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

Slowing of Electrical Activity in Ventricular Fibrillation is Not Associated with Increased Defibrillation Energies in the Isolated Rabbit Heart

Prolonged out-of-hospital ventricular fibrillation (VF) arrests are associated with reduced ECG dominant frequency (DF) and diminished defibrillation success. Partial reversal of ischemia increases ECG DF and improves defibrillation outcome. We have investigated the metabolic components of ischemia responsible for the decline in ECG DF and defibrillation success. Isolated Langendorff-perfused rabbit hearts were loaded with the voltage-sensitive dye RH237. Using a photodiode array, epicardial membrane potentials were recorded at 252 sites (15 mm × 15 mm) on the anterior surface of the left and right ventricles. Simultaneously, a global ECG was recorded. VF was induced by burst pacing, and after 60s, perfusion was either reduced to 6 ml/min or the perfusate composition changed to impose hypoxia (95% N(2)/5% CO(2)), pH 6.7 (80% O(2)/20% CO(2)), or hyperkalemia (8 mM). Using fast Fourier transform, power spectra were created from the optical signals and the global ECG. The optical power spectra were summated to give a global power spectrum (pseudoECG). At 600 s the minimum defibrillation voltage (MDV) was determined by step-up protocol. During VF, the ECG and pseudoECG DF were reduced by low-flow ischemia (9.0 ± 1.0 Hz, p < 0.01, n = 5) and raised [K(+)](o) (12.2 ± 1.3 Hz, p < 0.05, n = 7) compared to control (19.2 ± 1.5 Hz, n = 20), but were unaffected by acidic pH(o) (16.7 ± 1.1 Hz, n = 11) and hypoxia (14.0 ± 1.2 Hz, n = 10). In contrast, the MDV was raised by acidic pH (156.1 ± 26.4 V, p < 0.001) and hypoxia (154.1 ± 22.1 V, p < 0.01) compared to control (65.6 ± 2.3 V), but comparable changes were not observed in low-flow ischemia (61.0 ± 0.5 V) or raised [K(+)](o) (56 ± 3 V). In summary, different metabolites are responsible for the reduction in DF and the increase in defibrillation energy during ischemic VF.

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.

Prolongation of the QTc interval is seen uniformly during early transmural ischemia

OBJECTIVES

In order to more clearly understand the electrocardiographic manifestations of early transmural ischemia, we studied electrocardiograms (ECGs) in patients undergoing balloon angioplasty.

BACKGROUND

Decisions regarding reperfusion strategies in patients with acute myocardial infarction rely largely on the presence of ST-segment elevation (STE) in the ECG, consequently with significant limitations. Studies of the "ischemic cascade" show that ST-segment changes occur well after the onset of wall motion abnormalities.

METHODS

We prospectively analyzed ECGs obtained at 20-s intervals in 74 patients undergoing elective balloon angioplasty. The ECGs were analyzed using 3 methodologies. In 74 patients, the ST-segment, the T-wave, and the QT-interval were analyzed using the MUSE (General Electric HC, Menomonee Falls, Wisconsin) automated system (MUSE). Fifty patients were also analyzed using the Interval Editor automated system (IE; General Electric HC). In 20 patients, measurements were made manually.

RESULTS

Transmural ischemia prolonged the QTc interval (using the Bazett's formula) in 100% of patients. In all 74 patients analyzed with MUSE, QTc interval prolonged from 423 +/- 25 ms to 455 +/- 34 ms (p < 0.001). In the 50 patients analyzed with IE, QTc interval prolonged in 50 of 50 (100%) patients (from 424 +/- 27 ms to 458 +/- 33 ms [p < 0.001]). Mean time to maximal QTc interval prolongation, changes in T-wave polarity, > or =1 mm STE, and ST-segment depression (STD) were 22, 24, 29, and 35 s, respectively. Although QTc interval prolonged in 100% of patients, T-wave changes, STE, and STD (> or =1 mm) occurred in 7%, 15%, and 7%, respectively.

