Effects of catecholamines on the AV nodal pacemaker in situ after destroying the SA node

EUROPEAN JOURNAL OF PHARMACOLOGY 19 (1972) 351-356. NORTH-HOLLAND PUBLISHING COMPANY

EFFECTS

OF CATECHOLAM1NES AFTER

ON THE AV NODAL PACEMAKER

DESTROYING

IN SITU

THE SA NODE

Shigetoshi CHIBA, Kazuhiro OHKUDA and Koroku HASHIMOTO Department of Pharmacology, Tohoku University School of Medicine, Sendai, Japan 980 Received 24 February 1972

Accepted 15 May 1972

S. CHIBA, K. OHKUDA and K. HASHIMOTO, Effects of catecholamines on the A V nodal pacemaker in situ after destroying the SA node, European J. Pharmacol. 19 (1972) 351-356. Perfusion of the AV node artery in 15 dogs was performed in situ. After destroying the SA node by selective injection of ethanol into the sinus node artery, administration of catecholamines (dopamine, norepinephrine, epinephrine and isoproterenol) into the sinus node artery produced an acceleration of AV nodal rhythm. During the course of recovery, a sudden deceleration or oscillatory change in the AV nodal rhythm was usually observed. This change was not influenced by treatment with atropine, tetrodotoxin or phenoxybenzamine, while both the acceleration and the postaccelerator change were completely blocked by propranolol. These results suggest that postaccelerator changes, i.e., deceleration or oscillation of the AV nodal rhythm, are characteristics of the AV nodal pacemaker itself. Catecholamines Oscillation AV node artery

AV node Tetrodotoxin

1. INTRODUCTION Previously, we frequently observed the induction of a deceleration o f sinus rhythm when naturally occurring catecholamines were injected selectively into the sinus node artery of the in situ dog heart. We concluded that the decelerator responses were due to an adrenergic-cholinergic interaction at the peripheral neuron, because they were blocked by atropine, tetrodotoxin, hexamethonium and phenoxybenzamine (Hashimoto and Chiba, 1969; Hashimoto et al., 1970). Such interactions, observed by many investigators (Burn and Rand, 1965; Burn 1968; Leaders, 1963), have recently been reviewed by Levy (1971). It was of interest to know the response of the AV node to these catecholamines when they were given into the AV node artery. In this case an acceleration occurred in every trial but an oscillation o f nodal rhythm was observed during the return course to the initial level of nodal rhythm. Such oscillation was

Pacemaker Phenoxybenzamine

first observed by Van der Pol and Van der Mark in 1929, and the concept of relaxation oscillation was introduced to cardiac physiology (Van der Pol and Van der Mark, 1929). Thereafter, oscillatory phenomena have been studied by many authors (Bozler, 1943; Reiter, 1962; Braveny et al., 1966) not only on rhythm but also on contraction. In this study we analysed pharmacologically the oscillatory responses of AV nodal rhythm and found that these oscillations could be ascribed to relaxation oscillation but not to an interaction between autonomic nerve fibers.

2. MATERIALS AND METHODS 15 mongrel dogs, of both sexes, weighing 1 0 - 1 6 kg were anesthetized with i.v. sodium pentobarbital, 30 mg/kg. A tracheal tube was inserted and artificial respiration with air was performed by an A I K A respirator. The chest was opened in the right fifth inter-

352

S. Chiba et al., A V nodal pacemaker and catecholamines

costal space. Both vagi were cut but the sympathetic nerves were left intact. The sinus node artery was isolated as already described (James and Nadeau, 1962; Hashimoto et al., 1967). Then the AV node artery was isolated from its origin beneath the terminal portion of the coronary sinus. Sodium heparin, 500 U/kg, was given i.v. at the beginning of perfusion and 200 U/kg added at 1-hr intervals. First, the AV node artery and then the sinus node artery were cannulated. Arterial blood from a femoral artery was driven by a peristaltic pump through both cannulae. Both arteries were perfused under constant pressure at 100 mm Hg (Nadeau and Amir-Jahed, 1965; Chiba and Hashimoto, 1968, 1970). Blood flow to the AV node artery was measured by an electromagnetic flowmeter (Nihon Kohden MF-2). The average flow rate of the AV node artery was 4.2 + 0.8 (mean + S.E.M.) ml/min at 100 mm Hg of perfusion pressure (n = 11). Two electromanometers (Nihon Kohden RP-2) were arranged to measure perfusion pressure and systemic blood pressure. The ECG (lead II) was recorded on an electrocardiograph (Nihon Kohden ME-20-TR), and the heart rate was continuously registered by use of a cardiotachograph (Nihon Kohden RT-2), which was triggered by the R wave of lead 11. The drugs used were dopamine hydrochloride (Tokyo Kasei), d,l-norepinephrine hydrochloride (Sankyo), d,l-epinephrine hydrochloride (Sankyo), 1isoproterenol (Nikken), phenoxybenzamine (S.K. and F), atropine sulfate, propranolol (Sumitomo) and tetrodotoxin (Sankyo). The volume of drug solution injected was 0.01-0.05 ml which was given in a period of 4 sec

