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Telmisartan is an angiotensin II receptor blocker and partial peroxisome proliferator-activated receptor gamma agonist that modulates the renin-angiotensin-aldosterone system. It is used primarily to manage hypertension, diabetic nephropathy, and congestive heart failure. Recent studies have reported that myocardial infarction (MI) has occurred in telmisartan-treated patients. The purpose of the study was to investigate the specific conditions and underlying mechanisms that may result in telmisartan-induced MI. We evaluated the effect of telmisartan on whole hearts, cardiomyocytes, and cardiac sarcolemmal ion channels. Hearts of 8-week-old male Sprague-Dawley rats were perfused with 3, 10, 30, or 100 μM telmisartan or losartan or with normal Tyrode's solution (control) for 3 h. We found that telmisartan induced myocardial infarction, with an infarct size of 21 % of the total at 30 μM (P < 0.0001) and 63 % of the total area at 100 μM (P < 0.001). Telmisartan also induced cardiac dysfunction (e.g., decreased heart rate, diminished coronary flow, hypercontracture, and arrhythmia). Confocal microscopy demonstrated that 30 μM telmisartan significantly elevated the intracellular Ca(2+) level, leading to hypercontracture and cell death. Patch clamp analysis of isolated cardiomyocytes revealed that telmisartan induced Na(+) overload by slowing the inactivation of voltage-gated Na(+) current (I (Na)), activating the reverse mode of Na(+)-Ca(2+) exchanger activity, and causing Ca(2+) overload. Telmisartan significantly delayed the inactivation of the voltage-gated Na(+) channel, causing cytosolic Na(+) overload, prolonged action potential duration, and subsequent Ca(2+) overload. Above 30 μM, telmisartan may potentially cause cardiac cell death and MI.

The mechanism of enhancement of the L-type Ca current by a P2-purinergic agonist adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) was studied by recording single channel activity from cell-attached patches on rat isolated ventricular cells using patch pipettes containing 110 mM Ba2+. The application of ATP gamma S to the patch membrane through the pipette solution did not affect single channel activity. The addition of ATP gamma S to the bath containing a depolarizing solution was ineffective due to the voltage dependence of the purinergic stimulation. Bath application of ATP gamma S (100 microM) to control 4-(2-hydroxyethyl)-1-piperazine-ethanesulphonic acid (HEPES) solution increased the amplitude of ensemble average currents both by decreasing the probability of a blank sweep occurring and by increasing the number of openings per non-blank sweep. The single channel conductance (17 pS) was not changed by ATP gamma S. Both activation and inactivation curves were shifted towards hyperpolarized potentials by about 10 mV under P2-purinergic stimulation. Since ATP gamma S increased channel activity when applied via the bath, it must be supposed that a diffusible messenger is involved.

The action of the blood clotting enzyme thrombin on single channel and whole cell Ca(2+)-currents was studied in isolated mammalian cardiac myocytes. Thrombin, at a concentration of 10(-8) mol/l, increased the Ca(2+)-channel activity in cell-attached patches. The mean open probability of the channel was enhanced, while the number of sweeps without openings, which reflects the availability of the channel, was significantly reduced. Neither the single channel conductance nor the activation curve were affected by thrombin. Thrombin was added to the bath solution, and its effect is therefore indirect and probably mediated via a second messenger. However, thrombin did not affect whole-cell Ca(2+)-currents, whereas a beta-adrenergic stimulation in the same cell increased the Ca(2+)-current. It is concluded that thrombin affects an intracellular mechanism for Ca2+ channel current regulation, which is still unknown and which is rapidly lost during conventional whole-cell Ca2+ current measurements.

Currents through DPI 201-106 modified single sodium channels have been measured in cell-free inside-out patches from guinea-pig ventricular myocytes. Single-channel conductance and reversal potential of the sodium channel have been calculated at different intracellular sodium concentrations [( Na+]i) from microscopic I-V curves, which were obtained by application of linear voltage ramps. The relation between the reversal potential and [Na+]i could be fitted with a modified Goldman-Hodgkin-Katz equation with a relative permeability for K+ over Na+ ions of 0.054. The zero-current conductance of the Na channel as a function of [Na+]i shows a plateau value at low Na concentrations, and increases in a sigmoidal manner at higher concentrations. It is concluded that the Na channel can carry outward currents and that its conductance depends on [Na+]i.

