Slow Action Potentials Research Articles

Electrophysiology and ultrastructure of eustachian ridge from cat right atrium: a comparison with SA node

Intracellular microelectrode and electron microscopic techniques were used to investigate and correlate the electrophysiology of subsidiary pacemaker activity with the presence of cells having ultrastructural characteristics of pacemaker cells i.e. P cells, in Eustachian ridge tissue isolated from cat right atrium. In addition, the electrophysiological characteristics of subsidiary pacemaker activity and the ultrastructural characteristics of P cells in Eustachian ridge were compared to those of SA node obtained from the same hearts. Action potential recordings and morphological analysis were restricted to the endocardial site of earliest activation. Electrophysiological recordings revealed that spontaneously active Eustachian ridge tissues generate slow response action potentials with pacemaker characteristics similar, although not identical, to those of SA node. These included a relatively steep diastolic slope, low maximum diastolic potential (-70 mV), rate of rise (5.5 V/s), take-off potential (-52.5 mV), a relatively large overshoot potential (+7.7 mV) and a spontaneous cycle length (948 ms) about twice as long as SA node (434 ms). Morphological analysis revealed cells with ultrastructural characteristics of P cells, that were restricted to the endocardial site of earliest pacemaker activation. Morphological measurements indicate that Eustachian ridge P cells are not significantly different from P cells in SA node of the same hearts. However, Eustachian ridge P cells exhibit a unique apposition of subsarcolemmal cisternae between cells not seen in SA node. We conclude that pacemaker cells within the Eustachian ridge generate stable, spontaneous activity via slow response pacemaker action potentials. Cells responsible for this subsidiary pacemaker activity are most likely P cell types that are similar, although not identical, to P cells in SA node.

Contributions of cellular electrophysiology to the understanding of the electrocardiogram

The understanding of cardiac action potential and membrane currents has broadened the theoretical foundation and enhanced the clinical usefulness of the electrocardiogram. An improved understanding of the morphology of the electrocardiographic waveform has resulted from: correlations between Vmax of depolarization and QRS complex, plateau of the ventricular action potential and S-T segment, terminal repolarization and T-wave, from definitions of action potential differences responsible for the T-wave, and recordings of action potential alternans. Cellular electrophysiology has contributed to the understanding of certain mechanisms of cardiac standstill. Many disturbances of conduction and refractoriness associated with ventricular arrhythmias can be attributed to the following derangements at the cellular level: slowing of terminal repolarization, development of diastolic depolarization in fibers with stable resting membrane potential, after-depolarizations, currents of injury resulting from non-uniform polarization, increased dispersion of action potential durations, and co-existence of slow conduction and short premature action potentials.

Modification of the cardiac transmembrane potentials and currents by polyamines

Spermine up to 10 −5 M increased the resting potential (RP) and maximum rate of rise (V max) of the action potential (AP) and accelerated the initial (20, 50%) repolarization in isolated guinea-pig and cat atria. These effects were not modified by pindolol (4 × 10 −7 M) or atropine (3 × 10 −7 M) but were prevented by indomethacin (10 −6 M). The right papillary muscle of guinea-pig only showed the acceleration of repolarization while the other parameters were not changed. Spermine did not influence the inward membrane currents under voltage clamp conditions in frog sinoatrial fibers. Spermidine up to 10 −5 M increased RP, V max and the duration of AP in guinea pig papillary muscle. RP, V max and AP duration were moderately enhanced by spermidine in guinea pig atrium and were not changed in cat atrium. Spermidine was found to stimulate the inward membrane current. Its stimulatory effect was confined to the fast component of the inward current. Neither spermidine nor spermine were able to induce the Ca 2+-dependent slow AP in 25 mM K +-depolarized atrium. It is suggested that polyamines influence cardiac membrane events which are responsible for AP and ionic currents.

Voltage‐dependent conductances of solitary ganglion cells dissociated from the rat retina.

