Enhancement of the coronary dilator effect of intravenous and intracoronary administered adenosine by hexobendine in dogs

EUROPEAN JOURNAL OF PHARMACOLOGY 13 (1971) 312-319. NORTH-HOLLAND PUBLISHING COMPANY

ENHANCEMENT

OF THE CORONARY

AND INTRACORONARY

ADMINISTERED

DILATOR EFFECT OF INTRAVENOUS A D E N O S I N E BY H E X O B E N D I N F

IN DOGS

G. RABERGER and O. KRAUPP Institut fffr Pharmakologie und Toxikologie der Ruhr-Universitiit Bochum, Germany

Received 8 June 1970

Accepted 12 October 1970

G. RABERGER and O. KRAUPP, Enhancement o f the coronary dilator effect of intravenous and intracoronary administered adenosine by hexobendin e in dogs European J. Pharmacol. 13 ( 1971) 312- 319. The effects of i.v. and i.cor, administered adenosine on coronary vascular resistance were investigated before and following intraduodenal administration of 1 or 2 mg/kg hexobendine in anaesthetized dogs. The following results were obtained: 1) The intraduodenal application of hexobendine (2 mg/kg) led to a significant potentiation of the coronary dilator effects of i.v. administered adenosine (relative potency = 18) for a period of more than 3 hr. The coronary dilator effects of i.cor, administered adenosine (manifest at one hundredth of the i.v. effective dosage) were less-markedly enhanced (relative potency = 2.6) by pretreatment with hexobendine. 2) Potentiation of the coronary dilator effect of i.v. administered adenosine reached its maximum 40-50 min following hexobendine pretreatment, whereas the effects of i.cor, administered adenosine reached maximum potentiation only 150-180 min following the intraduodenal application of hexobendine, at a time when the potentiation of i.v. injected adenosine had already declined. From this significant difference in the temporal course of the potentiation of the effects of adenosine administered by different routes, it may be concluded that specific myocardial binding of hexobendine occurs, leading to a direct effect of hexobendine on the myocardial site of adenosine action. Adenosine potentiation

Coronary vasodllation

1. INTRODUCTION Potentiation of the coronary dilator effects of adenosine by long-acting coronary dilator substances was first established for dipyridamol (Bretschneider et al., 1959) and subsequently for hexobendine (Kraupp et al., 1965), lidoflazine (Afonso et al., 1968; Jageneau and Schaper, 1969) and carbochromen (Bretschneider et al., 1963). The question arose as to whether an actual increase in the blood level of free adenosine was the only reason for the observed potentiation of the coronary effects of adenosine by coronary dilators, a view proposed by Lochner (1963) on the basis of findings that the effect of intracoronary (i.cor.) administered adenosine were only slightly enhanced by pretreatment

Hexobendine

with dipyridamole. The present investigations were carried out in order to establish the quantitative difference between the adenosine potentiation by hexobendine for the i.v. and i.cor, routes of administration of adenosine in dogs. By using intraduodenal application of hexobendine in doses which did not per se produce any coronary dilator effects, it was possible to follow up the time course of the adenosine potentiating effect without any interference.

2. METHODS 2.1. Experimental procedure The investigations were carried out on 6 mongrel dogs weighing between 25 and 36 kg, which had been

