The relationship among intracellular sodium activity, calcium, and strophanthidin inotropy in canine cardiac Purkinje fibers.

The role of sodium and calcium ions in strophanthidin inotropy was studied by measuring simultaneously the electrical, mechanical, and intracellular sodium ion activities in electrically driven cardiac Purkinje fibers under conditions that change the intracellular sodium or calcium level (tetrodotoxin, strophanthidin, high calcium, and norepinephrine). Tetrodotoxin (TTX; 1-5 X 10(-6)M) shifted the action potential plateau to more negative values, shortened the action potential duration, and decreased the contractile tension and the intracellular sodium ion activity (aiNa). The changes in tension and in aiNa caused by TTX appear to be related since they had similar time courses. Strophanthidin (2-5 X 10(-7)M) increased tension and aiNa less in the presence of TTX, and, for any given value of aiNa, tension was less than in the absence of TTX. Increasing extracellular calcium (from 1.8 to 3.3-3.6 mM) or adding norepinephrine (0.5-1 X 10(-6)M) increased tension and decreased aiNa less in the presence than in the absence of TTX. When two of the above procedures were combined, the results were different. Thus, during the increase in aiNa and tension caused by strophanthidin in the presence of TTX, increasing calcium or adding norepinephrine increased tension markedly but did not increase aiNa further. In a TTX-high calcium or TTX-norepinephrine solution, adding strophanthidin increased both tension and aiNa, and the increase in tension was far greater than in the presence of TTX alone. The results indicate that: (a) the contractile force in Purkinje fibers is affected by a change in aiNa; (b) a decrease in aiNa by TTX markedly reduces the inotropic effect of strophanthidin, possibly as a consequence of depletion of intracellular calcium; (c) increasing calcium influx with norepinephrine or high calcium in the TTX-strophanthidin solution produces a potentiation of tension development, even if aiNa does not increase further; and (d) when the calcium influx is already increased by high calcium or norepinephrine, strophanthidin has its usual inotropic effect even in the presence of TTX. In conclusion, the positive inotropic effect of strophanthidin requires that an increase in aiNa be associated with suitable calcium availability.

The pacemaker current in cardiac Purkinje myocytes.

It is generally assumed that in cardiac Purkinje fibers the hyperpolarization activated inward current i(f) underlies the pacemaker potential. Because some findings are at odds with this interpretation, we used the whole cell patch clamp method to study the currents in the voltage range of diastolic depolarization in single canine Purkinje myocytes, a preparation where many confounding limitations can be avoided. In Tyrode solution ([K+]o = 5.4 mM), hyperpolarizing steps from Vh = -50 mV resulted in a time-dependent inwardly increasing current in the voltage range of diastolic depolarization. This time-dependent current (iKdd) appeared around -60 mV and reversed near EK. Small superimposed hyperpolarizing steps (5 mV) applied during the voltage clamp step showed that the slope conductance decreases during the development of this time-dependent current. Decreasing [K+]o from 5.4 to 2.7 mM shifted the reversal potential to a more negative value, near the corresponding EK. Increasing [K+]o to 10.8 mM almost abolished iKdd. Cs+ (2 mM) markedly reduced or blocked the time-dependent current at potentials positive and negative to EK. Ba2+ (4 mM) abolished the time-dependent current in its usual range of potentials and unmasked another time-dependent current (presumably i(f)) with a threshold of approximately -90 mV (> 20 mV negative to that of the time-dependent current in Tyrode solution). During more negative steps, i(f) increased in size and did not reverse. During i(f) the slope conductance measured with small (8-10 mV) superimposed clamp steps increased. High [K+]o (10.8 mM) markedly increased and Cs+ (2 mM) blocked i(f). We conclude that: (a) in the absence of Ba2+, a time-dependent current does reverse near EK and its reversal is unrelated to K+ depletion; (b) the slope conductance of that time-dependent current decreases in the absence of K+ depletion at potentials positive to EK where inactivation of iK1 is unlikely to occur. (c) Ba2+ blocks this time-dependent current and unmasks another time-dependent current (i(f)) with a more negative (> 20 mV) threshold and no reversal at more negative values; (d) Cs+ blocks both time-dependent currents recorded in the absence and presence of Ba2+. The data suggest that in the diastolic range of potentials in Purkinje myocytes there is a voltage- and time-dependent K+ current (iKdd) that can be separated from the hyperpolarization-activated inward current i(f).