1. Ganglion cells were dissociated from the enzyme-treated rat retina, identified with specific fluorescent labels, and maintained in vitro. Electrophysiological properties of solitary retinal ganglion cells were investigated with both conventional intracellular and patch-clamp recordings. Although comparable results were obtained for most measurements some important differences were noted. 2. The input resistance of solitary retinal ganglion cells was considerably higher when measured with 'giga-seal' suction pipettes than with conventional intracellular electrodes. Under current-clamp conditions with both intracellular and patch pipettes, these central mammalian neurones maintained resting potentials of about -60 mV and displayed action potentials followed by an after-hyperpolarization in response to small depolarizations. The membrane currents during this activity, analysed under voltage clamp with patch pipettes, consisted of five components: Na+ current (INa), Ca2+ current (ICa), and currents with properties similar to the delayed outward, the transient (A-type), and the Ca2+-activated K+ currents (IK, IA and IK(Ca), respectively). 3. Ionic substitution, pharmacological agents, and voltage-clamp experiments revealed that the regenerative currents were carried by both Na+ and Ca2+. 100 nM-1 microM-tetradotoxin (TTX) reversibly blocked the fast spikes carried by the presumptive INa, which under voltage-clamp analysis had classical Hodgkin-Huxley-type activation and inactivation. 4. Single-channel recordings of the Na+ current (iNa) permitted comparison of these 'microscopic' events with the 'macroscopic' whole-cell current (INa). The inactivation time constant (tau h) fitted to the averaged single-channel recordings of iNa in outside-out patches was slower than the tau h obtained during whole-cell recordings of INa. 5. In the presence of 1-40 microM-TTX and 20 mM-TEA, slow action potentials appeared in intracellular recordings and were probably mediated by Ca2+. The potentials were abrogated by 3 mM-Co2+ or 200 microM-Cd2+; conversely, increasing the extracellular Ca2+ concentration from 2.5 to 10-25 mM or substitution of 1 mM-Ba2+ for 2.5 mM-Ca2+ enhanced their amplitude. ICa was measured directly in whole-cell recordings with patch pipettes after blocking INa with extracellular 1 microM-TTX and K+ currents with intracellular 120-mM Cs+ and 20 mM-TEA. 6. During whole-cell recordings with patch electrodes, extracellular 20 mM-TEA suppressed IK and, to a lesser extent, IA. Extracellular 5 mM-4-AP or a pre-pulse of the membrane potential to -40 mV prior to stronger depolarization completely blocked IA.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of direct cardiostimulating actions of hydralazine

The vasodilator, hydralazine, was reported to also exert a direct positive inotropic effect on the myocardium at high concentrations. In the present study we investigated the mechanism of this positive inotropic action by using the ventricular myocardium of isolated perfused chick hearts. Hydralazine (10 −3 M) enhanced contractile force and heart rate, and elevated the myocardial cyclic AMP level. To study the Ca 2+-dependent slow action potentials, the fast N + channels were voltage-inactivated with elevated K + (25 mM), resulting in a loss of electrical excitability. Hydralazine (10 −4 M) rapidly (< 3 min) allowed the generation of slow action potentials and accompanying contractions by electrical stimulation. These effects of hydralazine were only partially prevented by propranolol. The results suggest that the increase of myocardial contractility produced by hydralazine is the result, at least in part, of a direct effect on the myocardium to increase Ca 2+ inflow. The increased Ca 2+ influx and inward slow current is due partly to activation of β-adrenoceptors, with resultant elevation of cyclic AMP, and partly to another mechanism.

Abnormal pacemaking is modulated by sarcoplasmic reticulum in partially-depolarized myocardium from dilated right atria in humans

