronary vascular bed,although adjustments in the vascular beds atthe peripheral level have not been ruled out.In view of the great increase of coronary flowby the combinations of ATP and dipyridamoleit is suspected that they may be acting at

the level of myocardial microcirculation aswell as at the coronary arterial level. Con-comitant sampling of coronary sinus bloodfor arteriovenous differences would be de-sirable to assess alterations in myocardialmetabolism; however, such sampling was nottechnically feasible in this study. Althoughthe combination of ATP and dipyridamoleseems to have a selective coronary vasodi-lator action, additional knowledge is necessarybefore its use can be suggested in clinicalsituations.

References1. BRETSCHNEIDER, H. J., FRANK, A., BERNARD,

U., KOCHSIEK, K., AND SCHF.I.KR, F.: Die

Wirkung eines Pyrimidopyrimidin-Derivatesauf die Sauerstoffversorgung des Herzmus-kels. Arzneimittel-Forsch. 9: 49, 1959.

2. ROWE, G. C , AFONSO, S., CURTNER, H. P.,

CHELJUS, C. J., LOWE, W. C , CASTILLO, C.

A., AND CRUMPTON, C. W.: Systemic andcoronary hemodynamic effects of adenosinetriphosphate and adenosine. Am. Heart J.64: 22, 1962.

3. KUBLER, W., AND BHETSCHNEIDER, H. J.: {Competi-

tive Hemmung der Katalysierten Adenosindiffu-sion als Mechasismus der coronarerweiterndenWirkung eines Pyrimido-pyrimidin-Derivates.Pfluegers Arch. Ges. Physiol. 280: 141, 1964.

4. AFONSO, S.: A thermodilution flowmeter. J.Applied Physiol. 21: 1883, 1966.

5. Koss, F. W., BEISENHEBZ, G., AND MAERKISCH,

R.: Die Eliminierung von Adenosin aus demBlut unter dem Einfluss von 2,6-Bis(diathan-olamino)-4,8-dipiperidino-pyrimido (5,4-d) py-rimidin und Papaverin. Arzneimittel-Forsch.12: 1130, 1962.

6. BUNAC, R. D., DOUGLAS, C. R., IMAI, S., AND

BERNE, R. M.: Influence of a pyrimidopyrim-idine derivative on deamination of adenosineby blood. Circulation Res. 15: 83, 1964.

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SKODA AFONSO and GEORGE S. O'BRIENEnhancement of Coronary Vasodilator Action of Adenosine Triphosphate by Dipyridamole

Print ISSN: 0009-7330. Online ISSN: 1524-4571 Copyright © 1967 American Heart Association, Inc. All rights reserved.is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Circulation Research

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