CONCLUSIONS

The QTc interval prolongs in 100% of patients with early transmural ischemia. When compared with clinically accepted indexes of transmural ischemia (i.e., STD and STE [> or =1 mm]) it is the earliest ECG abnormality.

The electrogenic Na+/HCO3- cotransport modulates resting membrane potential and action potential duration in cat ventricular myocytes

Perforated whole-cell configuration of patch clamp was used to determine the contribution of the electrogenic Na+/HCO3- cotransport (NBC) on the shape of the action potential in cat ventricular myocytes. Switching from Hepes to HCO3- buffer at constant extracellular pH (pH(o)) hyperpolarized resting membrane potential (RMP) by 2.67 +/- 0.42 mV (n = 9, P < 0.05). The duration of action potential measured at 50% of repolarization time (APD50) was 35.8 +/- 6.8% shorter in the presence of HCO3- than in its absence (n = 9, P < 0.05). The anion blocker SITS prevented and reversed the HCO3- -induced hyperpolarization and shortening of APD. In addition, no HCO3- -induced hyperpolarization and APD shortening was observed in the absence of extracellular Na+. Quasi-steady-state currents were evoked by 8 s duration voltage-clamped ramps ranging from -130 to +30 mV. A novel component of SITS-sensitive current was observed in the presence of HCO3-. The HCO3- -sensitive current reversed at -87 +/- 5 mV (n = 7), a value close to the expected reversal potential of an electrogenic Na+/HCO3- cotransport with a HCO3-:Na+ stoichiometry ratio of 2: 1. The above results allow us to conclude that the cardiac electrogenic Na+/HCO3- cotransport has a relevant influence on RMP and APD of cat ventricular cells.

Functional diversity of electrogenic Na+-HCO3- cotransport in ventricular myocytes from rat, rabbit and guinea pig

The Na(+)-HCO(3)(-) cotransporter (NBC) is an important sarcolemmal acid extruder in cardiac muscle. The characteristics of NBC expressed functionally in heart are controversial, with reports suggesting electroneutral (NBCn; 1HCO(3)(-) : 1Na(+); coupling coefficient N= 1) or electrogenic forms of the transporter (NBCe; equivalent to 2HCO(3)(-) : 1Na(+); N= 2). We have used voltage-clamp and epifluorescence techniques to compare NBC activity in isolated ventricular myocytes from rabbit, rat and guinea pig. Depolarization (by voltage clamp or hyperkalaemia) reversibly increased steady-state pH(i) while hyperpolarization decreased it, effects seen only in CO(2)/HCO(3)(-)-buffered solutions, and blocked by S0859 (cardiac NBC inhibitor). Species differences in amplitude of these pH(i) changes were rat > guinea pig approximately rabbit. Tonic depolarization (-140 mV to -0 mV) accelerated NBC-mediated pH(i) recovery from an intracellular acid load. At 0 mV, NBC-mediated outward current at resting pH(i) was +0.52 +/- 0.05 pA pF(-1) (rat, n= 5), +0.26 +/- 0.05 pA pF(-1) (guinea pig, n= 5) and +0.10 +/- 0.03 pA pF(-1) (rabbit, n= 9), with reversal potentials near -100 mV, consistent with N= 2. The above results indicate a functionally active voltage-sensitive NBCe in these species. Voltage-clamp hyperpolarization negative to the reversal potential for NBCe failed, however, to terminate or reverse NBC-mediated pH(i)-recovery from an acid load although it was slowed significantly, suggesting electroneutral NBC may also be operational. NBC-mediated pH(i) recovery was associated with a rise of [Na(+)](i) at a rate approximately 25% of that mediated via NHE, and consistent with an apparent NBC stoichiometry between N= 1 and N= 2. In conclusion, NBCe in the ventricular myocyte displays considerable functional variation among the three species tested (greatest in rat, least in rabbit) and may coexist with some NBCn activity.