The pharmacological modification of the thrombin effect on the mechanical and electrical responses of frog heart was examined in the Straub heart preparation and in single ventricular cells. Associated with the positive inotropic action thrombin increases voltage and duration of action potentials of isolated frog ventricular cells. As found by the patch-clamp technique in the cell-attached mode, thrombin stimulates single L-type Ca2+ channels, presumably mediated by a second messenger. The enhancement of contractility by thrombin depends on the proteolytic activity of the enzyme because enzymatically inactivated thrombin has no effect on frog hearts. The positive inotropic effect of thrombin as well as its stimulation of Ca2+ channel currents were inhibited by the protein kinase C blocker 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H7). However, phorbol 12-myristate 13-acetate (PMA), a known stimulator of protein kinase C, was ineffective in stimulating the inotropic action of thrombin. The inhibition of the thrombin-induced enhancement of contractility by indometacin indicates an involvement of arachidonic acid in the action of thrombin on frog heart.

Currents through single cardiac sodium channels have been measured in inside-out patches from guinea pig ventricular cells. To abolish the fast inactivation, Na channels were modified by DPI 201-106. In symmetrical Na solutions, a diminution of outward sodium currents can be observed that depends on the intracellular magnesium concentration and the membrane potential. Inward currents were not altered by the concentrations of magnesium used (between 0 and 22.5 mmol/l). In Mg free solutions a linear current-voltage relation can also be measured in the range of outward Na currents. At +60 mV (symmetrical Na solutions, single channel conductance 24 pS) a half maximal block of cardiac Na channels by intracellular magnesium was found at 2.1 mmol/l. From the analysis of single channel current-voltage relationships the concentration and voltage-dependent block by intracellular magnesium of cardiac sodium channels could be described as binding of Mg at one site with a Kd value of 5.1 mmol/l at 0 mV. The site is located at an electrical distance of 0.18 from the inside.

The dependence of the repolarization phase of the action potential in the atrial myocardium on the pacing pattern was studied in normoxic and hypoxic conditions. Intracellular action potentials were recorded by means of the conventional microelectrode technique from isolated left atrial trabeculae of the rabbit. By varying the pacing pattern and the oxygen supply changes of the configuration of action potential and of the membrane current as well as a conductance measure gK were obtained. In all pacing patterns the duration of action potentials is shortened, and the net outward current is enhanced by hypoxia. In steady state and for premature action potentials, gK is enlarged by hypoxia, but in action potentials after a pacing pause it is diminished more in hypoxic than in normoxic conditions. The findings suggest inhibition of an inward current in hypoxia as well as a dependence of a potassium background conductance on the inner Ca-concentration in normoxic and hypoxic conditions.

A new antiarrhythmic acting drug (Bonnecor, AWD 19-166, GS 015, 3- Carbethoxyamino-5-dimethyl-amino-acetyl-iminodibenzyl -hydrochloride) was tested electrophysiologically on isolated sinus node preparations and right ventricular papillary muscle of the rabbit. Transmembrane potentials were measured using a standard microelectrode technique. The substance was tested within a concentration range between 0.1 and 10 mg/l (2.5 X 10(-7) to 2.5 X 10(-5) mol/l). AWD 19-166 exerted negative chronotropic effects due to decreasing of the slope of the diastolic depolarization in sinus node cells. In ventricular myocardium AWD 19-166 increased the threshold strength of stimulation, prolonged the duration of action potentials at 90% but shortened it at 25% repolarization. The maximum upstroke velocity was found to be depressed. The maximum overshoot potential was diminished inspite of a drug-induced hyperpolarization of the resting transmembrane potential. Analysis of membrane responsiveness due to premature stimulation showed a concentration-dependent delay in the restitution of the maximum upstroke velocity of the premature action potentials and both a flattening and a shift towards more negative potentials of the potential-responsiveness relationship. AWD 19-166 decreased both the maximum upstroke velocity and the duration of Ca-mediated action potentials in concentrations between 2 and 8 mg/l. It is concluded that AWD 19-166 is able to exert potent antiarrhythmic effects in influencing both the fast and slow channel activity.

Pacemaker activity of sino-atrial (SA) node cells is explained by reconstructing the pacemaker potentials using a Hodgkin-Huxley type model which based on voltage clamp data. The new point of the reconstruction is the consideration of kinetic data of a novel "transient" (T-type) Ca current. It is demonstrated that the diastolic depolarization is closely correlated to the activation of the T-type Ca current. Overshooting SA node action potentials, however, are evoked by activation of the conventional L-type Ca current triggered by a depolarization induced by the T-type Ca current.

