Acute diabetes modulates response to ischemia in isolated rat heart.

Diabetic hearts are suggested to exhibit either increased or lower sensitivity to ischemia. Detrimental effects of prolonged ischemia can be attenuated by preconditioning, however, relatively little is known about its effects in the diseased myocardium. This study was designed to test the susceptibility to ischemia-induced arrhythmias and the effect of preconditioning in the diabetic heart. Rats were made diabetic with streptozotocin (45 mg/kg, i.v.). After 1 week, isolated Langendorff-perfused hearts were subjected to 30 min occlusion of LAD coronary artery without or with preceding preconditioning induced by one cycle of 5 min ischemia and 10 min reperfusion. Glycogen and lactate contents were estimated in the preconditioned and non-preconditioned hearts before and after ischemia. Diabetic hearts were more resistant to ischemia-induced arrhythmias: incidence of ventricular tachycardia (VT) decreased to 42% and only transient ventricular fibrillation (VF) occurred in 17% of the hearts as compared to the non-diabetic controls (VT 100% and VF 70% including sustained VF 36%; p < 0.05). Preconditioning effectively suppressed the incidence and severity of arrhythmias (VT 33%, VF 0%) in the normal hearts. However, this intervention did not confer any additional protection in the diabetic hearts. Despite higher glycogen content in the diabetic myocardium and greater glycogenolysis during ischemia, production of lactate in these hearts was significantly lower than in the controls. Preconditioning caused a substantial decrease in the accumulation of lactate in the normal hearts, whereby in the diabetic hearts, this intervention did not cause any further reduction in the level of lactate. In conclusion, diabetic rat hearts exhibit lower susceptibility to ischemic injury and show no additional response to preconditioning. Reduced production of glycolytic metabolites during ischemia can account for the enhanced resistance of diabetic hearts to ischemia as well as for the lack of further protection by preconditioning.

Na,K-ATPase in the myocardium: molecular principles, functional and clinical aspects.

The role that Na,K-ATPase plays in Na+ and K+ antiport through the sarcolemma, in cation-homeostasis in cardiomyocytes as well as in excitation-contraction coupling and cell signalling in the myocardium is now widely recognized. It was its key importance for the cell membrane function that kept this enzyme intensively studied during the last three decades and finally brought to its discoverer the deserved Nobel Prize. Almost weekly are appearing new data concerning structure, function, regulation and role of the Na,K-ATPase in different physiological and pathological conditions. The special importance of the enzyme for heart function as well as the great amount of data that is concerned specifically with the heart Na,K-ATPase and accumulated since yet, started to call for setting them in order. The present paper updates basically important data on the cardiac Na,K-ATPase in relation to its specific properties, molecular mechanisms of function, mode of action, humoral and pharmacological modulation, adaptability, physiological role and clinical aspects.

Salutary effect of tedisamil on post-ischemic recovery rat heart: involvement of sarcolemmal (Na,K)-ATPase.

The in vitro effect of tedisamil on the specific activity and kinetic parameters of the sarcolemmal (Na,K)-ATPase as well as its ex vivo effect on the (Na,K)-ATPase in the isolated, perfused rat hearts was determined. Five micromol/l of tedisamil was added 5 min before the onset of 30 min global normothermic ischemia followed by 10 min reperfusion. At the conditions of its maximal cardioprotective effect (heart rate reduction, improved postischemic recovery of left ventricular developed pressure), the hearts were immediately used for isolation of sarcolemmal vesicles. In vitro, 1-100 micromol/l of tedisamil produced a concentration-dependent stimulatory effect on (Na,K)-ATPase activity, with a peak seen at 20 micromol/l (p < 0.01), while Mg-dependent ATPase was almost unchanged. Kinetic analysis revealed a significant increase in the affinity of the Na-binding sites on ATPase molecule at 20 micromol/l of tedisamil. These biochemical findings were confirmed by cytochemistry. Moreover, ex vivo experiments revealed that tedisamil rendered the sarcolemmal (Na,K)-ATPase activity to be a more resistant to detrimental effects of ischemia. In conclusion, the cardioprotective action of tedisamil was accompanied with a better preservation of the specific activity of (Na,K)-ATPase.

Postischemic reperfusion of the spinal cord: prolonged reperfusion alleviates the metabolic alterations induced by 25 min ischemia in the cervical and thoracolumbal segments.