Desbutylhalofantrine: evaluation of QT prolongation and other cardiovascular effects after intravenous administration in vivo

Desbutylhalofantrine (Hfm) is an active and equipotent metabolite of halofantrine (Hf). Both compounds are effective in the treatment of sensitive and multidrug-resistant and In vitro data and interpretation of some clinical studies of Hf have suggested that, unlike Hf, Hfm may be devoid of adverse cardiac effects. The aim of these investigations was to provide the first in vivo examination of the intrinsic capacity of Hfm to affect repolarization in the heart, using an anesthetized rabbit model. Using a dose-rising regimen, Hfm was administered IV at doses of 1, 1, 2, 4, and 8 mg/kg and the baseline rate-corrected QT interval (QTc) value of 377 +/- 13 ms rose to 394 +/- 16, 396 +/- 12, 429 +/- 18, 433 +/- 16, and 489 +/- 15 ms, respectively. There were no significant changes in blood pressure, heart rate, or PR or QRS intervals. The Hfm plasma concentrations were quantitated after high-performance liquid chromatographic analysis, the results indicating a significant correlation between Hfm plasma concentration and QT(c) prolongation. The study also identified a concentration-dependent hemolysis of erythrocytes after administration of Hfm. The conclusions from this study are that IV administration of Hfm does cause a significant prolongation of the QT(c) interval in a rabbit model.

Generation of intracellular pH gradients in single cardiac myocytes with a microperfusion system

This study describes the use of a microperfusion system to create rapid, large regional changes in intracellular pH (pH(i)) within single ventricular myocytes. The spatial distribution of pH(i) in single myocytes was measured with seminaphthorhodafluor-1 fluorescence using confocal imaging. Changes in pH(i) were induced by local external application of NH(4)Cl, CO(2), or sodium propionate. Local application was achieved by simultaneously directing two parallel square microstreams, each 275 microm wide, over a single myocyte oriented perpendicular to the direction of flow. One stream contained the control solution, and the other contained a weak acid or base. End-to-end, stable pH(i) gradients as large as 1 pH unit were readily created with this technique. This result indicates that pH within a single cardiac cell may not always be spatially uniform, particularly when weak acid or base gradients are present, which can occur, for example, in regional myocardial ischemia. The microperfusion method should be useful for studying the effects of localized acidosis on myocyte function, estimating intracellular ion diffusion rates, and, possibly, inducing regional changes in other important intracellular ions.

Modulation of repolarization in rabbit Purkinje and ventricular myocytes coupled by a variable resistance

Purkinje-ventricular junctions (PVJs) have been implicated as potential sites of arrhythmogenesis, in part because of the dispersion of action potential duration (APD) between Purkinje (P) and ventricular (V) myocytes. To characterize electrotonic modulation of APD as a function of junctional resistance (Rj), we coupled single isolated rabbit P and V myocytes with an electronic circuit. In seven of eight PV myocyte pairs, both APDs shortened on coupling at Rj = 50 MOmega. This was in contrast to modulation of APD in paired ventricular myocytes, which demonstrated APD shortening of the intrinsically longer action potential and APD prolongation of the intrinsically shorter action potential. Companion computer simulations, performed to suggest possible mechanisms for the paradoxical shortening of the V action potential in paired P and V myocytes, showed that the difference in intrinsic peak plateau potentials (Vpp) of the P and V myocytes determined whether the V action potential shortened or prolonged on coupling. This difference in Vpp caused a large, repolarizing coupling current to flow to the V myocyte, contributing to early inactivation of the L-type calcium current and early activation of the inward rectifier current. These results suggest that intrinsic differences in phase 1 repolarization could yield differing patterns of APD shortening or prolongation in the network of subendocardial PVJs, leaving some PVJs vulnerable to conduction of premature stimuli while other PVJs remain refractory.

