Chemosensitivity of cardiac muscle

The living state: How cellular excitability is controlled by the thermodynamic state of the membrane

The thermodynamic (TD) properties of biological membranes play a central role for living systems. It has been suggested, for instance, that nonlinear pulses such as action potentials (APs) can only exist if the membrane state is in vicinity of a TD transition. Herein, two membrane properties in living systems - excitability and velocity - are analyzed for a broad spectrum of conditions (temperature (T), 3D-pressure (p) and pH-dependence). Based on experimental data from Characean cells and a review of literature we predict parameter ranges in which a transition of the membrane is located (15-35°C below growth temperature; 1-3pH units below pH7; at ∼800atm) and propose the corresponding phase diagrams. The latter explain: (i) changes of AP velocity with T,p and pH.(ii) The existence and origin of two qualitatively different forms of loss of nonlinear excitability ("nerve block", anesthesia). (iii) The type and quantity of parameter changes that trigger APs. Finally, a quantitative comparison between the TD behavior of 2D-lipid model membranes with living systems is attempted. The typical shifts in transition temperature with pH and p of model membranes agree with values obtained from cell physiological measurements. Taken together, these results suggest that it is not specific molecules that control the excitability of living systems but rather the TD properties of the membrane interface. The approach as proposed herein can be extended to other quantities (membrane potential, calcium concentration, etc.) and makes falsifiable predictions, for example, that a transition exists within the specified parameter ranges in excitable cells.

Acidosis and contractility of heart muscle

The contractility of heart muscle is sensitive to small and physiological changes of extracellular pH. The reduction of contractility associated with an acidosis is determined by the fall of pH in the intracellular fluid. The function of many organelles within the cardiac cell is affected by hydrogen ions. The tension generated by isolated myofibrils at a fixed calcium concentration is reduced at low pH. The dominant mechanism for the reduction of contractility in whole tissue is competitive inhibition of the slow calcium current by hydrogen ions. The reduction of the slow calcium current is similar when the same fall of developed tension is induced by acidosis or by a reduction of extracellular calcium concentration. Measurement of tissue pH with fast-responding extracellular electrodes show that, in myocardial ischaemia, tissue acidosis develops at the same time or only seconds before the onset of contractile failure. Much of the reduced contractility can be accounted for by the severity of the acidosis. Although a mild acidosis can delay or prevent damage to the myocardium from ischaemia or hypoxia, a severe acidosis is not beneficial and may even cause tissue necrosis.

Anesthetic formulation of tumescent solutions

There is no standard or official recipe for the tumescent anesthetic solutions. The actual concentrations of lidocaine and epinephrine should depend on the areas to be treated and clinical situation. This article discusses the safe usage of tumescent solutions and the proper procedures and precautions to take when mixing these solutions.

Antiarrhythmic drug classifications and the clinician: a gambit in the land of chaos

Several classifications of antiarrhythmic drugs have appeared and all suffer considerable limitations. Recently a Task Force of the Working Group on Arrhythmias of the European Society of Cardiology proposed a novel classification of antiarrhythmic drugs (the so-called Sicilian Gambit) based on their action on the most vulnerable parameter of an arrhythmogenic mechanism. The present article attempts a critical reappraisal of the antiarrhythmic drug actions and the relationship of vulnerable parameters with cellular mechanisms such as ion channels. The clinical applicability of these concepts, the implications of the new classification in the pharmacologic therapy of arrhythmias, and its potential limitations are discussed.

