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Single Na channel currents were studied in cell-attached patches from guinea-pig ventricular myocytes in the presence of DPI 201-106. In addition to openings with a single channel conductance of 15 pS, two other types of events occur that are characterized by a conductance of 5 and 8 pS. The low conductance state is less sensitive to block by TTX than the higher conductance states. The medium conductance state has a reversal potential which is similar to the high conductance channel. It is argued that the low conductance channel is a second type of Na channel, whereas the medium conductance channel appears to be a low conductance substate of the normal Na channels.

Effects of the plant alkaloid Aconitine on the kinetics of sodium channels were studied in enzymatically isolated single cells of the mouse ventricular myocardium. Aconitine (1 mumol/l) induced a prolongation of the 90% repolarization of action potentials from 52.4 +/- 3.7 ms to 217.0 +/- 12.5 ms. Delayed terminal repolarization and oscillatory afterpotentials preceded spontaneous activity with high frequencies. Peak sodium currents were diminished from 28.0 +/- 9.0 to 14.0 +/- 6.0 nA. The reversal potential of the sodium current was shifted from 16.0 +/- 11.0 to -8.0 +/- 6.0 mV (52.5 mmol/l extracellular sodium concentration) suggesting a decreased selectivity of the Aconitine-modified Na channels. The m-affinity-curves were shifted 31 mV towards more negative potentials at a constant slope. The h affinity-curves were shifted in the same direction by 13 mV. The slope parameter of the h affinity-voltage relationship was enlarged from 9.1 +/- 2.2 mV to 15.6 +/- 4.4 mV. Shifts in m affinity and h affinity resulted in an increased "window". The alkaloid modified channels inactivated extremely slowly at potentials negative to -40 mV, but showed a fast and complete inactivation at potentials positive to -40 mV.

1. External Ca2+ block of Na+ channels was studied by a gigaohm-seal patch clamp technique in single cardiac ventricular cells from guinea-pig. Single-channel currents were recorded from cell-attached patches. 2. Increasing external Ca2+ concentrations in the patch pipette from 0.1 to 20 mM reduced the single-channel conductance of normal Na+ channels from 27 to 14 pS without causing flickering (obtained from linear regression, eight patches). 3. Exposed to external Ca2+ concentrations of 20 mM, the single-channel currents decreased at potentials negative to -60 mV in spite of an increased driving force for inward Na+ currents. 4. An external concentration of 35 mM-Mg2+, which is supposed to exert a screening of surface charges nearly equal to that of 20 mM-Ca2+ (Hille, Woodhull & Shapiro, 1975), reduced the single-Na+-channel conductance only from 26 (1 mM-Mg2+) to 20 pS (linear regression, eight patches). A weaker voltage-dependent block at potentials negative to -50 mV was observed in 35 mM-Mg2+ than in 20 mM-Ca2+. Therefore, surface charge effects cannot explain the obvious reduction of the conductance of single Na+ channels found when the external Ca2+ concentration was increased. 5. Single Na+-channel currents increased with an increase in the external Na+ concentration [( Na+]o) but showed saturation. The Na+o-single-channel current relationship could be described by i = imax/(1 + kd/[Na+]o) with imax = 5.4 pA and kd = 359 mM (seventeen patches). 6. The mean open time of Na+ channels varied between 0.18 and 0.59 ms (potentials between -80 and -20 mV). No significant changes in the mean open time could be obtained when Ca2+ was varied between 0.1 and 20 mM. 7. The piperazinylindole compound DPI 201-106 was used as a tool to prolong the open time of single Na+ channels. If the external Ca2+ concentration was increased from 0.1 to 20 mM the currents through the modified channels were reduced. The reduction of single-channel currents was accentuated at potentials negative to -60 mV (20 mM-Ca2+) similar to the control channels. 8. In contrast to non-modified Na+ channels, the mean open time of DPI 201-106-modified channels proved extremely voltage and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Cs+ (15-20 mM) decreases the electrotropic vagal effects on an isolated vagal innervated rabbit atrium. By means of investigating the action potentials and the phase plane trajectories of trabeculae from the rabbit atrium using a modified single sucrose gap technique the anomalous rectification disappears and also the effect of acetylcholine (ACh) on action potential duration. We presume that the anomalous rectification should be a necessary condition of electrotropic vagal (ACh) action on the rabbit atrium.