In recent years, increasing amount of information has indicated that in some tissues the main damage due to oxidative stress does not occur during reperfusion but during the ischemic episode of the ischemia/reperfusion event. In this respect, serious doubts were also expressed about the origin of the increased amounts of free radicals which were believed to form and reported to appear in the perfusate during the first minutes of reperfusion. Moreover, speculative explanations were only available for a second increase in lipid peroxidation which was reported to occur after postischemic reperfusions exceeding 60 min. For this reasons, the present paper reports the results of investigation of ischemia/reperfusion injury to the cervical (CE) and thoracolumbal (ThL) segments of the spinal cord (SP) after an acute 25 min occlusion of the abdominal aorta, followed by 60-120 min reperfusion of the ischemic areas in rabbits. In CE and ThL segments of the SP, the ischemia induced: 1) a decrease in activities of superoxide dismutase (SOD), from 57.35+/-6.36 to 45.27+/-5.45 U x mg(-1) x min(-1) (S.E.M., 20.92%), p < 0.01, and from 58.36+/-5.45 to 33.00+/-4.55 U x mg(-1) x min(-1) (S.E.M., 43.46%), p < 0.001; 2) a significant decrease in gamma-glutamyl transpeptidase (gamma-GTP), from 114.66+/-1.45 to 99.88+/-4.4 micromol p-nitroaniline x mg(-1) x h(-1) (S.E.M. 12.89 %), p < 0.05 and from 112.24+/-1.20 to 95.09+/-2.40 micromol p-nitroaniline x mg(-1) x h(-1) (S.E.M., 16.26%), p < 0.05; 3) a considerable depression in Na,K-ATPase activity, from 7.14+/-0.58 to 5.08+/-0.32 micromol Pi x mg(-1) x h(-1) (S.E.M., 28.86%), p < 0.01, and from 7.23+/-0.11 to 5.09+/-0.31 micromol Pi x mg(-1) x h(-1) (S.E.M., 30.00%), p < 0.01. The Na,K-ATPase activity became decreased by ischemia and remained depressed significantly (all p < 0.01) throughout the experiment. After 60 min of reperfusion, SOD activity in the CE segment and that of gamma-GTP in the CE as well as ThL segments recovered, even slightly surpassing the control values, wheras SOD activity in the ThL segment became stabilized again close to its post-ischemic value. Prolonged, reperfusion for 120 min resulted in a further increase in gamma-GTP activity in the CE and ThL segments (to 132.79 and 132.30%, p < 0.01), and this was accompanied by a slight (p > 0.05) elevation in the content of conjugated dienes as well as by a new wave of depression of the SOD activity (p < 0.05) in both the CE and the ThL segment. From our results it could be concluded that all considerable damage to the spinal cord occurred during the ischemic period. In the period of reperfusion reparative changes started to predominate. This is in accordance with the recent discoveries indicating that, when coupled with an increase in tissue gamma-GTP activity, the post-ischemic reparative changes comprise a replenishment of the cell glutathione pool. This process is accompanied with a gradual increase in H2O2 production that results in repeatead inhibition of the SOD activity and a tendency to conjugated dienes formation.

Free oxygen radicals contribute to high incidence of reperfusion-induced arrhythmias in isolated rat heart.

Early period of reperfusion of ischemic myocardium is associated with a high incidence of severe tachyarrhythmias including ventricular tachycardia and fibrillation (VT and VF). Free oxygen radicals (FOR) have been identified as one of the principal factors responsible for reperfusion-induced events. However, their role in arrhythmogenesis is not clear. In the present study, in isolated Langendorff-perfused rat hearts subjected to 30 min global ischemia, the onset of reperfusion induced 100% incidence of both VT and VF with their gradual cessation over 5 min of reperfusion. Generation of H2O2 in the myocardium in the first minutes of reperfusion was visualized by means of cerium cytochemistry and confirmed by X-ray microanalysis. The mechanism of the arrhythmogenic effect of FOR may involve inhibition of the sarcolemmal Na+/K+-ATPase, as demonstrated in the rat heart sarcolemmal fraction subjected to FOR-generating system (H2O2 + FeSO4).

Prevention of processes coupled with free radical formation prevents also the development of calcium-resistance in the diabetic heart.

