H. Tritthart

Calcium-movement controlling cardiac contractility. II. Analog computation of cardiac excitation-contraction coupling on the basis of calcium kinetics in a multi-compartment model ☆ ☆☆

Abstract A computer model for excitation-contraction coupling in mammalian cardiac cells was designed based on calcium movements in a multicompartment system. The model includes an extracellular space of constant calcium concentration from which calcium crosses the membrane and reaches the myoplasma by a voltage and time dependent process. The free myoplasmic calcium content is considered to reflect the intensity of the active state. Calcium can be taken up either by a carrier-sustained two step reaction into a compartment representing some parts of the longitudinal system or into another pool representing mitochondria. Calcium can leave the cell or can be released again into the myoplasma by a second order reaction. The behaviour of the model is compared to that of living cardiac ventricular muscle. The model correctly predicts the following groups of inotropic phenomena. Steady state and dynamic force-frequency relationships, positive and negative staircases after both changes of frequency and AP-duration, mechanical transients after a period of rest and aftercontractions.

Right ventricular hypertrophy in the cat--an electrophysiological and anatomical study.

Abstract In right ventricular hypertrophy induced by pulmonary artery coarctation the transmembrane electrical activity of papillary muscles and trabeculae has been studied and correlated to anatomic and electrocardiographic findings. The results of an experimental group with mild hypertrophy (group B, 25 days of exposure, 10% increase of free wall/body weight ratio) and severe hypertrophy (group C, 35 days of exposure, 23% increase) were compared with those of the control group. The average cell volume increased significantly and width/length ratio was augmented by an expansion of cell diameter. There was no obvious dilatation of the right ventricle and no increase in sarcomere length. Connective tissue increased in the subendocardial muscle layers of the right ventricle. The resting potential was augmented in both groups of hypertrophy; the overshoot of the action potential was unchanged in group B and increased in C. The rate of rise of the action potential was progressively reduced with increasing degree of hypertrophy. This reduction was more marked if action potentials with identical resting potentials were compared in normal and in hypertrophied fibres. The conduction velocity of excitation decreased in group B probably as a result of the reduced rate of rise. Surprisingly, the conduction velocity was again at or above control level in group C despite the further depressed rate of rise. This is probably due to the outstanding increase in cell diameter in group C. The QRS duration of the heart in situ was also prolonged in B and normal or below normal in C. The action potential duration was augmented in both groups of hypertrophy, this was not reflected as a QT prolongation in situ.

Calcium-mediated action potentials in mammalian myocardium

SummaryCat papillary muscles were depolarized by raising the external K concentration so that resting potential went from about −80 mV to −50 mV. This caused inactivation of the transmembrane Na system. The remaining Ca-mediated action potentials showed greater overshoot, reduced upstroke velocity and action potential duration, and higher threshold of excitation. A graded electrical response to stimulous strength was also present. These action potentials depended mainly on the extracellular Ca concentration [16]. Further, the specific Ca-antagonistic compounds verapamil and D 600 [10] or Ca-with-drawal completely abolished the ability of the K-depolarized fibres to respond to stimulation with propagated electrical activity. Conversely, extra Ca, epinephrine or isoproterenol overcame this inhibitory effect by increasing the Ca transmembrane influx. Also, within 1 min after returning treated muscles to normal K0 Tyrode solution, the Na system was reactivated and apparently normal electrical activity possible. In this case, Ca-withdrawal and the Ca-antagonists lose their electrical inhibitory influence, while excitation-contraction uncoupling still persist.

