Alpha adrenoceptors and arrhythmias

J Mol Cell Cardiol 18, (Supplement 5) 59-68 (1986)

ALPHA ADRENOCEPTORS AND A R R H Y ~ A S

D.J. SHERIDAN

Department of Cardiology, St. Mary's Hospital, Praed Street,

London W2 INY

Alpha blocking drugs such as phentolamine have been known to possess antiarrhythmic properties for many years.

Leimdorfer (I) demonstrated theanti-

arrhythmicpotential of phentolamine against experimental arrhythmias as early as 1953; its effect against arrhythmias associated with experimental myocardial ischaemia and reperfusion has been known since 1960 (2).

At this time, it was

generally believed that adrenergic actions on the heart were mediated by beta adrenoceptors and the anti-arrhythmic action of phentolamine was generally attributed to a direct myocardial effect (it was some time later that myocardial alpha adrenoceptors were demonstrated, 3,4,5).

Renewed interest in myocardial alpha adreno-

ceptors was aroused in 1980, following the demonstration that alpha adrenoceptor blocking drugs could prevent arrhythmias associated with myocardial ischaemia and reperfusion (6,7).

Both phentolamine (alpha I and alpha 2 blockade) and prazosin

(alpha blockade) were effective in preventing arrhythmias associated with LAD ligation in the chlorolosed anaesthetised cat and during subsequent reperfusion (6). This anti-arrhythmic effect of alpha adrenoceptor blockade has now been confirmed for a number of agents in different animal models (7,8,9).

MECHANISM OF ACTION Alpha adrenoceptor blocking drugs such as phentolamine exert several actions on the cardiovascular system, which could contribute to its anti-arrhythmic action.

In

addition to its alpha blocking action on the heart, phentolamine has been shown to have a direct electrophysiological action on isolated superfused myocardium (10). In addition, alpha adrenoceptor blockade reduces systemic vascular resistance which could also be beneficial during myocardial ischaemia, by reducing energy requirements. Thus, while there is little doubt that alpha adrenoceptor blocking drugs have a potent anti-arrhythmic effect, the mechanism of action remains uncertain. To investigate the possibility that haemodynamic manipulation might be involved, cats were studied under chlorolose anaesthesia during LAD ligation and subsequent reperfusion.

In these experiments infusions of dextran were used to reverse the

effect of phentolamine on left ventricular filling pressure.

This maneouvre did not

attenuate the anti-arrhythmic effect of the drug, suggesting that its mode of action could not be explained by a reduction and filling pressure (6).

Similarly, the

haemodynamic effects of phentolamine were mimicked by infusing nitroprusside and in these experiments, establishing the same haemodynamic profile as that which resulted from phentolamine pretreatment, had no anti--arrhythmic effect (6), again suggesting that the anti-arrhythmic effect observed with phentolamine could not be explained by

0022-2828/86/$50059+ 10 $03.00/0

9 1986 Academic Press Inc. (London) Limited

60

D.J. Sheridan

its haemodynamic action.

The possibility that alpha adrenoceptor blockade may

modify myocardial blood flow with a net increase to the ischaemic zone, has also been raised as a possible explanation for its anti-arrhythmic action.

There is no

direct evidence to support this suggestion and in studies using radio labelled microspheres to

measure

blood flow in ischaemic and non-ischaemic zones, phentolamine

did not significantly alter myocardial blood flow to the ischaemic zone during ischaemia or reperfusion (6).

These findings indicate that the anti-arrhythmic

action cannot be satisfactorily explained on this basis. therefore,

Available evidence

suggests that the mechanism of action for the anti-arrhythmic effect of

alpha blocking drugs during myocardial ischaemia and reperfusion is not based on haemodynamic alterations and that alternate mechanisms must be involved. Are the anti-arrhythmic effects mediated by an adrenergic mechanism or by direct myocardial action?

Alpha blocking drugs such as phentolamine undoubtedly, possess

direct myocardial electrophysiological effects.