On the mechanisms underlying cardiac standstill: factors determining success or failure of escape pacemakers in the heart.

The mechanisms underlying cardiac standstill in health and disease are considered. Ventricular standstill results from failure of impulse formation or transmission in the ventricles. In the healthy heart, idioventricular automaticity is not brought into play and instead is suppressed by the sinus node by virtue of its faster rate (overdrive suppression). However, should the sinus node activity be suppressed or atrioventricular (AV) conduction blocked, overdrive suppression no longer persists. For this reason, the ventricular pacemakers activate the ventricles at a slow rate and under the regulatory activity of the sympathetic system. In the diseased heart, the idioventricular pacemakers or the regulatory mechanism can be altered structurally or functionally. This can be the result of the disease, compensatory mechanisms or therapeutic interventions. Disease may affect the idioventricular pacemakers directly or indirectly through anoxia, a change in ionic environment or an alteration of sympathetic innervation. Compensatory mechanisms may affect reflex actions, blood supply or heart rate. Drug administration may alter autonomic balance, block the action of neuromediators on their receptors or modify diastolic depolarization or its ability to attain the threshold. Because of these different direct and indirect actions, a sudden cessation of sinus node activity or sudden AV block may result in the diseased heart in a prolonged and even fatal cardiac standstill, especially if the tolerance to ischemia of other organs (notably the brain) is decreased.

Interaction between overdrive excitation and overdrive suppression in canine Purkinje fibres.

The interaction between overdrive excitation and overdrive suppression was studied in canine Purkinje fibres perfused in vitro with a solution containing noradrenaline (10(-6) mol . litre-1) and/or high calcium (8.1 mmol . litre-1). The following results were obtained: 1) in the presence of either high calcium or noradrenaline, a fast drive induced at times an acceleration of the spontaneous rhythm; 2) the simultaneous administration of high calcium and noradrenaline quite often allowed the onset of overdrive excitation that showed different patterns; 3) short (5 to 10 s) and fast (60 to 180 min) drives elicited a rhythm that was fastest immediately after overdrive, gradually slowed toward control and could be followed by suppression; 4) in other instances, overdrive caused little acceleration but failed to induce suppression; 5) drives longer than 15 s usually induced only suppression, although there could be a few beats before suppression; 6) intermittent drive led to an initial excitation and then to suppression; 7) driving at a rate slower than the spontaneous rate caused a temporary suppression of the spontaneous discharge which resumed even if the slow drive continued; 8) excitation was not induced by overdriving in low calcium, even in the presence of noradrenaline. It is concluded that many of the in vivo features of overdrive excitation can be reproduced in vitro in small strands of Purkinje fibres. The relationship between overdrive excitation and overdrive suppression involves the opposing actions of drive on the oscillatory potential and on diastolic depolarisation.

Acetylcholine induces overdrive excitation in sheep Purkinje fibres.