G.Raberger, O.Kraupp, Adenosine potentiation by hexobendine

starved for 24 hr. One hour after pretreatment with 2 mg/kg morphine, anaesthesia was induced with 3.5 ml/kg urethane/chloralose (urethane: 25%, chloralose: 2% (w/v)). Following intubation the animals were artificially ventilated with a mixture of N~ 0 / 0 2 (3:1) using an EngstrSm respirator, and the end tidal CO~ concentration (URAS 4, Hartmann and Braun) was maintained constant at 4.5 vol% by varying the respiratory volume. Blood pressure was recorded electromanometrically in the iliac artery (Hewlett Packard bridges) and heart rate was simultaneously integrated from the ECG lead II (cardio tach unit). The coronary sinus was catheterized under X-ray control from the left jugular vein according to Goodale et al. (1948) and coronary sinus outflow was measured electromagnetically using a Lochner-Oswald (1964) flow-tip catheter. Heart frequency, blood pressure and coronary sinus outflow were recorded after electrical integration on an 8 channel 7700-Hewlett Packard recorder. The animals were placed in the supine position and a special catheter (developed for coronary angiography by Cordis CO, Miami) was introduced under X-ray control from the right femoral artery into the left coronary artery. A control injection of Endographin ® showed the correct position of the catheter tip, not only by the distribution of the radio-opaque fluid into the ramifications of the left coronary artery, but also by a significant transient increase in coronary sinus outflow. For the intraduodenal application of hexobendine a catheter was inserted into the duodenum (tip near the pylorus) after median laparotomy, 2.2. Calculations The coronary vascular resistance was calculated from the mean arterial blood pressure and the mean coronary sinus outflow as resistance units (mm Hg min/ml). As a parameter of the intensity and duration of the coronary dilator effect of adenosine, the percentage changes in coronary resistance following the individual injections were integrated planimetrically and presented as percentage minute values in figs. 3 - 5 . The significance of changes in coronary resistance between the adenosine injection before and following the intraduodenal application of hexobendine was assessed by means of the Student's t test. For the dose-action curves in fig. 5 the test for parallelism was carried out by a comparison of the regression coefficients according to Brownlee (1953).

313

3. RESULTS The investigations were carried out with two intraduodenal doses of hexobendine (1 mg/kg and 2 mg/ kg, three dogs for each dose). Adenosine was given alternately by the i.cor, and i.v. routes in 3 - 4 increasing doses before, and at different time intervals following the intraduodenal application of hexobendine. The effects of 100/ag/kg adenosine i.v. on heart rate, mean arterial blood pressure and mean coronary sinus outflow before, and at 8 different intervals following the duodenal application of 2 mg/ kg hexobendine are presented as the original registration curves obtained in one dog in fig. 1, those of an i.cor, injection of 1 ~g/kg adenosine in fig. 2. It can be seen that the i.v. administration of 100/~g/kg adenosine led only to a slight and shortlasting increase in coronary flow and heart rate and to a decrease in mean arterial blood pressure (fig. l). Following pretreatment with 2 mg/kg hexobendine intraduodenally, the effects of adenosine on coronary flow, heart rate and blood pressure increased steadily reaching maximal potentiation within 4 0 - 5 0 min after the application of hexobendine (figs. 1 and 4). The i.cor, administration of 1/~g/kg adenosine produced a threefold increase in coronary blood flow without any change in blood pressure and heart rate (fig. 2). Pretreatment with hexobendine only caused a moderate enhancement of the coronary dilator effect of i.cor, administered adenosine. Moreover, no changes in blood pressure and heart rate were obtained during the whole experiment (fig. 2). For an exact comparison of the degree and temporal course of the potentiating effect of intraduodenal hexobendine in the coronary dilator action of i.cor, and i.v. injected adenosine, the relative changes in coronary resistance were calculated and integrated over the entire period of action for each test dose of adenosine (see methods). Figs. 3 and 4 show mean values (3 dogs) of this parameter (decrease in coronary vascular resistance in percentage minute values) plotted versus time for i.v. (100 t~g/kg) and i.cor. (1 ~g/kg) administered adenosine. Following the intraduodenal application of hexobendine the coronary dilator action of adenosine was enhanced about 3 fold for the i.cor, and about 20 fold for the i.v. route of administration. The degree of potentiation was the same following 1 or 2 mg/kg hexobendine, but more prolonged with the higher dosage.

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Interestingly, a different time course of the effect of hexobendine treatment on the coronary dilator action of adenosine was obtained for i.cor, and i.v. administration. Maximum potentiation of i.v. aden•sine occurred 3 0 - 4 0 rain following intraduodenal application of hexobendine whereas maximal enhancement of i.cor, administered adenosine occurred considerably later ( 6 0 - 1 2 0 min)(figs. 3 and 4).