Evidence for an electrogenic Na+-HCO3- symport in rat cardiac myocytes

1. The perforated whole-cell configuration of patch clamp and the pH fluorescent indicator SNARF were used to determine the electrogenicity of the Na+-HCO3- cotransport in isolated rat ventricular myocytes. 2. Switching from Hepes buffer to HCO3- buffer at constant extracellular pH (pHo) hyperpolarized the resting membrane potential (RMP) by 2.9 +/- 0.4 mV (n = 9, P < 0.05). In the presence of HCO3-, the anion blocker SITS depolarized RMP by 2.6 +/- 0.5 mV (n = 5, P < 0.05). No HCO3--induced hyperpolarization was observed in the absence of extracellular Na+. The duration of the action potential measured at 50 % of repolarization time (APD50) was 29.2 +/- 6.1 % shorter in the presence of HCO3- than in its absence (n = 6, P < 0.05). 3. Quasi-steady-state currents were evoked by voltage-clamped ramps ranging from -130 to +30 mV, during 8 s. The development of a novel component of Na+-dependent and Cl--independent steady-state outward current was observed in the presence of HCO3-. The reversal potential (Erev) of the Na+-HCO3- cotransport current (INa,Bic) was measured at four different levels of extracellular Na+. A HCO3-:Na+ ratio compatible with a stoichiometry of 2:1 was detected. INa,Bic was also studied in isolation in standard whole-cell experiments. Under these conditions, INa,Bic reversed at -96.4 +/- 1.9 mV (n = 5), being consistent with the influx of 2 HCO3- ions per Na+ ion through the Na+-HCO3- cotransporter. 4. In the presence of external HCO3-, after 10 min of depolarizing the membrane potential (Em) with 45 mM extracellular K+, a significant intracellular alkalinization was detected (0.09 +/- 0. 03 pH units; n = 5, P < 0.05). No changes in pHi were observed when the myocytes were pre-treated with the anion blocker DIDS (0.001 +/- 0.024 pH units; n = 5, n.s.), or when exposed to Na+-free solutions (0.003 +/- 0.037 pH units; n = 6, n.s.). 5. The above results allow us to conclude that the cardiac Na+-HCO3- cotransport is electrogenic and has an influence on RMP and APD of rat ventricular cells.

Changes in ventricular repolarization during acidosis and low-flow ischemia

Myocardial ischemia, primarily a metabolic insult, is also defined by altered cardiac mechanical and electrical activity. We have investigated the metabolic contributions to the electrophysiological changes during low-flow ischemia (7.5% of the control flow) using 31P NMR spectroscopy to monitor metabolic parameters, suction electrodes to study epicardial monophasic action potentials, and 86Rb as a tracer for K+-equivalent efflux during low-flow ischemia in the Langendorff-perfused ferret heart. Shortening of the action potential duration at 90% repolarization (APD90) was most marked between 1 and 5 min after induction of ischemia, at which time it shortened from 261 +/- 4 to 213 +/- 8 ms. The period of marked APD90 shortening was accompanied by a fivefold increase in the rate of 86Rb efflux, both of which were inhibited by the ATP-sensitive K+ (KATP)-channel blockers glibenclamide and 5-hydroxydecanoate (5-HD), as well as by a significant fall in intracellular pH (pHi) from 7.14 +/- 0.02 to 6.83 +/- 0.03 but no change in intracellular ATP concentration ([ATP]i). We therefore investigated whether a fall in pHi could be the metabolic change responsible for modulating cardiac KATP channel activity in the intact heart during ischemia. Both metabolic (30 mM lactate added to extracellular solution) and respiratory (PCO2 increased to 15%) acidosis caused an initial lengthening of APD90 to 112 +/- 1.5 and 113 +/- 0.9%, respectively, followed by shortening during continued acidosis to 106 +/- 1.2 and 106 +/- 1.4%, respectively. The shortening of APD90 during continued acidosis was inhibited by glibenclamide, consistent with acidosis causing activation of KATP channels at normal [ATP]i. The similar responses to metabolic (induced by adding either l- or d-lactate) and respiratory acidosis suggest that lactate has no independent metabolic effect on action potential repolarization.

