Hexyl)-Xanthine (Pentoxifylline)Was Obtained by Alkylation of Theobromine.1) It Has Been Suggested That the Effects of Methylxanthinesmay Be Mediated In

Cardiovascular Effects of Aminophylline and Pentoxifylline on Intact Dogs and Isolated Dog Atria

Hidehiko WATANABE, M.D., Yasuyuki FURUKAWA, M.D., and Shigetoshi CHIBA,M.D.

SUMMARY The effects of aminophylline and pentoxifylline were investi- gated on the blood pressure and heart rate in the intact dog and on the atrial rate and contractile force in isolated atrial muscle which was perfused with arterial blood of the donor dog. Firstly, each drug was intravenously administered to the donor dog. Both compounds produced a dose-dependent hypotension. Aminophylline frequently caused bradycardia in intact dogs and positive chrono- and inotropic effects in isolated atria. However, pentoxifylline usually produced only tachycardia in the donor dog and positive chrono- and inotropic effects on the isolated atrium. When the compounds were administered into the sinus node artery, positive chrono- and inotropic effects were induced, with aminophylline being approximately 3-10 times more potent than pentoxifylline. Larger doses of aminophylline or pentoxifylline when administered intravenously might cause a release of catecholamine, be- cause increased sinus rate and developed tension in isolated atria which was perfused with donor's blood were partially, but significantly, suppressed by propranolol treatment. In the isolated atrium, aminophylline, but not pentoxifylline, significantly inhibited the negative chronotropic and inotropic effects of adenosine.

Additional Indexing Words: Aminophylline Pentoxifylline Adenosine Intact dog Isolated atrium

HE xanthines, caffeine, aminophylline, and theobromine are colsely re- T lated vasoactive alkaloids. The water soluble 3,7-dimethyl-1-(5-oxo- hexyl)-xanthine (pentoxifylline)was obtained by alkylation of theobromine.1) It has been suggested that the effects of methylxanthinesmay be mediated in

From the Department of Pharmacology, Faculty of Medicine, Shinshu University, Matsumoto 390, Japan. Received for publication March 25, 1981. 235 Jpn. Heart J. 236 WATANABE, FURUKAWA, AND CHIBA March 1982

part by the augmented release of catecholamines from the sympathoadrenal system.2),3) In 1975, Kalsner et al4) reported that aminophylline enhanced beta-adrenergic receptor-mediated responses via a blockade of catecholamine uptake, in addition to its inhibitory action of phosphodiesterase. It has been also reported that caffeine and aminophylline competitively antagonize the effects of adenosine in the heart.5)-8) Moreover, it is well known that methylxanthines stimulate vagal and vasomotor centers in the brain stem.9)-12) However, it is still not clear how methylxanthines exert their effects on the cardiovascular system and little informations are available concerning the action of pentoxifylline. In the present study, we made an attempt to investigate direct and in- direct cardiac effects of aminophylline and pentoxifylline, using the isolated atrial preparation perfused with the donor dog's blood.13),14)

MATERIALS AND METHODS

Eighteen mongrel dogs, weighing 10-20Kg, were anesthetized with sodium pentobarbital, 30mg/Kg, i.v. After treatment with 2,000 units of sodium heparin i.v., the right atrium was excised and plunged into a cold

Tyrode solution at 4-10•Ž. The sinus node artery was cannulated via the right coronary artery and perfused with blood conducted from the carotid artery of the donor dog by aid of a peristaltic pump (Harvard Apparatus,

Model 1210). The perfusion pressure was constantly maintained at 100 mmHg. The flow rate was 4.5-5.0ml/min. The atrium was suspended in a bath filled with blood at a constant temperature of 37•Ž. The upper part of the crista terminalis of the atrium was connected directly to the force dis- placement transducer (Grass FTO3B) by a silk thread. Bipolar platinum electrodes were placed into contact with the atrial epicardium. The atrial rate was measured with a cardiotachograph triggered by the signal from the atrial depolarization of an electrogram. The isometric tension was recorded on an ink-writing rectigraph (Sanei Sokki Instrument) through a carrier preamplifier (Nihon Kohden RP-3), RC circuit and a high gain amplifier

(Nihon Kohden RPH-2). The donor dogs, weighing 9-16Kg, were also anesthetized with 30mg/

