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Electrophysiological effects of hypoglycaemic sulphonylureas on rabbit heart
European Journal of Pharmacology, 67 (1980) 0 Elsevier/North-Holland Biomedical Press
ELECTROPHYSIOLOGICAL RABBIT HEART
EFFECTS
333
333-338
OF HYPOGLYCAEMIC
SULPHONYLUREAS
ON
GABOR POGATSA and MIKLGS NBMETH Research Unit, National Institute of Cardiology, 1450 Budapest, P.O.B. 9-88, and Department of Pharmacology, Medical University School of Szeged, 6701 Szeged, P.O.B. 115, Hungary Received 28 May 1980, accepted 7 July 1980 G. POGkTSA and M. NhMETH, Electrophysiological effects of hypoglycaemic sulphonylureas on rabbit heart, European J. Pharmacol. 67 (1980) 333-338. The hypoglycaemic sulphonylurea drugs glibenclamide and carbutamide were compared as to their influence on the electrical activity of isolated rabbit heart preparations. Carbutamide (1O-s-1O-3 M) was found to depress slightly the nomotopic and junctional automaticity, while in Purkinje fibres it enhanced automaticity and conduction velocity. On the other hand, glibenclamide (10-6-10-3 M) had no significant influence on the nomotopic and junctional automaticity, and markedly depressed automaticity and conduction velocity in the Purkinje fibres. The atrial conduction system was not much influenced by the sulphonylureas tested. Carbutamide depressed the electrical threshold in Purkinje fibres. It follows that the hypoglycaemic sulphonylurea compounds carbutamide and glibenclamide differ considerably with respect to their influences on cardiac electrical activity. Positive and negative chronotropism Positive and negative dromotropism
Hypoglycaemic sulphonylureas Isolated rabbit heart
Negative bathmotropism Glibenclamide
Carbutamide
1. Introduction
2. Materials and methods
It is generally accepted that the hypoglycaemic sulphonylureas increase arterial blood pressure and myocardial contractile force (Crass et al., 1973; Curtis et al., 1975; Lasseter et al., 1972; Levey et al., 1971,1974; Roth et al., 1971; Wales et al., 1971), but opinions diverge about their influence on cardiac electrical activity (Curtis et al., 1975; Lasseter et al., 1972). It has been shown in this laboratory that the new sulphonylurea derivative, glibenclamide, influenced myocardial contractile force and arterial blood pressure differently from the traditional sulphonylureas, e.g. from carbutamide which is very widely used in Hungary (Pogatsa and Dubecz, 1977). This prompted comparative investigations into the effects of glibenclamide and carbutamide on the electrical activity of the heart.
Forty-nine albino rabbits of either sex and weighing 1.0-1.7 kg were used in the experiments. The animals were killed by a blow on the neck, the heart was removed rapidly and placed in an oxygenated ,Krebs-Henseleit solution at 32°C. For examination of the nomotopic and heterotopic automaticity of the rabbit heart, the right atrium, the atrioventricular septal region and the right ventricular papillary muscles along with their Purkinje fibre supply were dissected from the immersed tissue. The three tissue preparations were then transferred to fresh Krebs-Henseleit solution and were fixed next to each other with thin needles on a non-toxic rubber sheet (Silicoset 105, ICI) which was placed on the bottom of the jar; spontaneous action was then allowed to take place. The tissue preparations were always brought to spontaneous action for 60-90 min before addition of the compounds to be tested. The electric activity of the spontaneous
334
G. POGATSA, M. NBMETH
myocardial contractions was recorded extracellularly on a three-channel ECG recorder (3 NEK-1, RFT, Dresden). For examination of the atrial conduction system, the left atrium was removed from the rabbit heart and mounted on the bottom of the jar as described above. The left atrium was then stimulated electrically at three times electrical threshold, for 1 msec, at 100 imp /min from a biological stimulator (Disa Multistim 13G04), using bipolar electrodes. Conduction velocity was then measured at a constant rate. The electrical threshold, conduction velocity and effective (local) refractory period were determined by the method of Szekeres et al. (1976), and were recorded with an oscilloscope (EMG TR-4602). The distal conduction system in the isolated right ventricle was prepared by the method of Myerburg et al. (1970) from rabbit heart. Electrical threshold, conduction velocity, effective (local) refractory period
and transmembrane potentials were recorded from the free-running Purkinje fibres (false tendons) using a standard microelectrode technique as described previously (Papp and Vaughan Williams, 1969; ‘Szekeres et al., 1976). Composition of the Krebs-Henseleit solution was (in mM): Na’ 135.0, K’ 5.0, Ca” 2.5, MG’+ 1.2, Cl- 126.0, HCO; 24.7, SO,‘1.2, POJ3- 1.2, glucose 5.0. The solution was oxygenated by a continuous supply of 95% O2 -5% C02. The pH of the solution was adjusted to 7.4, and its temperature was maintained at 32°C. Carbutamide (Bucarban, Chinoin) was dissolved in methyl glycamate and polyethylene glycol, glibenclamide (Gilemal, Chinoin) in polyethylene glycol and glycerinphormal at 20 and 0.2% concentration, respectively. An appropriate solvent control group was set up with each test. The results were evaluated by means of Student’s paired t-test.
