Cardiac glucosides: correlations among Na+ K+-ATPase, sodium pump and contractility in the guinea pig heart

Evaluation of the Effectiveness of Dantrolene Sodium against Digoxininduced Cardiotoxicity in Adult Rats

BACKGROUND

Digoxin poisoning commonly occurs in people treated with digoxin. It has been suggested that treatment with dantrolene may be a suitable strategy for digoxin-induced cardiotoxicity.

OBJECTIVE

The aim of this study was to evaluate the protective effect of dantrolene on digoxininduced cardiotoxicity in male rats.

METHODS

This study was approved by the ethics committee of Birjand University of Medical Sciences (Ethical number: IR.BUMS.REC.1400.067). Forty-two Wistar rats weighing between 300- 350 gr were randomly allocated to 7 groups (n = 6) as follows: Normal Saline (NS) group, Normal Saline + Ethanol (NS + ETOH) group, Normal Saline + dantrolene 10 mg/kg (NS + Dan 10) group, Digoxin (Dig) group), Digoxin + dantrolene 5 mg/kg (Dig + Dan 5) group, Digoxin + dantrolene 10 mg/kg (Dig + Dan 10) group, Digoxin + dantrolene 20 mg/kg (Dig + Dan 20) group, Dig was injected intravenously at 12 mL / h (0.25 mg / mL). Dan (5, 10 and 20 mg/kg) was injected intravenously at 5-8 min/mL. After 1 hour, blood samples were obtained from the animals' cavernous sinus and each animal's heartremoved. The blood sample was rapidly centrifuged at 2,500 rpm for 10 minutes and the serum was separated for measurement of creatine phosphokinase (CPK), potassium (K), sodium (Na), calcium (Ca), and magnesium (Mg). The samples were stored at -20°C. The heart samples were fixed in formalin 10% for histopathological evaluation.

RESULTS

K levels slightly increased in the dig group versus the NS group. A significant increase in the K levels was observed in the Dig + Dan 20 group versus the NS group (p < 0.001). Dig slightly decreased Ca levels in the treated group versus the NS group. The levels of Ca significantly increased in the Dig + Dan 10 group versus the Dig group (p < 0.05). Histological examination of the heart tissue in the dig group showed cardiomyocyte degeneration, increased edematous intramuscular space associated with hemorrhage, and congestion. Focal inflammatory cell accumulation in the heart tissue was also seen. Cardiomyocytes were clear and arranged in good order in the Dig + Dan 10 group.

CONCLUSION

dantrolene (10 mg/kg) was cardioprotective in a model of digoxin-induced cardiotoxicity, secondary to cardiac remodeling and hyperkalemia. However, further research is necessary to determine dantrolene's cardioprotective and cardiotoxic doses in animal models.

Effect of chronic pretreatment of guinea-pigs with beta-adrenoceptor agonists on the Na+,K(+)-pump in skeletal muscle

1. The Bmax values for the radioligand [3H]-ouabain in gastrocnemius muscle membranes from guinea-pigs pretreated with vehicle (0.9% saline, 0.1% ascorbic acid), isoprenaline (50 micrograms kg-1) or terbutaline (125 micrograms kg-1) subcutaneously three times daily for 7 days were 5.3 +/- 0.6; 3.1 +/- 0.4 (P less than 0.05) and 2.0 +/- 0.2 pmol mg-1 protein (P less than 0.01) (means +/- S.E.M.) respectively. Thus chronic pretreatment of the guinea-pigs with isoprenaline or terbutaline reduced the density of [3H]-ouabain binding sites by 40 and 60% respectively. 2. The ouabain-sensitive uptake of 86Rb in intact soleus muscles from control and terbutaline pretreated animals were 10.8 +/- 1.2 and 9.0 +/- mmol g-1 wet tissue 20 min-1 respectively. Pretreatment of guinea-pigs with terbutaline did not therefore significantly alter ouabain-sensitive uptake of 86Rb (P greater than 0.1). 3. The results support a close relationship between regulation of beta-adrenoceptors and [3H]-ouabain binding sites. 4. Additional investigations are required to confirm further the dissociation between the function of the pump and the ouabain binding sites measured under the present experimental conditions.