Whole cell clamp experiments and single channel measurements were used to study whether the T-type Ca channel in guinea-pig ventricular myocytes is sensitive to beta-adrenergic stimulation. In whole cell clamp experiments we determined the T-type Ca current by measuring the difference between the currents at a test potential of -30 mV in Na-free solution, starting from a holding potential either at -90 or at -50 mV. Application of 1 microM isoproterenol (ISO) did not change the difference current, while the L-type Ca current was enhanced during application of the drug. In cell attached patch experiments with 110 mM CaCl2 in the pipette solution, single channel currents were observed which were due to opening of the T-type Ca channel. Application of 4 microM ISO to the bath solution did neither change the appearance of the channel openings, nor the ensemble averaged currents. It is concluded that the T-type Ca channel in guinea-pig ventricular cells is insensitive to ISO.

Single channel currents flowing through Na channels were studied in cell-attached patches from guinea-pig ventricular myocytes after removal of the fast component of inactivation by the S enantiomer of DPI 201-106. In addition to openings with a single channel conductance of 15 pS, two other types of events occur. A low conductance state has a single channel conductance of 5 pS, while a medium conductance state can sometimes be seen at 8 pS. These different conductance states can coexist in the same patch. The low conductance state appears to have the same kinetic properties as the high conductance state. However it is less sensitive to block by TTX. Averaged currents obtained from exclusively low conductance openings also show a decay (time constant at -30 mV of 0.8 +/- 0.4 s) which is not significantly different from the decay of the high conductance channel. The average open and closed times are also similar to the values for the high conductance channel. The medium conductance state has a single channel conductance of 8 pS and a reversal potential which is similar to the high conductance channel. Closings of the high conductance state to the medium conductance state can be observed as well as openings from the medium to the high conductance state. The average current of sweeps showing only the medium conductance state has the same time course and shows a similar TTX-sensitivity as the average current of the high conductance state. Also the voltage range for steady-state activation and inactivation for the medium conductance channel matches that of the high conductance channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Laser diffraction patterns were investigated from enzymatically isolated, unattached myocardial cells of guinea-pigs and mice. Experiments were performed at 2.5 mM Ca2+ and room temperature. The mean sarcomere length of resting guinea-pig and mouse myocardial cells amounted to about 1.83 micron and 1.75 micron, respectively. When paced with alternating intervals by field stimulation carefully selected ventricular cells showed transient phenomena. (1) The staircase following a rested state contraction was positive in the case of guinea-pig and negative in the case of mouse myocardial cells; (2) The rested state as compared to the steady state sarcomere shortening of guinea-pig and mouse cardiac myocytes amounted to 35% and 600%, respectively; (3) The interval strength curve of guinea-pig myocardial cells passed through a maximum which was 0.26 +/- 0.06 micron (mean +/- S.D.) at a pacing interval of 2 s whereas myocardial cells of mice showed a rise of shortening with increasing intervals reaching a maximum at the rested state (0.24 +/- 0.08 micron). Results were similar to those obtained from multicellular preparations. We conclude therefore, dynamic properties of multicellular preparations are nicely reflected at the sarcomere level.

Using a microelectrode technique the influence of experimental conditions which are known to alter the intracellular free Ca concentration upon the repolarization phase in the rabbit atrial myocardium was studied. Under all the tested conditions (increased external Ca concentration, application of ouabain, decreased external K concentration) a positive inotropic effect was observed. The action potentials showed a rapid initial repolarization. The positive inotropic effect was found to be well correlated with the effect of the action potential's shortening at 25% repolarization. After pacing pauses a positive correlation between the velocity of the initial rapid repolarization and the potentiation of the atrial contraction was established. It is discussed that all the conditions were able to increase the internal Ca concentration and that in consequence of it a CA-sensitive outward current at potentials about 40 mV more positive than the resting potential is activated.

Investigated are rabbit atrial trabeculae (2-4 mm length, 150-800 micrometers diameter). When using a standard microelectrode technique, the action potentials after an intracellular 4-AP and TEA load are prolonged. Only the TEA load significantly diminishes the resting potential of the beating preparations. The typical configuration of the action potential following a period of rest (rapid initial repolarization, secondary depolarization and a late retarded repolarization) is not changed qualitatively. During a repetitive stimulation after a pacing pause the action potentials' restitution is accelerated at 4-AP. After 4-AP and TEA load the secondary depolarization of the first post rest action potential is more accentuated. By reconstructing the post rest action potentials a 4-AP sensitive current is determined. At constant drive this current shows a threshold potential at -40 and a reversal potential at -10 mV. The current is inward directed between -40 and -10 mV and is outward directed at more positive potentials than -10 mV. After a pacing pause the current-voltage relation is shifted to more negative potentials. The 4-AP sensitive current is modelled by consideration of a slow activated (during the action potential) and deactivated (during the pacing pause) conductivity. Processes of accumulation are discussed.