Fifty human atrial specimens removed at time of cardiac surgery were studied in vitro. Thirty-four samples were selected as presenting partial cell depolarization and exhibiting slow response action potentials. Twenty of these preparations were automatic whereas 14 were not. Neither mean maximum diastolic potential (-52.8 +/- 1.3 mV and -49.3 +/- 2.2 mV respectively) nor maximum rate of depolarization (Vmax) (1.1 +/- 0.1 V/s and 1.3 +/- 0.8 V/s) significantly differed between these two groups. Abnormal automaticity due to phase 4 depolarization occurred in 12/13 preparations dissected from markedly dilated atria whereas it occurred in only 2/10 preparations sampled from non-dilated atria. A statistically significant relationship between in vitro abnormal pacemaking and atrial dilatations was found. We investigated the effects on abnormal pacemaker depolarization and automaticity of a reduction in the extracellular Na and Ca and of sarcoplasmic reticulum (SR) inhibitors. Abnormal pacemaker depolarization appeared to be much more sensitive to a reduction in the extracellular Na than in the extracellular Ca. Both Sr and Mg slowed the automatic rate. Ryanodine 3 X 10(-6) M, a specific SR inhibitor, irreversibly lengthened the spontaneous basic cycle duration to about 300% of control. Epinephrine up to 10(-4) M was ineffective in accelerating the residual spontaneous rhythm that persists after ryanodine action, although epinephrine markedly enhanced the overshoot and Vmax of the slow responses. It is concluded that, in the human atrial myocardium, abnormal pacemaking that develops at low level of membrane potential: is promoted by chamber dilatation; is strongly modulated by SR-dependent processes.

Somatostatin effects in isolated human atrial fibres

Effects of hypothalamic hormone somatostatin on the action potential and contractile force of 54 human atrial preparations obtained at cardiac surgery were studied with standard microelectrode techniques. In the atrial fibres responding to electrical stimuli with fast response action potentials in 4 mM [K]0 Tyrode solution (maximum rate of phase 0 depolarization greater than 50 V/s), somatostatin (10(-7) to 10(-6) M) reduced slightly the duration of action potential and decreased twitch force dose-dependently. In spontaneously active atrial fibres, somatostatin (10(-7) to 10(-6) M) abolished the action potentials in the preparations with low maximal diastolic potential (MDP, -48 +/- 2.8 mV), but induced only mild suppressive effect in the preparations with high MDP (-70 +/- 2.4 mV). When the fibres were depolarized in 27 mM [K]0 Tyrode solution, somatostatin decreased the maximum rate of depolarization and amplitude of the slow response action potentials induced by electrical stimuli. The delayed afterdepolarizations or triggered action potentials induced by high-frequency electrical drive in the presence of epinephrine were also suppressed by somatostatin. The above findings suggest that somatostatin may suppress the abnormal automaticity and the triggered activity in human atrial fibres through a reduction of cellular calcium.

Influence of phosphodiesterase inhibition and of carbachol on inotropic effects of 8-substituted cyclic AMP analogues

The influence of phosphodiesterase inhibitors and of carbachol on the positive inotropic effect of 8-substituted cyclic AMP analogues was studied on isometrically contracting guinea-pig papillary muscles driven at a rate of 0.2 Hz. In muscles from reserpine-pretreated animals, the phosphodiesterase inhibitors 3-isobutyl, 1-methyl xanthine (IBMX; 20 mumol/l) and papaverine (10 mumol/l) shifted the concentration-effect curves of 8-substituted cyclic AMP benzyl esters to the left, decreasing the EC50 by a factor of 10 to 25. In the presence of IBMX (5 and 20 mumol/l) or papaverine (10 mumol/l), the slopes of the concentration-effect curves of 8-substituted cyclic AMP benzyl esters became flatter. The positive inotropic effect and the increase in Vmax, overshoot and duration of slow action potentials induced by cyclic AMP analogues were not affected by carbachol (0.1-10 mumol/l). In the presence of IBMX (20 mumol/l), however, carbachol (3 mumol/l) antagonized the positive inotropic effect of 8-substituted cyclic AMP derivatives, shifting the EC50-values by a factor of 3 to the right. Cyclic AMP content determined by radioimmunoassay in individual papillary muscles was raised 1.22 and 1.63-fold in the presence of 3 and 20 mumol/l IBMX. Isoprenaline (0.1 mumol/l) induced an increase in cyclic AMP content which was not significantly different from that produced by 20 mumol/l IBMX, but in contrast to the phosphodiesterase inhibitor enhanced force of contraction by 17.7 mN as compared to 1.5 mN obtained with 20 mumol/l IBMX. The findings are consistent with a model that describes the interaction between IBMX and cyclic AMP analogues as an additive effect with only endogenously accumulated cyclic AMP (due to phosphodiesterase inhibition) being involved in the negative inotropic effect of carbachol. From the failure of carbachol to affect the positive inotropic effect of cyclic AMP analogues, it is concluded, that cyclic AMP derivatives do not act as phosphodiesterase inhibitors, and that the well-known negative inotropic effect of carbachol in the presence of cyclic AMP-elevating drugs does not occur at a step beyond cyclic AMP accumulation.