The hypothesis that in the presence of acetylcholine overdrive may induce spontaneous repetitive activity (overdrive excitation) was tested by recording the transmembrane potentials and contractile force in sheep cardiac Purkinje fibres perfused in vitro. The results were: (a) acetylcholine 10(-8) to 10(-4) mol.litre-1 increased the action potential duration, the diastolic depolarisation slope and amplitude, and the twitch amplitude in a dose dependent manner; (b) at concentrations of 10(-5) to 10(-4) mol.litre-1 the interruption of a 60 beat.min-1 drive often induced repetitive spontaneous activity which was fastest immediately after the drive, slowed gradually, and ceased abruptly when an oscillatory potential failed to attain the threshold; (c) in the presence of acetylcholine, in quiescent preparations, applying drives of different durations and at different frequencies resulted in a steeper and larger diastolic depolarisation, oscillatory potentials, and a prolonged afterdepolarisation; (d) when overdrive excitation was induced it was faster and longer after faster or longer overdrives; (e) the cessation of overdrive was often associated with an aftercontraction; (f) in the presence of acetylcholine and a high extracellular calcium concentration (10.8 mmol.litre-1) overdrive excitation was preceded by a short period of inhibition; (g) strophanthidin (5 x 10(-8) mol.litre-1) facilitated acetylcholine induced overdrive excitation (without an initial inhibition); (h) lowering the extracellular sodium concentration (-50% NaCl) antagonised overdrive excitation; and (i) atropine 10(-5) mol.litre-1 prevented acetylcholine induced overdrive excitation. It is concluded that acetylcholine induces overdrive excitation by causing a larger diastolic depolarisation, oscillatory potentials, and a transient prolonged afterdepolarisation; that overdrive excitation requires sodium as a charge carrier underlying the electrical events leading to the attainment of the threshold; and that acetylcholine induces excitation by acting on a muscarinic receptor.

Reversal of strophanthidin negative inotropy by metabolic substrates in cardiac Purkinje fibres.

The action of substrates and metabolic inhibitors on strophanthidin inotropy was studied in canine cardiac Purkinje fibres perfused in vitro. Both membrane potentials and contractile force were recorded. The results show that: 1) pyruvate (1 to 10 mmol X litre-1) decreases and then increases contractile force in Tyrode solution or in the presence of a low concentration of strophanthidin without modifying the action potential magnitude or configuration; 2) during the decreasing (but not the increasing) phase of strophanthidin (2 to 5 X 10(-7) mol X litre-1) inotropy, pyruvate increases contractile force; 3) beta-hydroxybutyrate has effects similar to those of pyruvate; 4) glucose increases contractile force and more so during the descending phase of strophanthidin inotropy; 5) the positive inotropic effect of pyruvate during declining strophanthidin inotropy is not prevented by glucose-lack and/or by iodoacetate but is markedly reduced in a hypoxic solution; 6) in contrast, glucose under similar conditions has the usual effects in hypoxia but not in the presence of iodoacetate; 7) the combination of hypoxia and glucose-lack leads to a decline in contractile force and to contracture: strophanthidin increases both contractile and resting force whereas O2 and/or glucose relax the contracture; 8) pyruvate has a positive inotropic effect also in the presence of high (greater than 10 mmol X litre-1) calcium. It is concluded that the decline in contractile force during exposure to strophanthidin or high calcium can be reversed by providing suitable substrates and therefore may involve a deficit in the production of high energy phosphates.

Inhibitory action of acetylcholine on potassium uptake of the sinus node.

The effect of endogenously released and exogenously administered acetylcholine on potassium uptake was studied in the sinus node of the guinea pig under different conditions. The results suggest the following conclusions: (1) acetylcholine is released in small amounts in the sinus node perfused in vitro and more so during electrical stimulation; (2) when the muscarinic receptors are blocked, an inhibitory action of endogenously-released acetylcholine on potassium uptake is revealed; (3) the inhibition requires small concentrations of acetylcholine, for 10(-5) mol/i eserine, by preventing acetylcholine hydrolysis, turns the inhibition into a stimulation of potassium uptake; and (4) the inhibitory action of acetylcholine on potassium uptake can be reproduced by administering small amounts of exogenous acetylcholine.

Role of membrane potential in Ba2+ induced automaticity in guinea pig cardiac myocytes.