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 novel dihydropyridine compound BAY-K 8644 on Ca-dependent slow action potentials in guinea pig papillary muscles were studied. The maximum upstroke velocity Umax of slow action potentials (sAP) increased in concentrations between 2 X 10(-9) and 2 X 10(-7) mol/1 BAY-K whereas at higher concentrations Umax decreased again. Half maximum effects were obtained at 2.5 X 10(-8) mol/l (20 mol/L K+). The increase in Umax could be antagonized by nifedipine in a similar range of concentrations. The duration of sAP was changed depending on the pacing interval. At short pacing intervals (up to 2 s) sAP were abbreviated by BAY-K. BAY-K also induced a small hyperpolarization in 20 mmol/l K+ and lowered the threshold voltage distinctly. Washing out of BAY-K resulted always in an overshooting lengthening of sAP. BAY-K strongly increased Umax at 40 mmol/l K+. The relationship between Umax and the membrane potential was shifted to increased Umax values. At concentrations higher than 6 X 10(-7) mol/l BAY-K produced oscillatory afterdepolarizations indicating a drug-induced Ca overload. All the experimental findings are consistent with the hypothesis that the novel dihydropyridine compound BAY-K 8644 activates Ca-channels in the mammalian ventricular myocardium.

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.

Potentiation of right-ventricular strips obtained from normal rats (NT-group) and from rats showing hypertrophic hearts as a result of long-time swimming (HT-group) has been investigated. Strength-interval relationships, poststimulation potentiation and the potentiation by paired stimulation has been tested. The behavior of the action potential in both groups showed no significant differences. Standardized strength-interval relationships of HT-group were steeper than those of the NT-group. Maximal potentiation in the HT-group occurred at shorter delay-intervals. Potentiation after long-time pauses was greater in the HT-group. The results are discussed on the basis of model considerations of electro-mechanical coupling.

Electrical properties of frog atrial myocardium under Ca-free conditions (30 min superfusion with 1 mM EGTA) were examined using a microelectrode and voltage clamp technique. Action potentials recorded from atrial trabeculae were extremely prolonged, up to more than 2 s. The resting potential was shifted towards more positive values (-60 +/- 9 mV (Ca-free); -75 +/- 4 mV (control); n = 6). Slow inward currents inactivated extremely slow showing time constants between 2 and 3 s. The voltage-dependence of the time constant of inactivation was found to be U-shaped in Ca-containing but bell-shaped in Ca-free solutions. After a 30 min Ca-free superfusion, the surfaces of the atrial trabeculae studied by scanning electron microscopy changes from being rough to extremely smooth, supposedly indicating a change in the surface coat. Using electron-microprobe analysis within a surface layer of about 0.2 micron thickness direct measurements of the Ca concentration showed a reduction under Ca-free conditions by more than 200-fold. Electrophysiological measurements, morphological findings and measurements of the Ca concentration in the surface region directly suggest the existence of a Ca-sensitive inactivation of slow channels in frog atrial myocardium.

Action potentials of constantly driven isolated trabeculae of the rabbit's atrial myocardium, being depressed by acetylcholine (ACh) at concentrations between 0.2 and 20 mumol/l, recovered during a constant drug application. The sensitivity of the atrial myocardium to ACh after a prolonged drug application was decreased but found to be restored about 7 min after switching off the ACh superfusion. The recovery during drug application was accelerated in Ca++ excess solutions but decelerated in K+ excess solutions. All the effects of ACh were blocked by atropine (1 mumol/l) completely. The results suggest that the muscarinic ACh receptor in the mammalian myocardium, responsible for the electrotropic effects, shows desensitization to ACh similar to the nicotinic receptor in other tissues.

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.

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 (1). 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 on frog heart of thrombin.