In canine Purkinje fibres,

phentolamine reduces upstroke velocity and prolongs action potential duration and refractory period (10).

These actions could theoretically provide an explanation

for its anti-arrhythmic properties during myocardial ischaemia,

however,

it may be

an oversimplification to relate such findings to the situation which pertains during myocardial ischaemia and reperfusion.

It would seem wrong to assume that electro-

physiological effects demonstrated in myocardial preparations superfused with oxygenated buffer will pertain to perfused hearts during ischaemis.

In addition, the

concentrations at which direct electrophysiological effects have been demonstrated with phentolamine, are approximately ten times greater than those required to produce a potent anti-arrhythmic effect during ischaemia.

No consistent electrophysiologlcal

action could be demonstrated (10) at concentrations <10 -5 M, whereas it is potently anti-arrhythmic during myocardial ischaemia at 5 x 10-6 M (8).

Attempts to clarify

this question have concentrated on three areas; a) studies of the effects of catecholamine depletion, b) studies of the electrophysiological effects of alpha adrenoceptor blockade, and c) studies of the electrophysiological and arrhythmogenic effects of alpha adrenoceptor stimulation. Several studies have reported that effective depletion of myocardial catecholamlnes significantly reduces ventricular arrhythmias during myocardial ischaemia and reperfusion (6,11,12,13).

Studies using isolated guinea pig hearts obtained from

animals which had been pretreated with 6-hydroxydopamine, indicate that myocardial catecholamine content must be reduced to about 5% of control to completely abolish arrhythmias (11).

The occurrence of arrhythmias in association with quite small

residual amounts of myocardial noradrenaline (IO-15% of control) may perhaps be best explained by the occurrence of noradrenaline release during ischaemia in

Alpha Adrenoceptors and Arrhythmias

61

association with beta adrenoceptor supersensitivity which is known to occur with catecholamine depletion.

This emphasises the importance of checking tissue

catecholamine levels in experiments in which this is used as a tool to examine mechanisms of action.

To assume for example, that adequate myocardial catecholamine

depletion will have occurred, 24 hours following a single dose of reserpine (19) cannot be regarded as reliable. Catecholamine depletion has been known for many years to increase myocardial glycogen content (14-18).

To examine this as a possible mechanism for its anti-

arrhythmic action, myocardial glycogen content was measured in hearts obtained from control and 6-hydroxydopamine pretreated guinea pigs, demonstrating a significant increase in myocardial glycogen content in the latter (11).

Fasting animals for 48

hours prior to the study however, reversed this effect on myocardial glycogen content with no reversal of the anti-arrhythmic action observed (11), suggesting that increased availibility of myocardial glycogen is unlikely to be the mechanism responsible.

~1"

~

I'l

"-5 80

~-.-I. . . . . . . . . . . . . .

~....

5O 4O 3O

(b) -

-

Ischeemlo

DI

Rel~etfutim~--~

Fig. I. Changes in (a) action potential amplitude and (b)Vmax in catecholamine depleted and phentolamine treated hearts. Catecholamine depletion blunted the ischaemia-induced reduction in action potential amplitude and Vmax. Phentolamine enhanced the reduction in Vmax during ischaemia, an effect which was reversed by cateeholamine depletion. In contrast phentolamine reduced action potential amplitude during isehaemia in control and catecholamine depleted hearts. Thus the action of phentolamine on Vmax appeared to be oatecholamine dependent, while that on amplitude was not [o--o control; Z ~ - - ~ phentolamine; o - - o 6.OHDA; M - - I 6.OHDA + phentolamine] (After Penny et al 1985)

Assessment of the electrophysiological

effects of drugs which possess adrenolytic

properties is complicated by uncertainty about how much of its effects reflect

D.J. Sheridan

62

adrenergic blockade compared with direct sarcolemmal action.

In order to clarify

the direct eleetrophysiological effects of phentolamine, we used hearts depleted of endogenous catecholamines to study cellular electrophysiological effects during normal and ischaemic perfusion.