Relationship between intracellular pH and tension development in resting ventricular muscle and myocytes

Simultaneous measurements of intracellular pH (pHi) and tension development were made in resting cat papillary muscles and resting ventricular myocytes (cat, guinea pig). pH microelectrodes and the fluorescent indicator carboxy-seminaphthorhodafluor-1 (SNARF-1) were used to measure pHi in muscles and myocytes, respectively. pHi-induced changes in isometric muscle tension and myocyte length were elicited by variations in PCO2, HCO3-, and [NH4Cl]. Increased pHi elevated resting tension and decreased resting cell length, whereas decreased pHi decreased tension and increased cell length. The tension-pHi and cell length-pHi relationships were nonlinear and displayed hysteresis. A reduction in extracellular [Ca2+] from 2.7 to 0.5 mM caused little or no change in the tension and cell length responses to elevated pHi. Ca2+ uptake and/or release by the sarcoplasmic reticulum (SR) does not appear to be involved in the tension response to intracellular alkalosis because the response was unaffected by a postpacing rest period and was not inhibited by ryanodine (5 microM), which depletes SR Ca2+ stores. The cross-bridge inhibitor 2,3-butanedione monoxime (15 mM), but not internal dialysis with 14 mM ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid, inhibited myocyte contractures elicited by elevated pHi. The latter finding suggests that factors other than pHi-induced increase in myofilament Ca2+ sensitivity may contribute to the resting contractile response to elevated pHi.

Beneficial effects of verapamil during metabolic acidosis in isolated perfused rat hearts

Metabolic acidosis was produced in two groups of isolated, glucose-perfused beating rat hearts. The first group (control) was untreated whereas the second group was pretreated for 48 h by the addition of verapamil (1.2 g/L) to the drinking water. Untreated hearts all developed asystole during a 30 min perfusion with an acidotic substrate (pH = 6.8) or during subsequent reequilibration with physiologic substrate (pH = 7.4). Prior to asystole, all untreated hearts showed evidence of severe mechanical and biochemical deterioration evaluated by 31 P NMR spectroscopy. In contrast, hearts of treated rats showed less mechanical and metabolic deterioration, and all recovered during reequilibration. The mechanism of protection of verapamil against the effects of metabolic acidosis is unclear but appears to be related to preserved mitochondrial function by the drug and not to a reduced demand for energy.

Use-dependent effects of lidocaine on conduction in canine myocardium: application of the modulated receptor hypothesis in vivo

Lidocaine is a commonly used antiarrhythmic drug that causes use-dependent blockade of sodium channels in vitro and reduces conduction velocity in vitro and in vivo. According to the modulated receptor hypothesis of antiarrhythmic drug action, lidocaine has a low affinity for rested sodium channels but a high affinity for open and inactivated channels. In the present experiments, we characterized use-dependent conduction slowing and recovery from slowing by lidocaine in anesthetized dogs. The His-to-ventricular conduction interval was used as the indicator of conduction velocity. We found that prolongation of conduction time was greater as the stimulation frequency was increased. Moreover, on abruptly changing the stimulation frequency, a new steady-state conduction time was approached in two to three depolarizations. On discontinuation of stimulation, the conduction time of progressively less premature extrastimuli shortened exponentially with a terminal phase time constant of 152 +/- 115 msec. These effects by lidocaine were enhanced during acidosis and enhancement was reversed by correction of the acidosis. It is concluded that the effects in vivo of lidocaine on conduction under several conditions of rate, rhythm, and pH are similar to its effects on the maximum upstroke velocity of the action potential in vitro. Although these experiments were not designed to validate the modulated receptor hypothesis, it appears that the modulated receptor hypothesis can predict the effects of lidocaine on conduction in vivo.

Cardiac cell action potential duration is dependent upon induced changes in free Ca2+ activity during pH changes in vitro