Lidocaine block of single cardiac sodium channels was studied in cell free inside-out patches of ventricular cells isolated from guinea-pig hearts. When applied to the inner surface of the membrane lidocaine depressed Na channel currents by decreasing the probability P of the channels to open measured from the peaks of the averaged currents. In parallel to the decrease in P the relative number of empty sweeps (nulls) was increased. Half maximum block of the activity of single Na channels was observed at 2.9 microM. Lidocaine affected the gating behaviour of Na channels by shortening of the mean open time tau 0 from 0.44 +/- 0.17 (control) to 0.19 +/- 0.13 (5 microM lidocaine, holding potential-120 mV, test potential -60 mV). Five micromolar lidocaine completely suppressed burst-like openings of Na channels and abolished the slow decaying phase of the averaged currents. A shift from - 120 towards - 160 mV exerted relief from the effect of both P and tau 0.

Potentiation of heart muscle stripes of the right ventricle was investigated in normal rats (NT-group) and in rats stressed by swimming (HT-group). The mean fibre diameter runs to 26 micrometer in HT-group and 16 micrometer in NT-group. The difference was significantly high, as well as that of relative heart weights. The potentiation by paired pulse stimulation was more marked in HT-group than in NT-group. The interval-strength-relationships increase with prolongation of the driving interval. Potentiation is greater in loaded rats than in normal rats. A mathematical model for Ca++-storage in sarcoplasmic reticulum is introduced. Decrease of basic-filling of Ca++-store is modelled to HT-group with support of morphological measurements and is discussed as reason for improved properties of potentiation of HT-group.

Effects of desipramine (DPr), a tricyclic antidepressant, on isolated trabeculae of the right rabbit atrial myocardium were studied using a standard microelectrode technique. Between 10(-7) and 10(-4) g/ml DPr reduced the maximum upstroke velocity Vmax of the action potentials. The membrane responsiveness was found to be depressed for DPr increased the time constant of the recovery of Vmax in a concentration-dependent manner distinctly. The voltage-Vmax relationships were shifted to more positive potentials and were flattend by increasing the concentration of DPr. DPr prolonged the duration of the action potentials on all levels of repolarization. Between 10(-7) and 10(-5) g/ml both resting and overshoot potential were found to be unchanged. At concentrations higher than 10(-5) g/ml slow response action potential insensitive to 10(-5) mol/l TTX were evoked, the resting transmembrane potential depolarized, spatial inhomogeneities in transmembrane potential and triggered activity could be observed. DPr may act antiarrhythmogenically in a quinidine-like manner at concentrations between 10(-7) and 10(-5) g/ml by (1) decreasing the fast channel conductance, (2) delaying the recovery of the fast channel. At concentrations higher than 10(-5) g/ml DPr could effect arrhythmogenic actions by a complete depression of fast response action potentials, unmasking of slow response action potentials and generation of severe spatial inhomogeneities.

Using a modified sucrose gap technique action potential and phase plane trajectories of trabeculae from the left rabbit's auricle were recorded. By means of a numerical treatment and some mathematical model interpretations from these measurements current-voltage-(I-V)-relationships and the amount of inward transported calcium are calculated. A time independent K+-current shows that a voltage region within the anomalous rectification should exist. By acetylcholine the anomalous rectification is abolished and the inward transported amount of Ca-ions by the action potential (Q) decreases. A conductance of the K+-current gK will be dimished by decreasing the extracellular calcium concentration. The amount of inward transported calcium does not change significantly by variation of the extracellular calcium concentration. The calcium amount Q should contribute 17% of the calcium necessary for maximum contraction.

Calcium influx is vital for several aspects of cardiac activity, so it is important to ask if heart cells possess a single or multiple types of Ca channel. Only one Ca channel type has been identified in patch-clamp studies of unitary current, despite suggestions to the contrary from whole-cell recordings in heart cells and unitary recordings from other cells. Here we describe a novel type of cardiac Ca channel with several properties that distinguish it from the hitherto-identified Ca channel in heart cells. Its conductance in isotonic Ba is small (8 pS), and is no larger in Ba than in Ca. It activates and inactivates at relatively negative potentials and remains functional long after patch excision. It is insensitive to dihydropyridines such as nimodipine and the Ca agonist Bay K 8644, and is more resistant to block by external Cd than the previously described type of cardiac Ca channel.

The effects of external Cd++ and Hg++ on the early outward current (IEO) in myocardial mouse cells were studied using voltage clamp with one suction pipette. Both Cd++ in the millimolar and Hg++ in the micromolar range shifted the half time to peak IEO to positive potentials more than the current amplitude after 50 ms. Incomplete blocking of IEO could be obtained by Cd++ concentrations between 10(-3) and 2 x 10(-2) mol/l and by Hg++ concentrations between 10(-6) and 5 x 10(-5) mol/l. The current-voltage relationships of IEO at peak time and at 50 ms were mainly shifted to the right by Cd++ whereas Hg++ mainly decreased the slope. At incomplete blocking concentrations Cd++ slowed down IEO activation more strongly than did Hg++. All Cd++ effects were completely reversible. The action of Hg++ was irreversible. It is concluded that both ions act directly on the gating machinery of the channels rather than simply binding to homogenous surface charges.