Morphologically intact single myocardial cells of the adult mouse show a length of 132 +/- 20 microns, a width of 21 +/- 5 microns, and a height of 10 +/- 4 microns (all mean +/- SD) and are brick-like in shape. A one suction pipette method is used for voltage clamp of those single cells. The determined time constant of capacitive current tau = 35 +/- 14 microseconds is very short. Series resistance rs, membrane resistance rm, and membrane capacity Cm are calculated to be 192 +/- 48 k omega, 6.1 +/- 1.1 M omega, and 186 +/- 92 pF (all mean +/- SD), respectively. Assuming the specific unit membrane capacitance of 1 microF/cm2, a total membrane area of 1.86 X 10(-4) cm2 is determined yielding a specific membrane resistance Rm of 1,134 omega cm2. Settling time of voltage clamp is 30 microseconds. TTX-block of sodium current is described by 1:1 binding with a KD value of 1.4 X 10(-6) M. Using a reduced extracellular sodium concentration the maximum Na current is between 25 and 40 nA at voltages between -40 and -30 mV. Currents of between +20 and +30 mV reverse in an outward direction. Inward currents are approximated by a m3h model. The time constant of activation decreases from 0.7 ms at -60 mV to 0.12 ms at +20 mV. The time constant of inactivation falls from 9.1 ms at -60 mV to 0.6 ms at + 20 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The dependence of contraction in the atrial myocardium on the stimulation pattern was studied in normoxic and hypoxic conditions. Isotonic contractions were measured from left atrial muscle strips of the rabbit. At rapid stimulation hypoxia produced a negative inotropic effect which disappeared at slower stimulation and became a positive inotropic one. The potentiation of a first beat after a pause in stimulation was enhanced by hypoxia because the contractions were stronger depressed at steady state than after a pause. At all pacing patterns the relative maximum rate of shortening and re-lengthening were independent of the oxygen-supply. After application of epinephrine in hypoxia they were enhanced and so was peak shortening. The findings suggest an impaired or altered function of the sarcolemma only under the degrees of hypoxia chosen.

A four compartments model describes the electrotropic and the inotropic vagal effect time courses during long time stimulation. The model realises electro-mechanical coupling. A differential equation for the coupling model is developed, solutions are given and a numerical treatment is described. The model shows the essential properties of effect adaptation: time delay and smaller extent of electrotropic effect adaptation.

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.

Action potentials were recorded from isolated atrial trabeculae of the rabbit using a standard microelectrode technique and analyzed by a phase-plane technique. Muscarinic effects of acetylcholine (ACh) were studied in verapamil-containing solutions (13.2 mumol/l). The shortening of the action potentials by ACh was accentuated in high K+ solutions but depressed in Ca++ excess solutions. ACh-induced conductance was found to be concentration-dependent. From the concentration conductance plots a Hill coefficient of 1 was obtained under all the conditions tested. The maximum conductance calculated from the concentration-conductance plots varied between 11.3 +/- 2.7 microS/cm2 (at 7.5 mmol/l Ca++, 2.7 mmol/l K+ in the external solution) and 46.4 +/- 5.0 microS/cm2 (at 0.1 mmol/l Ca++, 8.1 mmol/l K+). All the effects on the ACh-induced conductance could be described by the reaction scheme for the nicotinic ACh action. From both the experimental findings and the approximation of the measurements using the Katz-Miledi model it is suggested that an increase in the external K+ concentration increased, but an external Ca++ excess decreased, the conductance of the ACh-induced channel.

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.

At spontaneously beating rabbit atria in vitro longtime pauses of contraction were produced by vagal stimulation. After finishing the stimulation at once contractions took place; the first of them was potentiated. During the vagal pauses various beat interval programs were realized. After removing the sinusoidal node the same programs were repeated by variation of drive interval length. Their effects were compared to the effects in vagal pauses. All these effects were simulated by a two stores three phases model. This model allows description of potentiation and staircase phenomena as well as inotropic vagal effect and inotropic vagal escape. It suggests a connection between this two kinds of phenomena.