The interrelationship of calcium-mediated action potentials and tension development in cat ventricular myocardium

Abstract Studies of Ca-mediated action potentials and mechanical activity of cat papillary muscles and trabeculae partially depolarized and Na-inactivated by K-rich (13.7 m m ) solution were performed. Variation in strength of muscle stimulation resulted in gradation of rate of rise, overshoot and duration of Ca-mediated action potentials, peak tension and time to peak tension. At constant stimulus strength the rate of rise, overshoot and duration of the Ca-mediated action potentials were markedly depressed when the frequency of stimulation was increased. Two/second was maximum driving frequency. Within this range the frequency: tension relationship was similar to, although less than, the control. When steady state peak tension was low the Ca inflow was high—as indicated by d V d t max , overshoot and plateau positivity—and action potential prolonged. Following frequency change, d V d t max of the rising phase and overshoot decreased (increased) during the initial positive (negative) inotropic effect, and then tapered to steady-state by the end of the 1–2 min inotropic change. Hence, during staircase, as well as in steady state, the tension development was intimately interrelated to the Ca inflow, the underlying mechanism being discussed. Reducing [Ca]o to 0.25 m m completely inhibited Ca-mediated excitation as well as contraction within 15–20 min. Inhibitors of the slow Ca inward current (Verapamil, D 600) similarly inhibited Ca-mediated action potentials, but left resting state excitation and contraction (1/min) essentially unaltered.

Das mechanische und elektrische Verhalten isolierter embryonaler Herzmuskelzellen in Zellkulturen

SummaryA new photoelectrical device was constructed in order to investigate the mechanical and electrical activity of embryonic chick heart cells cultured in vitro. The results derived from single individual cells as well as from small cell clusters after 2 to 4 days in culture can be summarized as follows:1.Cultured beating heart cells can be divided into three groups: (i) ventricle-like cells with a large plateau-forming action potential followed by a correspondingly long-lasting (300 msec) mechanogram, (ii) atrium-like cells showing a significantly shorter action potential and a very brief (50 msec) contraction, (iii) pacemaker cells having all the typical signs of a frequent automatic impulse generation but a very small and shortlasting contractile activity.2.All types of cultured heart cells exhibit the phenomena of “staire-case”, of postextrasystolic and post-tetanic potentiation and of a frequency-depending diastolic tone.3.Increase of the extracellular Ca++ content (up to 4 mM/l) results in a positive inotropic effect as well as in a significant prolongation of the contraction courve. Under these circumstances the slow diastolic depolarizations becomes more and more effective in excitation-contraction coupling producing finally synchroneous activations of the contractile system.4.Further increase in the extracellular Ca++-concentration (5–10 mM/l) leads to an inhibition of the relaxation and, for a short period, to an irregular fibrillatory activity. After 10–20 min in the Ca++-rich medium, cells stop beating in a highly contracted state showing all the signs of an irreversible cell damage.5.Lowering the temperature (from 37 to 22° C) slows the rate of spontaneous beating (Q10=2,4 for cooling and 3,8 for rewarming), prolongs the duration of the action potential as well as the peak time and the relaxation time (Q10=2,3 for all three parameters in both directions), but do not produce any measurable inotropic effect.6.Cultured embryonic heart cells are completely insensitive to adrenergic substances. Although the whole embryonic heart (embryonic age 8–10 days) shows all the well known signs of an adrenergic responsiveness, even high doses of epinephrine, aludrine or norepinephrine produce neither an inotropic nor a chronotropic effect in the cultured chick heart cells. This result is tentatively explained by assuming a tryptic inactivation or digestion of the specific β-receptor.ZusammenfassungEs wird der Aufbau einer photoelektrischen Registriereinrichtung beschrieben, mit der gleichzeitig die mechanische und die elektrische Aktivität isolierter embryonaler Herzmuskelzellen aus Zellkulturen registriert werden kann. Die an Einzelzellen und kleineren Zellkomplexen vom Hühnchen gewonnenen Ergebnisse können wie folgt zusammengefaßt werden:1.Auch in der Zellkultur lassen sich drei Typen pulsierender Herzmuskelzellen unterscheiden. a) Ventrikelartige Zellen mit einem langdauernden, plateaubildenden Aktionspotential und einer entsprechend breiten Kontraktionskurve, b) vorhofartige Zellen mit erheblich kürzeren Aktionspotentialen und auffällig schmalen Spike-artigen Kontraktionen, c) Schrittmacherzellen mit allen Kennzeichen einer frequenten automatischen Erregungsbildung bei schwacher und kurzdauernder Kontraktionstätigkeit.2.Bei den drei Typen der beschriebenen Herzmuskelzellen lassen sich die Phänomene der “Treppe”, der postextrasystolischen und post-tetanischen Potenzierung und das Auftreten frequenzabhängiger diastolischer Kontraktionsrückstände nachweisen.3.Erhöhung der extracellulären Ca++-Konzentration (bis zu 4 mM Ca++/l) führt zu einer Amplitudenzunahme des Mechanogramms bei gleichzeitiger Verbreiterung der Kontraktionskurve. Zusätzlich werden in dieser Situation die langsamen diastolischen Potentialbewegungen an der Membran in zunehmendem Maße kontraktionswirksam, so daß die Kontraktionskurve schließlich ein nahezu synchrones Abbild des bioelektrischen Potentialverlaufs an der Membran wird.4.Weitere Erhöhung der extracellulären Ca++-Konzentration (5–10 mM Ca++/l) führt zu überdauernden Kontraktionsrückständen, kurzzeitiger fibrillatorischer Aktivität und schließlich zu einer irreversiblen Schädigungskontraktur der kultivierten Herzmuskelzellen.5.Temperaturerniedrigung (von 37 auf 22° C) verlangsamt die Pulsationsrate (Q10=2,4 bei Kühlung und 3,8 bei Wiedererwärmung), verlängert die Dauer von Aktionspotential, Gipfelzeit und Erschlaffungszeit (Q10=2,3 für alle drei Meßgrößen bei Variation der Temperatur in beiden Richtungen), führt jedoch nicht zu einem meßbarenpositiv-inotropen Effekt.6.Kultivierte embryonale Herzmuskelzellen sind vollständig unempfindlich gegenüber adrenergen Substanzen. Obwohl das intakte embryonale Hühnerherz (Brutalter 8–10 Tage) bereits alle Reaktionen einer adrenergen Stimulation zeigt, war selbst durch höchste Dosen von Adrenalin, Noradrenalin und Aludrin an kultivierten Herzzellen weder ein positiv-inotroper noch chronotroper Effekt zu erzielen. Es wird vermutet, daß diesem Befund eine tryptische Inaktivierung der spezifischen β-Receptoren zu Grunde liegt.

