- Email: [email protected]
Antagonism between adenosine and bromobenzoyl-methyladamantylamine, A K+ channel blocker, in atrial myocardium of guinea pig
European Journal of Pharmacology, 98 (1984) 265-268
265
Elsevier
Short communication
A N T A G O N I S M B E T W E E N A D E N O S I N E AND B R O M O B E N Z O Y L - M E T H Y L A D A M A N T Y L A M I N E , A K + C H A N N E L BLOCKER, IN ATRIAL M Y O C A R D I U M OF GUINEA P I G JANOS MI~SZAROS *, KAROLY KELEMEN *, VALI~RIA KECSKEMI~TI * and JOZSEF SZEGI
Department of Pharmacology, Medical University of Debrecen, H-4012 Debrecen, and * Department of Pharmacology, Semmelweis University of Medicine, H-1445 Budapest, Hungary Received 2 November 1983, accepted 13 December 1983
J. MI~SZAROS, K. KELEMEN, V. KECSKEMI~TI and J. SZEGI, Antagonism between adenosine and bromobenzoyl-methyladamantylamine, a K + channel blocker, in atrial myocardium of guinea pig, European J. Pharmacol. 98 (1984) 265-268. The effect of bromobenzoyl-methyladamantylamine (BMA) on the adenosine-induced changes in the electrical and mechanical activity of the atrial muscle, and the effect of adenosine on the slow action potentials induced by BMA in K+-depolarized atrial myocardium of guinea pig were studied. BMA was able to antagonize the adenosine-induced shortening of the action potential duration and negative inotropic effect. This action of BMA was potentiated by theophylline, and reduced by dipyridamole. Adenosine depressed the BMA-induced slow action potentials. The results suggest that there may be an antagonism between BMA and adenosine. Guinea pig heart
Adenosine
Adamantane
Action potential
1. Introduction
Adenosine, a physiological metabolite released from cardiac cells mainly during myocardial hypoxia, is known to exert a negative inotropic effect on atrial heart muscle (De Gubareff and Sleator, 1965). Schrader and his coworkers (1975) demonstrated that adenosine depressed the slow action potentials mediated mainly by slow Ca 2 ÷ channels, and suggested that adenosine-induced negative inotropic effect might be due to a decrease in the Ca 2÷ influx during excitation in guinea pig atrial heart muscle. In a previous study (Mrszfiros et al., 1982), we demonstrated that a new adamantane derivative, bromobenzoyl-methyladamantylamine (BMA), markedly prolonged the action potential duration, depolarized the membrane, reduced the m a x i m u m rate of depolarization (Vmax) and increased the amplitude of contractions in m a m -
* To whom all correspondence should be addressed at first address. 0014-2999/84/$03.00 © 1984 Elsevier Science Publishers B.V.
Slow responses
malian myocardium. In voltage clamp experiments, BMA reduced the outward K + current, but had no effect on either fast inward N a t or slow inward Ca 2+ currents, suggesting that BMA is capable of selectively blocking the myocardial K ÷ channels. The purpose of the present work was to investigate whether BMA could antagonize the adenosine-induced electrical and mechanical changes in atrial heart muscle of the guinea pig.