TABLE 1 Effects heart.
of glibenclamide
and carbutamide
Compound
on nomotopic
and heterotopic
automaticity
in the isolated rabbit
Concentration (M)
Sinoatrial node (min-’ )
Junctional area (min-’ )
Purkinje fibre (min-’ )
Glibenclamide
n=9
0 1o-6 1o-5 1o-4 1o-3 Washout
125 f 125 + 122 f 116f 114 + 125 f
3 9 6 6 3l 9
70f 74+ 75 f 71+ 60 + 71 f
5 6 6 7 3l 6
21+ 17 + 14 f 10 + 8f 22 +
2 2 4l 3l 2’ 5
Carbutamide
n=6
0 1o-5 1o-4 10 -3 Washout
137f4 134 + 130 + 125 f 134 f
5 4 3r 4
74f6 69 f. 65 + 64+ 73f
6 7 7’ 6
21 23 25 27 22
4 4 4l 4’ 5
116 115 115 114 116
9 9 9 9 9
64 f 63 f 63 f 62 * 63+
8 8 8 8 8
24 + 4 24+ 4 24 + 4 24*A 24 + 4
Solvents
n=6
(?*1-1 1 5 10 140
)
f + f f f
f f f + *
The values in the table are means + S.E.M. The significance relative to baseline values is indicated by: ’ P < 0.05.
ELECTROPHYSIOLOGICAL
EFFECTS
A-V NODE
SINUS- NODE ” !‘!~I’~‘Il!!i\III “rccr.~crrrlc~,uu
OF SULPHONYLUREAS
4“!\y’l’“ll’ HCe.
IIllI’IIIIlII
PURKINJE FIBER
-J--+.----4
rrr,rrrl,
137/min
IIIIIIlll
335
74/min
( Iill\
.,~,]~,~~l.,,~~.~~s._y
/
-8,8 o/or
2l/min
-13,5O/o/ _
;
;
;
125/min
64/min
27/min
138jnin
76/min
2O/min
;
1+28,6%
Fig. 1. Actual recordings of the effect of 10m3 M carbutamide on nomotopic and heterotopic automaticity of the isolated rabbit heart. The first column of recordings demonstrates the automaticity of the sinus node, the second one that of the junctional area and the third that of Purlcinje fibres. Upper level: control; middle: carbutamide, 10e3 M; lower: recovery (wash for 60 min).
A-V
SINUS- NODE
NODE
A--
wM 132/min
WRKINJE
IA_------
Y-
70/min
-83%
m-l&g%
FIBER
Zl/min
-+---A--71,4%
122/min
Gl/mio
d/mid
133/tnin
69/min
22/min 2sec
Fig. 2. Actual recordings of the effect of 10e3 M glibenclamide on nomotopic and heterotopic automaticity of the isolated rabbit heart. Legend as in fig. 1. Upper level: control; middle: glibenclamide, 10m3 M; lower: recovery (wash for 60 min).