Electrophysiologic Actions of Dibenzepin, a Tricyclic Antidepressant, on Canine Purkinje Fibers

Electrophysiologic effects of dibenzepin, a tricyclic antidepressant drug known experimentally to defibrillate the ventricles, were studied in canine Purkinje fibers with glass microelectrodes. In 11 normally perfused preparations the dose-response curve (10(-8)-10(-5) g/ml) revealed that the threshold concentration of this drug was 10(-7) g/ml, reducing the maximal rate of depolarization (MRD) from 930 to 876 V/s and overshoot from 32 to 30 mV, respectively (p less than 0.05). At 10(-6) g/ml, dibenzepin shortened action potential duration (APD) at 50 and 90% repolarization (APD50, APD90), respectively, from 290 to 248 ms and from 385 to 363 ms (p less than 0.001). At 10(-5) g/ml, it decreased the maximal diastolic potential (MDP) from -92 to -85 mV and further reduced MRD, overshoot and APD50 (p less than 0.01). The membrane responsiveness curve was shifted to more negative potentials, but its normalized curve (h infinity curve) remained unchanged. This drug did not alter slow response action potentials produced by 22 mM K+ and 10(-7) M isoproterenol. In six Purkinje fibers, hypoxia (100% N2) reduced MDP and MRD. Subsequent addition of dibenzepin further decreased MRD and shortened APD50. These results indicate that (a) dibezepin inhibits the fast Na+ current without changing its inactivation kinetics, thus suggesting a class 1 antiarrhythmic action; (b) it does not affect the slow inward current; and (c) the former action appears enhanced during hypoxia, possibly contributing to ventricular defibrillation.

Calcium Channel Blocking Properties of Amlodipine in Vascular Smooth Muscle and Cardiac Muscle In Vitro: Evidence for Voltage Modulation of Vascular Dihydropyridine Receptors

Amlodipine was twice as potent as nifedipine at inhibiting Ca2+-induced contractions in depolarised rat aorta (IC50 1.9 nM vs. 4.1 nM) but, unlike nifedipine, displayed a very slow onset of action. Contractions induced by depolarising steps with 45 mM K+ were much less potently blocked by amlodipine (IC50 19.4 nM), whereas the potency of nifedipine was little changed (IC50 7.1 nM). This difference may be explained by a modulated receptor hypothesis, similar to that described for cardiac muscle, in which block of vascular calcium channels by dihydropyridines is enhanced at depolarized membrane potentials, such voltage-dependence only being apparent with a slow-acting drug such as amlodipine. Recovery from amlodipine block of K+-responses in rat portal vein after drug washout was also very slow. Amlodipine and nifedipine blocked phenylephrine-induced contractions of the rat aorta with potencies similar to those against depolarisation-induced responses. Negative inotropic potencies of amlodipine and nifedipine in perfused guinea pig hearts were approximately one-tenth those against Ca2+-induced contractions in rat aorta. Amlodipine caused complete block of guinea pig papillary muscle single-cell slow action potentials at a concentration (5 microM) that had no effect on upstroke velocity of normal, fast potentials but reduced the duration of the plateau phase.