Significance of classifying antiarrhythmic actions since the cardiac arrhythmia suppression trial

The Cardiac Antiarrhythmic Suppression Trial (CAST) showed flecainide and encainide induced excess mortality compared with placebo. Labeling drugs as Class 1C is based on clinical observations, comprising measurements of the electrocardiographic parameters QRS. H-V and J-T intervals and of effective refractory period (ERP) as follows: 1--(QRS) wide, 2--(HV) long, 3--(ERP) unchanged, 4--(JT) unchanged. In vitro electrophysiology helped to explain the clinical findings. Flecainide and encainide rendered Na channels as nonconducting, but F and E were only slowly released from the channels after repolarization. At any given drug concentration, a proportion of total channels were eliminated, and the steady-state proportion increased at rising heart rate. It is not proven that the properties that lead to classification of a drug as 1C were those that caused excess deaths in the CAST. The proarrhythmic tendency of 1C drugs can be reduced by beta-blockade, and the mechanisms of adrenergic arrhythmogenicity are discussed. Propafenone is both a 1C drug and a beta-blocker, and its pharmacologic profile is reviewed to illustrate how it resembles and differs from flecainide and encainide. Some features of the CAST are assessed with particular reference to the extent to which conclusions drawn from the results may be justifiably extrapolated to other drugs classified as 1C.

Relevance of cellular to clinical electrophysiology in interpreting antiarrhythmic drug action

The usefulness of cellular electrophysiologic techniques in elucidating the fundamental actions of antiarrhythmic drugs is contrasted with their apparent lack of relevance to the selection of drugs for the treatment of particular arrhythmias. Clinical electrophysiologists employ different techniques, but their results may be explained in terms of cellular drug actions. The varying clinical effects of class IA, IB and IC agents are due to differences in the speed of their attachment to, and detachment from, sodium channels. The role of sympathetic activity in arrhythmogenesis is complex, but again readily explicable in terms of the electrophysiologic cellular actions of stimulation of the individual types of adrenoceptors (alpha 1, alpha 2, beta 1 and beta 2) and the distribution of these receptors, and of the longterm effects of sympathetic deprivation, either by antisympathetic drugs (class II) or by sympathetic denervation. Delayed repolarization (e.g., by class III drugs or prolonged beta blockade) is antiarrhythmic because it is homogeneous, despite the incidental prolongation of QT. If, however, QT is prolonged by heterogeneity of conduction or repolarization, or by partial sympathetic denervation (long QT syndrome or post myocardial infarction), this indicates increased risk of arrhythmia. Finally, the efficacy of calcium antagonists (class IV) in supraventricular arrhythmias is attributable to the cellular electrophysiologic characteristics of sinoatrial and atrioventricular nodal and transitional elements.

Effects of changes of intracellular pH on contraction in sheep cardiac Purkinje fibers

Intracellular pH (pHi) was measured with a pH-sensitive microelectrode in voltage-clamped sheep cardiac Purkinje fibers while tension was simultaneously measured. All solutions were nominally CO2/HCO3 free and were buffered with Tris. The addition of NH4Cl (5-20 mM) produced an initial intracellular alkalosis that was associated with an increase of twitch tension. At the same time, a component of voltage-dependent tonic tension developed. Prolonged exposure (greater than 5 min) to NH4Cl resulted in a slow recovery of pHi accompanied by a decrease of tension. Removal of NH4Cl produced a transient acidosis that was accompanied by a fall of force. In some experiments, there was then a transient recovery of force. If extracellular pH (pHo) was decreased, then pHi decreased slowly. Tension also fell slowly. An increase of pHo produced a corresponding increase of both force and pHi. The application of strophanthidin (10 microM) increased force and produced an intracellular acidosis. The addition of NH4Cl, to remove this acidosis partially, produced a significant increase of force. The above results show that contraction is sensitive to changes of intracellular but not extracellular pH. This pH dependence will therefore modify the contractile response to inotropic maneuvers that also affect pHi.

On the mechanism by which changes in extracellular pH affect the electrical activity of the rabbit sino-atrial node

1. The effect of altering the extracellular pH (pHo) on the electrical activity of the isolated rabbit sino-atrial (s.a.) node was studied using the two-micro-electrode voltage-clamp technique. 2. Alkalinization of the perfusate increased the maximum rate of rise and amplitude of the nodal action potential, and also the frequency of spontaneous beating of the s.a. node; acidification produced opposite effects. 3. A change in the extracellular pH from a value of 6.5 to 8.5 caused increases of the maximal conductance for both the slow inward current and the steady-state outward current systems without affecting the gating processes of the channels. 4. H+ could modify the electrical activity of s.a. nodal cells, not through altering the membrane surface potential but through the protonation of the ionic channels themselves. From the titration curves, the apparent pKa values for the slow inward current and the steady-state outward current were estimated to be 6.4 and 6.0, respectively.