Interpretation of [3H]ouabain binding in guinea-pig ventricular myocardium in relation to sodium pump activity

1. The attempt was made to analyse the complex [3H]ouabain binding curves obtained in intact cardiac ventricular preparations electrically stimulated at different frequencies. The result of this analysis was used to draw conclusions from the binding curves on the frequency dependence of sodium pump activity. 2. [3H]Ouabain binding to isolated, electrically stimulated (1.5 Hz) ventricular strips of guinea-pig hearts was investigated. The positive inotropic effects were studied in separate experiments. Specific [3H]ouabain binding barely reached an equilibrium within 3 h of incubation. A binding curve was constructed using the equilibrium values of specific [3H]ouabain binding obtained at different ouabain concentrations. This binding curve revealed a concentration-proportional component at positive inotropic concentrations and a saturating component at high, toxic concentrations. At very low, inotropically ineffective ouabain concentrations, however, binding values were higher than expected from a linear relationship between ouabain concentration and binding. 3. The peculiar shape of the binding curve could be largely accounted for by a mathematical model, which takes into consideration biochemical properties and physiological regulation of the sodium pump. The model predicts a concentration-proportional pattern of binding which takes place in the non-toxic ouabain concentration range. The slope of the concentration-proportional component of the binding curve should represent a measure of sodium pump activity. 4. Investigation of binding curves at various stimulation frequencies revealed that, as predicted by the model, the slope of the concentration-proportional component of the binding curves was increased and the maximum non-toxic equilibrium binding was decreased with increasing beat frequencies. 5. Quantitative evaluation of the binding curves led to the conclusion that sodium pump activity is a linear function of stimulation frequency in guinea-pig ventricular preparations, the activity in resting preparations amounting to about 15% of the maximum activity. Comparison of the present results with former studies on sodium pump function suggests that [3H]ouabain binding reflects steady-state sodium pump activity. If the complex pattern of binding curves is taken into consideration, [3H]ouabain binding measurements may serve as a means of studying sodium pump function.

Effects of ryanodine on inotropic and arrhythmogenic actions of cardiotonic steroids

In addition to its role in myocardial excitation-contraction coupling, the sarcoplasmic reticulum may be involved in arrhythmogenic actions of cardiotonic steroids. This study in isolated cardiac muscle demonstrates that ryanodine can prevent the arrhythmias and reduce the positive inotropic effect produced by cardiotonic steroids without affecting their specific binding to sites on Na,K-ATPase. These data suggest that the antagonism is mediated by ryanodine-induced alterations in Ca2+ release from the sarcoplasmic reticulum.

Can maximum ouabain-sensitive 86Rb+ uptake rate be obtained by increasing Na+ influx?

Increasing stimulation frequency and monensin were utilized in attempts to enhance Na+ influx and thereby elicit the maximum rate of ouabain-sensitive 86Rb+ uptake in rat atrial muscle. Increasing stimulation frequency between 0 and 9 Hz enhanced the rate of ouabain-sensitive 86Rb+ uptake, whereas the uptake rate appeared to decrease at higher frequencies. Monensin (1.0-5.0 microM) increased the rate of uptake in atrial preparations stimulated at 6 Hz. A higher concentration of monensin produced a significant decline in uptake rate. The maximum rate of ouabain-sensitive 86Rb+ uptake produced by monensin in preparations stimulated at 6 Hz was not significantly different from the maximum elicited by stimulation frequency alone. The results suggest that either method of increasing Na+ influx may be used to maximize the rate of ouabain-sensitive 86Rb+ uptake and thereby estimate the Na+ pump capacity of rat atrial muscle; however, it is possible that the inhibitory actions which were apparent at very high levels of monensin and stimulation frequency may mask the true maximum value.