We studied changes of the transmembrane resting and steady state action potentials in the rabbit atrial trabecular preparations (length 2-4 mm, diameter 150-800 micrometers, standard microelectrode techniques). Following cessation of rapid drive the transmembrane potential shows an initial depolarization and a terminal hyperpolarization. Increasing the stimulation frequencies before the cessation of driving the initial depolarization and the terminal hyperpolarization are more accentuated. When the stimulation frequency is abruptly reduced, the enddiastolic ("resting") potential increases (steady state resting potential - 82 +/- 3 mV at 1.0 s, -73 +/- 6 mV at 0.25 s driving interval). Decreasing the rate of stimulation the duration of the steady state action potential shortens at 25% and prolongs at 90% repolarization. A calculated background conductivity is diminished using a slower drive. From the experimental findings ensue some considerations about a K+ accumulation, the rate dependent sodium pump and the rate related shifts of the steady state outward current-voltage relationships.

A single glass micropipette voltage clamp technique with intracellular dialysis was used to study the effects of the trapidil derivatives AR 12-456 and AR 12-463 on Ca channel currents carried by Ba2+ in isolated ventricular cells from mice hearts. Inspite of a more potent inhibition of the cAMP phosphodiesterase from heart (Bartel et al. 1985) a reversible Ca channel blocking action of both compounds could be observed. The concentration of half maximal block was calculated to about 50 mumol/l for both derivatives tested. Neither a shift in the current-voltage relationships nor a significant change in the potential for half maximal activation was found. The maximal Ba2+-conductance was reduced. The steady state inactivation was shifted towards more negative potentials by application of 100 mumol/l AR 12-463. The decay of the Ba currents was accelerated in the range of the applied test potentials between -20 and +20 mV. It is concluded that the new trapidil derivatives with more potent inhibitory action on cardiac phosphodiesterase than trapidil can block myocardial Ca channels.

The action of acetylcholine (ACh) and verapamil (VePa) on the action potential (V(t)), phase plane trajectories of V(t) (dV/dt--V(t) -- plot) and isotonic contractions were investigated using an isolated vegal innervated preparation from rabbit atrium (method I) and investigating action potentials from atrial trabeculae by means a modified sucrose gap technique (method II). If the VePa-concentration increases to 4 mg/1 the duration of the action potential decreases at 20 and 90% repolarization (driving frequencies 2 s-1). In the VePa-solutions phase plane trajectories of the action potential did not change significantly. ACh application favours the disappearance of a region in the repolarization phase plane plot showing anomalous rectification (d(--dV/dt)/dV less than 0) both by control conditions and verapamil. The electrotropic ACh-and vagal effects will be unchanged by verapamil. The inotropic ACh-and vegal action (method I) increases by VePa (2 mg/1). The action of ACh and verapamil will be analysed using a mathematical model for reconstructing the repolarization phase of mammalian atrial myocardium action potentials.

1. The blocking mechanism of a disopyramide derivative Penticainide (2-alkyl-(4-(dialkylamino)-2-)pyridyl-butyramide) on cardiac Na+ channels has been studied using single-channel analysis in cell-attached and inside-out patches from guinea-pig ventricular cells. Penticainide was applied in concentrations between 3 and 100 microM. The S-enantiomer of DPI 201-106 (5 microM) was used as a tool to slow the inactivation, improve the time resolution by prolonging the mean open time, and to increase the number of openings per depolarization of the channel. 2. When in cell-attached or inside-out patch experiments up to 100 microM-Penticainide was applied to the bathing solution no significant effect was observed on the probability of the channel being open or on the mean open time. 3. In cell-attached patch experiments with 100 microM-Penticainide in the pipette, the open-state probability of the Na+ channel was much lower than in the absence of Penticainide. No significant changes were found in the potential of half-maximum activation or in the slope of the activation curve. The maximum open-state probability was reduced by a factor five in the presence of Penticainide. The single-channel conductance was not affected by the drug. 4. The decrease in the probability of the channel being open was mainly due to an increased probability of observing sweeps with no activity ('nulls'). 5. A dramatic relief from the block was observed when pauses were interposed into the normal activation pattern, or when the pacing rate was reduced. 6. The distribution of the open times of the bursting Na+ channel could be fitted with two exponentials. Penticainide in the patch pipette reduced the mean open time. Also the contribution of the number of long openings to the total number of openings was reduced. 7. Closed-time distribution was also fitted with two exponentials. Penticainide in the patch pipette prolonged the long mean closed time. The contribution of the number of short closings to the total number of closings was decreased. 8. Penticainide in the patch pipette did not significantly change the time constant of the decay of the ensemble-averaged currents measured at -30 mV. Because of the close correlation between the burst duration and the time constant of inactivation (Nilius, Vereecke & Carmeliet, 1988b), we conclude that no striking effect of Penticainide on the burst duration can be expected.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca-mediated action potentials (CaAP) from guinea pig papillary muscle have been reconstructed by means of a computer simulation. The reconstruction model is based as closely as possible on ionic currents which have been measured by voltage clamp methods. Three currents were formulated in terms of Hodgkin-Huxley type equations: a Ca inward current, a time-independent K outward current ("rectifier" current), and a time dependent slowly activated K outward current ("delayed rectifier" current). The maximum upstroke velocity of CaAP was compared with the Ca inward current. It turned out that the discrepancy between Vmax and maximum Ca current can be tremendous. A non-linearity between Vmax and gCa has been described. Furthermore, a twice peaking of the calcium current during CaAP cannot be excluded. The existence of outward currents in nearly the same size as the Ca current even during the upstroke of CaAP seriously question the use of CaAP as a measure of calcium current properties.