Intracellular pH and contraction of isolated rabbit and cat papillary muscle: effect of superfusate buffering

The influence of external buffering on surface pH (pHs), intracellular pH (pHi) and developed twitch tension was investigated in rabbit and cat papillary muscle. pHs and pHi were measured using single and double-barreled microelectrodes respectively. In 20 mM HEPES buffered solution, steady state pHi is close to that in control CO2/HCO-3 (25 mM HCO-3, 5% CO2) solution. pHs and developed tension also do not differ greatly from their control values. Decreasing the HEPES concentration to 5 mM, at constant external pH, lowers pHs considerably. The surface acidosis is associated with a small intracellular acidification; steady state pHi in 5 mM HEPES is always more acid than that in control CO2/HCO-3. A significant decrease in developed tension is also seen in 5 mM HEPES. Alteration of the superfusion velocity influences pHs only slightly. Stimulation of the muscle at high frequency is shown to increase surface acidification, the extent of which is dependent on the buffer concentration. The conclusion from the present experiments is that in papillary muscle external buffering influences intracellular pH and contraction via its effect on pHs.

Interaction of acidosis and increased extracellular potassium on action potential characteristics and conduction in guinea pig ventricular muscle

We studied the individual and combined effects of extracellular acidosis and increases in extracellular potassium on action potential characteristics and conduction in order to gain a better understanding of the effects of acute ischemia. At each level of potassium between 2.7 and 17 mm, acidosis induced by increasing Pco2 (respiratory acidosis) and by decreasing HCO3- (metabolic acidosis) decreased resting membrane potential, the maximum rate of rise of the action potential upstroke (Vmax), and slowed conduction. Metabolic acidosis consistently and significantly lengthened the steady state action potential duration whereas respiratory acidosis did not. Respiratory acidosis caused changes in resting membrane potential, Vmax, and conduction velocity; which occurred more rapidly and were of greater magnitude than the changes induced by metabolic acidosis. The changes in Vmax induced both types of acidosis were due to a change in the resting membrane potential-Vmax relationship as well as to the changes in the resting membrane potential. The conduction slowing induced by acidosis was greater when potassium was 9 and 13 mM than when potassium was 5.4 mm. Our results suggest that acidosis causes important changes in the electrophysiological properties of ventricular fibers and that many of the known electrophysiological effects of acute ischemia can be mimicked by the combined effects of extracellular acidosis and an increase in extracellular potassium.

The effects of nadolol on various cardiac tissues in normoxia, and on atrial muscle in simulated ischaemia

Nadolol, a non-selective beta-blocker with a long duration of action, reportedly is devoid of membrane stabilizing action. Since such action is augmented by ischaemic conditions, it was of interest to investigate whether, in simulated ischaemia, a direct membrane effect of nadolol might be revealed. We have confirmed that in normoxia nadolol had no effects on intracellulary recorded potentials in isolated rabbit atrial or ventricular muscle at a concentration of 4.84 microM, but significantly reduced action potential amplitude and maximum rate of depolarisation at 14.5 microM. In Purkinje cells a small reduction of action potential amplitude was produced by 1.61 microM nadolol, but the effect was not increased by nadolol 4.84 microM. There were no drug-induced changes in action potential duration (APD), spontaneous frequency, conduction velocity, contractions, electrical threshold, effective refractory period or the maximum frequency at which a stimulus could be followed. Nadolol 14.5 microM did not reduce the positive inotropic effect of increasing extracellular calcium concentrations. Nadolol was 25 times less potent than procaine as a local anaesthetic on desheathed frog nerve. In a solution simulating ischaemia (8 mM KCl, 10 mM NaHCO3, gassed with 20% O2, pH 7.0) nadolol had a significant class 1 action on atrial muscle, however, even at 4.84 microM and reduced the shortening of APD caused by the solution. It is concluded that nadolol has no class 1, 3 or 4 antiarrhythmic activity in normoxia, but could have an additional protective effect in ischaemic myocardium against the arrhythmogenic factor of shortened APD.