Effects of BAY K-8644 on inotropic and arrhythmogenic actions of digoxin

Myocardial intracellular 'Ca2+ overload' may be involved in the direct arrhythmogenic actions of cardiotonic steroids. This proposal was examined by determining if the sensitivity of guinea-pig atrial muscle to digoxin-induced arrhythmias was affected by BAY K-8644, a 1,4-dihydropyridine derivative which promotes Ca2+ influx via slow channels. BAY K-8644 significantly reduced both the time required for a given concentration of digoxin to produce arrhythmias and the amount of digoxin bound to atrial muscle at the onset of arrhythmias. In addition, BAY K-8644 increased the maximum developed tension observed in the presence of digoxin before the onset of arrhythmias. Similar results were obtained with increasing concentrations of buffer Ca2+. In contrast, A23187, a Ca2+ ionophore, enhanced the sensitivity to digoxin-induced arrhythmias without affecting maximum developed tension. These results suggest that increases in intracellular Ca2+ enhance cardiac sensitivity to digoxin-induced arrhythmias and that the arrhythmogenic action may involve Ca2+ overload at a pool other than that which activates contractile proteins.

Activities of novel polyhydroxylated cardiotonic steroids purified from nuchal glands of the snake, Rhabdophis tigrinus

Seven novel polyhydroxylated steroids were isolated from the nucho-dorsal glands of the snake, Rhabdophis tigrinus. Biological activities of these steroids in inhibiting (Na+ + K+)ATPase and in producing positive inotropic action were examined in comparison with those of ouabain and gamabufotalin. Gamabufotalin was approximately 10 times more potent than ouabain in inhibiting (Na+ + K+)ATPase. Two compounds, compounds III and XIII, of the seven, produced nearly equipotent enzyme inhibitory activity to ouabain. The activity of the remaining five was relatively low among the compounds tested. All compounds exhibited more or less positive inotropic action in the papillary muscle preparations. The ranking order of the potency was: gamabufotalin greater than ouabain and compound IV greater than compound III and XIII greater than compound I, II, XII and XIV.

Interaction between theophylline and ouabain in the rabbit heart: effect on (Na+ + K+)-ATPase

In electrically stimulated rabbit ventricular strips, theophylline (0.3-30 mM) antagonized the increased contractility produced by ouabain (0.8 microM). Initial velocity of specific [3H]ouabain binding to homogenates prepared from the muscle strips was used to determine the fraction of binding sites occupied by ouabain. Theophylline decreased the binding of ouabain to (Na+ + K+)-ATPase. It is concluded that the effect of theophylline on ouabain-induced positive inotropism may be mediated by decreased ouabain binding to (Na+ + K+)-ATPase.

Stimulation of glucose transport in skeletal muscle by the sodium ionophore monensin

The tissue/medium distribution of the nonmetabolized glucose analog 3-O-methyl-D-glucose was measured in mouse diaphragm muscle and related to changes in 45Ca influx, Na+ content and Na+-pump activity. In the presence of external Ca2+ the sodium ionophore monensin greatly increased cellular Na+ content (and decreased K+ content) although 86Rb uptake, reflecting Na+-pump activity was increased. Concomitantly, 45Ca influx was stimulated, presumably through activation of Na+-Ca2+ exchange. In parallel to the rise in Ca2+ influx sugar transport was also increased. Sugar transport was also increased by monensin in the nominal absence of external Ca2+, when Ca2+ influx was minimal. To test if monensin releases Ca2+ from intracellular storage sites in the absence of external Ca2+, the ionophore was added to medium perfusing rat hind limb preparations and the total Ca content of muscle mitochondria was determined. When Ca2+ was present in the perfusate, monensin increased the mitochondrial Ca content. In the absence of Ca2+, the mitochondrial Ca content was lower and was further depressed by monensin, suggesting that elevation of internal Na+ by monensin may increase mitochondrial Ca2+ loss via activation of Na+-Ca2+ exchange across the mitochondrial membrane. The above results are consistent with the effect of monensin on sugar transport being due to alterations in Ca2+ distribution. They support the earlier conclusion that regulation of sugar transport in muscle is Ca2+ dependent.