2. Materials and methods
Experiments were carried out on isolated electrically driven left atrial myocardial preparations of the guinea pig. The animals were killed by a blow on the head. The heart was quickly removed and the atrium was dissected and mounted horizontally in an organ chamber. The preparations were pre-equilibrated for 30 min in Krebs solution of the following composition (mM): NaC1 118, KCI 4.7, CaC12 2.5, MgCI 2 1.2, N a H 2 P O 4 1.0, N a H C O 3 24.9, glucose 11.5. The solution was
266 aerated with a gas mixture of 95% 02 and 5% CO 2, the p H was 7.4 and the temperature was maintained at 37°C. The preparations were paced at a frequency of 2 Hz by rectangular pulses of 1 ms duration and 3 times the threshold voltage, via bipolar platinum electrodes placed on the tissue. Isometric contractions of the left atrial preparations were measured by means of an isometric force transducer. Resting and action potentials were reordered with conventional glass-microelectrodes filled with 3 M KC1 solution and having resistances of 5 - 1 0 MI2, and connected to a cathode follower and a DC amplifier (MIKI-1623) (Kelemen et al., 1968). The maximum rate of depolarization (~]max) of the action potential was measured by electronic differentiation (Papp and Vaughan Williams, 1969). The twitch contractions, the transmembrane potentials and the "Vm~xwere displayed on a dual-beam oscilloscope (EMG1555) and photographed on film. In some experiments, adenosine was tested for its effects on the slow action potentials induced by BMA (10 4 M ) in K+-depolarized atria in which the fast N a + channels were inactivated (Pappano, 1970). In our experiments, this was achieved by reducing the resting potential from - 8 0 mV to about - 4 0 mV with elevated K + (26 mM) Krebs solution (isosmolar substitution of K + for Na+). Under such conditions, the myocardium was only excitable with intense electrical stimulation (0.5
AD 10-5M
Hz and 10 times threshold voltage). The drugs used were adenosine, dipyridamole (Boehringer, Mannheim, F.R.G.) and bromobenzoyl-methyladamantylamine (BMA; EGYT, Budapest, Hungary) and theophylline (Reanal, Budapest, Hungary). Data are given as m e a n + S.E.M. Statistical analysis was performed using Student's t-test. A P value of less than 0.05 was considered significant.
3. Results
3.1. Effect of BMA on the adenosine-induced changes in the electrical and mechanical activity of atrial myocardium A typical experiment is shown in fig. 1. The control action potentials obtained in 5 preparations had the following parameters: resting potential (RP) - 86.4 4- 1.7 mV, overshoot (OS) + 23.7 _+ 0.6 mV, "V'max196.2 _+ 1.8 V/S, action potential duration at 50% repolarization (APDs0) 32.6 _+ 0.9 ms. Addition of adenosine (10 5 M) slightly hyperpolarized the cell membrane to - 9 1 . 6 + 1.8 mV, had no significant effect on OS, slightly increased the Vmax to 214.5 _+ 1.6 V/S, markedly shortened the APDs0 to 14.2_+ 1.3 ms and decreased the contractile force to 22.7 _+ 2.6% of the control value. Application of BMA (10 -4 M) was
• BMA
Fig. 1. Effect of BMA (10 -4 M) on adenosine (AD; 10-5 M) induced changes of transmembrane potential (upper panels) and contractile force (lowerpanels) in left atrial heart muscle of the guinea pig. A,A': control. B,B': 5 n-finafter adding 10-5 M adenosine (AD). C,C': 5 min after adding 10-4 M BMA.
267 K*
Control
26ran
BHA
i.
Z.
AD ~OmV 1200V/$ (A) 20V/s (B-E)
Fig. 2. Effect of adenosine (AD) on the slow action potential induced by BMA in K+-depolarized left atrium of the guinea pig. A: control action potential. B: Application of K ÷ rich (26 raM) Krebs solution depolarized the membrane to about -40 mV and abolished the excitability. C: 10 -4 M BMA-induced slow action potential. D: Addition of adenosine (10 -5 M) decreased the Vmaxand amplitude and shortened the duration of the slow action potential within 5 rain. E: Adenosine, at a higher concentration (10 -4 M), practically abolished the membrane electrogenesis. Pindolol (5×10 7 M) was present throughout the experiment to block the ~-adrenoceptors.