EXP. 60 SBP 1 '

'200-150 100

through a 27 gauge needle by a micrometerdrive syringe. Each drug solution was injected by means of its own microsyringe in order to avoid contamination.

3. RESULTS

3.1. Mode of appearance of" A V nodal rhythm after the injection of ethanol into the sinus node artery When ethanol was injected into the sinus node artery, the P wave disappeared and an AV nodal rhythm took place. The pacemaker shifted from the SA node to the AV node. At the beginning of the pacemaker shift, the AV nodal rhythm usually showed oscillation as shown in fig. 1. The amplitude of the oscillatory changes decreased gradually and a stable AV nodal rhythm was established 2 - 3 rain later. 3.2. Ejyects of catecholamines on A V nodal pacemaker Dopamine, norepinephrine, epinephrine and isoproterenol induced a dose-dependent acceleration of nodal rhythm. Table 1 shows a comparison of the accelerated response to catecholamines on AV nodal pacemaker activity. During the return course to its inital rate, the accelerated nodal rhythm was suddenly interrupted by a deceleration for a short period of about 10-20 sec. Fig. 2 shows a typical response to 1.0 /~g of norepinephrine on the AV nodal pacemaker. This appearance of a sudden deceleration occurred repetitively and such oscillatory changes continued until complete recovery to the initial nodal rhythm occurred (fig. 3).

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HR

1 ,.

1

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~-

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ETHANOL 0.5 ml

Fig.1. Oscillatory phenomenon of AV nodal rhythm just after the injection of 0.5 ml ethanol into the sinus node artery. SBP, systemic blood pressure. HR, heart rate.

S. Chiba et al., A V nodal pacemaker and catecholamines 3.3. Absence o f a blocking effect on deceleration and/or the oscillatory phenomenon by atropine, tetrodotoxin and phenoxybenzamine The deceleration (fig. 2) or oscillation (fig. 3) induced by catecholamines was not blocked by treatment with atropine, tetrodotoxin or phenoxybenzamine. Fig. 4 shows that the oscillatory changes in-

200-- l 1 50--1

o* -1E

353

duced by 1/~g of norepinephrine were not blocked by 1 and 10 ~g of atropine, a dose which completely blocked the effects of 1 /2g of acetylcholine for about 2 0 - 3 0 rain. Fig. 5 demonstrates that 10/~g oftetrodotoxin itself produced a depressive effect on AV nodal rhythm but did not abolish the decelerator phenomenon. This amount of tetrodotoxin complete-

SBP

30 sec

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r

'

1 50-- I HR

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B

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ECG

NOREPINEPHRINE

1.0pg

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A

I

C

t

r

|

I

i

I

Fig.2. An acceleration response of the A V nodal pacemaker to I tag of norepinephrine and the sudden interruption by deceleration during the course of recover. SBP, systemic blood pressure. I-JR, heart rate.

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30 sec

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ISdPROTERENOL 0.1 ug Fig.3. Effect of 10 tag of dopamine, 3 tag of norepinephrine and 0.1 tag of isoproterenol. Oscillatory changes in AV nodal rhythm are shown during the course of recovery after each drug. SBP, systemic blood pressure. HR, heart rate.