We examined how changes in solution pH alter myocardial cell action potentials (AP) with and without changes in free [Ca2+] caused by pH induced effects on calcium binding. Guinea pig ventricular tissue was isolated, superfused either with Krebs-Ringer (K-R) bicarbonate, phosphate buffered solution, or with Hepes buffered solution, and electrically paced during control (5% CO2 in O2), acidic (12% CO2), and alkalotic (0% CO2) conditions. Action potentials were recorded with intracellular microelectrodes. Extracellular free [Ca2+] was measured with a calcium ion selective electrode and total soluble calcium was measured by ultrafiltration and spectrophotometry. With a total [CaCl2] of 2.5 mM in the K-R solution, we found a free [Ca2+] of 2.14 mM at pH 7.44 (control), 2.48 mM at pH 6.97 and 1.60 mM at pH 8.19; total soluble calcium concentration was 2.00 mM at pH 8.19. In the Hepes solution, free [Ca2+] was only slightly altered (2.42 to 2.55 mM) within this pH range. Equivalent acidosis of either K-R or Hepes suffusate significantly, and similarly, prolonged the AP and its refractory period. Alkalosis of the Hepes suffusate shortened the AP; but equivalent alkalosis of the K-R suffusate prolonged the AP as did a reduction of [CaCl2] in Hepes suffusate from 3.0 to 1.5 mM at pH 7.43. Our study demonstrates that a paradoxical increase in APD occurs because free Ca2+ ion activity falls in K-R solution and overrides the effect of alkalosis alone to decrease APD.(ABSTRACT TRUNCATED AT 250 WORDS)

Pace-maker current changes during intracellular pH transients in sheep cardiac Purkinje fibres

Intracellular acidosis, at constant extracellular pH, hyperpolarizes the resting potential and reduces the diastolic depolarization rate of cardiac Purkinje fibres. With alkaline pHi, the fibre depolarizes and spontaneous firing is observed. Intracellular pH transients induced either by superfusion with Tyrode buffered with 5% CO2/23 mM HCO3- or 16% CO2/61 mM HCO3-, or with solutions containing weak undissociated acids, transiently shifted the half-maximum activation potential E0.5 of the pace-maker current. Similar transients were observed when NH4Cl was added and subsequently withdrawn from the solution. Simultaneous pHi measurements demonstrate a close relation between the time course of the pHi and E0.5 variations. Acid pHi shifts E0.5 to more negative and alkaline pHi's to less negative potentials. These pace-maker current activation voltage shifts are interpreted as the direct consequence of fixed charges titration at the inside of the sarcolemma. Other effects, like the slowing-down and reduction of the pace-maker current by acid pHi, presumably result from other interactions of protons with the pace-maker channel.

Lipid solubility modulates pH potentiation of local anesthetic block of Vmax reactivation in guinea pig myocardium

Current theories envision recovery from local anesthetic block of sodium channels via slow hydrophilic and fast hydrophobic paths. Extracellular pH reduction which increases cationic/neutral anesthetic form should especially prolong recovery kinetics of highly lipid soluble compounds that could readily exit via the hydrophobic pathway at normal extracellular pH. To test this hypothesis, we compared the effects of three related compounds with similar pKa on the time course of Vmax reactivation in guinea pig papillary muscle at pHo 7.4 and 6.95. The compounds were lidocaine and its two desethylation products, monoethylglycinexylidide and glycinexylidide. Judged from the octanol:water partition coefficient, lidocaine was the most lipid soluble (log partition coefficient 2.39 +/- 0.10), followed by monoethylglycinexylidide (log partition coefficient 1.32 +/- 0.09) and glycinexylidide was the least lipid soluble (log partition coefficient 0.41 +/- 0.09). At 30 microM and pHo 7.4, the potency order for Vmax depression at zero diastolic interval was lidocaine (53 +/- 6%), monoethylglycinexylidide (17 +/- 3%), and then glycinexylidide (7.8 +/- 1.9%). The decay of Vmax block appeared monoexponential, and the time constant of recovery was dose independent. Most important is the fact that there were significant differences in the tau r increase with extracellular pH reduction (P less than 0.05; Scheffé contrasts). The increase was greatest with lidocaine [73 +/- 28% (mean +/- SD)], less with monoethylglycinexylidide (42 +/- 15%), and least with glycinexylidide (13 +/- 17%). The simplest interpretation of the differences in extracellular pH-dependence of recovery kinetics was that recovery from block due to the neutral form of these ionizable local anesthetics depended on lipid solubility, whereas recovery from block due to the protonated form depended on molecular weight.

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.

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.