336
G. POGATSA, M. NBMETH
3. Results Carbutamide, tested in the range of 10e5 10e3 M, slightly depressed nomotopic and junctional automaticity in the isolated rabbit heart, but enhanced automaticity in the Purkinje fibres and in close correlation to its concentration. Glibenclamide, tested in the range of 10-6-10-3 M, had a slight negative effect on nomotopic and junctional automaticity but depressed dose-relatedly the automaticity of the Purkinje fibres (table 1). Actual recordings are shown in figs.1 and 2. Carbutamide, in the same doses, depressed in close correlation to its concentration both the maximal velocity of membrane depolarization and the electrical threshold. It also prolonged both the phase of partial recovery of transmembrane potential and the time of
impulse conduction in the Purkinje fibres. On the other hand, glibenclamide, in the same dose range, in contrast to carbutamide increased the electrical threshold and shortened the time of impulse conduction but similarly to carbutamide it depressed the maximal velocity of membrane depolarization and the phase of partial recovery of the transmembrane potential in the Purkinje fibres (table 2). Neither carbutamide nor glibenclamide affected the resting membrane potential. The recovery of the action potential was measured in all cases at 50, 75 and 90% recovery. Since the values at 50 and 75% were not significantly different from those at 90%, only the values at 90% are recorded in the table. Fig. 3 demonstrates actual recordings of action potentials in Purkinje fibres and the modifications induced by the two drugs.
TABLE 2 Effects of glibenclamide and carbutamide on electrophysiological
activity of isolated Purkinje fibres.
Compound
Concentration (M)
Electrical threshold (%)
Conduction velocity (%)
Phase of partial (90%) recovery of action potential (msec)
Maximal velocity of membrane depolarization (V.sec-’ )
Glibenclamide
0 lo+ 1o-5 1o-4 1o-3 Washout
loo+ 892 95 + 122 * 311+ lOO*
0 9 11 21 48l 6
loo+ 0 95* 7 111*10 126*13 162 f 16l 102 * .4
216.0 210.3 200.3 198.5 197.5 216.1
k f + f * +
1.1 1.0’ 1.6l 1.3l 2.1’ 1.0
258.5 250.3 210.4 200.1 207.1 245.3
f f + f + +
Carbutamide
0 1o-5 1o-4 1o-3 Washout
100 f 88+ 8Of 72? 100 *
0 5 9 8’ 2
100 f 0 84+ 6 78 + 10 752 9l 98k 2
216.8 199.2 194.5 188.4 216.1
f f + + +
1.1 0.7l 1.3l 1.0’ 1.0
258.5 202.5 191.6 199.2 245.3
+_6.3 2 2.B1 + 6.7l + 2.6l f 3.4
100 f 100 f 100 + 99+ 992
0 1 1 1 1
100 + 100 + 100f 992 99+
216.8 216.7 217.1 216.9 216.7
f f f f +
1.1 0.7 1.0 0.6 0.5
258.5 255.3 257.3 258.7 257.9
f f f + +
Solvents
(?*l 1 5 10 140
-l)
0 1 1 1 1
6.3 3.4 3.1’ 2.6’ 1.6l 3.4
6.3 4.2 5.1 6.1 5.8
The values in the table are means f S.E.M. The investigations were carried out in the case of glibenclamide on 7, in the case of carbutamide and of solvents on 6 tissue preparations. The values of intracellular action potentials were counted in the case of glibenclamide from 54, in the case of carbutamide from 62 and in the case of solvents from 48 measurements. The significance relative to baseline values is indicated by: ‘P < 0.05.
ELECTROPHYSIOLOGICAL
EFFECTS OF SULPHONYLUREAS
CONTROL
337
225 msec
\ CARBUTAMIDE
,
-\ \
GLIBSN
to-’ M 190 rnsec
-I
i-
200 rwec
RECOVERY I Wash for 60 m,n 1
Fig. 3. Actual recordings of action potentials amide and low3 M glibenclamide.
in Purkinje
Carbutamide increased, whilst glibenclamide decreased automaticity and conduction velocity in the Purkinje fibres. At the same time, carbutamide decreased and glibenclamide increased the electrical threshold in this tissue. It follows that carbutamide and glibenclamide act differently on the electrophysiological parameters in the isolated rabbit heart. Neither carbutamide, nor glibenclamide affected the effective (local) refractory period in Purkinje fibres or in the atrial myocardium, and none of the sulphonylureas investigated markedly affected the electrical threshold and the conduction velocity in the auricle. The electrophysiological parameters of the isolated rabbit heart were not affected by the sulphonylurea solvents themselves. Moreover, no spontaneous change occurred in the isolated tissue preparations when they were observed for 180-240 min (tables 1 and 2).
fibres and the modifications
induced by 10e3 M carbut-
Furthermore, it was found that the effects of glibenclamide and carbutamide differed not only with different dose levels, adjusted to the hypoglycaemic action of each agent, but also when practically identical molar levels were used.