able to antagonize these changes. B M A depolarized the m e m b r a n e to - 6 8 . 3 + 1.9 mV, caused no significant changes in OS, but reduced the '~/max to 129.8 + 1.9 V/S, prolonged the APDs0 to 73.5 + 1.7 ms and increased the contractile force to the 118.1 + 2.4% of the control value. M o o d y and Burnstock (1982) have demonstrated that adenosine inhibits twitch responses to transmural stimulation of guinea pig ileum via P]-purinoreceptors. Theophylline, a P]-purinoceptor blocker, can block this effect, and dipyridamol, an adenosine uptake blocker, can potentiate it. Therefore we examined the anti-adenosine effect of B M A in a series of experiments, in the presence of theophylline and dipyridamole. It was found (in 5 atria), that adenosine (10 - s M) reduced the contractile force to 25.9 + 3.1%, and application of B M A (10 -4 M) and theophylline (6.5 x 10 -5 M) increased it to 194.6 + 2.7% of the control value. In another experiment (in 5 atria), adenosine (10-5 M) reduced the contractile force to 20.4 + 3.2%, and appliation of B M A (10 -4 M) and dipyridamole (5 × 10 -7 M) increased it to 6 2 . 9 + 3.5% only. It must be mentioned that B M A (10 -4 M) alone increased the contractile force to 255.3 + 5.1% of the control value. The results were obtained under steady state conditions. The incubation time was 5 min in the case of adenosine and B M A and theophylline, and 20 min in the case of B M A + dipyridamole. The concentrations were chosen at which the characteristic effects of the drugs developed.
3.2. Effect of adenosine on the slow action potentials induced by BMA in atrial myocardium Fig. 2 shows a typical experiment. In this series of experiments, we wanted to see whether adenosine depressed the BMA-induced slow action potentials. In 5 atrial muscle preparations, the m y o c a r d i u m m e m b r a n e was depolarized from - 8 5 . 3 + 1.6 mV to - 4 0 . 9 + 2.4 mV in order to voltage-inactivate the fast N a ÷ channels by using elevated K ÷ (26 m M ) Krebs solution. Preparations so treated were only excitable with intense electrical stimulation (0.5 Hz and 10 times threshold voltage). Pindolol (5 × 10-7 M) was added so as to block the fl-adrenoceptors. In all preparations, B M A (10 -4 M) rapidly, i.e. within 2 - 3 min, induced slow action potentials accompanied by contractions. Pindolol did not prevent this action, indicating that the slow response-evoked effect of B M A was not mediated by catecholamine release. The BMA-induced slow action potentials had a m e a n ~Tm~x of 13.4 _ 1.6 V/S, OS of 19.8 + 0.6 mV, and APDs0 of 52.7 + 1.3 ms. After addition of adenosine (10-5 M) the ~/max of the slow responses was significantly reduced to 5.1 + 0.7 V / s , the ADPs0 was shortened to 24.3 + 0.9 ms, and OS was completely abolished. At a higher concentration (10 -4 M), adenosine totally depressed the B M A - i n d u c e d slow responses.
268
4. Discussion
The results presented here provide evidence that BMA, a selective K + channel blocker (M6szfiros et al., 1982), antagonizes the mechanical and electrophysiological changes induced by adenosine in atrial heart muscle of the guinea pig. The adenosine-induced large shortening of the action potential duration suggest that the drug, besides inhibiting slow inward Ca 2+ current (Schrader et al., 1975), may increase the outward K + current (Goto et al., 1977). This increased outward current shortens the action potential and prevents the slow inward Ca 2 + current from running its normal time course, thus producing a negative inotropic effect, as also described in the case of acetylcholine (Ten Eick et al., 1976). BMA, by decreasing the outward K + current, lengthens the action potential duration, i.e. prolongs the time during which the slow inward Ca 2+ current can be operative, thus causing a positive inotropic effect (M6szfiros et al., 1982). Moreover, adenosine depressed the slow action potentials induced by BMA in K+-de polarized atrial preparations. This finding suggests that adenosine increases the K + conductance, since in these preparations, BMA restores the electrical activity by decreasing the high K +-induced 'shunting conductance' (M6sz~ros et al., 1982), like Ba 2+ ions (Ehara and Inazawa, 1980). We suppose that BMA rather inhibits the effect of adenosine at the myocardial K + channels than at the Pl-purinoceptors. This hypothesis seems to be justified by the fact that dipyridamole, an adenosine uptake blocker, can reduce the anti-adenosine effect of BMA, whereas theophylline, a Pt-purinoceptor blocker, and potentiate it. The use of BMA, a selective K + channel blocker, offers a new chemical tool for the analysis of the
mechanism of action of adenosine which plays an important role in ischaemic heart disease.