Kg of sodium pentobarbital i.v. and artificially ventilated with room air by use of a Harvard respirator. Sodium heparin, 500units/Kg, was administered intravenously at the beginning of the perfusion and 200units/Kg was added at 1-h intervals. The systemic blood pressure of the donor dog was measured from the cannulated left femoral artery with a pressure transducer

(Nihon Kohden RP-2), and heart rate was measured with a cardiotachograph Vol.23 No.2 CARDIOVASCULAR EFFECTS OF METHYLXANTHINES 237 triggered by the R wave of the lead II. Details of the isolated, blood-perfused canine atrial preparations were described in previous papers.13),14)

The drugs used in these experiments were aminophylline (Eisai), pentoxifylline (Hoechst), adenosine (Boehringer & Sohn), (•})-propranolol hydrochloride (Sumitomo Chemicals), and (•})-norepinephrine hydrochloride

(Sankyo). Each drug was administered into the jugular vein of the donor dog, and into the sinus node artery of the isolated atrium. A continuous infusion was performed by use of an infusion pump (Harvard Apparatus

Model 901).

RESULTS

Effects of aminophylline and pentoxifylline injected into the jugular vein of the donor dog

Aminophylline or pentoxifylline was injected into the jugular vein of the donor dog. The systemic blood pressure decreased dose-dependently by each drug. The hypotensive effect of aminophylline was usually more potent than that of pentoxifylline. Pentoxifylline dose-dependently caused an increase in

the heart rate. On the other hand, aminophylline caused not only tachycardia but also frequently bradycardia with relatively larger doses of 3 and

10mg/Kg (expressed as theophylline base), although it caused slight tachycardia at a relatively smaller dose of 1mg/Kg. Fig. 1 shows a typical effect

Fig. 1. A comparison of effects of aminophylline and pentoxifylline injected into the jugular vein of the donor dog at a dose of 10mg/Kg on the blood pressure and heart rate in the donor dog (A), and the developed tension and atrial rate in the isolated atrium (B). JP n. Heart J. 238 WATANABE, FURUKAWA, AND CHIBA March 1982 of aminophylline and pentoxifylline in the same preparation. In the isolated atrium which was perfused with the donor's arterial blood, the developed tension and atrial rate were increased dose-dependently by each drug. In the isolated atrium, effects on atrial rate and developed tension occurred approximately 2.5min after the onset of the injection, since the time to pass through the circuit was about 2.5min. The positive chronotropic and iso- tropic effects of aminophylline were much more potent than those of pentoxifylline. The results are summarized in Fig. 2.

After the treatment with 100ƒÊg/Kg of propranolol i.v., the effects of aminophylline on the blood pressure and heart rate were not influenced, but pentoxifylline-induced tachycardia was significantly suppressed. On the other hand, the positive chrono- and inotropic responses to both drugs were significantly suppressed but not completely. The results are summarized in

Fig. 3.

Effects of aminophylline and pentoxifylline injected into the sinus node artery of the isolated right atrium

When aminophylline or pentoxifylline was given into the sinus node artery of the isolated right atrium, the developed tension and atrial rate were dose-dependently increased. Effects of aminophylline were roughly about

10 times more potent than those of pentoxifylline in a dose range of 10-300

Fig. 2. Effects of aminophylline and pentoxifylline injected into the

jugular veins of the donor dogs at doses of 1-10mg/Kg in 9 experiments. The control blood pressure was 102•}6mmHg (mean•}S.E.M.), the control

heart rate was 142•}10beats/min (mean•}S.E.M.), the control developed

tension was 2•}0.1g (mean•}S.E.M.), and the control atrial rate was 110•}6

beats/min (mean•}S.E.M.). Vertical lines represent the standard errors of the mean. Vol.23 No.2 CARDIOVASCULAR EFFECTS OF METHYLXANTHINES 239

Fig. 3. Effects of propranolol injected into the jugular veins of donor dogs at doses of 100ƒÊg/Kg on actions of aminophylline and pentoxifylline

when given i.v. (10mg/Kg) to donor dogs in 5 experiments. Vertical lines represent the standard errors of the mean.