STUDY OBJECTIVE: The aim was to study in isolated myocardial cells the role of membrane potential in barium induced spontaneous activity and the ionic mechanism of the underlying pacemaker current. DESIGN: The membrane potential and resistance of single myocytes were studied at different voltage levels by means of current and voltage clamp steps in the absence and presence of barium (Ba). EXPERIMENTAL MATERIAL: The membrane potentials and currents of single guinea pig ventricular myocytes were recorded by means of an intracellular microelectrode through which current could also be passed. MEASUREMENTS AND MAIN RESULTS: In the presence of Ba (0.1-0.2 mM), stepwise depolarisations induced a transient overshoot and initiated action potentials followed by an undershoot, diastolic depolarisation and spontaneous discharge. During progressive depolarisations, membrane resistance (Rm) increased, decreased transiently at the end of the action potential, and reincreased during diastole. Stepwise repolarisations had opposite effects. Hyperpolarisations reversed diastolic depolarisation and could unmask oscillatory potentials (Vos). Voltage clamp steps to +20 mV were followed by outward tail currents during which Rm increased. Larger or longer depolarisations were followed by larger outward tail currents at resting potential level. The outward tail current reversed at potentials negative to EK. CONCLUSIONS: In the presence of Ba, applied depolarisation facilitates the induction of spontaneous activity through an interplay between voltage dependent and time dependent Ba block and unblock of gK1, voltage dependent increase in Rm, increased potassium driving force, and negative shift in the slow inward current threshold and sometimes Vos. The pacemaker potential underlying spontaneous activity is due to the slow re-establishment of Ba block of IK1 during diastole.

On the mechanism of barium induced diastolic depolarisation in isolated ventricular myocytes.

Barium can induce spontaneous activity in cardiac non-pacemaker cells. The mechanism of barium induced diastolic depolarisation was studied in isolated ventricular myocytes, using a microelectrode technique. Barium (0.05-0.2 mmol.litre-1) decreased resting potential and caused the membrane potential at the end of the action potential to undershoot the diminished resting value temporarily, thereby inducing diastolic depolarisation. Resting membrane resistance was increased by Ba but at the end of phase 3 repolarisation the resistance temporarily decreased below its steady state diastolic value. In presence of Ba, hyperpolarisation abolished or reversed diastolic depolarisation. At the end of phase 3 repolarisation, membrane resistance was decreased, whether diastolic depolarisation was present, absent or reversed. A high [K]o (15.4 mmol.litre-1) decreased Ba effects on action potential, membrane resistance and diastolic depolarisation. Caesium decreased the Ba induced diastolic depolarisation and the associated increase in membrane resistance, but had little effect on spontaneous activity at depolarised levels. Barium induced an oscillatory potential, with increased membrane resistance. Noradrenaline plus low [Ba]o, and high [Ba]o alone (1-5 mmol.litre-1), can induce spontaneous activity. Thus, in myocardial cells barium induces diastolic depolarisation at polarised levels by a voltage and time dependent block of potassium conductance, which is modulated by action potential voltage changes. However, as [Ba]o is increased, spontaneous activity at a depolarised level may be related to the decay of potassium currents and to oscillatory potentials.

Electrical and mechanical effects of strontium in sheep cardiac Purkinje fibres.

The electrical and mechanical effects of strontium were studied in sheep cardiac Purkinje fibres perfused in vitro. In a nominally calcium free solution, strontium (1.35-10.8 mmol.litre-1): (1) caused a time, rate and concentration dependent shift of the plateau to more positive potentials, prolonged the action potential and decreased the maximum diastolic potential; (2) increased the time to peak and amplitude of the twitch and caused a tonic force which relaxed only on repolarisation; (3) was rapidly overcome in its effects by calcium (1.35-2.7 mmol.litre-1); (4) was antagonised by manganese (1 mmol.litre-1) and cadmium (0.1-0.2 mmol.litre-1); (5) was potentiated by noradrenaline (0.1 mumol.litre-1); (6) could induce action potentials in 27 mmol.litre-1 [K]o; (7) induced a tail following the action potential when the pacemaker potential had been blocked by caesium; (8) could induce a tail in 8 mmol.litre-1 [K]o which sustained force development and was reduced by calcium antagonists; (9) if applied to a quiescent fibre, induced a prolongation of the first resumed action potential and tonic force but a small twitch, and these effects were antagonised by calcium and manganese; and (10) induced a strong twitch after a period of quiescence in low [Na]o. It is concluded that the pronounced and progressive electrical and mechanical effects of strontium in cardiac Purkinje fibres are due to an enhanced strontium influx (due to inability of strontium to substitute for calcium in the inactivation of Isi) and to strontium extrusion through an electrogenic Na-Sr exchange.