In these experiments, myocardial ischaemia (coronary

flow reduced to 10% of control) produced ~ sharp reduction in Vmax during the first ten minutes with subsequent recovery during the latter part of ischaemia.

Addition

of phentolamine to these hearts accentuated the early reduction in Vmax and impaired recovery during the latter part of ischaemia (Figure I).

Myocardial catecholamine

depletion blunted the early reduction in Vmax with little recovery during the latter part of ischaemia.

Addition of phentolamine to catecholamine depleted hearts

produced no significant additional effects (Figure I), suggesting that its effects on Vmax during ischaemia are dependent on the presence of catecholamines and that its mode of action is therefore adrenergic. Myocardial isohaemia reduced action potential duration (Figure 2).

~E 2 0 0 w

~"

160

~

120

o

IO0

;

80

200.

== ,2o1:

- =

-

'~176

V

60 I-

( b )

I

I

I

I

i

t

5

{0

J5

20

25

30

Ischoemlo

Time (rain)

;:,

k__

55 Repeffusion

I

40

Fig. 2. Changes in (a) action potential duration and (b) refractory period in control, 6-hydroxydopamine pretreated and phentolamine treated hearts. Both phentolamine and catecholamine depletion prolonged action potential duration and refractory period during normal and ischaemic perfusion. When added to catecholamine depleted hearts, phentolamine had no significant additional effect suggesting that its mode of action during ischaemia is mediated via an adrenergic rather than a direct myocardial action. Note marked reduction in action potential duration and refractory period during early reperfusion in control hearts, an effect which is attenuated or abolished by phentolamine and catecholamine depletion [o--o sham; ~ - - ~ phentolamine; o - - e 6.0HDA; R - - U 6.0HDA + phentolamine] (After Penny et al 1985)

Both myocardial catecholamine depletion and phentolamine prolonged action potential duration during normal perfusion and attenuated shortening during ischaemia.

Alpha Adrenoceptors and Arrhythmias

63

Addition of phentolamine to catecholamine depleted hearts resulted in no significant additional effects.

A similar pattern of responses was observed for refractory

period (Figure 2).

The absence of significant effects of phentolamine in hearts

which have been depleted of endogenous cateeholamines suggests that electrophysiological effects observed during isohaemia are dependent on the presence of catecholamines and that its mode of action is adrenergic.

Opposing conclusions to

those outlined above were drawn by Northover (19) from experiments in which the electrophysiological effects of phentolamine were studied in hearts obtained from animals pretreated with reserpine.

Those conclusions were based on the findings

that the electrophysiological effects of phentolamine were unchanged 24 hours following reserpine treatment.

However,

in those studies the degree of myocardial

catecholamine depletion was not measured, and the electrophysiological effects of reserpine treatment (20) were ignored. If the anti-arrhythmic action of drugs such as phentolamine, indoramin and prazosin is indeed due to alpha blockade,

stimulation of myocardial alpha adreno-

captors should be arrhythmogenic during myocardial ischaemia.

To examine this, the

effects of methoxamina (10-6 M) in isolated guinea pig hearts were studied during normal and ischaemie (coronary flow reduced 10% of control) perfusion.

To avoid

complications from release of endogenous catecholamines, these studies were carried out in catecholamine depleted hearts.

As previously, myocardial catecholamine

depletion significantly reduced the incidence of ventriculartachycardia and fibrillation during ischaemia and reperfusion, compared with sham treated controls (Figure 3)-

Perfusion of catecholamine depleted hearts with methoxamine reversed

this anti-arrhythmic effect during ischaemia and ~eperfusion, increasing the incidence of VT and VF,

It is important to emphasise however,

that perfusion of

hearts with methoxamine during normal perfusion was never arrhythmogenic.

Thus,

alpha adrenoceptor stimulation is capable of inducing VT and VF during ischaemia and reperfusion; its arrhythmogenicity being peculiarly related to the presence of ischaemia.

