The 'sodium pump lag' revisited

Force-frequency relation in frog-ventricle is dependent on the direction of sodium/calcium exchange in diastole

AIM

Force of contraction increases with stimulus-frequency in mammalian and amphibian hearts under control conditions. Here, we have analysed the mechanism of the force-frequency relation (FFR) in frog-ventricle.

METHODS

Circular strips of frog-ventricle were subjected to field-stimulation with frequencies in the range 0.03-0.2 Hz and force recorded on a chart-recorder. In another protocol, varying rest-periods were imposed while the preparation beat steadily at 0.2 Hz and the effect of rest on post-rest beat amplitude was noted.

RESULTS

Under control conditions, a positive FFR and a rest-induced decay of contraction amplitude (RID) were seen in the frequency range 0.03-0.2 Hz. With cadmium, nifedipine, nickel (40 micromol L(-1)), ryanodine and adrenaline (all drugs at 10 micromol L(-1) concentration, except nickel), the positive FFR and RID seen under control conditions persisted. When the bathing solution contained ouabain (10 micromol L(-1)) or low external sodium (40 mmol L(-1)), or high external calcium (5 mmol L(-1)), the FFR turned negative in the frequency range stated above and there were rest-induced potentiations (RIP).

CONCLUSION

When the conditions favour a net leak of calcium in diastole from intracellular stores via the calcium-extrusive mode of sodium-calcium exchanger (NCX), FFR is positive. An increase in frequency lessens the diastolic interval and therefore the diastolic calcium leak, thereby augmenting force. On the other hand, interventions which favour the calcium-acquisitive mode of NCX during diastole, changed the pattern of RID to RIP and converted FFR from positive to negative. With net diastolic calcium uptake, there is better store-filling and therefore higher force at lower frequencies.

Cardiac impairment and nitric oxide synthase activity in the chronic portal vein-stenosed rat

Decreased cardiac contractility and beta-adrenergic responses have been observed in the chronic portal vein-stenosed (PVS) rat. Because nitric oxide (NO) may be increased in PVS and has been recognized as a negative inotropic agent, we investigated the induction of NO synthase (NOS2) and/or changes in constitutive NOS (NOS3) as factors in the cardiac dysfunction of the PVS rat. Ten to twelve days after portal vein stenosis or sham operation, cardiac function was evaluated in paced left ventricular papillary muscles (LVPM) and right ventricular strips (RV). To determine if NO modulation of contractile function was altered in PVS, we examined the increase in developed tension produced by the effect of Nomega-nitro-L-arginine (L-NNA) on the myocardial force-frequency relationship. Cardiac tissue NOS2 and NOS3 activities were assayed, Western blot analyses of NOS2 and NOS3 expression were performed, and circulating nitrate-nitrite (NOX) levels (an indicator of in vivo NOS activity) were assayed. Basal LVPM and RV contractile indexes were significantly reduced in PVS (30-50%), without a change in the relaxation rate. No between-group differences in the cardiac NOS2 or NOS3 enzymatic activities of PVS and sham-operated (SO) rats were observed. Western blots revealed no cardiac NOS2 expression in either SO or PVS rats. In contrast, NOS3 was expressed in both SO and PVS rats, but there was no quantitative difference in expression between the two groups. Changes in the cardiac force-frequency relationship (staircase effect) after L-NNA were consistent with NOS3 modulation of contractile function in both SO and PVS rats, but there was no between-group difference in the modulation. Circulating NOX concentrations did not differ between SO and PVS rats. In conclusion, protein expression data, enzymatic assays, end-product assays, and functional data indicate that between-group differences in NOS2 and NOS3 activity are not responsible for the cardiac impairment that has been observed in the chronic PVS rat.