Pacemaker current, membrane resistance, and K+ in sheep cardiac Purkinje fibres.

OBJECTIVE: The pacemaker current in cardiac Purkinje fibres has been attributed to either a decrease in potassium conductance or an increase in a non-specific (Na-K) conductance. The former mechanism would be associated with an increase in membrane resistance (Rm) and the latter with a decrease in Rm. The aim of this study was to obtain evidence in support of one or other mechanism by measuring Rm during the pacemaker current (Idd) under conditions where there is a small or no extracellular potassium depletion. METHODS: Hearts were obtained from anaesthetised sheep and thin strands of ventricular Purkinje fibres were shortened to less than or equal to 1.6 mm. Purkinje fibres were voltage clamped to potentials positive and negative to the potassium equilibrium potential (EK) using a two microelectrode technique. Small current pulses were superimposed on Idd to measure Rm changes. Procedures were used that decrease either the background potassium current IKl or Idd in order to dissect changes in Rm due to K depletion from those due to Idd. RESULTS: Rm increased during Idd, whether the pacemaker current increased or decreased as a function of time. Increasing [K]o from 2.7 to 5.4 mmol.litre-1 decreased Rm and during hyperpolarising steps increased the instantaneous current but did not change Idd amplitude. In 2.7 mmol.litre-1 K, caesium (Cs, 2 mmol.litre-1) increased the holding current (Ih), had little effect on the instantaneous current, and eliminated Idd and associated Rm changes. In 5.4 and 10.8 mmol.litre-1 K, Cs increased Ih and decreased Idd amplitude and in 10.8 mmol.litre-1 K Cs decreased the instantaneous current on hyperpolarisation. If the current was reversed, Cs decreased but did not abolish it. In normal [K]o, barium (Ba, 0.05-0.5 mmol.litre-1) increased Ih and Rm, reduced the instantaneous current but did not increase Idd amplitude. In high [K]o, Ba instead increased the amplitude and rate of development of Idd. When Cs was applied in the presence of Ba, Idd was reduced or eliminated depending on [K]o. CONCLUSIONS: The changes in membrane resistance during the pacemaker current cannot be accounted for by K depletion and suggest that in the range of diastolic depolarisation the pacemaker current results predominantly from a time dependent decrease in K conductance.

Mechanism of caffeine-induced arrhythmias in canine cardiac Purkinje fibers.

The effects of caffeine were studied in canine cardiac Purkinje fibers perfused in vitro. The study revealed that (1) caffeine induces an oscillatory potential (Vos) superimposed on early diastolic depolarization in driven fibers; (2) Vos magnitude increases (within limits) with the concentration of caffeine (0.5 to 3 mM) and as a function of time of exposure; (3) if the drive is interrupted, Vos may attain the threshold and initiate spontaneous repetitive activity; (4) caffeine increases the rate of discharge in spontaneously active fibers, also through a Vos; (5) Vos shows the same characteristics as under other conditions of calcium overload; that is, it disappears after a long pause, increases after a shorter pause, can be suppressed by overdrive and can initiate spontaneous discharge at normal or depolarized levels; (7) Vos is dependent on cellular calcium as it appears at lower caffeine concentrations in fibers that are loaded with calcium by increasing extracellular calcium concentration [Ca]o, by decreasing extracellular sodium concentration [Na]o or by administering strophanthidin; (8) caffeine-induced Vos is made to peak sooner and to initiate fast spontaneous rhythms by norepinephrine; (9) Vos is reduced by low [Ca]o but not by propranolol. It is concluded that caffeine causes an oscillatory potential that is modulated by cellular calcium, and this Vos can induce arrhythmias.