pH aspects of transient changes in conduction velocity in isolated heart fibers after partial replacement of chloride with organic anions

Conduction velocity in isolated rabbit atrial fibers was continuously measured in solutions having a different anionic composition. When 20mmol/l of chloride was replaced by 20 mmol/l lactate or other anions of weak organic acids at constant pH 6.8, biphasic initial transient changes in conduction velocity were observed. The produced transient changes had a greater amplitude with organic acids which have a greater pK and lipid/water partition ratio. The magnitude of the transients was also greater at pH 6.8 than at pH 7.5, and also when the buffering capacity of the superfusion solution was smaller. Measurements of intracellular pH (pHi) in sheep Purkinje fibers and of pH at the surface (pHs) of sheep Purkinje and rabbit atrial fibers with pH sensitive microelectrodes, showed a transient increase of pH and a sustained decrease of pHi on replacement of 20mmol/l chloride by organic anions of weak acids (at constant pH of the superfusion solution). A combined influence of the transient pHs change and the sustained pHi modification seems to be important in the explanation of the biphasic changes in conduction velocity.

Effects of acid-base changes on excitation--contraction coupling in guinea-pig and rabbit cardiac ventricular muscle

1. Respiratory and metabolic acid-base changes caused similar steady-state changes in the contractility of cardiac ventricular muscle, but the rate of response was more rapid with the former intervention. 2. Variations in extracellular pCO2 and [HCO3-] at constant pH caused only a transient change in contractility. 3. An intracellular pH change can describe the above events. 4. The changes in contractility caused by extracellular acid-base changes could be explained by competition between Ca2+ and H+ ions for a single process. 5. Assuming an electroneutral scheme whereby one extracellular Ca2+ ion or two intracellular H+ ions compete for a binding site, the interior of ventricular cells must be better buffered than the extracellular fluid. 6. H+ ions evoked a release of Ca2+ ions from a mitochondrial suspension with a time course similar to the partial recovery of tension observed during a respiratory acidosis. 7. Respiratory and metabolic acidosis depressed the action potential plateau and prolonged repolarization. 8. The resting potential and the maximum rate of depolarization were unaffected by the above acid-base changes. 9. An acidosis depressed Ca2+ influx through the slow inward channel by an amount sufficient to account for the observed contractility changes. 10. It is concluded that between pH 7.6 and 6.6 the major physiological effect of an acidosis is to depress the slow inward current as a result of an intracellular pH change.

The effects of acidosis and bicarbonate on action potential repolarization in canine cardiac Purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of acidosis and bicarbonate (HCO3) on action potential repolarization. Extracellular pH (pHe) was reduced from 7.4 to 6.8 by increasing carbon dioxide (CO2) concentration from 4 to 15% in a HCO3-buffered solution or by NaOH titration in a Hepes-buffered solution. Both types of acidosis produced a slowing of the rate of terminal repolarization (i.e., period of repolarization starting at about -60 mV and ending at the maximum diastolic potential) with an attendant increase in action potential duration of 10--20 ms. This was accompanied by a reduction in the maximum diastolic potential of 2--8 mV. In contrast, if the same pH change was made by keeping CO2 concentration constant and lowering extracellular HCO3 from 23.7 to 6.0 mM, in addition to the slowing of terminal repolarization, the plateau was markedly prolonged resulting in an additional 50- to 80-ms increase in action potential duration. If pHe was held constant at 7.4 and HCO3 reduced from 23.7 mM to 0 (Hepes-buffered solution), the changes in repolarization were nearly identical to those seen in 6.0 mM HCO3 except that terminal repolarization was unchanged. This response was unaltered by doubling the concentration of Hepes. Reducing HCO3 to 12.0 mM produced changes in repolarization of about one-half the magnitude of those in 6.0 mM HCO3. These findings suggest that in Purkinje fibers, HCO3 either acts as a current that slows repolarization or modulates the ionic currents responsible for repolarization.