Recently it was shown that besides their negative role in pathogenesis of diabetes, reactive oxygen species (ROS) and particularly the products of non-enzymatic glycation of proteins (NEGP) may also participate in some processes of adaptation of the myocardium to diabetes, such as in the mechanism of development of calcium resistance of the heart. Our study revealed that the hearts of rats with experimentally induced diabetes (single dose of streptozotocin, 45 mg/kg i.v., 6 U/kg insulin daily) develop considerable resistance against calcium overload (induced by means of Ca-paradox). On the day 63 after the beginning of experiment, when the diabetic cardiomyopathy became fully developed but the hearts were still not failing, their calcium resistance was increased to 83.33%. Our results provide evidence that, when applied in a special regimen, resorcylidene aminoguanidine (RAG, 4 mg/kg) prevented both, the formation of fructosamine (a source of ROS generation), and also that of the advanced Maillard products, in the heart sarcolemma of diabetic rats. The effect of RAG was accompanied by a decrease in calcium resistance in the group of rats with chronic diabetes (63 days) from 83.3 to 46.7%. It is concluded that NEGP and ROS formation are inevitably needed for development of calcium resistance in the diabetic hearts.

Estradiol modulates the sodium pump in the heart sarcolemma.

Cardiovascular effects of estrogens and particularly that of estradiol involve protection of the heart against ischemia. These effects were believed to be mainly indirect, mediated via changes in the blood and blood vessels. In the present paper a direct action of estradiol on the heart is demonstrated. Estradiol stimulates (p < 0.001) the Na,K-ATPase activity of cardiac sarcolemmal membranes by stimulating in an allosteric manner, the activation of the enzyme by potassium. The latter activation involves also an increase in affinity to potassium of the potassium binding sites on the enzyme molecule, but remains without any effect on the capacity and KD value of specific ouabain binding to the Na,K-ATPase. Estradiol is also antagonizing the depression of Na,K-ATPase activity that may be caused by ischemia and it is stimulating (p < 0.01) the ouabain-sensitive uptake of 86Rb into the heart cells. Our results indicate, that in addition to the known indirect effects of estradiol on the heart, the hormone also stimulates the activity and improves the kinetics of interaction of cardiac sarcolemmal Na,K-ATPase with ATP as well as with Na+ and K+ ions. This direct action may also account for the cardioprotective effects of estradiol.

Mechanisms that may be involved in calcium tolerance of the diabetic heart.

In diabetes the hearts exhibit impaired membrane functions, but also increased tolerance to Ca2+ (iCaT) However, neither the true meaning nor the molecular mechanisms of these changes are fully understood. The present study is devoted to elucidation of molecular alterations, particularly those induced by non-enzymatic glycation of proteins, that may be responsible for iCaT of the rat hearts in the stage of fully developed, but still compensated diabetic cardiomyopathy (DH). Insulin-dependent diabetes (DIA) was induced by a single i.v. dose of streptozotocin (45 mg.kg-1). Beginning with the subsequent day, animals obtained 6 U insulin daily. Glucose, triglycerides, cholesterol and glycohemoglobin were investigated in blood. ATPase activities, the kinetics of activation of (Na,K)-ATPase by Na+ and K+, further the fluorescence anisotropy of diphenyl-hexatriene as well as the order parameters of membranes in isolated heart sarcolemma (SL) were also investigated. In addition, the degree of glycation and glycation-related potency for radical generation in SL proteins were determined by investigating their fructosamine content. In order to study calcium tolerance of DH in a 'transparent' model, hearts were subjected to calcium paradox (Ca-Pa, 3 min of Ca2+ depletion; 10 min of Ca2+ repletion). In this model of Ca(2+)-overload, Ca2+ ions enter the cardiac cells in a way that is not mediated by receptors. Results revealed that more than 83% of the isolated perfused DH recovered, while the non-DIA control hearts all failed after Ca-Pa. DH exhibited well preserved SL ATPase activities and kinetics of (Na,K)-ATPase activation by Na+, even after the Ca-Pa. This was considered as a reason for their iCaT. Pretreatment and administration of resorcylidene aminoguanidine (RAG 4 or 8 mg.kg-1) during the disease prevented partially the pathobiochemical effects of DIA-induced glycation of SL proteins. DIA-induced perturbations in anisotropy and order parameters of SL were completely prevented by administration of RAG (4 mg.kg-1). Although, the latter treatment exerted little influence on the (Na,K)-ATPase activity, it decreased the calcium tolerance of the DH. Results are supporting our hypothesis that the glycation-induced enhancement in free radical formation and protein crosslinking in SL may participate in adaptive mechanisms that may be also considered as 'positive' and are responsible for iCaT of the DH.