The role of calcium channel in the effect of nicotine on contractility in isolated toad ventricle

The mechanism of the positive inotropic effect produced by nicotine (6.2 X 10(-5) mol/l to 4.9 X 10(-4) mol/l) on electrically driven toad ventricles was investigated. The response to nicotine was not affected by 6-hydroxydopamine pretreatment, bretylium (2.4 X 10(-4) mol/l) exposure or tyramine tachyphylaxis. Following desensitisation by isoprenaline (4.2 X 10(-6) mol/l) of the beta-adrenoceptor in the ventricles, the response to nicotine was no affected. However, the response was antagonised by ethylene diamine tetraethyl acetate (2.3 X 10(-4) mol/l), verapamil (0.4 X 10(-5) mol/l) or calcium-free Ringer. Nicotine prolonged the action potential duration and enhanced the force of contraction. Nicotine induced slow action potentials in partially depolarized (in high potassium solution) ventricles and this was antagonised by verapamil (0.4 X 10(-5) mol/l). These results suggest that the effects of nicotine are mediated by a direct interaction with the Ca2+ channels at the cell surface.

Effects of KW-3049, a newly developed calcium antagonist, on rabbit sinus nodes and guinea pig ventricular muscles.

The effects of KW-3049, a newly developed dihydropyridine-type calcium antagonist, were studied on the electrophysiological properties of isolated rabbit sinus node tissues and guinea pig ventricular muscles. In spontaneously firing sinus node pacemaker cells, KW-3049 (greater than or equal to 10(-8) M) prolonged the cycle length, and decreased the maximum upstroke velocity (Vmax) and overshoot (OS) of action potential. KW-3049 (greater than or equal to 10(-6) M) produced a shortening of action potential duration (APD) of normally polarized ventricular muscles without affecting other parameters. Vmax, OS, and APD of slow action potentials, which had been induced by isoproterenol or by barium in K+-depolarized ventricular muscles, were reduced dose-dependently by KW-3049 (greater than 10(-9) M). The relative potency of KW-3049 for the Vmax inhibition of slow responses was 20 times higher than verapamil, but 0.32 times less than nifedipine. In voltage calmp experiments on ventricular muscles, KW-3049 (10(-6) M) caused a decrease of slow inward current by 52.7%, but did not affect the net outward current. These findings suggest that KW-3049 may have a potent and highly selective inhibitory action on cardiac slow channels.

O-149 - Developmental changes in the muscarinic inhibition of Ca-dependent slow action potentials in rat ventricular muscles

O-149 - Developmental changes in the muscarinic inhibition of Ca-dependent slow action potentials in rat ventricular muscles

Further characterization of the receptors involved in the positive inotropic effect of histamine on rabbit papillary muscle

The effects of histamine on the force of contraction and calcium-dependent action potentials were studied in rabbit ventricular papillary muscles. The positive inotropic effect of histamine seems to be dependent on stimulation of H1 and H2 receptors. The H1 antagonist chlorpheniramine produced a competitive blockade of the positive inotropic effects of histamine. Cimetidine produced a competitive blockade, which was apparent only after blockade of H1 receptors. Histamine increased the maximum upstroke velocity of slow action potentials. This effect can be entirely accounted for by stimulation of H2 receptors. The phosphodiesterase inhibitor 3-isobutyl-methyl-xanthine potentiated the H2 receptor mediated effects of histamine on the force of contraction and slow action potentials. We conclude that rabbit ventricular muscle possesses both H1 and H2 receptors that mediate the positive inotropic effect of histamine. The H2-mediated effect seems to be causally related to an increase in the calcium slow inward current and is probably linked to an enhanced cellular cyclic adenosine monophosphate content. The mechanism of the H1-mediated positive inotropic effect remains unknown.

The effects of doxorubicin on ventricular tachycardia.