Sodium-calcium relationships and cardiac function during coronary bolus perfusion

The present review deals with the side-effects of contrast media (CM) on cardiac function during coronary angiography. A physiological approach is used to redefine existing concepts of CM osmotoxicity and chemotoxicity in terms of osmolal, ionic and molecular effects. The main idea conveyed is that purely ionic effects are of central importance during and immediately following the transit of a brief coronary bolus. Ionic effects result largely from rapid transient washout of normal extracellular ions, but are also influenced by ions present in the CM. In particular, the calcium (Ca) and sodium (Na) ions controlling cardiac function are easily affected. The myocardial Na-Ca exchange, which is mainly a physiological mechanism for cellular Ca efflux during cardiac relaxation, is therefore highlighted in detail. The importance of avoiding a potential Na-Ca mismatch is shown by examples from basic physiology, cardiac surgery and coronary angiography and by results of experiments with Visipaque. In the isomolal and isotonic CM Visipaque, which is based on the dimer isodixanol (320 mg I/ml), an available osmolal space is filled with an appropriately balanced supplement consisting of NaCl (19mM) and CaCl2 (0.3 mM).

Effect of amiloride on inotropic and toxic actions of ouabain in guinea-pig left atria

The effect of amiloride on the positive inotropic and toxic effects of ouabain in guinea-pig left atria has been studied. In atria driven at 1 Hz, amiloride (0.3 and 0.5 mM) decreased the EC50 but did not affect the maximal tension developed by ouabain. At 0.1 Hz, amiloride did not change either the EC50 or the maximal tension developed by ouabain. Ouabain toxicity (onset of arrhythmias) was not changed by amiloride at either frequency of stimulation. Therefore, amiloride did not antagonize either the positive inotropic or the toxic effect of ouabain. The positive inotropic effect of amiloride has been ascribed to the inhibition of the Na+/Ca2+ exchanger. Since amiloride inhibits also the Na+/H+ exchanger, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an amiloride derivative which selectively inhibits the Na+/H+ exchange, has been tested to evaluate the role of the Na+/H+ exchange in the amiloride-ouabain interaction. EIPA increased the EC50 values of ouabain and decreased the maximal developed tension by the glycoside in atria driven at 0.1 and 1 Hz, but did not antagonize the toxic response (arrhythmias) of atria to ouabain. It is suggested that the inhibition of Ca2+ exit through the Na+/Ca2+ exchange by amiloride and ouabain may explain the observation that the positive inotropic effects of amiloride and ouabain are additive.

K+ shifts of skeletal muscle during stepwise bicycle exercise with and without beta-adrenoceptor blockade

1. K+ efflux rate and control of K+ reuptake rate in exercising muscle cells was examined in six healthy female volunteers. 2. A K(+)-selective electrode in the femoral vein continuously monitored K+ concentration ([K+]fv) during bicycling. Power was increased stepwise 5-6 times by 30-40 W every fourth minute until exhaustion before and after I.V. administration of propranolol. Leg blood flow was measured by bolus injections of Cardiogreen. 3. [K+]fv increased from about 4.3 to 6.8 mmol l-1 at exhaustion both before and after propranolol administration, but after drug infusion endurance was reduced from 22.2 +/- 0.6 to 19.7 +/- 1.1 min, so [K+]fv rose more rapidly. 4. The exercise-induced efflux rate of K+ from the muscle cells was estimated to be about 11 mumol kg-1s-1 at exhaustion both before and after propranolol administration. 5. As an indicator of rate of net loss of K+ from the leg, veno-arterial concentration differences ([K+]fv-a) during first, fourth and fifth power increments were high after 15 and 40 s, but declined toward the end of each power step. Propranolol accentuated [K+]fv-a only after 15 and 40 s of the first and fourth increments. 6. The exercise-induced increase in reuptake rate of K+ in the muscle, estimated at exhaustion, was not significantly changed by propranolol and was about 10 mumol kg-1s-1, corresponding to about 15% of maximum Na(+)-K+ pump capacity in man. 7. Extracellular accumulation and loss of K+ from muscle during bicycle exercise is due to Na(+)-K+ pump lag. The higher [K+]fv during propranolol is mainly due to impaired redistribution outside the exercising muscles. In addition at low powers, beta-adrenoceptor blockade caused a transiently increased net loss due to an accentuated Na(+)-K+ pump lag.