Suppression of positive inotropic and toxic effects of cardiac glycosides by amiloride

Effects of amiloride on the inotropic and toxic actions of cardiac glycosides were examined using left atrial muscle isolated from guinea pig heart. Preincubation of atrial muscle with amiloride significantly decreased the maximum positive inotropic effect of dihydrodigoxin but failed to reduce that of isoproterenol. Amiloride prevented the contracture and significantly reduced the incidence of arrhythmias induced by 2 microM digoxin. Similar experiments examining 5 microM digoxin-induced arrhythmias showed that amiloride increased both the time required to produce arrhythmias and the fractional occupancy of sarcolemmal Na,K-ATPase by digoxin at the onset of arrhythmias. The antagonism of cardiac glycoside actions was best observed during the decline in developed tension elicited by amiloride subsequent to its initial positive inotropic effect. Amiloride had no effect on binding site concentration for ATP-dependent [3H]ouabain binding but decreased affinity of the binding sites for ouabain in membrane preparations obtained from guinea pig heart. Furthermore, amiloride inhibited Na,K-ATPase activity and increased the IC50 value for ouabain inhibition of the enzyme. These results indicate that amiloride antagonizes the positive inotropic and toxic effects of cardiac glycosides. Possible mechanisms for the antagonism include inhibition of sarcolemmal Na+/Ca2+ or Na+/H+ exchange.

Assay of muscarinic acetylcholine receptor function in cultured cardiac cells by stimulation of 86Rb+ efflux

An assay for the increase in potassium permeability mediated by muscarinic acetylcholine receptors (mAChR) in cultured cardiac cells is described, using the K+ ion substitute 86Rb+ as the tracer ion. Cardiac cells accumulate 86Rb+ from the extracellular medium in a Na+/K+ ATPase-dependent manner. Subsequent efflux of 86Rb+ in the absence and presence of muscarinic agonists follows kinetics similar to those previously reported for 42K+. The mAChR agonist carbamylcholine (carbachol) stimulated 86Rb+ efflux with an EC50 of 50 nM. The half-time for efflux is reduced by greater than 40% at maximally effective concentrations of agonist. Stimulation of 86Rb+ efflux by carbachol is blocked by the mAChR antagonist atropine with an IC50 of 15 nM. The stimulation of 86Rb+ efflux by carbachol is not affected by the presence of the Na+/K+ ATPase inhibitor ouabain. This assay provides a method for quantitating the mAChR-mediated increase in K+ permeability in cardiac cells without the use of 42K+.

Comparative studies of some semisynthetic K-strophanthins with natural cardiac glycosides

In this study the pharmacological effects of seven semisynthetic cardenolides have been investigated and compared with those of 11 natural cardiac glycosides. These compounds are of different potency on electrically driven isolated guinea-pig atria. Their concentration response curves showed different slopes, which could be an indication of varying therapeutic range. The pharmacodynamics of these compounds on isolated guinea-pig atria are in good correlation with the data obtained from binding studies on guinea-pig ventricular homogenate as well as that obtained from comparative experiments on Na+,K+-ATPase activity inhibition.