Prevention by 7-oxo-prostacyclin of the calcium paradox in rat heart: role of the sarcolemmal (Na,K)-ATPase.

It is demonstrated a fast and significant depression in the sarcolemmal (Na,K)-ATPase activity that occurs as early as 25 sec after the onset of Ca2+ depletion, and participates in the development of Ca(2+)-paradox in the rat heart. Pretreatment of the animals with 7-oxo-prostacyclin (PGI2) 24-48 h prior to the experiment prevented fairly the Ca(2+)-depletion-induced depression in (Na,K)ATPase activity and the accompanying structural and functional damage to the heart and sarcolemma during Ca(2+)-depletion as well as the development of Ca(2+)-paradox during the subsequent Ca(2+)-repletion. Pretreatment with PGI2 was chosen intentionally because previous experiments revealed, that in its late effect the drug is acting via stabilizing the membranes due induction of high activity of (Na,K)-ATPase that has increased affinity to ATP. From results obtained the following may be concluded: If during the phase of Ca(2+)-deprivation, the capability of heart sarcolemma to maintain sodium extrusion remains preserved, the expected aggravation of Ca(2+)-overload injury to Ca(2+)-paradox that would develop during Ca(2+)-repletion, may be definitely prevented. Sufficiently preserved (Na,K)-ATPase activity, hand in hand with stabilized sarcolemmal structure, may prevent an accumulation of sodium beneath the sarcolemma and consequently also an overexcessive entry of Ca2+ into the myocytes.

The membrane effect of benfluron: modulation of the heart sarcolemmal (Na+, K+)-ATPase and Mg(2+)-ATPase activities.

The effect of Benfluron on the heart sarcolemmal (Na+, K+)-ATPase and Mg(2+)-ATPase activities was studied in crude membrane fraction as well as in purified sarcolemmal membranes isolated from rat heart. Benfluron in concentration range 10(-7) -5 x 10(-5) mol.l-1 did not exert any effect on ATPase activities studied. 10(-4) mol.l-1 Benfluron was stimulatory towards (Na+, K+)-ATPase, while Mg(2+)-ATPase activity was depressed. Kinetic analysis of interaction of Benfluron with (Na+, K+)-ATPase revealed an increase in the Vmax and decrease in the K(m) values for ATP. The possible mechanism of interaction of the drug with (Na+, K+)-ATPase is discussed.

Enzyme kinetics and the activation energy of Mg-ATPase in cardiac sarcolemma: ADP as an alternative substrate.

Increasing concentrations of Mg within a range between 0.1-5.0 mmol/l step-by-step activated the Mg-dependent ATPase and ADPase in rat heart sarcolemma. Both Mg-dependent activities were influenced by NaN3 in a similar way. Also, activation of both enzymes by their substrates, ADP and ATP, were affected by NaN3 in a similar mode. It appears that both enzyme activities are secured by the same system which is capable of ADP hydrolysis during ATP insufficiency. In the absence of naN3 the enzyme revealed higher affinity to ATP than to ADP. The activation energy was lower for ATP hydrolysis. The above findings indicate that at non limiting concentrations of Mg2+ the enzyme is favoring ATP.

Adaptation of the heart to ischemia by preconditioning: effects on energy equilibrium, properties of sarcolemmal ATPases and release of cardioprotective proteins.

Ischemic preconditioning of the heart is referred as a manifest increase in tolerance of the myocardium to otherwise damaging ischemic insult, achieved by one or few consequent initial short exposures to ischemia, each followed by reperfusion of the ischemic area. Several mechanisms such as opening of collateral vessels, the action of catecholamines, inositol phosphates, G-proteins and/or adenosine; inhibition of mitochondrial ATPase, the effects of different endogenous protective substances like heat stress or shock proteins, etc., are believed to cooperate in the mechanism of induction of preconditioning or in maintaining its effect. The present study is an attempt to extend the present knowledge about preconditioning from two aspects: i.) the peculiarities of energy equilibrium in preconditioned myocardium including adaptation of cardiac sarcolemmal ATPases to ischemia and/or hypoxia, and ii) participation of a new endogenous cardioprotective substance in the mechanism of preconditioning. The energy equilibrium in preconditioning is characterized by adaptation of cardiac energy demands to the capacity of energy production and delivery decreased by anaerobiosis and is manifested by constant ratios between ATP, ADP, AMP and the sum of ADN. Principles are proposed that may enable a prediction and mathematical modelling of the balanced energetic state in the preconditioned myocardium. These principles are based on thermodynamics and involve besides others a more economic handling of ATP by sarcolemmal ATPases. The latter enzymes adapt themselves to lowered availability of ATP by decreasing besides their Vmax also their values of Km (increase in the affinity) for ATP and some of them even adjust their activation energy (the anaerobiosis-induced elevation of Ea.t. is missing).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of global ischemia on the sarcolemmal ATPases in the rat heart.