Cellular Ca2+ loading and inotropic effects of doxorubicin in atrial muscle preparations isolated from rat or guinea-pig hearts

The inotropic effects of doxorubicin were examined under conditions that alter the Ca2+ loading of myocardial cells. Atrial muscle preparations isolated from rat or guinea-pig hearts were used. Cellular Ca2+ loading was altered by changing the temperature, extracellular Ca2+ concentrations or the frequency of electrical stimulation. In guinea-pig heart muscle preparations stimulated at 2 Hz at 30 degrees C in the presence of 1.2 mM CaCl2, 30 microM doxorubicin caused biphasic (early and late phase) positive inotropic effects. A higher concentration (200 microM) of doxorubicin caused a transient positive inotropic effect followed by a gradual decrease in developed tension. An increase in CaCl2 concentration from 1.2 to 2.4 mM decreased the positive inotropic effect of 30 microM doxorubicin and changed the inotropic effect of 200 microM doxorubicin from positive to negative after a transient increase in developed tension. At 0.5-Hz stimulation or 36 degrees C incubation, 30 or 200 microM doxorubicin produced remarkable late phase positive inotropic effects. In rat heart muscle preparations, the patterns of the inotropic effects of doxorubicin were similar to those observed with guinea-pig hearts; however, the negative inotropic effect observed at 200 microM doxorubicin was greater. These results indicate that the early and the late phase positive inotropic effects of doxorubicin have different mechanisms. The pattern of inotropic effects of doxorubicin is affected by conditions that alter cellular Ca2+ loading or the concentration of doxorubicin.

Effects of amiloride in guinea-pig and rat left atrial contraction as affected by frequency of stimulation and [Ca2+]0-[Na+]0 ratio: role of Na+/Ca2+ exchange

1. The effect of amiloride (0.5 mM) on guinea-pig and rat left atria driven at various rates of stimulation and different [Ca2+]0-[Na+]0 ratios has been studied. 2. Amiloride elicited a positive inotropic response in guinea-pig left atria driven at 0.1 Hz, 0.5 Hz and 1 Hz when [Ca2+]0 was 3.6 mM, 1.8 mM and 0.9 mM respectively but not when [Ca2+]0 was 2.7 mM at 0.1 Hz, 0.9 mM at 0.5 Hz and 0.45 mM at 1 Hz. 3. A positive inotropic response was obtained in guinea-pig left atria driven at 0.1 Hz and 1 Hz when [Ca2+]0-[Na+]0(2) was increased respectively from 8 x 10(-5) to 16 x 10(-5) and from 2 x 10(-5) to 8 x 10(-5). The positive inotropic effect was evident only when the ratio was increased by increasing [Ca2+]0 and not by decreasing [Na+]0. 4. In the presence of amiloride, the force of contraction of guinea-pig left atria decreased instead of increasing, when the rate of stimulation was lowered from 1 Hz to 0.01 Hz. Amiloride inhibited the post-rest potentiation. 5. In rat left atria amiloride was devoid of any effect in all the above-mentioned experimental conditions. 6. It is suggested that the pattern of cardiac actions of amiloride can be explained by the inhibition of the Na+/Ca2+ exchange system.