Ca-movement controlling myocardial contractility I

Summary1.Tension development and membrane currents have been measured in cat ventricular fibres applying a double sucrose-gap voltage clamp technique.2.A well defined mechanical threshold was found at which a considerable all-or-non response occurred. This threshold obviously coincided with the threshold potential of the fast inward current (INa). Changes in [Cae] (0.8–7.5 mM) did not separate the two thresholds. Ramp clamps of decreasing steepness shifted the threshold of bothINa and that of tension development towards less negative potentials. Ramp clamps ineffective in elicitingINa did not produce phasic tension development. This suggests that a fast inward current normally triggers the internal Ca release.3.In rhythmically activated preparations the steady state voltage-tension relationship rose roughly in parallel toICa in a S-shaped manner up to about zero potential. With further depolarizations the steady-state tension development tended to decline again. It was concluded that this voltage dependent gain of contractile activation reflects the time integral of transmembrane Ca supply provided from the extracellular space.4.Under steady state conditions peak tension produced by square clamps of a given suprathreshold voltage rose with prolongation of the clamp duration up to a maximum at 500–700 msec. This maximum in turn depended on the clamp potential applied. From the results obtained, a tension-time-voltage relationship has been drawn. Even under very long lasting depolarizations (up to 10 sec) usually no tonic responses occurred.5.A muscle once activated by a suprathreshold square clamp did not respond to a second depolarization unless the membrane had been repolarized to a given level and for a given duration. The amplitude of contractile response to second clamp steps depended in a characteristic manner on both the voltage and the duration of the recovery period.6.Mechanical transients typically observed when either the voltage or the duration of clamp steps had been changed were analyzed in order to obtain information about the kinetics of intracellular Ca movements assumed to take place in a multicompartment model derived from structural and functional observations.