Direct measurement of the intracellular pH of mammalian cardiac muscle

1. The intracellular pH (pHi) of sheep heart Purkinje fibres and rat, ferret and guinea-pig ventricle has been measured using recessed-tip pH-sensitive micro-electrodes. 2. In the absence of CO2 the pHi was approximately 7-2 in all the preparations used. In 5% CO2 the mean pHi was 7-14 in rat and ferret ventricle and 7-02 in sheep Purkinje fibres. 3. The pHi response to an increase or a decrease in the CO2 level (at constant external pH) was biphasic with a large transient change followed by a partial recovery to a new sustained pHi. 4. The intracellular buffering capacity was 34-8 +/- 2-7 m-equiv H+/pH unit per l. (+/- S.E. of mean) in sheep Purkinje fibres, 76-6 +/- 13-6 in rat ventricle and approximately 69 in ferret ventricle. 5. The pHi of all the preparations tested indicated that H+ ions were not passively distributed across the cell membrane. There was also little or no pHi change produced by depolarization with high K solutions. 6. Short exposures to hypertonic solutions (100 mM sucrose or 50 mM-KCl) produced a decrease in pHi of approximately 0-1 pH units. 7. Acetazolamide slowed the pHi response to CO2 changes. 8. Restoration of the pHi after displacement by increasing the CO2 was not blocked by ouabain or SITS. 9. The relationship between pHi and cardiac contractility is discussed.

Effects of respiratory alkalosis and acidosis on myocardial excitation

In anesthetized dogs electrocardiogram and monophasic action potentials (MAPs) were recorded from the right atrium and the right ventricle by intracardiac suction electrode technique. The animals were subjected, by means of ventilation with CO2 and hyperventilation, to periods of respiratory acidosis and respiratory alkalosis, respectively. Pronounced respiratory acidosis induced an increased sympathetic activity followed by a decrease in heart rate and prolongation of the A-V conduction time whereas the shape and duration of the atrial and ventricular MAPs remained unaltered. Arterial hypoxia in combination with pronounced respiratory acidosis did not influence the MAP durations. Respiratory alkalosis resulted in an increased sympathetic influence on the heart activity whereas the shape and duration of the atrial and the ventricular MAPs remained unaffected. During pronounced hyperventilation with increasing central venous pressure an increased parasympathetic influence on the heart activity with decrease in the heart rate, prolongation of the A-V conduction time and shortening of the atrial MAP duration was recorded.

Acid-base changes and excitation-contraction coupling in rabbit myocardium. II. Effects on resting membrane potential, action potential characteristics and propagation velocity

The effects of changes in acid-base parameters on the resting membrane potential, action potential characteristics and propagation velocity were studied in isolated rabbit papillary muscles. Lowering extracellular pH from 7.4 to 6.7 in a bathing solution buffered with 10 mM histidine did not alter the resting membrane potential or action potential characteristics but casused slight reduction in propagation velocity. A parallel increase in HCO3-minus concentration (up to 47 mM)and PCO2 at a constant extracellular pH of 7.4 caused a substantial decrease in action potential duration but did not alter the resting membrane potential or propagation velocity. The decrease in action potential duration was caused by the increase in HCO3-minus concentration. Propionate (47nM) caused a shortening of the action potential which was of the same magnitude as for HCO3-minus but methylsulfate (47nM) did not have this effect. The possible influence of these changes on the inotropic state of the myocardium and the cellular mechanisms involved are discussed.