References De Gubareff, T. and W. Sleator, 1965, Effects of caffeine on mammalian atrial muscle and its interaction with adenosine and calcium, J.Pharmacol. Exp. Ther. 148, 202. Ehara, T. and M. Inazawa, 1980, Calcium-dependent slow action potentials in potassium-depolarized guinea pig ventricular myocardium enhanced by barium ions, Naunyn-Schmiedeb. Arch. Pharmacol. 315, 47. Goto, M., A. Yatani and Y. Tsuda, 1977, An analysis of the action of ATP and related compounds on membrane current and tension components in bullfrog atrial muscle, Jap. J. Physiol. 27, 81. Kelemen, K., V. Kecskem6ti, L. Scult6ty and T. Friedmann, 1968, Activity index of frog heart ventricle cells. Analysis of the correlation between resting potential and activity of the sodium-carrier system in the frog heart by means of celluline and adrenaline, Acta Physiol. Acad. Sci. Hung. 33, 269. M~sz/~ros, J., K. Kelemen, R. Mark6, V. Kecskem&i and J. Szegi, 1982, Inhibition of myocardial K + channels by bromobenzoyl-methyladamantylamine, and adamantine derivative, European J. Pharmacol. 84, 151. Moody, C.J. and G. Burnstock, 1982, Evidence for the presence of Pl-purinoceptors on cholinergic nerve terminals in the guinea-pig ileum, European J. Pharmacol. 77, 1. Papp, J.Gy. and E.M. Vaughan Williams, 1969, The effect of bretylium on intracellular cardiac action potentials in relation to its anti-arrhythmic and local anaesthetic activity, Br. J. Pharmacol. 37, 380. Pappano, A.J., 1970, Calcium dependent action potentials produced by catecholamines in guinea pig atrial fibers depolarized by potassium, Circular. Res. 27, 379. Schrader, J., R. Rubio and R. Berne, 1975, Inhibition of slow action potentials of guinea pig atrial muscle by adenosine: A possible effect on Ca 2÷ influx, J. Mol. Cell. Cardiol. 7, 427. Ten Eick, R., H. Nawrath, T.H. McDonald and W. Trautwein, 1976, On the mechanism of the negative inotropic effect of acetylcholine, Pfliagers Arch. 361,207.
265
Elsevier
Short communication
A N T A G O N I S M B E T W E E N A D E N O S I N E AND B R O M O B E N Z O Y L - M E T H Y L A D A M A N T Y L A M I N E , A K + C H A N N E L BLOCKER, IN ATRIAL M Y O C A R D I U M OF GUINEA P I G JANOS MI~SZAROS *, KAROLY KELEMEN *, VALI~RIA KECSKEMI~TI * and JOZSEF SZEGI
Department of Pharmacology, Medical University of Debrecen, H-4012 Debrecen, and * Department of Pharmacology, Semmelweis University of Medicine, H-1445 Budapest, Hungary Received 2 November 1983, accepted 13 December 1983
J. MI~SZAROS, K. KELEMEN, V. KECSKEMI~TI and J. SZEGI, Antagonism between adenosine and bromobenzoyl-methyladamantylamine, a K + channel blocker, in atrial myocardium of guinea pig, European J. Pharmacol. 98 (1984) 265-268. The effect of bromobenzoyl-methyladamantylamine (BMA) on the adenosine-induced changes in the electrical and mechanical activity of the atrial muscle, and the effect of adenosine on the slow action potentials induced by BMA in K+-depolarized atrial myocardium of guinea pig were studied. BMA was able to antagonize the adenosine-induced shortening of the action potential duration and negative inotropic effect. This action of BMA was potentiated by theophylline, and reduced by dipyridamole. Adenosine depressed the BMA-induced slow action potentials. The results suggest that there may be an antagonism between BMA and adenosine. Guinea pig heart
Adenosine
Adamantane
Action potential
1. Introduction
Adenosine, a physiological metabolite released from cardiac cells mainly during myocardial hypoxia, is known to exert a negative inotropic effect on atrial heart muscle (De Gubareff and Sleator, 1965). Schrader and his coworkers (1975) demonstrated that adenosine depressed the slow action potentials mediated mainly by slow Ca 2 ÷ channels, and suggested that adenosine-induced negative inotropic effect might be due to a decrease in the Ca 2÷ influx during excitation in guinea pig atrial heart muscle. In a previous study (Mrszfiros et al., 1982), we demonstrated that a new adamantane derivative, bromobenzoyl-methyladamantylamine (BMA), markedly prolonged the action potential duration, depolarized the membrane, reduced the m a x i m u m rate of depolarization (Vmax) and increased the amplitude of contractions in m a m -
* To whom all correspondence should be addressed at first address. 0014-2999/84/$03.00 © 1984 Elsevier Science Publishers B.V.