Doxorubicin, in concentrations that have no effect on fast or slow response action potentials, has been shown to suppress ouabain-induced delayed afterdepolarizations. In this study, we used standard microelectrode techniques to determine the effects of doxorubicin on isolated canine Purkinje fibers. We studied automaticity induced at normal and low membrane potentials, conduction in normal and K+-depolarized Purkinje fibers, and triggered activity induced by ouabain and by experimental myocardial infarction. Doxorubicin, 50 microM, suppressed the triggered activity and the delayed afterdepolarizations that induced it, but had no effect on the other variables. We then studied the effects of intravenous doxorubicin, 16 to 64 mg/m2 body surface area, on ouabain-induced ventricular tachycardia and the ventricular tachycardia that occurs 24 hr after ligation of the left anterior descending coronary artery in the intact dog. There was no effect on the infarct-induced arrhythmia, but concentrations of doxorubicin that had no other effect on the electrocardiogram suppressed those ouabain-induced arrhythmias that appeared to have been triggered. The automatic arrhythmias induced by ouabain were not affected. Both the latter mechanisms were verified in studies of isolated Purkinje fibers that were obtained on completion of the intact animal experiments. These results indicate that agents having high selectivity for specific arrhythmogenic mechanisms can be useful adjuncts in discriminating among the mechanisms responsible for arrhythmias in intact animals.

Electromechanical effects of A23187 on Guinea-pig ventricular muscle: A dual action of A23187

The effects of A23187 on electromechanical activity of guinea-pig papillary muscles were examined by microelectrode techniques. A23187 (10(-6) M) caused a marked positive inotropic effect (868% of control) on preparations driven at 0.2 Hz. This inotropic effect was accompanied by a significant prolongation of the action potential duration at an early repolarization phase (APD 10), but not at a late repolarization phase (APD 30 and APD 80). Other parameters of action potential were unaffected. In the presence of nifedipine (10(-6) M), Ca2+ channel blocker, A23187 still increased the contractile force (495% of control) but had no effect on APD 10. On the other hand, APD 30 and APD 80 were significantly shortened. In the presence of ryanodine (2 X 10(-6) M), an inhibitor of internal Ca2+ release, A23187 was also able to cause the positive inotropic effect (389% of control). This inotropic effect was accompanied by a prolonged APD 10. These electrical and mechanical effects of A23187 were completely blocked by the combined treatment with nifedipine and ryanodine, suggesting that A23187 has a dual action. In papillary muscles depolarized by 26 mM [K+]0, A23187 augmented the slow action potential. In voltage clamp experiments using a single sucrose-gap method, A23187 caused a marked increase in the slow inward current and the outward current. The effects of A23187 were not antagonized by a beta-adrenoceptor and a histamine (H2) receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitation-contraction coupling in myocardium of nonhibernating and hibernating chipmunks: effects of isoprenaline, a high calcium medium, and ryanodine.

The electromechanical responses to isoprenaline and a high calcium medium on cardiac muscles from nonhibernating and hibernating chipmunks were studied in the presence and the absence of ryanodine. In nonhibernating animal preparations, isoprenaline (5 X 10(-8) M) caused a marked positive inotropic effect with an increase in the amplitude of the action potential plateau and augmented the slow action potential in muscle depolarized with 26 mM K+. In hibernating animal preparations, isoprenaline failed to cause a positive inotropic effect in spite of an increase in the amplitude of action potential plateau and slow action potentials. Similar inotropic effects were caused in the two preparations when extracellular calcium was raised to 6 mM, but action potential plateau and slow action potentials of the two preparations were less affected by this procedure; only in nonhibernating animal preparations was the amplitude of the slow action potential slightly increased. Ryanodine (2 X 10(-6) M) partially inhibited the contraction but augmented the action potential plateau and slow action potentials in nonhibernating animal preparations, while in hibernating animal preparations, it eliminated the contraction and severely inhibited the action potential plateau and slow action potentials. The electrical effects of isoprenaline on the two preparations and of a high calcium medium on nonhibernating animal preparations were more pronounced in the presence of ryanodine than in the absence of it. However, the electrical activity on hibernating animal preparations was unaffected by a high calcium medium in either the presence or absence of ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent effects of YC-170, a dihydropyridine calcium channel modulator, in cardiovascular tissues