4. Discussion Lasseter and his coworkers (1972) established that hypoglycaemic sulphonylureas enhance automaticity in the Purkinje fibres of the dog heart, and speculated that this phenomenon might probably play a role in the cardiac arrhythmias of diabetics also suffering from coronary artery disease. In contrast, Curtis and his coworkers (1975) failed to observe any stimulator-y effect of tolbutamide on nomotopic automaticity in rabbit heart,
338
feline Purkinje fibres and intact dog heart, and even found a slight depressive influence of the compound on feline Purkinje fibres. Crass and his coworkers (1973) also reported a slight depression of automaticity by tolbutamide in rat heart. Simultaneous studies of the effect of sulphonylureas on all three cardiac automaticity systems have not yet been reported for any species. Our own investigations along this line have disclosed that carbutamide distinctly enhanced heterotopic automaticity in the rabbit heart. This correlates well with the observations of Lasseter and his group (1972) and attracts attention to the stimulation of heterotopic automaticity by the hypoglycaemic sulphonylurea preparations used for the treatment of adult onset diabetes. Carbutamide, moreover, increases conduction velocity in Purkinje fibres also. In earlier studies we have demonstrated a considerable difference between the effects of carbutamide and of glibenclami’de on arterial blood pressure and myocardial contractile force (Pogatsa and Dubecz, 1977). The present experiments have shown that the two sulphonylureas also influenced cardiac electrophysiological parameters dissimilarly. It is known that glibenclamide differs from the other sulphonylureas in other respects also. Unlike tolbutamide and chlorpropamide, glibenclamide increases rather than reduces diuresis in diabetics (Garcia et al., 1971) and polyuria in diabetes insipidus (Rado and Borb&y, 1971) but the reason why glibenclamide has such different effects is not yet known. However, the depressant action of glibenclamide on heterotopic automaticity can be regarded as an advantageous effect in the prevention of cardiac arrhythmias in diabetics who have ischaemic heart disease.
G. POGATSA,
M. NEMETH
References Crass, M.F. III, R.G. Stenheimer, D.B. Stone and R.J. II. Brown, 1973, Tolbutamide induced inotropic responses in the perfused working heart: effects of albumin, Proc. Sot. Exp. Biol. Med. 142, 861. Curtis, G.P., J. Setchfield and B.R. Lucchesi, 1975, The cardiac pharmacology of tolbutamide, J. Pharmacol. Exp. Ther. 194, 264. Garcia, M., M. Miller and A.M. Moses, 1971, Chlorpropamide-induced water retention in patients with diabetes mellitus, Ann. Int. Med. 75, 549. Lasseter, K.C., G.S. Levey, R.F. Palmer and J.S. McCarthy, 1972, The effect of sulphonylurea drugs on rabbit myocardial contractility, canine Purkinje fiber automaticity, and adenyl cyclase activity from rabbit and human hearts, J. Clin. Invest. 51,2429. Levey, G.S., K.C. Lasseter and R.F. Palmer, 1974, Sulphonylureas and the heart, Ann. Rev. Med. 25, 69. Levey, G.S., R.F. Palmer, K.C. Lasseter and J. McCarthy, 1971, Effect of tolbutamide on adenyl cyclase in rabbit and human heart and contractility of isolated rabbit atria, J. Clin. Endocrinol. Metabol. 33, 371. Myerburg, R.J., J.W. Stewart and B.F. Hoffmann, 1970, Electrophysiological properties of the canine peripheral A-V conducting system, Circ. Res. 26, 361. Papp, J.Gy. and S.E. Vaughan Williams, 1969, A of the antiarrhythmic actions of comparison I.C.I. 50172 and (-)-propranolol and their effects on intracellular cardiac action potentials and other factures of cardiac function, Br. J. Pharmacol. 37, 391. Pogitsa, G. and E. Dubecz, 1977, The effect of hypoglycaemic sulphonylureas on myocardial contractile force and arterial blood pressure, Diabetologia 13, 515. Rado, J.P. and L. Borbily, 1971, Enhancement of polyuria by glibenclamide in diabetes insipidus, Lancet 2, 216. Roth, J., T.E. Prout, I.D. Goldfine, S.M. Wolfe, J. Muenzer, L.E. Grauzer and M.L. Marcus, 1971, Sulphonylureas: Effects in vivo and in vitro, Ann. Int. Med. 75, 607. Szekeres, L., J. Borbola, J.Gy. Papp, 1976, Cardiac actions of arachidonic acid, Acta Biol. Med. 36, 1119. Wales, J.K., A.M. Grant and F.W. Wolf, 1971, The effect of tolbutamide on blood pressure, J. Pharmacol. Exp. Ther. 178, 130.