Single voltage-activated Na+ channel currents were obtained from membrane patches of isolated ventricular cells of guinea pig hearts. The currents were compared when measured from cell-attached patches and from the same patch but at least 20 minutes after manual excision. The averaged currents showed a distinctly delayed decay in the excised patches due to the appearance of long lasting openings or bursts of openings. In contrast to control patches, the open time distribution in excised patches requires at least two exponentials. A short mean open time was voltage independent for cell-attached patches (0.38 ms +/- 0.07 ms between -60 and -20 mV, 6 cell-attached patches; and 0.41 +/- 0.1 ms, 7 excised patches). The long mean open time found in excised patches was clearly voltage dependent and increased from 0.48 +/- 0.14 ms (-80 mV) to 2.87 +/- 0.35 ms (-20 mV, regression coefficient +0.88, 7 patches). Sweeps with long openings appeared in clusters. The clustering of records with long openings, short openings, or without openings (nulls) was quantified by a runs analysis which showed a highly significant nonrandom ordering. The results show that in excised patches inactivation is temporally hibernating.

The modulation of Na channels by the vasoactive peptide angiotensin II (AT II) has been studied in isolated ventricular cells of guinea pigs using the patch clamp technique. In cell-attached patches the maximal probability of the channel being open was increased in a concentration range between 0.05 and 1 microM, but decreased at higher concentrations. A maximal increased of 2.5 +/- 0.86 was found at 1 microM AT II. The increase in the probability of the channel being open was due to a decrease in the number of nulls. In all affected cells (n = 17) we observed a delayed inactivation after application of AT II at concentrations between 0.05 and 10 microM. At -30 mV, the time constant of inactivation increased from 1.1 +/- 0.1 ms (controls) to 5.6 +/- 1.6 ms (10 microM AT II). This effect was due to an increased number of openings per sweeps. No significant effect on the mean open time and the first latency were observed. However, due to pronounced bursting, the averaged closed time was significantly increased from 0.8 +/- 0.1 ms to 1.3 +/- 0.1 ms in the presence of 1 microM AT II at -30 mV. An effect of AT II on cardiac Na channels via protein kinase C is discussed.

Inactivation of sodium currents is investigated in single myocardial mouse cells by the use of a one suction pipette voltage clamp technique. Semilogarithmic plots of the decay phase of sodium currents show two phases and analysis yields a fast (tau h1) and a slow (tau h2) time constant. At increased depolarization the contribution of tau h2 is decreased. Increasing the temperature from 22.0 +/- 0.5 degrees C to 36.5 +/- 0.5 degrees C influences tau h1 and tau h2 to the same extent. The Q10 values were 2.28 +/- 0.32 for tau h1 and 2.46 +/- 0.35 (mean +/- SD) for tau h2. Between -50 and -20 mV the time course of current decay is substantially faster than inactivation induced by prepulses. Onset of inactivation by the prepulse protocol needs two time constants (tau c1, tau c2) for a satisfactory description. Both decrease steeply with increasing depolarization. For small depolarizations, tau c2 values are in the range of several seconds. Recovery from inactivation by short prepulses (40 ms) could be described by two exponentials (tau r1, tau r2). For longer prepulses (1,000 ms) a very slow component, tau r3, was observed indicating a history dependence of inactivation. Delay time constants for the onset of inactivation (tau dc) are determined between -50 and -20 mV. The more depolarizing voltages generate smaller delay times (0.55 +/- 0.10 ms at -20 mV, mean +/- SD) but larger deviations from the exponential time course. Delay in recovery from inactivation (time constant tau dr) has the value 2.0 +/- 0.7 ms (mean +/- SD) at -80 mV and decreases at more hyperpolarizing potentials. The remaining series resistance at maximum compensation was calculated as 80 k omega. Its influence on the sensitive delay in the double pulse inactivation is discussed.