Ascorbic acid: an endogenous inhibitor of isolated Na+,K+-ATPase

During attempts to isolate and identify an endogenous ligand for the glycoside binding sites on Na+,K+-ATPase, bovine adrenal glands were found to contain a potent inhibitor of isolated Na+,K+-ATPase. The inhibitory principle was extracted from adrenal cortex, following homogenization in NaHCO3 solution and separation on a Sephadex G-10 column. The active principle was recovered from a fraction which eluted from the column after the 3H2O peak. The extract inhibited isolated Na+,K+-ATPase and the specific [3H]ouabain binding reaction. Sensitivity of the enzyme to the inhibitory action of the extract was species and tissue dependent; however, the pattern and the magnitude of the sensitivity were different from those of the digitalis glycosides. Moreover, the inhibitory principle failed to inhibit sodium pump activity, estimated from ouabain inhibitable 86Rb+ uptake by guinea pig brain slices. The activity of the extract to inhibit isolated Na+,K+-ATPase was stable under acidic condition but was lost rapidly at neutral pH, and could be eliminated by EDTA. In an acidic medium, the inhibitory principle had an absorption maximum at 244 nm which shifted to 264 nm and decayed rapidly at neutral pH. By using mass spectrometry, the principle was identified to be ascorbic acid, which has been shown previously to inhibit isolated Na+,K+-ATPase under appropriate conditions. Because ascorbic acid was incapable of inhibiting the sodium pump in intact cells, this inhibitor of the isolated enzyme does not appear to be the endogenous ligand which regulates sodium pump activity in vivo.

Enhanced ouabain sensitivity of the heart and myocardial sodium pump in aged rats

Tolerance to arrhythmogenic actions of digitalis decreases with advanced age. To determine if aged myocardium has reduced glycoside tolerance, ouabain sensitivity and associated biochemical changes were examined in young (3-month), adult (8-month) and aged (26-month old) Fisher 344 rats, because age-related changes in physiology and biochemistry are well characterized in this strain of rats. Although rats are uniquely tolerant to digitalis, basic mechanisms leading to inotropic and toxic actions of digitalis are not different from other species. In aged anesthetized rats, intravenous infusion of ouabain produced arrhythmias and cardiac arrest at lower doses. Aged myocardium had lower Na,K-ATPase activity and fewer high affinity [3H]ouabain binding sites. Affinity of the ouabain binding sites on Na,K-ATPase was not altered. Despite these findings, sodium pump activity, estimated from the ouabain-sensitive 86Rb+ uptake by ventricular muscle slices, was higher in the aged myocardium when the uptake was observed in the absence of ouabain and was more sensitive to inhibitory action of the glycoside. Differences in Na,K-ATPase and sodium pump data may be explained if sodium leak influx is increased in aged myocardium. Fewer sodium pumping sites and enhanced ouabain sensitivity of the sodium pump seem to be responsible for the reduced tolerance of aged heart to digitalis-induced toxicity.

Temperature-dependent development of ouabain action in isolated guinea pig heart: differences in sodium influx?

At 30 degrees C the development of the positive inotropic action of 0.3 microM ouabain observed in isolated left atrial muscle preparations of guinea-pig heart was significantly slower and more dependent on the frequency of electrical stimulation than it was at 37 degrees C. The hypothesis was tested that such differences are due to a variation in the rate of leak sodium influx, and ensuing differences in glycoside binding to sarcolemmal Na,K-ATPase. Monensin or grayanotoxin, agents which have been shown to increase sodium influx, caused faster development of the inotropic action of ouabain at 30 degrees C when muscle preparations were stimulated at 0.5 or 1 Hz but their effect was smaller at 2 or 3 Hz stimulation. The contracture caused by toxic concentrations of ouabain in quiescent preparations developed faster at the higher temperature. Ouabain binding to sarcolemmal Na,K-ATPase which occurred during exposure of atrial muscle preparations to the glycoside at 25 degrees C for 30 min was less than the corresponding value observed after a similar exposure at 37 degrees C; however, electrical stimulation increased glycoside binding to a greater extent at the lower temperature. The ouabain-sensitive 42K+ uptake was lower in quiescent preparations at 30 degrees C than at 37 degrees C. Moreover, the enhancement of the ouabain-sensitive 42K+ uptake caused by electrical stimulation was greater at 30 degrees C than at 37 degrees C. These results are consistent with the hypothesis that, at lower temperatures, the rate of sodium influx is low and is the factor determining the rate of development of the inotropic action of ouabain. At a higher temperature, a relatively high sodium leak influx seems to make the onset of ouabain action less dependent on the frequency of stimulation.