Effects of Ca-Free and EDTA-Containing Tyrode Solution on Transmembrane Electrical Activity and Contraction in Guinea Pig Papillary Muscle

SummaryGuinea pig papillary muscle was exposed to Tyrode solution to which no Ca had been added (Ca-free) and to various concentrations of EDTA in either Tyrode or Ca-free Tyrode. The results were as follows:1.During 15 to 20 min exposure to Ca-free Tyrode the force of contraction of papillary muscles initially declined rapidly and then at varying rates and to varying degrees. Action potential duration and resting potential were reduced but maximum upstroke velocity (dV/dtmax) was increased. The effects were essentially reversed upon Ca replacement.2.Exposure to 0.8 mM EDTA in Ca-free Tyrode or 2.5 mM EDTA in Tyrode produced similar effects as exposure to Ca-free Tyrode although the rates of change were more rapid and contraction was always abolished.3.Exposure to 2.5 or 5.0 mM EDTA in Ca-free Tyrode or 5 mM EDTA in Tyrode caused a very rapid loss of measurable contraction, a decrease in resting potential, a marked increase in action potential duration and an increase ind V/dtmax. Replacement of Ca caused a beat to beat decrease in action potential duration, an increase in resting potential, a decrease ind V/dtmax and often an increase in resting tension. Contraction was initiated within 15 to 20 sec associated with marked after-contractions. As the after-contraction amplitude declined there was a decrease in resting tension. Action potential duration and resting potential recovered during 10 to 15 min after Ca replacement altough plateau duration usually did not return to control value. The results indicate that the effects of Ca removal on guinea pig ventricular muscle depend on the rate of removal. Transmembrane electrical changes are proposed to be the result of membrane permeability changes resulting in an increase in both general and Na-specific leakage. After-contractions are considered to be the result of intracellular Ca overload due to increased membrane permeability to Ca.

Die spezifische Beschleunigung des Erschlaffungsprozesses durch sympathische Überträgerstoffe und die Hemmung dieses Effektes durch β-Receptoren-Blockade

SummaryAmong the agents with a positive inotropic effect epinephrine occupies a special position as far as it increases the velocity of relaxation in frogs and mammalian myocardium, with the consequence that the duration of contraction is significantly shortened. The acceleration of myocardial relaxation induced by epinephrine occurs in normal Tyrode solution but more distinctly in a Ca++-rich medium. In the latter case epinephrine increases the velocity of relaxation even if, in maximally Ca++-activated papillary muscles, the isometric peak tension cannot be further augmented. The influence of epinephrine on relaxation is not linked with its inotropic effects, neither with variations of the extracellular Ca++ concentration, nor with alterations of the action potential. The effects of epinephrine can be neutralized by sympathetic β-receptor blocking agents. The mechanism of action of epinephrine may be due to a co-catalytic activation of the Ca++-dependent granular ATP-ase of the sarcoplasmic reticulum, which seems to play a key-role in the relaxation process. Obviously under the influence of sympathetic transmitter substances a maximal cardiac output can only be achieved by the advantageous combination of three factors (a) increase in frequency, (b) augmentation of contractile forceand (c) acceleration of relaxation. Without factor (c) the ventricular filling period is shortened at high heart rates to such a degree, that it becomes the limiting factor for the stroke volume.ZusammenfassungAdrenalin zeichnet sich unter den positiv inotropen Wirkstoffen durch die besondere Fähigkeit aus, den Erschlaffungsprozeß in Frosch- und Warmblütermyokard so sehr zu beschleunigen, daß die Gesamtdauer der Kontraktion signifikant verkürzt wird. Die Adrenalin-bedingte Beschleunigung der Erschlaffung tritt an Frosch- und Warmblütermyokard schon in gewöhnlicher Tyrodelösung besonders deutlich aber in Ca++-reichem Milieu in Erscheinung. Adrenalin steigert dabei die Erschlaffungsgeschwindigkeit maximal Ca++-aktivierter Papillarmuskeln auch dann noch, wenn es die Kontraktionskraft nicht mehr weiter erhöhen kann. Der Adrenalin-Einfluß auf den Erschlaffungsprozeß ist ebenso unabhängig von inotropen Effekten und von Variationen der Ca++-Konzentration, wie von Änderungen im Verlauf des Aktionspotentials. Durch β-Receptoren-Blockade werden die Adrenalin-Effekte auf den Erschlaffungsprozeß neutralisiert. Als Wirkungsmechanismus von Adrenalin wird eine “co-katalytische” Aktivitätssteigerung der Ca++-aktivierbaren ATPase der Erschlaffungsgrana diskutiert. Das erreichbare Maximum des Herzeitvolumens liegt unter dem Einfluß sympathischer Überträgerstoffe wegen der Kombination von Frequenzerhöhung, Kontraktionskraftssteigerungund Erschlaffungsbeschleunigung zweifellos höher als bei alleiniger Zunahme von Herzfrequenz und Kontraktionskraft; denn im letzteren Falle würde bei Frequenzzunahme die diastolische Füllzeit bald zu einem limitierenden Faktor.