ELECTROPHYSIOLOGICAL RABBIT HEART
EFFECTS
333
333-338
OF HYPOGLYCAEMIC
SULPHONYLUREAS
ON
GABOR POGATSA and MIKLGS NBMETH Research Unit, National Institute of Cardiology, 1450 Budapest, P.O.B. 9-88, and Department of Pharmacology, Medical University School of Szeged, 6701 Szeged, P.O.B. 115, Hungary Received 28 May 1980, accepted 7 July 1980 G. POGkTSA and M. NhMETH, Electrophysiological effects of hypoglycaemic sulphonylureas on rabbit heart, European J. Pharmacol. 67 (1980) 333-338. The hypoglycaemic sulphonylurea drugs glibenclamide and carbutamide were compared as to their influence on the electrical activity of isolated rabbit heart preparations. Carbutamide (1O-s-1O-3 M) was found to depress slightly the nomotopic and junctional automaticity, while in Purkinje fibres it enhanced automaticity and conduction velocity. On the other hand, glibenclamide (10-6-10-3 M) had no significant influence on the nomotopic and junctional automaticity, and markedly depressed automaticity and conduction velocity in the Purkinje fibres. The atrial conduction system was not much influenced by the sulphonylureas tested. Carbutamide depressed the electrical threshold in Purkinje fibres. It follows that the hypoglycaemic sulphonylurea compounds carbutamide and glibenclamide differ considerably with respect to their influences on cardiac electrical activity. Positive and negative chronotropism Positive and negative dromotropism
Hypoglycaemic sulphonylureas Isolated rabbit heart
Negative bathmotropism Glibenclamide
Carbutamide
1. Introduction
2. Materials and methods
It is generally accepted that the hypoglycaemic sulphonylureas increase arterial blood pressure and myocardial contractile force (Crass et al., 1973; Curtis et al., 1975; Lasseter et al., 1972; Levey et al., 1971,1974; Roth et al., 1971; Wales et al., 1971), but opinions diverge about their influence on cardiac electrical activity (Curtis et al., 1975; Lasseter et al., 1972). It has been shown in this laboratory that the new sulphonylurea derivative, glibenclamide, influenced myocardial contractile force and arterial blood pressure differently from the traditional sulphonylureas, e.g. from carbutamide which is very widely used in Hungary (Pogatsa and Dubecz, 1977). This prompted comparative investigations into the effects of glibenclamide and carbutamide on the electrical activity of the heart.
Forty-nine albino rabbits of either sex and weighing 1.0-1.7 kg were used in the experiments. The animals were killed by a blow on the neck, the heart was removed rapidly and placed in an oxygenated ,Krebs-Henseleit solution at 32°C. For examination of the nomotopic and heterotopic automaticity of the rabbit heart, the right atrium, the atrioventricular septal region and the right ventricular papillary muscles along with their Purkinje fibre supply were dissected from the immersed tissue. The three tissue preparations were then transferred to fresh Krebs-Henseleit solution and were fixed next to each other with thin needles on a non-toxic rubber sheet (Silicoset 105, ICI) which was placed on the bottom of the jar; spontaneous action was then allowed to take place. The tissue preparations were always brought to spontaneous action for 60-90 min before addition of the compounds to be tested. The electric activity of the spontaneous
334
G. POGATSA, M. NBMETH
myocardial contractions was recorded extracellularly on a three-channel ECG recorder (3 NEK-1, RFT, Dresden). For examination of the atrial conduction system, the left atrium was removed from the rabbit heart and mounted on the bottom of the jar as described above. The left atrium was then stimulated electrically at three times electrical threshold, for 1 msec, at 100 imp /min from a biological stimulator (Disa Multistim 13G04), using bipolar electrodes. Conduction velocity was then measured at a constant rate. The electrical threshold, conduction velocity and effective (local) refractory period were determined by the method of Szekeres et al. (1976), and were recorded with an oscilloscope (EMG TR-4602). The distal conduction system in the isolated right ventricle was prepared by the method of Myerburg et al. (1970) from rabbit heart. Electrical threshold, conduction velocity, effective (local) refractory period
and transmembrane potentials were recorded from the free-running Purkinje fibres (false tendons) using a standard microelectrode technique as described previously (Papp and Vaughan Williams, 1969; ‘Szekeres et al., 1976). Composition of the Krebs-Henseleit solution was (in mM): Na’ 135.0, K’ 5.0, Ca” 2.5, MG’+ 1.2, Cl- 126.0, HCO; 24.7, SO,‘1.2, POJ3- 1.2, glucose 5.0. The solution was oxygenated by a continuous supply of 95% O2 -5% C02. The pH of the solution was adjusted to 7.4, and its temperature was maintained at 32°C. Carbutamide (Bucarban, Chinoin) was dissolved in methyl glycamate and polyethylene glycol, glibenclamide (Gilemal, Chinoin) in polyethylene glycol and glycerinphormal at 20 and 0.2% concentration, respectively. An appropriate solvent control group was set up with each test. The results were evaluated by means of Student’s paired t-test.