Digitalis receptors affinity labelling and relation with positive inotropic and cardiotoxic effects

Affinity labelling of the digitalis receptor has indicated that it is situated on the N-terminal part of the alpha-subunit of the (Na+,K+)ATPase. Biochemical and pharmacological properties of the (Na+,K+)ATPase studied on intact chick embryonic hearts and under heart cell culture conditions have indicated the existence of two families of ouabain binding sites i.e.: a low affinity binding sites with a dissociation constant (Kd) of 2-6 microM for the ouabain-receptor complex and a high affinity binding site with a Kd of 26-48 nM. High and low affinity sites also are present at all embryonic stages studied. Inhibition of 86Rb+ uptake in cultured cardiac cells and increase in intracellular Na+ concentration, due to (Na+,K+)ATPase blockade, occur in an ouabain concentration range corresponding to the saturation of the low affinity ouabain site. Ouabain stimulated 45Ca2+ uptake increases in parallel with the increase in the intracellular Na+ concentration. It is suppressed in Na+ free medium or when Na+ is replaced by Li+ suggesting that the increase is due to the indirect activation of the Na+/Ca2+ exchange system in the plasma membrane. Dose-response curves for the inotropic effects of ouabain on papillary muscle and on ventricular cells in culture indicate the development of the cardiotonic properties is parallel to the saturation of the low affinity binding site for ouabain. Therefore, inhibition of the cardiac (Na+,K+)ATPase corresponding to low affinity ouabain binding sites seems to be responsible for both the cardiotonic and cardiotoxic effects of the drug.

How increased sodium influx enhances digoxin-induced arrhythmias in guinea-pig atrial muscle

The hypothesis that digitalis-induced arrhythmias occur when Na,K-ATPase inhibition exhausts the sodium pump reserve capacity, producing an accumulation of intracellular Na+, was tested by reducing the reserve capacity in isolated left atrial muscle of guinea pig heart and estimating specific digoxin binding and Na,K-ATPase activity in atrial muscle homogenized at the onset of digoxin-induced arrhythmias. Reductions in reserve capacity were produced by either increasing the stimulation frequency of the atrial muscle or adding a sodium ionophore, monensin, to the media bathing the tissue. As stimulation frequency was increased, both the time required to produce arrhythmias with a given concentration of digoxin and the amount of digoxin bound to sarcolemmal Na,K-ATPase at the onset of arrhythmias were reduced. Similarly, monensin treatment produced reductions in the time to arrhythmia and in digoxin binding and Na,K-ATPase inhibition observed at the onset of arrhythmias. These results support the above proposal suggesting that a decrease in reserve capacity of the sodium pump enhances cardiac sensitivity to digitalis-induced arrhythmias.

Consequences of specific [3H]ouabain binding to guinea pig left atria and cardiac cell membranes