To elucidate the effect of global ischemia on the energy utilizing processes, regarding the molecular principles, the kinetic and thermodynamic properties of the sarcolemmal ATPases were investigated in the rat heart. The activation energy for hydrolysis of ATP during ischemia was higher when the reaction was catalyzed by Ca-ATPase or Mg-ATPase. For the Na,K-ATPase reaction, no changes in the activation energy were observed. With respect to the enzyme kinetics, ischemia in a time-dependent manner induced important alterations in KM and Vmax values of Na,K-ATPase, Ca-ATPase and Mg-ATPase. The Vmax value decreased significantly already after 15 min of ischemia, and it also remained low after 30, 45 and 60 min for all 3 enzymes. The significant diminution of KM values occurred later in the 30th min for Ca-ATPase, in the 45th min for Na,K-ATPase. The observed drop in KM indicates the increase in the affinity of the enzymes to substrate, suggesting thus the adaptation to ischemic conditions on the molecular level. This effect could be attributed to some conformational changes of the protein molecule in the vicinity of the ATP-binding site developing after longer duration of ischemia.

Enzyme kinetics and the activation energy of (Na,K)-ATPase in ischaemic hearts: influence of the duration of ischaemia.

Hearts from male rats were incubated at 37 degrees C for variable periods of global ischaemia. Estimation of kinetic parameters of (Na,K)-ATPase at 37 degrees C in the presence of increasing concentrations of ATP revealed a significant decrease of Vmax in the first 15 minutes of ischaemia with further stabilization at the lowest level in 45-60 minutes of ischaemia. The changes in ATP binding site occurred later after 45 minutes of ischaemia as showed by the decrease of the Km value. As to the activation energy, there were no significant differences between control and ischaemic hearts.

High arrythmogenesis during early reperfusion of ischaemic myocardium: participation of oxygen free radicals.

The early period of reperfusion of ischaemic myocardium leads to a high incidence of severe tachyarrhythmias including ventricular fibrillation (VF), accompanied by a sudden transitional dysfunction. Oxygen free radicals (OFR) have been identified as one of the principal factors responsible for reperfusion-induced events. However, direct evidence for participation of OFR in the arrhythmogenic mechanisms upon reperfusion is still lacking. In the present study, in isolated Langendorff-perfused rat hearts subjected to 30 min global ischaemia, the onset of reperfusion induced 100% incidence of both ventricular tachycardia (VT) and VF with their gradual cessation during 5 min of reperfusion. Generation of H2O2 in the myocardium in the first minutes of reperfusion was demonstrated by means of cerium cytochemistry. There was an increased density of cerium perhydroxide precipitate distributed throughout the myocytes and endothelial cells, confirmed by X-ray microanalysis. The mechanism of the arrhythmogenic effect of OFR may involve the inhibition of the sarcolemmal Na+/K+ ATPase activity, as was revealed by subjecting the isolated sarcolemmal fraction of rat heart to the action of an oxy-radical generating system (H2O2 + FeSO4).

Kinetic and thermodynamic properties of membrane bound Ca-ATPase with low affinity to calcium in cardiac sarcolemma; response to global ischemia of the heart.

Thermodynamic and kinetic properties of membrane bound Ca-ATPase with low affinity to calcium in cardiac sarcolemma were investigated with respect to the effect of global ischemia on the heart. Energy barrier for ATP hydrolysis catalyzed by the Ca-ATPase was slightly higher in hearts subjected to ischemia, as it was evident from increased values of activation energy. Ischemia also induced a time dependent decrease in the activity and maximum velocity (Vmax) value of Ca-ATPase. The depression of enzyme activity was evident as early as 15 minutes after the onset of ischemia. After 30 minutes of ischemia the decrease in Vmax slowed down, probably due to an "adaptational" decrease of the Km value for Ca-ATPase. This phenomenon may be interpreted as a mechanism by which the enzyme attempts to keep working in a situation when the supply of ATP is insufficient.