TABLE 1 Effects heart.
of glibenclamide
and carbutamide
Compound
on nomotopic
and heterotopic
automaticity
in the isolated rabbit
Concentration (M)
Sinoatrial node (min-’ )
Junctional area (min-’ )
Purkinje fibre (min-’ )
Glibenclamide
n=9
0 1o-6 1o-5 1o-4 1o-3 Washout
125 f 125 + 122 f 116f 114 + 125 f
3 9 6 6 3l 9
70f 74+ 75 f 71+ 60 + 71 f
5 6 6 7 3l 6
21+ 17 + 14 f 10 + 8f 22 +
2 2 4l 3l 2’ 5
Carbutamide
n=6
0 1o-5 1o-4 10 -3 Washout
137f4 134 + 130 + 125 f 134 f
5 4 3r 4
74f6 69 f. 65 + 64+ 73f
6 7 7’ 6
21 23 25 27 22
4 4 4l 4’ 5
116 115 115 114 116
9 9 9 9 9
64 f 63 f 63 f 62 * 63+
8 8 8 8 8
24 + 4 24+ 4 24 + 4 24*A 24 + 4
Solvents
n=6
(?*1-1 1 5 10 140
)
f + f f f
f f f + *
The values in the table are means + S.E.M. The significance relative to baseline values is indicated by: ’ P < 0.05.
ELECTROPHYSIOLOGICAL
EFFECTS
A-V NODE
SINUS- NODE ” !‘!~I’~‘Il!!i\III “rccr.~crrrlc~,uu
OF SULPHONYLUREAS
4“!\y’l’“ll’ HCe.
IIllI’IIIIlII
PURKINJE FIBER
-J--+.----4
rrr,rrrl,
137/min
IIIIIIlll
335
74/min
( Iill\
.,~,]~,~~l.,,~~.~~s._y
/
-8,8 o/or
2l/min
-13,5O/o/ _
;
;
;
125/min
64/min
27/min
138jnin
76/min
2O/min
;
1+28,6%
Fig. 1. Actual recordings of the effect of 10m3 M carbutamide on nomotopic and heterotopic automaticity of the isolated rabbit heart. The first column of recordings demonstrates the automaticity of the sinus node, the second one that of the junctional area and the third that of Purlcinje fibres. Upper level: control; middle: carbutamide, 10e3 M; lower: recovery (wash for 60 min).
A-V
SINUS- NODE
NODE
A--
wM 132/min
WRKINJE
IA_------
Y-
70/min
-83%
m-l&g%
FIBER
Zl/min
-+---A--71,4%
122/min
Gl/mio
d/mid
133/tnin
69/min
22/min 2sec
Fig. 2. Actual recordings of the effect of 10e3 M glibenclamide on nomotopic and heterotopic automaticity of the isolated rabbit heart. Legend as in fig. 1. Upper level: control; middle: glibenclamide, 10m3 M; lower: recovery (wash for 60 min).
336
G. POGATSA, M. NBMETH
3. Results Carbutamide, tested in the range of 10e5 10e3 M, slightly depressed nomotopic and junctional automaticity in the isolated rabbit heart, but enhanced automaticity in the Purkinje fibres and in close correlation to its concentration. Glibenclamide, tested in the range of 10-6-10-3 M, had a slight negative effect on nomotopic and junctional automaticity but depressed dose-relatedly the automaticity of the Purkinje fibres (table 1). Actual recordings are shown in figs.1 and 2. Carbutamide, in the same doses, depressed in close correlation to its concentration both the maximal velocity of membrane depolarization and the electrical threshold. It also prolonged both the phase of partial recovery of transmembrane potential and the time of
impulse conduction in the Purkinje fibres. On the other hand, glibenclamide, in the same dose range, in contrast to carbutamide increased the electrical threshold and shortened the time of impulse conduction but similarly to carbutamide it depressed the maximal velocity of membrane depolarization and the phase of partial recovery of the transmembrane potential in the Purkinje fibres (table 2). Neither carbutamide nor glibenclamide affected the resting membrane potential. The recovery of the action potential was measured in all cases at 50, 75 and 90% recovery. Since the values at 50 and 75% were not significantly different from those at 90%, only the values at 90% are recorded in the table. Fig. 3 demonstrates actual recordings of action potentials in Purkinje fibres and the modifications induced by the two drugs.