Effects of caffeine on intracellular sodium activity in cardiac Purkinje fibres: relation to force.

1. An increase in cytoplasmic calcium by caffeine would lead to Ca extrusion via the Na/Ca exchange. The hypotheses were investigated that, as a consequence, caffeine might increase intracellular sodium activity (aiNa) and that the relation between aiNa and force might be conditioned by the Ca load. 2. Action potential, aiNa and contractile force were recorded in sheep Purkinje fibres during exposure to caffeine under conditions that decrease or increase the Ca load by different mechanisms. 3. In Tyrode solution, caffeine (8 mM) increased aiNa from 8.05 +/- 0.20 to 10.52 +/- 0.40 mM (+30.5%) and had a triphasic effect on force: an initial transient increase (+93.6%), a subsequent decrease (-37.1%) (negative inotropy) and slow partial recovery (+8.9%). 4. Decreasing the Ca load by means of manganese (1 mM) decreased aiNa and force. Adding caffeine re-increased aiNa and no longer caused a negative inotropic action. Cadmium (0.2 mM) also decreased aiNa, and caffeine reincreased it although far less than in Tyrode solution. 5. High [K]o (10 mM) and tetrodotoxin (5 microM) decreased aiNa as well as force. In their presence, caffeine re-increased aiNa and no longer had a negative inotropic action. 6. Increasing the Ca load by means of high [Ca]o (8.1 mM) increased force (+195%) and decreased aiNa, (-20.3%). Adding caffeine re-increased aiNa (+28.1%), but immediately decreased force (-32.3%). 7. Addition of pyruvate (10 mM) to caffeine increased force, as it does in the presence of Ca overload. 8. Noradrenaline (0.1-1 microM) decreased aiNa and increased contractile force. In its presence, caffeine decreased aiNa further and increased force. 9. It is concluded that caffeine increases aiNa, even during the negative inotropic effect. The decrease in force appears to depend on Ca load. Thus, caffeine no longer decreases force under conditions that decrease Ca load (Mn, high [K]0, TTX) and immediately decreases force when the Ca load is increased(high [Ca]0). However, in the presence of noradrenaline, caffeine decreases aiNa and markedly increases force, as the Ca load is increased, but Ca can be removed from the cytoplasm into the SR.

Role of Intracellular Sodium Activity in the Control of Contraction in Cardiac Purkinje Fibers.

The role of intracellular sodium activity (a(i)(Na)) in the control of force was studied in sheep cardiac Purkinje fibers exposed to norepinephrine (NE) and high [Ca](o) in the absence and presence of overdrive or of a low concentration of strophanthidin. Both NE and high [Ca](o) decrease a(i)(Na) and increae force, while overdrive increases and low strophanthidin decreases both parameters. In the presence of NE, overdrive increases a(i)(Na) less than force and is followed by a more pronounced undershoot in a(i)(Na) and force. In contrast, in high [Ca](o) overdrive increases a(i)(Na) more than force and is followed by a less pronounced undershoot in a(i)(Na) and force than in NE. High [Ca](o) increases force to a peak, but then the decreasing a(i)(Na) reduces force. In all these conditions, a(i)(Na) determines force changes during recovery from overdrive. NE and high [Ca](o) decrease a(i)(Na) less and increase force more in low strophanthidin. Thus, changes in a(i)(Na) modulate the increase in force due to increased Ca influx and control force development when Ca influx is either unchanged (low strophanthidin) or has reached a steady state (high [Ca](o), recovery from overdrive). Copyright 1994 S. Karger AG, Basel

Role of the Inward K Rectifier in the Repetitive Activity at the Depolarized Level in Single Ventricular Myocytes.