Slow responses
malian myocardium. In voltage clamp experiments, BMA reduced the outward K + current, but had no effect on either fast inward N a t or slow inward Ca 2+ currents, suggesting that BMA is capable of selectively blocking the myocardial K ÷ channels. The purpose of the present work was to investigate whether BMA could antagonize the adenosine-induced electrical and mechanical changes in atrial heart muscle of the guinea pig.
2. Materials and methods
Experiments were carried out on isolated electrically driven left atrial myocardial preparations of the guinea pig. The animals were killed by a blow on the head. The heart was quickly removed and the atrium was dissected and mounted horizontally in an organ chamber. The preparations were pre-equilibrated for 30 min in Krebs solution of the following composition (mM): NaC1 118, KCI 4.7, CaC12 2.5, MgCI 2 1.2, N a H 2 P O 4 1.0, N a H C O 3 24.9, glucose 11.5. The solution was
266 aerated with a gas mixture of 95% 02 and 5% CO 2, the p H was 7.4 and the temperature was maintained at 37°C. The preparations were paced at a frequency of 2 Hz by rectangular pulses of 1 ms duration and 3 times the threshold voltage, via bipolar platinum electrodes placed on the tissue. Isometric contractions of the left atrial preparations were measured by means of an isometric force transducer. Resting and action potentials were reordered with conventional glass-microelectrodes filled with 3 M KC1 solution and having resistances of 5 - 1 0 MI2, and connected to a cathode follower and a DC amplifier (MIKI-1623) (Kelemen et al., 1968). The maximum rate of depolarization (~]max) of the action potential was measured by electronic differentiation (Papp and Vaughan Williams, 1969). The twitch contractions, the transmembrane potentials and the "Vm~xwere displayed on a dual-beam oscilloscope (EMG1555) and photographed on film. In some experiments, adenosine was tested for its effects on the slow action potentials induced by BMA (10 4 M ) in K+-depolarized atria in which the fast N a + channels were inactivated (Pappano, 1970). In our experiments, this was achieved by reducing the resting potential from - 8 0 mV to about - 4 0 mV with elevated K + (26 mM) Krebs solution (isosmolar substitution of K + for Na+). Under such conditions, the myocardium was only excitable with intense electrical stimulation (0.5
AD 10-5M
Hz and 10 times threshold voltage). The drugs used were adenosine, dipyridamole (Boehringer, Mannheim, F.R.G.) and bromobenzoyl-methyladamantylamine (BMA; EGYT, Budapest, Hungary) and theophylline (Reanal, Budapest, Hungary). Data are given as m e a n + S.E.M. Statistical analysis was performed using Student's t-test. A P value of less than 0.05 was considered significant.
3. Results
3.1. Effect of BMA on the adenosine-induced changes in the electrical and mechanical activity of atrial myocardium A typical experiment is shown in fig. 1. The control action potentials obtained in 5 preparations had the following parameters: resting potential (RP) - 86.4 4- 1.7 mV, overshoot (OS) + 23.7 _+ 0.6 mV, "V'max196.2 _+ 1.8 V/S, action potential duration at 50% repolarization (APDs0) 32.6 _+ 0.9 ms. Addition of adenosine (10 5 M) slightly hyperpolarized the cell membrane to - 9 1 . 6 + 1.8 mV, had no significant effect on OS, slightly increased the Vmax to 214.5 _+ 1.6 V/S, markedly shortened the APDs0 to 14.2_+ 1.3 ms and decreased the contractile force to 22.7 _+ 2.6% of the control value. Application of BMA (10 -4 M) was
• BMA
Fig. 1. Effect of BMA (10 -4 M) on adenosine (AD; 10-5 M) induced changes of transmembrane potential (upper panels) and contractile force (lowerpanels) in left atrial heart muscle of the guinea pig. A,A': control. B,B': 5 n-finafter adding 10-5 M adenosine (AD). C,C': 5 min after adding 10-4 M BMA.