Fig. 4. Positive chrono- and inotropic effects of aminophylline (10-300

ƒÊ g) and pentoxifylline (10-1000ƒÊg) injected into the sinus node artery of the isolated atrium in 5 experiments. Vertical lines represent the standard errors

of the mean. Jpn. Heart J. 240 WATANABE,FURUKAWA, AND CHIBA March 1982

Fig. 5. Effects of propranolol (3ƒÊg) injected into the sinus node arteries of 5 isolated atria on actions of aminophylline (300ƒÊg), pentoxifylline (300

ƒÊ g), and norepinephrine (0.1ƒÊg). Vertical lines represent the standard errors of the mean.

Fig. 6. Effects of a continuous infusion of aminophylline (125ƒÊg/min)

and pentoxifylline (250ƒÊg/min) into the sinus node artery of the isolated

atrium on action of adenosine (1-30ƒÊg) when given into the sinus node artery

of the isolated atrium. Vol.23 No.2 CARDIOVASCULAR EFFECTS OF METHYLXANTHINES 241

ƒÊ g. Fig. 4 shows dose-response curves of aminophylline (10-300ƒÊg) and

pentoxifylline (10-1000ƒÊg). After the treatment with 3ƒÊg of propranolol, the positive chrono- and inotropic effects of aminophylline and pentoxifylline

were not significantly suppressed although those of norepinephrine (0.1ƒÊg)

were significantly suppressed as shown in Fig. 5.

Effects of aminophylline and pentoxifylline on action of adenosine in the isolated

atrium

When adenosine was injected into the sinus node artery of the isolated

atrium at doses of 1-30ƒÊg, both the developed tension and atrial rate de-

creased dose-relatedly as previously reported.15) An continuous infusion of

aminophylline (125ƒÊg/min) or pentoxifylline (250ƒÊg/min) produced slight

increase in the developed tension and atrial rate. Aminophylline significantly

suppressed the negative ino- and chronotropic effects of adenosine, but pen-

toxifylline did not modify these effects of adenosine as shown in Fig. 6. Sum-

marized data are shown in Fig. 7.

Fig. 7. Effects of a continuous infusion of aminophylline (125ƒÊg/min)

and pentoxifylline (250ƒÊg/min) on actions of adenosine (1-30ƒÊg) when given

into the sinus node arteries of isolated atria in 5 experiments. Vertical lines represent the standard errors of the mean.

DISCUSSION

Aminophylline is known to exert central nervous stimulation as well as direct myocardial stimulation and relaxation of smooth muscle including blood vessels.9)-12) Thus, the effects of aminophylline on the heart rate are various Jpn. Heart J. 242 WATANABE, FURUKAWA, AND CHIBA March 1982 in doses given i.v.16) In the present study, we demonstrated that a large dose of aminophylline caused a bradycardia or tachycardia in intact dogs, showing markedly positive chrono- and inotropic effects on the isolated heart. On the other hand, pentoxifylline usually produced only tachycardia, although the positive chrono- and inotropic effects were produced rather weaker than those of aminophylline on the isolated heart. Therefore, it was demonstrated that bradycardia by aminophylline might be not due to its direct effect on the SA node but due to its central vagal stimulation. These results support the re- ports that cardiac effects of pentoxifylline are weaker than those of aminophylline and pentoxifylline has little effect on central nervous system.16),17) It has been reported that the effects of methylxanthines may be mediated in part by the augmented release of catecholamines from the adrenal medulla,2),18),19)and sympathetic nerve terminals.3),20) In the present study, positive inotropic and chronotropic effects of aminophylline and pentoxifylline when injected into the sinus node artery of the isolated atrium were not significantly changed by the treatment with propranolol, while the effects of norepinephrine were markedly suppressed. Furthermore, positive inotropic and chronotropic effects of them when administered to donor dogs were suppressed by the treatment with propranolol although they were not complete. Therefore, it is assumed that the ino- and chronotropic effects of methylxanthines are a summation of their direct action on the heart and a release of catecholamine from the adrenal medulla directly or reflexly owing to hypotension. It is reported that either caffeine or theophylline antagonized the vasodilating action of adenosine in the coronary circulation.5),7) Popendiker6) reported that aminophylline inhibited the sinus bradycardia of the rat and the atrioventricular block of the guinea pig heart by adenosine, but that pentoxifylline did not inhibit them when administered intravenously in the intact animal. In the present study, we used isolated atria and confirmed that aminophylline significantly inhibited adenosine-induced negative chronotropic and inotropic effects but pentoxifylline never modified adenosine-induced effects.