354

S. Chiba et al., A V nodal pacemaker and catecholamines Table 1 Effect of catecholamines on AV nodal pacemaker activity.

Each value is the mean with S.E.M. Catecholamine

(ug)

No. of dogs

Control AV nodal rate

Maximal increase in nodal rhtyhm

Percentage increase in nodal rate

Dopamine

0.1 1.0

3 3

53 -+ 3.3 53 ± 3.3

77 ± 6.6 121 +- 1.7

45 ± 5.0 128 ± 1.8

Norepinephrine

0.1 1.0

4 6

55 -+ 2.9 57±3.3

110 ± 10.2 127-+ 4.7

100 +- 9.8 123±4.6

Epinephrine

0.1 1.0

4 4

52±2.5 55-+2.9

113± 11.1 128± 4.8

117± 11.2 132+- 4.9

lsoproterenol

0.01 0.1

5 3

54 ± 2.4 53 -+ 3.3

113 ± 7.7 137 ± 3.3

109 +- 7.5 158 -2_ 3.9

~'

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I

30

sec

i

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HR 100 50

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1 pg

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1 50

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ATROPINE

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NOREPI1.Opg Fig.4. Absence of a blocking effect of atropine on the oscillatory changes in AV nodal rhythm. SBP, systemic blood pressure. HR, heart rate.

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-7

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TTX

i

10 ~ g

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1 pg

NOREPINEPHRINE 1 ~g

Fig.5. Absence o f a blocking effect o f t e t r o d o t o x i n ( T T X ) on the decelerator phenomenon on A V nodal rhythm. E f f e c t on 1 #g of norepinephrine was not changed 1 rain after 10/~g of TTX, SBP, systemic blood pressure. HR, heart rate.

355

S. Chiba et al., A V nodal pacemaker and catecholamines

200

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Fig.6. Absence of a blocking effect of phenoxybenzamine on the decelerator phenomenon during the course of recovery from an acceleration of AV nodal rhythm. Effect of l u g of norepinephrine was not affected 1 min after 10 ug of phenoxybenzamine. SBP, systemic blood pressure. HR, heart rate.

ly blocked the effect of electrical stimulation of either the right vagus or the right steilate ganglion for more than 30 rain (Hashimoto and Chiba, 1969). Likewise, 10 ~tg of phenoxybenzamine did not influence the oscillatory change, as shown in fig. 6. Propranolol inhibited both acceleration and postaccelerator changes.

4. DISCUSSION It is known that the AV node is the principal alternative pacemaker of the heart. If sinus arrest or complete AV block occurs, the AV node assumes an important role as the pacemaker instead of the SA node. In this study, we investigated the nature of AV nodal pacemaker activity after destroying the SA node, especially to determine the effects of the catecholamines on the AV node. When the SA node was destroyed, sinus arrest occurred and then an AV nodal rhythm was initiated. The initial rate of AV nodal rhythm was rather slow but it showed oscillatory changes which finally reached a constant rate of about 60 beats/rain. Effects of catecholamines on AV nodal pacemaker activity were compared after the AV nodal rhythm stabilized. The accelerating effect of the 4 catecholamines was in the following order, isoproterenol > epinephrine = norepinephrine > dopamine, with potency ratios of approximately 10:1: 1: 1/2. A similar order for the positive chronotropic effect of catecholamines was observed on the SA node after atropine treatment (unpublished data). Oscillation of the nodal rhythm was induced during the recovery course of nodal rhythm after the catecholamines. Previously we described induction of a de-

celeration response of the SA node pacemaker to naturally occurring catecholamines when they were given into the sinus node artery (Hashimoto et al., 1970). This decelerator response was blocked by atropine, tetrodotoxin, phenoxybenzamine, hexamethonium and hemicholinium. In contrast, oscillation of the AV nodal pacemaker activity was not inhibited by atropine, tetrodotoxin and phenoxybenzarnine. From these results, the oscillatory response of the AV nodal pacemaker to catecholamines can be ascribed to an inherant property of the AV nodal pacemaker. When the ventricle is driven at a higher rate, the ventricular automaticity is suppressed and this is apparently observed at the moment of the interruption of stimulation (Linenthal et al., 1960; Chardack et al., 1961; Cammilli et al., 1964). Such inhibition of pacemaker activity might be attributed to a rise in potassium (Lu et al., 1965). Vassalle et al. reported that the potassium concentration of coronary sinus blood increased when the ventricles were driven at a rate faster than the spontaneous ventricular rate (Vassalle et al., 1967). Therefore it may be considered that the oscillatory or the sudden decelerator phenomenon resulted from changes in potassium levels. Whether the effect of catecholamines on the AV node would be similar to that of electrical stimulation at a high rate, however, cannot be concluded yet. ACKNOWLEDGEMENTS These experiments were supported in part by Grants from the Pharmacological Research Foundation, the Sankyo Central Laboratories and Squibb Institute for Medical Research. The authors express their hearty thanks to Professor J.l. Moore for her criticism. The authors thanks Mr. S. Hayashi for his technical assistance.

356

S. Chiba et al., A V nodal pacemaker and eatecholamines

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