Correlation of Na-withdrawal effects on Ca-mediated action potentials and contractile activity in cat ventricular myocardium

Summary1.Cat ventricular muscle fibers were bathed in K-rich (13.7 mM) Tyrode solution so that action potentials were mediated by the slow Ca inward current.2.Sucrose osmotically balanced Tyrode solutions with 50, 25 and 8.3% of normal Na concentration were used in all experiments. The basic effects of Na withdrawal are decrease of action potential upstroke velocity, overshoot and duration, and increase peak and resting tension. Threshold of stimulation also increased.3.The time courses of these Na withdrawal effects were significantly related in that action potential impairment always closely followed the positive inotropy. Furthermore, this time-dependence on inotropic state resembled that of electrical and mechanical effects from experiments of Cae variation in K-rich Tyrode solution.4.It was found that reduced frequency and rest periods enabled the muscles to recover from the electrical and mechanical effects of Na withdrawal. The reappearance of all full Na withdrawal effects after rest periods followed the same time courses, though slightly faster, as in Na withdrawal treatment of continuously stimulated muscles. If stimulation was stopped before Na withdrawal and not resumed until 10 or 25 min after, the initial action potentials were nearly full sized, there was no visible inotropic effect, and full effects required nearly as much time as in continuously stimulated muscles.5.The evidence shows clearly that Na does not detectably contribute to the slow inward current. In addition, flux studies showing that increased intracellular free Ca results from Na withdrawal, when considered with the relationship between action potential impairment and positive inotropy, suggest as a necessary conclusion that it is a reduced transmembrane Ca gradient which is responsible for impairment of Ca-mediated action potentials.

Spezifische Hemmung des Erschlaffungsprozesses am stark gekühlten Warmblütermyokard (1°C–10°C)

SummaryIn isolated papillary muscles or auricles or guinea pigs membrane excitability and impulse conductivity will persist for some time even at a temperature as low as 0°C if the Ca-content of the medium is increased to 14.4 mM/l. This peculiarity allowed the study of temperature dependence of action potential, isometric tension development and relaxation velocity of mammalian myocardium in a range between 0°C and 10°C which was hitherto reserved for frog muscle physiology. The principal result was that below 8°C the relaxation process of mammalian myocardium is particularly slowed so that a maximalQ10 of about 9 could be observed whereas theQ10 for the rising phase of the mechanogram amounted to about 2 (±0.2) over the whole range between 1°C and 14°C. It is concluded that in a Ca rich medium of low temperature the Ca-binding capacity of the vesicular components of the sarcoplasmic reticulum becomes insufficient by two reasons: (a) The passive influx or liberation of free Ca ions will be enhanced in the cold due to the prolongation of action potential. (b) The active reabsorption of Ca ions into the vesicles by means of an ATP-driven Ca pump—which is essential for relaxation—is depressed by cooling.