Nitrendipine modulates the sodium binding site of cardiac (Na,K)-ATPase.

The present study deals with the effect of calcium entry blocking agent, nitrendipine, on the rat heart sarcolemmal (Na,K)-ATPase. Nitrendipine (1 mumol/l) exerted a stimulatory effect on (Na,K)-ATPase activity. Kinetic analysis of enzyme activation by increasing concentrations of sodium in the presence of nitrendipine revealed a 67% increase in Vmax value and a decrease of the apparent Ka value to 33% of that in control. The observed influence on (Na,K)-ATPase may contribute indirectly to the calcium antagonistic effect of nitrendipine.

[Protective effect of 7-oxo-prostacyclin on myocardial calcium overload in the isolated rat heart]. / Protektívny úcinok 7-oxo-prostacyklínu na pret'azenie myokardu vápnikom v izolovanom srdci potkana.

The protective effect of a stable derivative of prostacyclin (7-oxo PGI2) was studied on the model of calcium overload (Ca2+ paradox) 48 h after i.m. administration of the drug in the dose of 50 micrograms/kg. In the isolated rat heart perfused at 37 degrees C and a constant perfusion pressure of 75 Torr (Langendorff preparation) Ca2+ paradox was induced by 3 min perfusion with calcium-free Krebs-Henseleit solution and a subsequent 10 min perfusion with a normal calcium-containing solution. The late protective effect of 7-oxo PGI2 was manifested by improved recovery of heart function (increase of contractility by 50%) and by better preservation of the content of macroergic phosphates (70% sigma ADN) during the Ca2+ repletion phase and of myocardial ultrastructure (sarcolemma) already during the Ca2+ depletion phase. The protective effect of 7-oxo PGI2 can be accounted for by stimulation of Na, K-ATPase activity, otherwise decreased during calcium depletion phase, and by the consequent prevention of alterations in sodium and calcium homeostasis. (Tab. 1, Fig. 4, Ref. 41.)

Increased activity of sarcolemmal (Na+K+)-ATPase is involved in the late cardioprotective action of 7-oxo-prostacyclin.

The delayed effects of 7-oxo-prostacyclin, protecting the heart against extrasystoles, ventricular fibrillation, and cardiac arrest induced by high doses of ouabain or in ischemia and postischemic reperfusion, have already been described; but little is known about the molecular mechanisms involved. In this study, 50 micrograms.kg-1 7-oxo-prostacyclin administered intramuscularly significantly stimulated the activity of (Na+K+)-ATPase in rat heart sarcolemma 24 and 48 hours after application (p less than 0.01 and p less than 0.001, respectively). Kinetic analysis revealed a mixed type of stimulation of ATPase activity, with increased Vmax and decreased Km values. Cycloheximide (1 mg.kg-1) applied together with 7-oxo-prostacyclin, significantly antagonized the stimulatory effect of 7-oxo-prostacyclin, and had a modulatory effect on the kinetics of the (Na+K+)-ATPase both 24 and 48 hours after administration. The results show that protein synthesis is involved in the mechanism of the increase in enzyme activity.

Influence of calcium antagonists on heart sarcolemmal (Na+ + K(+)-ATPase.

The effect of calcium entry blocking agents nitrendipine and flunarizine on Mg(2+)-ATPase, (Mg2+ + Na(+)-ATPase, (Na+ + K(+)-ATPase and (Mg2+ + Ca(2+)-ATPase activities was studied. Nitrendipine (1 mumol/l-1) exerted a stimulatory effect on (Mg2+ + Na(+)-ATPase activity. Kinetic analysis of this effect revealed a two-fold rise in Vmax value and lowered Km value for activation of the enzyme by Na+ ions. In concentrations 10(-7) and 10(-5) mol.l-1 flunarizine behaved as a non-specific inhibitor of all sarcolemmal ATPases investigated. Nevertheless, in concentration of 10(-6) mol.l-1 flunarizine inhibited selectively the (Mg2+ + Na(+)-ATPase and (Na+ + K(+)-ATPase activities of myocardial sarcolemma. These observations provided evidence that both flunarizine and nitrendipine, in the concentration 10(-6) mol.l-1, modulate the (Mg2+ + Na(+)-ATPase and (Na+ + K(+)-ATPase activities in cardiac sarcolemma. These side effects of the above drugs particularly that of nitrendipine might have potential physiological relevance.