267 K*
Control
26ran
BHA
i.
Z.
AD ~OmV 1200V/$ (A) 20V/s (B-E)
Fig. 2. Effect of adenosine (AD) on the slow action potential induced by BMA in K+-depolarized left atrium of the guinea pig. A: control action potential. B: Application of K ÷ rich (26 raM) Krebs solution depolarized the membrane to about -40 mV and abolished the excitability. C: 10 -4 M BMA-induced slow action potential. D: Addition of adenosine (10 -5 M) decreased the Vmaxand amplitude and shortened the duration of the slow action potential within 5 rain. E: Adenosine, at a higher concentration (10 -4 M), practically abolished the membrane electrogenesis. Pindolol (5×10 7 M) was present throughout the experiment to block the ~-adrenoceptors.
able to antagonize these changes. B M A depolarized the m e m b r a n e to - 6 8 . 3 + 1.9 mV, caused no significant changes in OS, but reduced the '~/max to 129.8 + 1.9 V/S, prolonged the APDs0 to 73.5 + 1.7 ms and increased the contractile force to the 118.1 + 2.4% of the control value. M o o d y and Burnstock (1982) have demonstrated that adenosine inhibits twitch responses to transmural stimulation of guinea pig ileum via P]-purinoreceptors. Theophylline, a P]-purinoceptor blocker, can block this effect, and dipyridamol, an adenosine uptake blocker, can potentiate it. Therefore we examined the anti-adenosine effect of B M A in a series of experiments, in the presence of theophylline and dipyridamole. It was found (in 5 atria), that adenosine (10 - s M) reduced the contractile force to 25.9 + 3.1%, and application of B M A (10 -4 M) and theophylline (6.5 x 10 -5 M) increased it to 194.6 + 2.7% of the control value. In another experiment (in 5 atria), adenosine (10-5 M) reduced the contractile force to 20.4 + 3.2%, and appliation of B M A (10 -4 M) and dipyridamole (5 × 10 -7 M) increased it to 6 2 . 9 + 3.5% only. It must be mentioned that B M A (10 -4 M) alone increased the contractile force to 255.3 + 5.1% of the control value. The results were obtained under steady state conditions. The incubation time was 5 min in the case of adenosine and B M A and theophylline, and 20 min in the case of B M A + dipyridamole. The concentrations were chosen at which the characteristic effects of the drugs developed.
3.2. Effect of adenosine on the slow action potentials induced by BMA in atrial myocardium Fig. 2 shows a typical experiment. In this series of experiments, we wanted to see whether adenosine depressed the BMA-induced slow action potentials. In 5 atrial muscle preparations, the m y o c a r d i u m m e m b r a n e was depolarized from - 8 5 . 3 + 1.6 mV to - 4 0 . 9 + 2.4 mV in order to voltage-inactivate the fast N a ÷ channels by using elevated K ÷ (26 m M ) Krebs solution. Preparations so treated were only excitable with intense electrical stimulation (0.5 Hz and 10 times threshold voltage). Pindolol (5 × 10-7 M) was added so as to block the fl-adrenoceptors. In all preparations, B M A (10 -4 M) rapidly, i.e. within 2 - 3 min, induced slow action potentials accompanied by contractions. Pindolol did not prevent this action, indicating that the slow response-evoked effect of B M A was not mediated by catecholamine release. The BMA-induced slow action potentials had a m e a n ~Tm~x of 13.4 _ 1.6 V/S, OS of 19.8 + 0.6 mV, and APDs0 of 52.7 + 1.3 ms. After addition of adenosine (10-5 M) the ~/max of the slow responses was significantly reduced to 5.1 + 0.7 V / s , the ADPs0 was shortened to 24.3 + 0.9 ms, and OS was completely abolished. At a higher concentration (10 -4 M), adenosine totally depressed the B M A - i n d u c e d slow responses.