The role of the inward K(+) rectifier in the repetitive activity at depolarized levels was studied in guinea pig single ventricular myocytes by voltage- and current-clamp methods. In action potentials arrested at the plateau by a depolarizing current, small superimposed hyperpolarizing currents caused much larger voltage displacements than at the resting potential and sometimes induced a regenerative repolarization. Around -20 mV, sub- and suprathreshold repetitive inward currents were found. In the same voltage range, small hyperpolarizing currents reversed their polarity. During depolarizing voltage-clamp ramps, around -20 mV there was a sudden decrease in the outward current (I(ns): current underlying the negative slope in the inward K(+) rectifier steady state I-V relation). During repolarizing ramps, the reincrease in outward current was smaller and slower. During depolarizing and repolarizing current ramps, sudden voltage displacements showed a similar asymmetry. Repetitive I(ns) could continue as long as the potential was kept at the level at which they appeared. Depolarizing voltage-clamp steps also caused repetitive I(ns) and depolarizing current steps induced repetitive slow responses. Cadmium and verapamil reduced I(ns) amplitude during the depolarizing ramp. BRL 34915 (cromakalim), an opener of the ATP-sensitive K(+) channel, eliminated the negative slope and I(ns), whereas barium increased I(ns) frequency (an effect abolished by adding BRL). Depolarization-induced slow responses persisted in an NaCl- Ca-free solution. Thus, the mechanism of repetitive activity at the depolarized level appears to be related to the presence of the negative slope in the inward K(+) rectifier I-V relation. Copyright 1994 S. Karger AG, Basel

Cesium, Na(+)-K(+) Pump and Pacemaker Potential in Cardiac Purkinje Fibers.

The mechanisms of the hyperpolarizing and depolarizing actions of cesium were studied in cardiac Purkinje fibers perfused in vitro by means of a microelectrode technique under conditions that modify either the Na(+)-K(+) pump activity or I(f). Cs(+) (2 mM) inconsistently increased and then decreased the maximum diastolic potential (MDP); and markedly decreased diastolic depolarization (DD). Increase and decrease in MDP persisted in fibers driven at fast rate (no diastolic interval and no activation of I(f)). In quiescent fibers, Cs(+) caused a transient hyperpolarization during which elicited action potentials were followed by a markedly decreased undershoot and a much reduced DD. In fibers depolarized at the plateau in zero [K(+)](o) (no I(f)), Cs(+) induced a persistent hyperpolarization. In 2 mM [K(+)](o), Cs(+) reduced the undershoot and suppressed spontaneous activity by hyperpolarizing and thus preventing the attainment of the threshold. In 7 mM [K(+)](o), DD and undershoot were smaller and Cs(+) reduced them. In 7 and 10 mM [K(+)](o), Cs(+) caused a small inconsistent hyperpolarization and a net depolarization in quiescent fibers; and decreased MDP in driven fibers. In the presence of strophanthidin, Cs(+) hyperpolarized less. Increasing [Cs(+)](o) to 4, 8 and 16 mM gradually hyperpolarized less, depolarized more and abolished the undershoot. We conclude that in Purkinje fibers Cs(+) hyperpolarizes the membrane by stimulating the activity of the electrogenic Na(+)-K(+) pump (and not by suppressing I(f)), and blocks the pacemaker potential by blocking the undershoot, consistent with a Cs(+) block of a potassium pacemaker current. Copyright 1995 S. Karger AG, Basel

Overdrive Excitation in the Guinea Pig Sinoatrial Node Superfused in High [K(+)](o).