64

D. J. Sheridan

n • 19

Ce, m ~

n = 19

[3&q

o

%

n=28

Fig. 3. Incidence of VT and VF during ischaemia and reperfusion in sham injected controls, eateeholamine depleted hearts and oateeholamine depleted hearts. perfused with methoxamine 10-b M. Note that eateoholamine depletion significantly reduced VT and VF during isehaemia and reperfusion, but that perfusion with methoxamine reversed this effect increasing VT and VF during isehaemia and reperfusion.

n=28

100 Catedmlamine depleted + Methoxamlne (104M) 80] n=13 n=13

The electrophysiologieal effects of methoxamine were also to reverse the actions of myocardial catecholamine depletion.

Thus, myocardial eateeholamine depletion

prolonged action potential duration and refractory period during normal and ischaemic perfusion (Figures 4 and 5).

Addition of methoxamine effectively reversed this with

action potential duration and refractory oeriod restored virtually to the control state (Figure 4).

These findings are in sharp contrast to previous studies of the

effects of alpha adrenoceptor agonists.

Giotti et al (3) observed alpha adreno-

ceptor mediated prolongation in action potential duration in isolated Purkinje fibres.

Similar changes have been observed in atrial action potential (4) duration

(5) and refractory period.

These differences emphasise the potential importance of

ischaemia in modifying myocardial electrophysiological responsiveness. There is now strong evidence that myocardial alpha adrenoeeptor blockade does possess powerful anti-arrhythmic properties during ischaemia and reperfusion.

The

data presented above sUggests that the anti-arrhythmic action of alpha blocking compounds reflects adrenergic rather than direct myocardial effects.

Electro-

physiological effects of anti-arrhythmie agents are traditionally assessed by measuring action potential characteristics in superfused myocardial preparations; however, these techniques may be poorly suited for the investigation of anti-

65

Alpha Adrenoceptors and Arrhythmias arrhythmic mechanisms during myocardial ischaemia and reperfusion.

~

9 SHAM (n =19) 9 60HDA ( n . 28) 9 60HDA "l- Methoxamine (10-6M)(n=13)

180

._~ 160 84

~-

120 100

"=

80-

60. 411

0

4

8

12

16

20

24

Ischaemia

28

2

6

10

, ~-Reperiuslon T i m e (min)

Fig. 4. Effects of methoxamine 10-6 M on action potential duration during ischaemia and reperfusion in catecholamine depleted hearts. Catecholamine depletion prolonged action potential duration during ischaemia and reperfusion. Addition of methoxamine reversed this action and as noted previously, also increased the incidence of VT and VF during ischaemia and reperfusion. [e = sham; I = 6.OHDA; A : 6.0HDA + methoxamine]

It cannot be assumed that electrophysiological effects observed in myocardial preparations superfused with oxygenated buffer reflect changes which would occur during myocardial ischaemia.

The changes in action potential upstroke velocity

during ischaemia for example, may be biphasic (Figure I) and successful antiarrhythmic interventions such as myocardial catecholamine depletion appear to work by modifying the electrophysiological effects of ischaemia rather than by exerting tradition pharmacological actions.

This is consistent with the view that endogenous

catecholamines contribute to the arrhythmogenic effects of ischaemia.

This concept

of course, suggests that anti-arrhythmic agents which act directly on the cell membrane may be less effective in preventing arrhythmias associated with myocardial ischaemia and reperfusion and there is some evidence to support this (21,22).

For

this reason, it is important to study the electrophysiological properties of anti-arrhythmic drugs during myocardial ischaemia and reperfusion and not to limit our interest to substances which can be classified according to their actions on

D. J, Sheridan

66

isolated superfused preparations, since the latter may be misleading in relation to ischaemia and reperfusion (23).

180

"~ ,6o "o "{ 140

*=l D.

,2o

m6OHDA (n.2B) 9 &OFIDA '~Melhoxam/ne (10-6M)(n.13)

0

.