Using a conventional microelectrode technique premature action potentials of left auricular trabeculae of the rabbit were studied. Varying the delay between a regular stimulus and the following premature stimulus the duration of the premature action potential (A. P.) is decreased during the relative refractory period and increased after this time at each level of the repolarization. Verapamil suppresses completely this augmentation of the A. P. duration. Turning to account the A. P. measurements an estimation of a potassium conductivity gK yields an enlargement of gK during the premature A. P. elevating the Ca+ concentration or prolonging the steady state pacing interval the increase of gK is more accentuated. Low Ca++ solutions and verapamil suppress the augmentation of gK during the premature A.P. lead to a membrane current responsible for the premature changes of the A. P. This current is inward directed at potentials more positive than --30 mV and outward directed at more negative potentials. By application of high-calcium solutions and in slow pacing both the inward and outward component are increased. The findings suggest an augmented Ca++ inward current during the premature action related to the postextrasystolic potentiation.

Properties of cardiac Ca channels have come into sharper focus with the advent of single cell preparations and suction pipette recording methods. We briefly summarize our present picture of the gating and permeation properties of the conventional, dihydropyridine-sensitive type of Ca channel (L-type). Distinctive features of a second type of voltage-gated Ca channel (T-type) are discussed.

A numerical method is described suitable to reconstruct the repolarization phase of the action potential measured from the mammalian atrial myocardium. The method allows an estimation of the current-voltage relationship of a time-independent potassium outward current and a voltage- and time-dependent calcium current, an inactivation term of the calcium current and an estimation of the calcium and potassium amounts transported during the action potential. The numerical evaluation needs some pairs of voltage-time values obtained from the measured action potentials and the first time derivative of the action potential. An example of the method's usefulness is demonstrated. The described method is suitable to reconstruct the late repolarization phase of the atrial action potentials sufficiently.

Repetitive activity of the isolated rabbit atrium was induced by trains of premature stimuli. Analyzing the interval-histograms of the recorded atrial action potentials a quantitative description of this arrhythmia is demonstrated. An atrial flutter or fibrillation is evoked 110 to 140 ms after a regular driving pulse. Vagal stimulation increases the electrical vulnerability. The degree of arrhythmia was found to be dependent on the frequency of stimulation. During increased vagal activity premature stimuli applied 40 to 190 ms after a regular driving pulse generate arrhythmias. By vagal stimulation a preexisting arrhythmia is converted into an atrial fibrillation. In spite of a constant pacing, the atrial arrhythmia disappears at atrial masses of below 55 mg if initiated by early coupled beats. During an application of acetylcholine this critical mass is reduced.

1. The kinetic properties of T-type Ca2+ channels were examined in single ventricular cells from guinea-pig hearts using the cell-attached configuration of the patch-clamp technique. 2. T-type Ca2+ channel activity has been observed in 44 out of 139 patches. The density of these channels was estimated at 0.1-0.3 micron-2. The T-type Ca2+ channel responds to a depolarizing voltage step either with a burst of openings which appears with a distinct delay or with no openings at all. The mean number of bursts per record for the records showing channel activity is 1.1. The probability of observing a blank sweep is high and amounts to 0.65 +/- 0.02 (n = 26). 3. With 110 mM-Ca2+ in the pipette solution, the slope conductance calculated from the current-voltage relationship of the single-channel current in the range between -50 and +10 mV is 6.8 pS. 4. Openings to a subconductance level of about 50% of the main level could be resolved. All possible transitions between the subconductance and the main level were observed, indicating that the cardiac T-type Ca2+ channel possesses a substate. 5. The macroscopic steady-state activation and inactivation, as determined from ensemble-averaged currents, could be described by Boltzmann functions. Half-maximal activation and inactivation occur at -14 and -60.7 mV, the slope parameters of these curves are 10.8 and 5.6 mV respectively. The maximum (peak) open probability is 0.15. 6. The ensemble-averaged current decays monoexponentially. The time constant is strongly voltage dependent and decreases at less negative potentials. 7. The open times are monoexponentially distributed. The mean open time of the channel does not depend on either the holding or the test potential, and has a mean value of 1.4 ms. The distribution of the closed times is biexponential. The fast mean closed time is also voltage independent with a mean value of 0.48 ms. The slow mean closed time increases with voltage from 1.9 ms at -40 mV to 8.8 ms at 0 mV. The mean burst duration also increases with voltage from a value of 4.9 ms at -40 mV to 13.9 ms at -10 mV. 8. The convolution of the first-latency distribution with that of the burst duration closely fits the open probability calculated from the ensemble-averaged current. The mean first latency is also closely correlated with the macroscopic time constant of inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical effects of the new antiarrhythmic compound Bonnecor (AWD 19-166, GS 015) were studied under conditions known to increase the intracellular Ca2+ activity (extracellular K+-deprivation, application of glycosides). In K+ free solutions right ventricular papillary muscles showed an extreme acceleration of the initial repolarization indicating an intracellular Ca overload. AWD 19-166 could not prevent the initial shortening of action potentials and seems to be less effective in K+ free than in K+ containing solutions. In partially depolarized (22 mmol/lK+) right ventricular papillary muscles oscillatory afterdepolarizations were not suppressed by AWD 19-166. However, the compound prevented the generation of irregular action potentials triggered by the afterdepolarizations.