An analysis of [3H]ouabain binding to electrically stimulated, contracting guinea pig left atria gave the following results. (1) A non-linear Scatchard plot with at least two binding sites: a high-affinity site (KD 1.1 X 10(-6) M) with about 430 receptors/micron2 related to positive inotropy, and a low-affinity site (KD' 2.1 X 10(-4) M) with about 18,000 receptors/micron2, possibly related to (Na+ + K+)ATPase inhibition. A crude left atrial homogenate gave about 530 receptors/micron2. (2) Half-maximal positive inotropic effects occurred at about 4 X 10(-7) M. (3) 86Rb+-uptake was significantly increased at all inotropic ouabain concentrations (10(-7) - 10(-6) M). Toxic concentrations (above 2 X 10(-6) M) inhibited 86Rb+-uptake (half-maximal inhibition at about 5 X 10(-6) M). [3H]Ouabain binding to partly purified guinea pig cardiac cell membranes showed: (a) linear Scatchard plots for (Mg2+, Pi)- and (Na+, ATP, Mg2+)-supported binding (KD 1.18 X 10(-7) M and 1.49 X 10(-7) M, respectively); (b) non-linear Scatchard plots for (Tyrode + ATP)-supported binding (KD 4.7 X 10(-7) M; KD' 6 X 10(-6) M); and (c) half-maximal [3H]ouabain binding occurred at a lower concentration (about 3.2 X 10(-7) M) than half-maximal inhibition of (Na+ + K+)ATPase activity (about 7.2 X 10(-7) M). Thus, we conclude that there may be more than one type of ouabain binding site in guinea pig left atria, and that measurable inhibition of (Na+ + K+)ATPase is not necessarily related to positive inotropy in the guinea pig.

On the binding of a 3-alpha-methylated digitoxigenin-glucoside to ouabain receptors in heart muscle homogenate

3-alpha-Methyl substituted digitoxigenin-3-beta-glucoside (methyl-dtg-gluc) displays unusual features, e.g. a high dissociation rate constant from its binding site leading to rapid reversibility of the inotropic and toxic effects, and a flat dose-response curve attaining higher inotropic maxima thus indicating an increased therapeutic index in animal experiments. In order to check whether or not methyl-dtg-gluc is specifically bound to the same receptors as classic cardiac glycosides we compared binding of ouabain and of methyl-dtg-gluc to guinea-pig heart muscle homogenate. For both compounds specific binding required the addition of ATP (2.5 mM). The binding curve for ouabain yielded half maximum binding at 1.3 x 10(-7) M and maximum number of binding sites of 6 pmole/mg protein; the corresponding values for methyl-dtg-gluc amounted to 1.4 x 10(-6) M and 6 pmole/mg protein, respectively. A mutual competition could be demonstrated between the two compounds. Since the provided data are equilibrium values they do not exclude higher turnover rates of methyl-dtg-gluc in comparison with ouabain at a given glycoside-ATPase complex concentration which can be expected from the fast dissociation rate constant of the methyl-dtg-gluc-ATPase complex. The results are briefly discussed with respect to the molecular mole of action of cardiac glycosides.

Ouabain sensitivity of Rb+ uptake in canine Purkinje fibre and ventricular muscle

Ouabain inhibitable active Rb+ uptake was compared in canine Purkinje fibre and the left ventricular muscle to define the possible difference in electrophysiological sensitivity to ouabain in the two tissues. Ouabain inhibitable, active uptake of Rb+ was calculated from the difference between total Rb+ uptake assayed in the absence of ouabain and nonspecific Rb+ accumulation assayed in the presence of 10(-4) mol/l ouabain. A similar degree of active Rb+ uptake was inhibited by ouabain in both Purkinje fibre and the left ventricular muscle from the same animal, even though the Purkinje fibre took up more 86Rb+ than the ventricle. The specific tissue binding of ouabain in vitro was also not significantly different in the functionally different two tissues. It is concluded that there is no differential ouabain sensitivity of Rb+ uptake in canine Purkinje fibre and ventricular muscle.

Correlation between catecholamine release and sodium pump inhibition in the perfused adrenal gland of the cat