Acid-base changes and excitation-contraction coupling in rabbit myocardium. I. Effects on isometric tension development at different contraction frequencies

The effects of changes in acid-base parameters on the active force of isolated rabbit papillary muscles were studied at contraction frequencies of 12, 60 and 120/min. When extracellular pH was lowered from 7.4 to 7.0 and 6.7 in a bathing solution buffered with 10 mM histidine, the active force decreased at all contraction frequencies studied. After parallel increases of HCO3-minus concentration (up to 47 mM) and PCO2 at a constant extracellular pH of 7.4 the active force of the muscle increased at low and decreased at high contraction frequencies. None of these effects can be attributed to catecholamine release or to altered extracellular concentration of ionized calcium. The inotropic effects produced by bicarbonate were not reproducible by methyl sulfate (47 mM) or propionate (47 mM). It is concluded that: 1. a lowering of the extracellular pH has a negative inotropic effect at all frequencies, 2. HCO3-minus has a positive inotropic effect that is most pronounced at low contraction frequencies and 3. CO2 has a negative inotropic effect exceeding that produced by the mere reduction in extracellular pH. The cellular mechanisms involved in the various inotropic effects are discussed.

Contractility in isolated mammalian heart muscle after acid-base changes

In-vitro experiments performed in cat papillary muscles and strips of rat right ventricle suggest that the changes in myocardial contractility that follow acid-base disturbances are not a function of extracellular pH. Simultaneous changes in Pco 2 and NaHCO 3 concentration, with extracellular pH constant, decreased developed tension and maximal rate of rise of the tension (dT/dt) without significant changes in the time to peak tension when the muscle was exposed to the solution with higher Pco 2 and NaHCO 3 concentration. At an extracellular pH of 7.40, developed tension decreased 0.51 ± 0.13 g/mm 2 ( P < 0.02) and dT/dt decreased 1.29 ± 0.50 g/sec ( P < 0.05) with no significant change in time to peak tension (0.038 ± 0.022 sec). Changes in pH produced by increasing Pco 2 at constant NaHCO 3 concentration were followed by a significant decrease in contractility. A change of Pco 2 from 20 to 90 mm Hg that produced a change in extracellular pH from 7.60 to 7.00 was accompanied by a decrease in developed tension of 0.67 ± 0.14 g/mm 2 ( P < 0.01), in dT/dt of 2.63 ± 0.54 g/sec ( P < 0.01) with no changes in time to peak tension (0.0017 ± 0.10 seconds). We were unable to show significant variations in contractility when extracellular pH was changed at a constant Pco 2 of approximately 21 mm Hg (NaHCO 3 7.5, 15, and 30 m M ) or at a Pco 2 of approximately 95 mm Hg (NaHCO 3 15, 30, 60, 80 and 120 m M ). Only when extracellular pH reached a value as high as 8.0 (Pco 2 21 mm Hg, NaHCO 3 80 m M ) a small but significant increase in contractility was evidenced. Either Pco 2 or intracellular pH could be the major determinants of the changes in myocardial contractility that follow acid-base alterations.

The effect of bretylium on intracellular cardiac action potentials in relation to its anti-arrhythmic and local anaesthetic activity

1. The initial effect of bretylium tosylate on isolated rabbit atria was to increase conduction velocity, contraction heights, spontaneous frequency and maximum driven frequency, and to reduce electrical threshold. At concentrations of 200 mg/l. or less, these were the only effects, and were consistent with the known sympathomimetic actions of bretylium.2. At extremely high concentrations, 1,200 and 2,400 mg/l., the initial actions were succeeded by weak quinidine-like effects; reduced conduction velocity, spontaneous and maximum driven frequencies, and rate of rise of action potential. The electrical threshold was raised, but contraction heights were not reduced.3. The local anaesthetic activity of bretylium, measured by reductions in the frog nerve action potential, was 1/90 that of procaine and 1/300 that of propranolol, on a molar basis.4. Acute pretreatment with bretylium, 20 mg/kg intravenously, significantly increased the amount of infused ouabain required before the appearance of the first signs of atrial arrhythmia in anaesthetized guinea-pigs, but did not prevent ventricular arrhythmias.5. Pretreatment with bretylium 30 mg/kg subcutaneously 24 hr, and again 4 hr before ouabain infusion, increased the dose of ouabain inducing atrial irregularity and slightly but significantly reduced the incidence of ventricular fibrillation.