TABLE 2 Effects of glibenclamide and carbutamide on electrophysiological
activity of isolated Purkinje fibres.
Compound
Concentration (M)
Electrical threshold (%)
Conduction velocity (%)
Phase of partial (90%) recovery of action potential (msec)
Maximal velocity of membrane depolarization (V.sec-’ )
Glibenclamide
0 lo+ 1o-5 1o-4 1o-3 Washout
loo+ 892 95 + 122 * 311+ lOO*
0 9 11 21 48l 6
loo+ 0 95* 7 111*10 126*13 162 f 16l 102 * .4
216.0 210.3 200.3 198.5 197.5 216.1
k f + f * +
1.1 1.0’ 1.6l 1.3l 2.1’ 1.0
258.5 250.3 210.4 200.1 207.1 245.3
f f + f + +
Carbutamide
0 1o-5 1o-4 1o-3 Washout
100 f 88+ 8Of 72? 100 *
0 5 9 8’ 2
100 f 0 84+ 6 78 + 10 752 9l 98k 2
216.8 199.2 194.5 188.4 216.1
f f + + +
1.1 0.7l 1.3l 1.0’ 1.0
258.5 202.5 191.6 199.2 245.3
+_6.3 2 2.B1 + 6.7l + 2.6l f 3.4
100 f 100 f 100 + 99+ 992
0 1 1 1 1
100 + 100 + 100f 992 99+
216.8 216.7 217.1 216.9 216.7
f f f f +
1.1 0.7 1.0 0.6 0.5
258.5 255.3 257.3 258.7 257.9
f f f + +
Solvents
(?*l 1 5 10 140
-l)
0 1 1 1 1
6.3 3.4 3.1’ 2.6’ 1.6l 3.4
6.3 4.2 5.1 6.1 5.8
The values in the table are means f S.E.M. The investigations were carried out in the case of glibenclamide on 7, in the case of carbutamide and of solvents on 6 tissue preparations. The values of intracellular action potentials were counted in the case of glibenclamide from 54, in the case of carbutamide from 62 and in the case of solvents from 48 measurements. The significance relative to baseline values is indicated by: ‘P < 0.05.
ELECTROPHYSIOLOGICAL
EFFECTS OF SULPHONYLUREAS
CONTROL
337
225 msec
\ CARBUTAMIDE
,
-\ \
GLIBSN
to-’ M 190 rnsec
-I
i-
200 rwec
RECOVERY I Wash for 60 m,n 1
Fig. 3. Actual recordings of action potentials amide and low3 M glibenclamide.
in Purkinje
Carbutamide increased, whilst glibenclamide decreased automaticity and conduction velocity in the Purkinje fibres. At the same time, carbutamide decreased and glibenclamide increased the electrical threshold in this tissue. It follows that carbutamide and glibenclamide act differently on the electrophysiological parameters in the isolated rabbit heart. Neither carbutamide, nor glibenclamide affected the effective (local) refractory period in Purkinje fibres or in the atrial myocardium, and none of the sulphonylureas investigated markedly affected the electrical threshold and the conduction velocity in the auricle. The electrophysiological parameters of the isolated rabbit heart were not affected by the sulphonylurea solvents themselves. Moreover, no spontaneous change occurred in the isolated tissue preparations when they were observed for 180-240 min (tables 1 and 2).
fibres and the modifications
induced by 10e3 M carbut-
Furthermore, it was found that the effects of glibenclamide and carbutamide differed not only with different dose levels, adjusted to the hypoglycaemic action of each agent, but also when practically identical molar levels were used.