Na/Ca exchange and tension development in vascular smooth muscle: effect of amiloride

1. The potassium-sparing diuretic, amiloride, has been shown to inhibit the Na/Ca exchange system in various preparations. The effects of this drug have been investigated on the contractions of guinea-pig aortic strips elicited by reduction of external K, by addition of ouabain and by removal of external Na. 2. Amiloride (5 X 10(-6) M-5 X 10(-4) M) inhibited the mechanical responses when it was added before giving the stimulus for contractions, but was not effective in relaxing the contracted strips. The drug shifted to the right the dose-response curve for Ca in low K solution. 3. The calcium antagonist diltiazem had no effect on the ouabain-, low K- and Na-free-induced contractions. 4. Amiloride decreased the rate of relaxation of aortic strips induced by removal of the low K solution. 5. The pattern of amiloride action on ouabain-, low K- and Na-free-induced contractions suggests that the drug interferes with Ca influx. The effect of amiloride on the relaxation rate of low K-contracted aortic strips is consistent with an interference with Ca efflux. 6. It is suggested that amiloride prevents Ca fluxes through the Na/Ca exchange system of guinea-pig aortic strips.

Carp (Cyprinus carpio) heart has a high sensitivity to the positive inotropic effect of strophanthidin despite negative force-frequency relationships

1. The relationship between response of the heart to increased stimulation frequency and digitalis sensitivity was examined comparing the positive inotropic effect of strophanthidin and [3H]ouabain binding to sarcolemmal Na+, K+-activated adenosine triphosphatase (Na+, K+-ATPase) in carp heart, which showed a negative force-frequency relationship, and in guinea-pig heart, which has a positive relationship. 2. In ventricular muscle preparations isolated from carp heart, strophanthidin increased developed tension with a half-maximal effect observed at 0.31 microM, indicating a relatively high digitalis sensitivity of this preparation. 3. The positive inotropic effect was not altered by concentrations of propranolol sufficient to block beta-adrenergic receptors. 4. Specific binding of [3H]ouabain to homogenates obtained from ventricular muscle of carp heart showed a single class of binding sites with a Kd value of 26 nM. 5. Potency of strophanthidin to produce the positive inotropic effect and affinity of the binding sites for [3H]ouabain were both higher in carp heart compared to those in guinea-pig heart. 6. These results demonstrate a clear dissociation between the force-frequency relationship and the sensitivity of heart muscle to the positive inotropic effect of cardiotonic steroids. 7. The latter is primarily determined by affinity of sarcolemmal Na+, K+-ATPase for the cardiotonic steroids.

The red blood cell: a model for ouabain receptor regulation in the heart?

The assumption that the red blood cell can be used as a model for ouabain receptor regulation in heart muscle has been tested using isolated tissues from humans, guinea pigs, and chickens. The following results were obtained: The affinity of the ouabain receptor was similar in both human erythrocytes and right atrial appendage, but the density of binding sites was much lower on the erythrocytes. There was no correlation between the binding capacity in both tissues. Ouabain receptor occupation was closely correlated with inhibition of Na+/K+-transport in human erythrocytes and chick heart nonmuscle cells in culture. In contrast, in chick heart muscle cells, an occupation of 40% of the receptors decreased the Na+/K+-transport rate by only 10%. In hypokalemia, the ouabain binding capacity was increased in human and guinea pig erythrocytes but not in guinea pig heart muscle. Such increases were seen in chick heart nonmuscle cells in moderate hypokalemia but in heart muscle cells only after severe hypokalemia. Incubation of chick heart muscle cells in toxic but not in "therapeutic" ouabain concentrations increased the number of ouabain receptors. Increases in receptor number attenuated the positive inotropic and toxic actions of ouabain. These variations between ouabain receptor regulation in red blood cells and heart muscle of several species may be attributable to the lack of a "sodium pump reserve" in erythrocytes and heart nonmuscle cells. Such variations indicate that the human erythrocyte is not a suitable model for the ouabain receptor in the human heart.