The aim of the present experiments was to study the characteristics and mechanisms of the rhythm induced by overdrive ('overdrive excitation', ODE) in the sinoatrial node (SAN) superfused in high [K(+)](o) (8-14 mM). It was found that: (1) overdrive may induce excitation in quiescent SAN and during a slow drive; (2) in spontaneously active SAN, overdrive may accelerate the spontaneous discharge; (3) immediately after the end of overdrive, a pause generally precedes the onset of the induced rhythm; (4) during the pause, an oscillatory potential (V(os)) may be superimposed on the early diastolic depolarization (DD); (5) during the subsequent late DD, a different kind of oscillatory potential appears near the threshold for the upstroke (ThV(os)) which is responsible for the initiation of spontaneous activity; (6) once started, the induced rhythm is fastest soon after overdrive; (7) faster drives induce longer and faster spontaneous rhythms; (8) the induced action potentials are slow responses followed by DD with a superimposed V(os), but ThV(os) is responsible for ODE; (9) the induced rhythm subsides when ThV(os) miss the threshold and gradually decay; (10) low [Ca(2+)](o) abolishes ODE; (11) in quiescent SAN, high [Ca(2+)](o) induces spontaneous discharge through ThV(os) and increases its rate by enhancing V(os) and shifting the threshold to more negative values, and (12) tetrodotoxin abolishes ODE as welll as the spontaneous discharge induced by high [Ca(2+)](o). In conclusion, in K(+)-depolarized SAN, ODE may be present in the apparent absence of calcium overload, is Ca(2+)- and Na(+)-dependent and is mediated by ThV(os) and not by V(os). Copyright 1997 S. Karger AG, Basel

Strophanthidin and force regulation by intracellular sodium activity in cardiac Purkinje fibers.

The role of intracellular sodium activity (aiNa) in the inotropy of a low concentration of strophanthidin (5 X 10(-8 M) was studied in sheep cardiac Purkinje fibers by recording contractile force, aiNa and transmembrane potentials under conditions that vary aiNa. High [Na]O, strophanthidin and tetrodotoxin (TTX) changed force and aiNa in a closely related manner: on logarithmic coordinates, the data were well fitted by a single line obtained through the regression equation F = b (aiNa)s where b represents the intercept and s the slope of the relation. With low strophanthidin, force increases as a linear function of (aiNa) approximately 5 and with high [Na]O as a linear function of (aiNa) approximately 6. However, the combined administration of high [Na]O and strophanthidin results in a potentiated inotropic effect as force becomes a linear function of (aiNa) approximately 14. This potentiation and its abolition by TTX suggests that factors other than aiNa powerfully modify the inotropy of a low strophanthidin concentration.

Quinacrine decreases the slow inward current and force in guinea pig ventricular tissue.

In guinea pig isolated ventricular myocytes, quinacrine (40 microM) decreases the action potential amplitude and duration, and markedly decreases the slow inward current (Isi). A lower quinacrine concentration (20 microM) has similar but smaller effects. In guinea pig papillary muscles, quinacrine decreases contractile force reversibly. Thus, in myocardial fibers the decrease in Isi by quinacrine appears to predominate over the inhibition of Na-Ca exchange found in membrane vesicles.

Role of intracellular Na+ activity in the negative inotropy of strophanthidin in cardiac Purkinje fibers.

The relation between intracellular sodium activity (aNai) and different phases of strophanthidin inotropy was studied in sheep cardiac Purkinje fibers superfused in vitro. Strophanthidin (1 microM) progressively increases aNai whereas increases and then decreases contractile force, induces contracture ('mechanical toxicity') and arrhythmias ('electrical toxicity'). Contractile force begins to decrease at approximately 11 mM aNai. Force and aNai show a positive correlation during the increasing and a negative correlation during the decreasing phase of strophanthidin inotropy. In high [K]o (8, 12 and 16 mM), strophanthidin increases aNai and force to a smaller peak and fails to induce toxicity. In high [Na]o (+18.5%), strophanthidin increases aNai and force to a larger peak and induces electrical toxicity below and mechanical toxicity above a higher aNai value (approximately 15 mM). In higher [K]o, high [Na]o restores the ability of strophanthidin to induce mechanical toxicity. Thus, mechanical toxicity begins when aNai increases past a critical value and the continuing aNai increase correlates with decrease in contractile force and contracture. The critical value of aNai is modified by Ca load related to changes in membrane potential or to Na electrochemical gradient.