4

8

12

16

20

24

28

Is,~aemia

2

6

10

*. ~ e ~ Tune (nv~)

Fig. 5. Methoxamine 10-6 M on refractory period during ischaemia and reperfusion in catecholamine depleted hearts. While catecholamine depletion prolonged refractory period during ischaemia and reperfusion, perfusion with the alpha adrenoceptor agonist methoxamine reversed this in association with an increased incidence of VT and VF during ischaemia and reperfusion Additionally, in experiments designed to study the direct myocardial electrophysiological effects of alpha adrenoceptor blockade it is important to avoid contaminating influences from release of endogenous catecholamines.

The

experiments outlined above using this approach suggest that the electrophysiological effects of phentolamine which are relevant to its anti-arrhythmic action during ischaemia and reperfusion are dependent on the presence of catecholamines. Further evidence in support of alpha adrenoceptor mediated arrhythmogenic effects during myocardial ischaemia and reperfusion comes from the observation that myocardial catecholamine depletion is anti-arrhythmic in these circumstances, independent of changes in myocardial glycogen availability (11). Furthermore, direot alpha adrenoceptor stimulation is capable of inducing ventricular fibrillation during myocardial ischaemia and reperfusion, but importantly, not during normal perfusion.

These results suggest that isohaemia and reperfusion increase myocardial

susceptibility to the arrhythmogenic effects of alpha stimulation.

This is supported

by previous studies in which increased myocardial responsiveness to neural and humoral stimulation was observed during reperfusion (6).

The cellular electro-

Alpha Adrenoceptors and Arrhythmias

67

physiological effects of alpha adrenoceptor stimulation associated with this arrhythmogenio action were a reduction in action potential duration and refractory period.

This differs from previous studies which used isolated superfused canine

Purkinje fibres (3) and demonstrated a net prolongation in action potential duration.

The basis for this discrepency is not clear, although it is likely to

reflect the presence of myocardial ischaemia and/or the models used. emphasised previously,

As

the kind of model used and the circumstances in which it

is used may have an important bearing on the responses observed.

Once again,

these findings emphasise that responses to eleetrophysiologically active substances may differ depending on the preparation used to study them and that these circumstances need to be considered when interpreting data, particularly in relation to the presence of myocardial ischaemia and reperfusion. REFERENCE LIST I)

Leimdorfer, A. (1953). Abolition of cardiac arrhythmias by regitine (parasympatholytio effects of regitine). Archs int. Pharmacodyn. Ther., 94, 119 and 249

2)

Stephenson, S.E., Cole, R.K., Parrish, T.F., et al. Ventrieular fibrillation during and after coronary artery occlusion: incidence and protection afforded by various drugs. Am. J. Cardiol., 5: 77-87, 1960

3)

Giotti, A., Ledda, F., Mannaioni, P.F. (1973). Effects of noradrenaline and isoprenaline, in combination with alpha and beta receptor blocking substances on the action potential of cardiac Purkinje fibres. J. Physiol. Lond., 229, 99-113

4)

Govier, W.C., Mosel, N.C., Whittington, P., Broom, A.H. (1966). Myocardial alpha and beta adrenergie receptors as demonstrated by atrial functional refractory period changes. J. Pharmac. exp. Ther., 154, 255-63

5)

Pappano, A.J. (1971)- Propranolol-insensitive effects of epinephrine on action potential repolarisation in electrically driven atria of the guinea pig. J. Pharmac. exp. Thor., 177, 85-95.