268
4. Discussion
The results presented here provide evidence that BMA, a selective K + channel blocker (M6szfiros et al., 1982), antagonizes the mechanical and electrophysiological changes induced by adenosine in atrial heart muscle of the guinea pig. The adenosine-induced large shortening of the action potential duration suggest that the drug, besides inhibiting slow inward Ca 2+ current (Schrader et al., 1975), may increase the outward K + current (Goto et al., 1977). This increased outward current shortens the action potential and prevents the slow inward Ca 2 + current from running its normal time course, thus producing a negative inotropic effect, as also described in the case of acetylcholine (Ten Eick et al., 1976). BMA, by decreasing the outward K + current, lengthens the action potential duration, i.e. prolongs the time during which the slow inward Ca 2+ current can be operative, thus causing a positive inotropic effect (M6szfiros et al., 1982). Moreover, adenosine depressed the slow action potentials induced by BMA in K+-de polarized atrial preparations. This finding suggests that adenosine increases the K + conductance, since in these preparations, BMA restores the electrical activity by decreasing the high K +-induced 'shunting conductance' (M6sz~ros et al., 1982), like Ba 2+ ions (Ehara and Inazawa, 1980). We suppose that BMA rather inhibits the effect of adenosine at the myocardial K + channels than at the Pl-purinoceptors. This hypothesis seems to be justified by the fact that dipyridamole, an adenosine uptake blocker, can reduce the anti-adenosine effect of BMA, whereas theophylline, a Pt-purinoceptor blocker, and potentiate it. The use of BMA, a selective K + channel blocker, offers a new chemical tool for the analysis of the
mechanism of action of adenosine which plays an important role in ischaemic heart disease.
References De Gubareff, T. and W. Sleator, 1965, Effects of caffeine on mammalian atrial muscle and its interaction with adenosine and calcium, J.Pharmacol. Exp. Ther. 148, 202. Ehara, T. and M. Inazawa, 1980, Calcium-dependent slow action potentials in potassium-depolarized guinea pig ventricular myocardium enhanced by barium ions, Naunyn-Schmiedeb. Arch. Pharmacol. 315, 47. Goto, M., A. Yatani and Y. Tsuda, 1977, An analysis of the action of ATP and related compounds on membrane current and tension components in bullfrog atrial muscle, Jap. J. Physiol. 27, 81. Kelemen, K., V. Kecskem6ti, L. Scult6ty and T. Friedmann, 1968, Activity index of frog heart ventricle cells. Analysis of the correlation between resting potential and activity of the sodium-carrier system in the frog heart by means of celluline and adrenaline, Acta Physiol. Acad. Sci. Hung. 33, 269. M~sz/~ros, J., K. Kelemen, R. Mark6, V. Kecskem&i and J. Szegi, 1982, Inhibition of myocardial K + channels by bromobenzoyl-methyladamantylamine, and adamantine derivative, European J. Pharmacol. 84, 151. Moody, C.J. and G. Burnstock, 1982, Evidence for the presence of Pl-purinoceptors on cholinergic nerve terminals in the guinea-pig ileum, European J. Pharmacol. 77, 1. Papp, J.Gy. and E.M. Vaughan Williams, 1969, The effect of bretylium on intracellular cardiac action potentials in relation to its anti-arrhythmic and local anaesthetic activity, Br. J. Pharmacol. 37, 380. Pappano, A.J., 1970, Calcium dependent action potentials produced by catecholamines in guinea pig atrial fibers depolarized by potassium, Circular. Res. 27, 379. Schrader, J., R. Rubio and R. Berne, 1975, Inhibition of slow action potentials of guinea pig atrial muscle by adenosine: A possible effect on Ca 2÷ influx, J. Mol. Cell. Cardiol. 7, 427. Ten Eick, R., H. Nawrath, T.H. McDonald and W. Trautwein, 1976, On the mechanism of the negative inotropic effect of acetylcholine, Pfliagers Arch. 361,207.