Radial glial (Müller) cells were isolated from rabbit retinae by papaine and mechanical dissociation. Regional membrane properties of these cells were studied by using the patch-clamp technique. In the course of our experiments, we found three distinct types of large K+ conducting channels. The vitread process membrane was dominated by high conductance inwardly rectifying (HCR) channels which carried, in the open state, inward currents along a conductance of about 105 pS (symmetrical solutions with 140 mM K+) but almost no outward currents. In the membrane of the soma and the proximal distal process, we found low conductance inwardly rectifying (LCR) channels which had an open state-conductance of about 60 pS and showed rather weak rectification. The endfoot membrane, on the other hand, was found to contain non-rectifying very high conductance (VHC) channels with an open state-conductance of about 360 pS (same solutions). These results suggest that mammalian Müller cells express regional membrane specializations which are optimized to carry spatial buffering currents of excess K+ ions.

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.

A single sucrose gap techniques has been used to study action potentials and phase plane trajectories of them in atrial trabeculae of the rabbit. Using polynomial representations of current-voltage relationships a model of membrane action potential of atrial myocardial fibres is described and allows an interpretation of recording data from the phase plane trajectories. Our findings show: 1. Increasing extracellular calcium concentration increases a potassium conductivity of the atrial membrane. 2. An anomalous rectification concerning repolarizing currents in atrial fibres decreases with increasing extracellular calcium. 3. Acetylcholine (3.10(-4) g.cm-3) abolishes the anomalous rectification. These results are discussed in relation to previous electrophysiological studies of negative electrotropic effects of acetylcholine in cardiac muscle.

To evaluate a Ca-sensitive component of the transient outward current, we studied action potentials, isometric tension, and membrane currents (single sucrose gap voltage clamp) of rabbit ventricular heart muscle in the presence of Ryanodine that selectively blocks the intracellular Ca release. Ryanodine (1 microM) prolonged the post rest (10 s) action potential, abolished the "notch" preceding the plateau phase, and depressed the isometric tension. With the same protocol a diminished transient outward current was measured. Application of 1 mM 4-aminopyridine in the presence of Ryanodine further prolonged the duration of the action potential, decreased the transient outward current but increased the maximal tension. It is concluded that in rabbit ventricular myocardium both a Ca-dependent component and a 4-aminopyridine sensitive component compose the transient outward current; however, the 4-aminopyridine sensitive component is dominating.

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.

Isolated left rabbit atria were tested. Preparations are vagal innervated and driven by a constant frequency. Isovolumetric contractions, volumes of the auricles and intraatrial pressures were recorded. With increasing intraatrial pressure the effectivity of peripheral nerve-ending stimulation decreases. There exists a critical value of atrial area. The vagal effectivity will decrease rapidly, if the atrial surface increases higher than this critical value by stretching the auricles.

The inotropic effects of the new antiarrhythmic compound AWD 19-166 have been studied at constantly driven isolated left atria of guinea pig (weight between 250 and 350 g). AWD 19-166 exerted dose-dependently negative inotropic effects. Half maximum effects were measured at 1.7 mg/l (n = 7). At concentrations higher than 5 mg/l the negative inotropic effects were found to be irreversible.

A mathematical model of pre- and postganglionic parasympathetic nerve fiber excitation transfer is developed. This model gives a measure Q of acetylcholine (ACh) release from presynaptic preganglionic boutons and postganglionic varicosities. When increasing Ca++ the measure Q increases too. Na-ions exert a competitive inhibition. The relationship between Q and the Ca/Na2-quotient is a hyperbolic one. Mn++ inhibits the release of ACh non-competitively. Q increases both by excess potassium and Cs+ depolarization. The ACh release is diminished by Mg++. Ba++ cannot replace the effect of Ca++ on ACh release in Ca++ depleting conditions. Q increases with decreasing pH-level. The ACh release is not significantly influenced by increasing pH, Verapamil (4 mg/l), prostaglandins E2 and F2alpha (20 ng/ml) and substitution of nonpermeable anions for Cl-.

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.

Müller (glial) cells were isolated from rabbit retinae by papaine and mechanical dissociation. In a special perfusion chamber, the cells were penetrated with a recording electrode. When high-K+ solutions were applied into the environment of the cells by means of a second micropipette, the cell membrane depolarized strongly. During prolonged application of high-K+ solutions, however, there occurred a marked repolarization, and after cessation of high-K+ application, a strong hyperpolarization was observed. Both effects disappeared under the influence of ouabain, suggesting the accumulation of intracellular K+ by an active membrane pump. The data were used for calculation of the membrane's Na+:K+ permeability ratio, the intracellular K+ concentration, the pump rate and the mean pump site density. The calculated values are in good agreement with published data from mammalian astrocytes and are compared with those from amphibian Müller cells.