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Fig. 5. Experiments on three dogs (urethane/chloralose/N20). Averagedose action regression lines for i.v. and i.cor, administered adenosine on coronary vascular resistance before, and 60 rain after the intraduodenal application of hexobendine (2 mg/kg). values as a combined parameter of intensity and duration of action) and are graphically presented in fig. 5. All four curves in fig. 5 gave a positive test for parallelism and hence it was possible to calculate relative potencies as follows: Adenosine i.cor. (control value) : Adenosine i.v. (control value) = 106:1. Adenosine i.cor. (following hexobendine) : Adenosine i.v. (following hexobendine)= 16:1 Adenosine i.cor. (control value) : Adenosine i.cor. (following hexobendine) = 1:2.6 Adenosine i.v. (control value) : Adenosine i.v. (following hexobendine) = 1:18.

4. DISCUSSION Following intraduodenal application of hexobendine to anaesthetized dogs, coronary vasodilatation was obtained only with doses higher than 4 - 5 mg/kg (Lorenz and Dell, 1969; Kraupp and Raberger, unpublished). In the present investigations, duodenal application of 1 or 2 mg/kg hexobendine led to a highly significant potentiation of the coronary dilator action of i.v. administered adenosine, although this hexobendine dosage per se was without influence on coronary resistance. Similar observations of adenosine potentiation by ineffective dosages of long-acting coronary dilator substances were obtained for dipyri-

damole (Afonso and O'Brien, 1967) and lidoflazine (Jageneau and Schaper, 1969; Afonso et al., 1968). The question arises as to whether the potentiation of the coronary dilator effects of i.v. administered adenosine by hexobendine is due only to inhibition of adenosine uptake and degradation by the erythrocytes (Stormann, 1966) and also by the lungs (Kolassa et al., 1970) as had been proposed by Scholtholt et al. (1965) for adenosine potentiation by dipyridamole or whether a direct action of long-acting coronary dilators on the cardiac site of the coronary dilator effect of adenosine is also of importance. This latter mode of action must be taken into consideration on the basis of the following findings: 1) adenosine accumulation in the hypoxic heart after pretreatment with dipyridamole (Gerlach and Deuticke, 1963); 2) enhancement of the coronary dilator action of adenosine on the isolated kitten heart perfused with erythrocyte-free McEwen's solution (Stafford, 1966); and 3) inhibition of adenosine uptake into the isolated perfused guinea-pig heart by hexobendine and dipyridamole (Pfleger et al., 1969). In the present experiments an enhancement of the coronary dilator effects of i.cor, injected adenosine was obtained after pretreatment with hexobendine, although the enhancement was significantly less marked than the potentiation of the effects of i.v. administered adeno-

318

G.Raberger, O.Kraupp, Adenosine potentiation by hexobendine

sine. The possibility cannot be excluded, that the relatively small enhancement of the effects of i.cor, injected adenosine (2.6 fold) was caused at least in part by diminished adenosine uptake by the erythrocytes within the coronary vessels. However, the different time course of adenosine potentiation obtained for the different routes of adenosine administration, especially pronounced with 2 mg/kg hexobendine, points to a difference in their mechanism o f action. The enhancement of the effects of i.cor, administered adenosine reached its maximum 150 rain after hexobendine application, at a time when potentiation of the effects o f i.v. adenosine had already declined (figs. 3 and 4). The gradual progressive development of the enhancement o f the effects of i.cor, injected adenosine very likely points to an increasing myocardial accumulation o f hexobendine, whereas the temporal course of the potentiation of i.v. administered adenosine may possibly be determined by simultaneous changes in the blood hexobendine level. These propositions are in accordance with results obtained in distribution studies with radioactivelylabelled hexobendine (Pfleger, personal communication). In those investigations a specific accumulation of i.v. administered hexobendine was observed in the rat heart (myocardial levels 10 fold higher than the corresponding blood levels). Moreover, myocardial elimination of hexobendine was found to occur at a singnificantly slower rate than the elimination o f hexobendine in the blood. It therefore seems justifiable to conclude that the observed prolonged ternporal course of the enhancement of the effects of i.cor, administered adenosine reflects a specific myocardial binding of hexobendine and, furthermore, points to a specific intracardiac component of the adenosine potentiating effects of hexobendine,

ACKNOWLEDGEMENTS The authors would like to thank Dr. L. Adler-Kastner for correction of the English text and gratefully acknowledge the technical assistance of Miss E. Tuisl and Mrs. S.A. Wrana.

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