Voltage-dependent effects of YC-170, a putative calcium channel activator, were examined and compared with those of Bay K 8644 in isolated guinea-pig cardiac tissues and rabbit aortae. In guinea-pig left atria superfused with a normal bathing solution (4 mmol/l K+), both YC-170 (10 mumol/l) and Bay K 8644 (1 mumol/l) produced a positive inotropic action accompanied by a prolongation of action potential durations (APDs). In normally-polarized guinea-pig papillary muscles Bay K 8644 increased force of contraction (fc) and APDs. However, YC-170 failed to increase fc in spite of a slight prolongation of APDs. In papillary muscles partially depolarized by 25 mmol/l K+ solution, Bay K 8644 enhanced the electrically-induced slow action potentials and contractile force. In contrast with Bay K 8644, YC-170 significantly depressed the slow action potentials and decreased fc. YC-170 also showed the depressant action on the slow action potentials induced by isoproterenol (0.1 mumol/l), histamine (3 mumol/l) and tetraethylammonium (10 mmol/l) plus high Ca2+ (4 mmol/l). In sinoatrial node cells of guinea-pig right atria Bay K 8644 produced a positive chronotropic action with increases in the maximum rate of rise (Vmax) and action potential amplitude (APA), whereas YC-170 produced a negative chronotropic action with decreases in Vmax and APA. In the rabbit aortic strips preincubated with bathing solution containing various concentrations of K+ (15, 20, 30 and 40 mmol/l), Bay K 8644 produced concentration-dependent contractions in a range of concentrations up to 0.3 mumol/l. However, when the concentration exceeded 1 mumol/l, Bay K 8644 caused a slight relaxation, irrespective of the K+ concentrations of bathing solution. YC-170 in concentrations of 10 and 30 mumol/l contracted the aortic strips placed in 5.9 or 15 mmol/l K+ bathing solution, but caused relaxation in 30 or 40 mmol/l K+ bathing solution. These results suggest that YC-170 is a dihydropyridine calcium channel modulator which behaves as a Ca2+ channel agonist in tissues of high membrane potentials, but as a Ca2+ channel antagonist in tissues of low membrane potentials.

Effect of methylated bepridil on slow action potentials in cardiac muscle and vascular smooth muscle

The anti-anginal agent bepridil blocks slow Ca 2+ channels in a variety of tissues. Since bepridil accumulates inside cells, the possibility exists that bepridil acts, at least partially, from inside the cell. To test this possibility, we examined the effects of a quaternary ammonium analog of bepridil, methylated bepridil, which presumably would enter the cells less readily, on the Ca 2+-dependent slow action potentials of guinea pig papillary muscles (in 25 mM [K +] 0) and rabbit pulmonary arteries (in tetraethylammonium chloride). In cardiac muscle, methylated bepridil had little effect on the slow action potentials at low stimulation frequencies (0.5 Hz), but at higher frequencies (1.0 and 2.0 Hz) the slow action potentials were depressed and/or the muscle was unable to follow each stimulation. These effects are similar to those obtained with bepridil, but bepridil was more potent than methylated bepridil. In vascular smooth muscle cells, methylated bepridil inhibited the slow action potentials at a somewhat lower dose than bepridil. We conclude that, in cardiac muscle, bepridil probably has two sites of action, one outside the cell (presumably on or associated with the slow Ca 2+ channel) and a second site inside the cell. On the other hand, in vascular smooth muscle cells, bepridil may act only on an external site.

PK 11195, an antagonist of peripheral type benzodiazepine receptors, modulates BAY K8644 sensitive but not β- or H 2-receptor sensitive voltage operated calcium channels in the guinea pig heart

In a partially depolarized guinea pig papillary muscle preparation, BAY K8644 stimulated voltage-operated calcium channels, promoting slow action potentials; this effect was dose-dependent over a concentration range of 3 × 10 −7 M to 3 × 10 −6 M. Isoproterenol and histamine also induced slow action potentials by stimulating β or H 2 receptors, respectively. PK 11195, the antagonist of peripheral type benzodiazepine receptors, inhibited the effect of BAY K8644, but not those of histamine or isoproterenol. Moreover, PK 11195 “dose-dependently” antagonized the ability of R05-4864 to inhibit the slow action potentials elicited by barium chloride. Thus, in the heart, PK 11195, an antagonist of peripheral type benzodiazepine receptors, can modulate voltage-operated calcium channels when they are activated directly, but not when they are activated by stimulation of neurotransmitter receptors.