REFERENCES

1. Mohler W, Soder A: Zur Chemie and Synthese von 3,7-Dimethyl-1-(5-oxo-hexyl)-xanthin.

Arzneim-Forsch 21: 1159, 1971

2. Atuk NO, Blaydes MC, Westervelt FB, Wood JE: Effect of aminophylline of urinary excre-

tion of epinephrine and norepinephrine in man. Circulation 35: 745, 1967

3. Aunis D, Mandel P, Miras-portugal MT, Coquillat G, Rohmer F, Warter JM: Changes of

human plasma dopamine-ƒÀ-hydroxylase activity after intravenous administration of theophylline. Brit J Pharmacol 53: 425, 1975 Vol.23 No.2 CARDIOVASCULAR EFFECTS OF METHYLXANTHINES 243

4. Kalsner S, Frew RD, Smith GM: Mechanism of methylxanthine sensitization of norepine-

phrine responses in a coronary artery. Am J Physiol 228: 1702, 1975 5. Afonso S: Inhibition of coronary vasodilating action of dipyridamole and adenosine by amino-

phylline in the dog. Circulat Res 26: 743, 1979 6. Popendiker K: Wirkung von 1-(5'-Oxohexyl)-3,7-dimethylxanthin auf die Herzfrequenz nach Gabe von Propranolol und Adenosine. Arzneim-Forsch 20: 1485, 1970

7. Bunger R, Haddy FJ, Gerlach E: Coronary responses to dilating substances and competitive inhibition by theophylline in the isolated perfused guinea pig haert. Pflug Arch ges Physiol

358: 213, 1975

8. Schrader J, Nees S, Gerlach E: Evidence for a cell surface adenosine receptor on coronary myocytes and atrial muscle cells. Pflug Arch ges Physiol 369: 251, 1977

9. Marcus ML, Skelton CL, Grauer LE, Epstein SE: Effects of theophylline on myocardial mechanics. Am J Physiol 222: 1361, 1972

10. Jacobs NH, Senior RM, Kessler G: Clinical experience with theophylline: relationships be-

tween dosage, serum concentration, and toxicity. JAMA 235: 1983, 1976 11. Kordash TR, Von Dellen RG, McCall JT: Theophylline concentrations in asthmatic pa- tients after administration of aminophylline. JAMA 238: 139, 1977

12. Ogilvie RI, Fernandez PG, Winsberg F: Cardiovascular response to increasing theophylline

concentrations. Eur J Clin Pharmacol 12: 409, 1977 13. Chiba S, Kimura T, Hashimoto K: Muscarinic suppression of the nicotinic action of acetyl-

choline on the isolated blood-perfused atrium of the dog. Naunyn-Schmiedebergs Arch Phar- macol 289: 315, 1975a

14. Chiba S, Yabuuchi Y, Hashimoto K: Comparison of the effects of norepinephrine and acetyl- choline between intraarterial and extravascular administration of the isolated, blood-perfused

canine atrium. Jpn J Pharmacol 25: 433, 1975b 15. Chiba S: Potentiation of the negative chronotropic and inotropic effects of adenosine by

dipyridamole. Tohoku J Exp Med 114: 45, 1974

16. Sakurai M, Komarek J: Comparative studies of 3,7-dimethyl-1-(5-oxo-hexyl)-xanthine and theophylline on cardiac function. Arch Int Pharmacodyn Ther 209: 342, 1974

17. Popendiker K, Boksay I, Bollmann V: Zur Pharmakologie des neuen peripheren Gefass- dilators 3,7-Dimethyl-1-(5-oxo-hexyl)-xanthin. Arzneim-Forsch 21: 1160, 1971

18. Westfall DP, Fleming WW: Sensitivity changes in the dog heart to norepinephrine, calcium

and aminophylline resulting from pretreatment with reserpine. J Pharmacol Exp Ther 159: 98, 1968

19. Peach MJ: Stimulation of release of adrenal catecholamine by adinosine 3',5'-cyclic mono-

phosphate and theophylline in the absence of extracellular Ca2+. Proc Natl Acad Sci USA 69: 834, 1972 20. Wooten GE, Thoa NB, Kopin IJ, Axelrod J: Enhanced release of dopamine ƒÀ-hydroxylase and norepinephrine from sympathetic nerves by dibutylyl cyclic-adenosine 3',5'-monophos-

phate and theophylline. Mol Pharmacol 9: 178, 1973