Effects of the proteolytic enzyme thrombin in the modulation of cardiac Ca-channel currents were examined in single ventricular cells from frog myocardium, using the whole-cell voltage clamp technique (Hamill et al. 1981). Application of 3.8.10(-9) M thrombin to the bath increased the peak of the Ca-channel current by 84 +/- 35% (8 cells). Hirudin (31.10(-9) M), a specific thrombin inhibitor, blocked the thrombin-induced increase of this current. The increase in the current can be made responsible for the measured positive inotropic effects of thrombin as well as for the shift of the plateau voltage of the action potentials towards more positive values.

The effects of cycle length (CL) alteration on action potentials of isolated trabeculae of the common right atrial working myocardium (M) and isolated preparation of the crista terminalis (CT) of the rabbit heart were studied. Prolonging the CL increased the steady state duration of action potentials in M but decreased it in CT fibres. The repolarization of the post-pause action potentials of the M preparations was characterized by (i) a rapid initial repolarization followed by (ii) a plateau phase and (iii) a retarded terminal repolarization. In CT fibres the retarded repolarization was not observed and the duration of the post-pause action potentials at 90% repolarization is decreased. Premature action potentials of the M preparations were shortened at 90%, but were prolonged at 25% repolarization. Premature action potentials of the CT fibres were prolonged. The post-drive resting transmembrane potential of the M preparations moved to more negative values depending on the CL stimulation. The shift of the post-drive transmembrane resting potential to negative values in CT preparations was preceded by an initial depolarization. The initial depolarization was absent in M preparations. Myofibril-poor pale cells were found consistently in CT, but never in M preparations.

Afterpotentials following a train of paced action potentials in isolated pectinate muscles bordering the crista terminalis in the rabbit atrial myocardium were analyzed. Electrical measurements were performed using a standard microelectrode technique. An ultrastructural stereological analysis was undertaken to study intercellular clefts within the preparations used. After cessation of a rapid drive, afterpotentials showed an initial depolarization followed by a delayed hyperpolarization. The initial depolarization could be blocked by verapamil (13.2 microM) leaving a monotonous hyperpolarization. This hyperpolarization was modelled supposing a K+-depletion out of morphological identified intercellular clefts. From best approximation of the measured after-hyperpolarization (1) the internal K+ activity was estimated to be in the range of 82 to 90 mM depending on the pacing interval Io. It decreased due to shortening of Io; (2) the accumulated "cleft" K+ activity was estimated to be about 2 mM at a pre-drive pacing interval of 250 ms; it decreased by increasing of Io, (3) decreasing of Io accelerated the K+-depletion, (4) a single beat accumulation of about 0.28 mM was estimated. The morphologically measured geometry of the intercellular clefts (mean cleft width 0.5 micron, ratio of cleft and cell volume 0.09 +/- 0.03) agreed well with the electrophysiological findings.

Long-time vagal stimulation induces a decrease of electrotropic and inotropic effects on the isolated rabbit atrium in spite of constant stimulation frequency. This decrease is defined as effect adaptation. To compensate this effect adaptation, the n. vagus was stimulated with increasing stimulation frequency. Compensation is possible for a certain period referred to as control time. The adaptation of the electrotropic effects is more delayed and weaker than the inotropic effect adaptation. The control times of the electrotropic effect are longer than those of the inotropic effect. Acetylcholine perfusion without vagal stimulation shows the same results. It was shown that the effect adaptaion does not result from depletion of acetylcholine stores or from co-stimulation of sympathetic nerve fibres. The effect adaptation is discussed as being the result of machano-electrical coupling.