Optical measurement of voltage-dependent Ca2+ influx in frog heart

Sarcolemmal Ca2+ movements in frog ventricular strips were measured by monitoring Ca2+ depletion from the extracellular space with an impermeant Ca indicator dye, antipyrylazo III. Ca2+ depletion was measured as a weighted average of light signals recorded simultaneously at three different wavelengths. This weighting procedure was designed to reduce the motion-induced light scattering and to enhance the Ca2+-related optical signals. Comparison of the time course of Ca2+ depletion signal with that of contraction showed that the rate of Ca2+ depletion was maximal immediately after the upstroke of the action potential but prior to the onset of tension. Peak Ca2+ depletion was reached toward the end of the action potential and amounted to a 10-50 microM decrease in the total extracellular Ca2+ concentration. The reaccumulation of extracellular Ca2+ seen after the action potential was 2-5 sec slower than the relaxation of tension. The rate of Ca2+ depletion had a bell-shaped voltage dependence and was enhanced by epinephrine, suggesting that Ca2+ influx occurred primarily through a slowly inactivating ionic channel. Ca2+ transport through the Na+-Ca2+ exchange system was not significantly altered in the presence of strophanthidin or with decrease of extracellular K+ concentration despite marked potentiation of tension by these agents. Ca2+ depletion measured at the end of a 1-sec clamp pulse had a voltage dependence noticeably different from that of the developed tension. This finding may suggest that a fraction of activator Ca2+ is released from membrane-bound Ca2+ pools in a voltage-dependent manner. Our results show that Ca2+ indicator dyes can be used not only to measure rapid changes in the extracellular Ca2+ concentration during contraction, but also to quantify the contribution of various sarcolemmal Ca2+ transport systems to the generation of tension in cardiac muscle.

Simulated calcium current can both cause calcium loading in and trigger calcium release from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell

Skinned canine cardiac Purkinje cells were stimulated by regularly repeated microinjection-aspiration sequences that were programmed to simulate the fast initial component of the transsarcolemmal Ca2+ current and the subsequent slow component corresponding to noninactivating Ca2+ channels. The simulated fast component triggered a tension transient through Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (SR). The simulated slow component did not affect the tension transient during which it was first introduced but it potentiated the subsequent transients. The potentiation was not observed when the SR function had been destroyed by detergent. The potentiation decreased progressively when the slow component was separated by an increasing time interval from the fast component. The potentiation was progressive over several beats under conditions that decreased the rate of Ca2+ accumulation into the SR (deletion of calmodulin from the solutions; a decrease of the temperature from 22 to 12 degrees C). In the presence of a slow component, an increase of frequency caused a positive staircase, and the introduction of an extrasystole caused a postextrasystolic potentiation. There was a negative staircase and no postextrasystolic potentiation in the absence of a slow component. These results can be explained by a time- and Ca2+-dependent functional separation of the release and accumulation processes of the SR, rather than by Ca2+ circulation between anatomically distinct loading and release compartments. The fast initial component of transsarcolemmal Ca2+ current would trigger Ca2+ release, whereas the slow component would load the SR with an amount of Ca2+ available for release during the subsequent tension transients.

Optical measurements of extracellular calcium depletion during a single heartbeat

The impermeant dye antipyrylazo III was used to measure depletion of extracellular calcium and net influx of calcium through the sarcolemma during the cardiac action potential. It was found that calcium entry occurs continuously during the action potential and is under direct control of the membrane potential. The inotropic action of epinephrine is accompanied by increased influx of calcium, while strophanthidin enhances the twitch without altering calcium influx during the action potential.

Potassium and the heart

The electrical stability of the heart is more sensitive to the extracellular than to the intracellular potassium concentration. During exercise, extracellular potassium varies rapidly. Catecholamines also modulate the plasma potassium concentration. Hypokalaemia of any cause can precipitate arrhythmias. Ischaemic myocardium loses potassium into the extracellular space within seconds and the cell becomes depolarized. The rise of the extracellular potassium ion concentration accounts for many of the early electrophysiological changes. Abrupt changes of plasma potassium concentration in normal myocardium and a high potassium concentration in ischaemic myocardium can set up electrical forces which initiate arrhythmias. The same phenomenon can account for changes on the electrocardiogram early after the cessation of an exercise test in a patient with ischaemic heart disease. Accumulation of potassium between cells in response to an increase of heart rate is a possible mechanism for false positive exercise tests and Syndrome X.