6)

Sheridan, D.J., Penkoske, P.A., Sobel, Corr, P.B. Alpha adrenergic contributions to dysrhythmia during myocardial ischaemia and reperfusion in cats. J. Clin. Invest., 65, 161-171 (1980)

7)

Stewart, J.R., Burmeister, W.E., Burmeister, J., Lucchesi, B.R. (1980) Eleetrophysiological and antiarrhythmic effects of phentolamine in experimental coronary artery occlusion and reperfusion in the dog. J. Cardiovasc. Pharmaool., 2, 77-91

8)

Penny, W.J., Sheridan, D.J. The effects of phentolamine on arrhythmias and cellular electrophysiology during myocardial isehaemia and reperfusion in the guinea pig. J. Physiol., 326, 64P (1982)

9)

Thandroyen, F.T., Worthington, M.G., Higginson, L.M., Opie, L.H. The effect of alpha and beta adrenoceptor antagonist agents on reperfusion ventricular fibrillation and metabolic status in the isolated perfused rat heart. J. Am. Coll. Cardiol., 14, 1056-1066 (1983)

68

D. J. S h e r i d a n

10)

Rosen, M.R., Gelband, H., Hoffman, B.E. Effects of phentolamine on electrophysiological properties of isolated canine Purkinje fibres. J. Pharmacol. Exp. Ther., 179, 586-593 (1971)

11)

Culling, W., Penny, W.J., Lewis, M.J., Middleton, K., Sheridan, D.J. Effects of myocardial catecholamine depletion on cellular electrophysiology and arrhythmias during ischaemia and reperfusion. Cardiovasc. Res., 18, 675-682 (1984)

12)

Ebert, P.A., Vanderbeck, R.B., Allgood, R.J., Sabiston, D.C. Effect of chronic cardiac denervation on arrhythmias after coronary artery ligation. Cardiovasc. Res., 4, 141-7 (1970)

13)

Sethi, V., Haider, B., Ahmed, S.S., Oldewurtel, H.A., Regan, T.J. Influence of beta blockade and chemical sympathectomy on myocardial function and arrhythmias in acute ischaemia. Cardiovasc. Res., 7, 740-7 (1973)

14)

Schwartz, A. Association of glycogenolysis with cardiac sarcoplasmic reticulum: II effect of glycogen depletion, Deoxycholate solubilization and cardiac ischaemia: evidence for a phosphorylase kinase complex. J. Mol. Cell. Cardiol., 9, 515-28 (1977)

15)

Gerken, G., Doting, V. The metabolism of the anoxic heart after surgical denervation or reserpine pretreatment. J. Mol. Cell. Cardiol., 5, 275-86 (1973)

16)

Sakai, K., Spieckermann, P.G. Effects of reserpine and propranolol on anoxiainduced enzyme release from the isolated perfused guinea pig heart. Naunyn-Schmiedebergs. Arch. Pharmacol., 291, 123-30 (1975)

17)

Scheuer, J., Stezoski, S.W. Protective effect of increased myocardial glycogen stores in cardiac anoxia in the rat. Circ. Res., 27. 835-49 (1970)

18)

Gauduel, y., Karagueuziau, H.S., De Leiris, J. Deleterious effects of endogenous catecholamines on hypoxic myocardial cells following reoxygenation. J. Mol. Cell. Cardiol., 11, 712-31 (1975)

19)

Northover, B.J. A comparison of the electrophysiological actions of phentolamine with those of some other anti-arrhythmic drugs on tissues isolated from the heart. Br. J. Pharmacol., 80, 85-93 (1983)

20)

Godin, D., Grimmond, C., Modeau, R.A., Leblanc, A.R. Electrophysiology of the chemically sympathectomised dog. Cardiovasc. Res., 16, 524-529 (1983)

21)

Corr, P.B., Helke, C., Gillis, R.A. Exacerbation of coronary occlusion induced ventricular arrhythmias by the vagolytic action of procainamide. Cardiovasc. Res., 12, 486-492 (1978)

22)

Naito, M., Michelson, E.L., Kmetzo, J.J., Kaplinsky, E., Dreifus, L.S. Failure of anti-arrhythmic drugs to prevent experimental reperfusion ventricular fibrillation. Circulation., 6, 70-9

23)

Culling, W., Penny, W.J., Sheridan, D.J. Effects of sotalol on arrhythmias and electrophysiology during myocardial ischaemia and reperfusion. Cardiovasc. Res., 18, 397-404 (1984)