4. Discussion Lasseter and his coworkers (1972) established that hypoglycaemic sulphonylureas enhance automaticity in the Purkinje fibres of the dog heart, and speculated that this phenomenon might probably play a role in the cardiac arrhythmias of diabetics also suffering from coronary artery disease. In contrast, Curtis and his coworkers (1975) failed to observe any stimulator-y effect of tolbutamide on nomotopic automaticity in rabbit heart,
338
feline Purkinje fibres and intact dog heart, and even found a slight depressive influence of the compound on feline Purkinje fibres. Crass and his coworkers (1973) also reported a slight depression of automaticity by tolbutamide in rat heart. Simultaneous studies of the effect of sulphonylureas on all three cardiac automaticity systems have not yet been reported for any species. Our own investigations along this line have disclosed that carbutamide distinctly enhanced heterotopic automaticity in the rabbit heart. This correlates well with the observations of Lasseter and his group (1972) and attracts attention to the stimulation of heterotopic automaticity by the hypoglycaemic sulphonylurea preparations used for the treatment of adult onset diabetes. Carbutamide, moreover, increases conduction velocity in Purkinje fibres also. In earlier studies we have demonstrated a considerable difference between the effects of carbutamide and of glibenclami’de on arterial blood pressure and myocardial contractile force (Pogatsa and Dubecz, 1977). The present experiments have shown that the two sulphonylureas also influenced cardiac electrophysiological parameters dissimilarly. It is known that glibenclamide differs from the other sulphonylureas in other respects also. Unlike tolbutamide and chlorpropamide, glibenclamide increases rather than reduces diuresis in diabetics (Garcia et al., 1971) and polyuria in diabetes insipidus (Rado and Borb&y, 1971) but the reason why glibenclamide has such different effects is not yet known. However, the depressant action of glibenclamide on heterotopic automaticity can be regarded as an advantageous effect in the prevention of cardiac arrhythmias in diabetics who have ischaemic heart disease.
G. POGATSA,
M. NEMETH
References Crass, M.F. III, R.G. Stenheimer, D.B. Stone and R.J. II. Brown, 1973, Tolbutamide induced inotropic responses in the perfused working heart: effects of albumin, Proc. Sot. Exp. Biol. Med. 142, 861. Curtis, G.P., J. Setchfield and B.R. Lucchesi, 1975, The cardiac pharmacology of tolbutamide, J. Pharmacol. Exp. Ther. 194, 264. Garcia, M., M. Miller and A.M. Moses, 1971, Chlorpropamide-induced water retention in patients with diabetes mellitus, Ann. Int. Med. 75, 549. Lasseter, K.C., G.S. Levey, R.F. Palmer and J.S. McCarthy, 1972, The effect of sulphonylurea drugs on rabbit myocardial contractility, canine Purkinje fiber automaticity, and adenyl cyclase activity from rabbit and human hearts, J. Clin. Invest. 51,2429. Levey, G.S., K.C. Lasseter and R.F. Palmer, 1974, Sulphonylureas and the heart, Ann. Rev. Med. 25, 69. Levey, G.S., R.F. Palmer, K.C. Lasseter and J. McCarthy, 1971, Effect of tolbutamide on adenyl cyclase in rabbit and human heart and contractility of isolated rabbit atria, J. Clin. Endocrinol. Metabol. 33, 371. Myerburg, R.J., J.W. Stewart and B.F. Hoffmann, 1970, Electrophysiological properties of the canine peripheral A-V conducting system, Circ. Res. 26, 361. Papp, J.Gy. and S.E. Vaughan Williams, 1969, A of the antiarrhythmic actions of comparison I.C.I. 50172 and (-)-propranolol and their effects on intracellular cardiac action potentials and other factures of cardiac function, Br. J. Pharmacol. 37, 391. Pogitsa, G. and E. Dubecz, 1977, The effect of hypoglycaemic sulphonylureas on myocardial contractile force and arterial blood pressure, Diabetologia 13, 515. Rado, J.P. and L. Borbily, 1971, Enhancement of polyuria by glibenclamide in diabetes insipidus, Lancet 2, 216. Roth, J., T.E. Prout, I.D. Goldfine, S.M. Wolfe, J. Muenzer, L.E. Grauzer and M.L. Marcus, 1971, Sulphonylureas: Effects in vivo and in vitro, Ann. Int. Med. 75, 607. Szekeres, L., J. Borbola, J.Gy. Papp, 1976, Cardiac actions of arachidonic acid, Acta Biol. Med. 36, 1119. Wales, J.K., A.M. Grant and F.W. Wolf, 1971, The effect of tolbutamide on blood pressure, J. Pharmacol. Exp. Ther. 178, 130.