We used the patch clamp technique to record unitary calcium (Ca2+) channel activity in freshly dissociated ventricular myocytes from adult guinea pigs. We found two types of Ca2+ channels with distinct permeation and gating properties and different sensitivity to pharmacological agents. One channel (T-type) requires negative membrane potentials to remove inactivation. It gives rise to a transient mean current and is not affected by dihydropyridines or isoproterenol. The other Ca2+ channel (L-type) has a larger unitary barium-conductance, activates at more positive potentials and its averaged current decays much more slowly. It shows a distinct gating pattern with different gating modes, the proportion of which is drastically altered by dihydropyridine Ca2+-channel agonists and antagonists. L-type channel activity is modulated by beta-adrenergic stimulation by a mechanism of action which differs from that of dihydropyridine Ca2+-channel agonists.

Single ventricular myocytes of adult mice were prepared by enzymatic dissociation for voltage clamp experiments with the one suction pipette dialysis method. After blocking the Na current by 10(-4) mol/l TTX early outward currents (IEO) with incomplete inactivation could be elicited by clamping from -50 mV to test potentials (VT) positive to -30 mV. Interfering Ca currents were very small (less than 0.6 nA at VT = 0 mV). The approximation of IEO by the q4r-model showed a pronounced decrease in the time constant of activation (tau q) to more positive potentials. At 50 ms test pulses the time course of the incomplete inactivation could be described by two exponentials and a constant. The time constant of the fast exponential (tau r1) showed a slight decline towards more positive test potentials (8.1 +/- 1.0 ms at -10 mV; 5.8 +/- 1.2 ms at +50 mV, mean +/- SD, n = 5) whereas the time constant of the slow exponential (tau r2) was voltage independent (41.1 +/- 7.9 ms, mean +/- SD, n = 5). The contributions of the fast exponential and the pedestal increased towards positive test potentials. The Q10 value for the time constants of activation and fast inactivation was 2.36 +/- 0.19 and 2.51 +/- 0.09 (mean +/- SD, n = 3), respectively. After an initial delay the recovery of IEO at a recovery potential of -50 mV could be fitted monoexponentially with a time constant of 16.3 +/- 2.9 ms (mean +/- SD, n = 3). The time course of the onset of inactivation determined with the double pulse protocol was slower than the decay at the same potential, and could be described as sum of a fast (tau = 18.4 +/- 6.0 ms) and a slow (tau = 62.1 +/- 19.9ms, mean +/- SD, n = 3) exponential. IEO could be blocked completely by 1 mmol/l 4-aminopyridine at potentials up to +20 mV. Stronger depolarizations had an unblocking effect.

A new antiarrhythmic compound (Bonnecor AWD 19-166, GS 015) was studied in its effects on the fast sodium channel in two types of cardiac preparations (rabbit ventricular myocardium, frog atrial trabecules). AWD 19-166 depressed the maximum upstroke velocity of the action potentials in ventricular myocardium (50% inhibition at 5 X 10(-6) mol/l) and decreased the fast inward current in frog atrial myocardium. High frequent trains of pacing pulses produced a pacing dependent extra block of both the maximal upstroke velocity and the fast inward current. It is concluded that AWD 19-166 blocks the fast sodium channel in a use dependent manner.

An isolated rabbit atrium, innervated by vagal nerve, was tested. Increased Mn++ concentrations decrease the areas of the action potentials and the amplitudes of contractions. At 2 mM Mn++ the contraction disappears and an extremely sharp action potential is recorded. By variation of both Ca++ and Mn++ concentrations one can observe parallely shifted concentration-response curves for the amplitudes of the contractions. This result is described by the model of a competitive-antagonistic Mn++-Ca++ action. At 1 mM Mn++ the inotropic and electrotropic effects disappear. Small concentrations of Mn++ (0.15 mM) increase the electrotropic vagal effects. At 0.5 mM Mn++ both vagal effects are diminished. The discussion of the results is based on the action of acetylcholine on the slow inward current of calcium and sodium ions.

Currents through single Na channels of ventricular cells from adult mouse and guinea pig hearts were studied using the patch clamp technique. Under control conditions the majority of openings is brief and a clustering at the beginning of the depolarizing pulse can be observed. Only between 1 and 8% of the sweeps show long lasting bursts. The bursting may account for a second slow phase of decay of macroscopic currents. In the presence of aconitine in the pipette up to 80% of the sweeps showed bursts underlying the slow inactivation of aconitine modified macroscopic currents. The mean open times are unchanged but the number of openings per sweep is dramatically increased due to aconitine. It is discussed that Na channels may function in different "modes". One mode is characterized by a fast transition into an absorbing state the other one by frequent reopenings ("bursts"). aconitine favours the "bursting" mode.