1 Ca(2+) reintroduction to retrogradely perfused and ouabain (10(-4) M)-treated cat adrenal glands caused a catecholamine secretory response which was greater the longer the time of exposure to the cardiac glycoside. Such a response was proportional to the external Na(+) concentration [Na(+)](o).2 A qualitatively similar, yet smaller response was observed when glands were perfused with Krebs solution lacking K(+) ions; thus, K(+) deprivation mimicked the secretory effects of ouabain. Catecholamine secretion evoked by Ca(2+) reintroduction in K(+)-free solution (0-K(+)) was also proportional to [Na(+)](o) and greater the longer the time of exposure of the gland to 0-K(+) solution.3 The ionophore X537A also mimicked the ouabain effects, since Ca(2+) reintroduction to glands treated with this agent (25 muM) caused a sharp secretory response. When added together with X537A, ouabain (10(-4) M) did not modify the response to the ionophore.4 N-ethylmaleimide (NEM), another Na(+), K(+)-ATPase inhibitor, did not evoke the release of catecholamines; on the contrary, NEM (10(-4) M) inhibited the catecholamine secretory response to high [K(+)](o), acetylcholine, Ca(2+) reintroduction and ouabain.5 Ouabain (10(-4) M) inhibited the uptake of (86)Rb into adreno-medullary tissue by 60%. Maximal inhibition had already occurred 2 min after adding the drug, indicating a lack of temporal correlation between ATPase inhibition and the ouabain secretory response, which took longer (about 30-40 min) to reach its peak. NEM (10(-4) M) blocked (86)Rb uptake in a similar manner.6 The results are further evidence in favour of the presence of a Na(+)-Ca(2+) exchange system in the chromaffin cell membrane, probably involved in the control of [Ca(2+)](i) and in the modulation of catecholamine secretion. This system is activated by increasing [Na(+)](i), either directly (ionophore X537A, increased [Na(+)](o)) or indirectly (Na(+) pump inhibition). However, the simple inhibition of Na(+) pumping does not always lead to a catecholamine secretory response; such is the case for NEM.

Ouabain effects on intracellular potassium activity and contractile force in cat papillary muscle

The relationship between the positive inotropic and toxic effects of cardiac glycosides and their effects on intracellular ionic composition is incompletely defined. We measured intracellular potassium activity (alpha ik), extracellular potassium activity (alpha ek), resting potential, action potential duration, and contractile force at 32 degrees C in paired papillary muscles from feline right ventricles exposed to ouabain. Muscles used for electrophysiological measurements were quiescent except for isolated stimulation to confirm impalement and record action potential duration. Muscles used for contractile force measurements were quiescent except for 4-min periods when force was measured at a cycle length of 1,400 ms. Muscle length was adjusted to achieve 50% of maximal tension at this cycle length before each experiment. In four experiments, alpha ik and contractile force were measured in the same muscle. Alpha iK was measured with single and double-barrel K-sensitive electrodes. At 10 nM ouabain, action potential duration is prolonged. Among the concentrations tested, the threshold for a clear positive inotropic effect is 0.1 microM ouabain. The threshold for decrease in alpha iK, increase in alpha eK, and decrease in membrane potential is 1 microM, at which concentration toxic signs develop, including arrhythmias, aftercontractions, and alteration in the staircase response of contractile force to repetitive stimulation. Ouabain need not change alpha iK to effect positive inotropy in ventricular muscle, a relationship different from that reported between [K]i (intracellular potassium concentration) and positive inotropy. Higher ouabain concentrations, which others have shown to clearly inhibit active Na and K transport, are shown to upset intracellular potassium activity homeostasis and to consistently produce toxicity.

Inotropic effects and Na+,K+-ATPase inhibition of ouabain in isolated guinea-pig atria and diaphragm

Effects of ouabain on force of contraction were compared in electrically driven isolated tissue preparations of guinea-pig left atria and diaphragm. A distinct and steady positive inotropic effect of ouabain was observed in atrial preparations, whereas in diaphragm preparations, ouabain produced only a slight and transient positive inotropic effect, followed by the negative inotropic phase. The transient positive inotropic effect of ouabain was observed even in the absence of extracellular calcium, but was markedly dependent on the extracellular sodium concentration. In vitro [3H]ouabain binding studies revealed that the affinity of Na+,K+-ATPase for ouabain was about eight times higher and tissue concentration of the enzyme was significantly lower in diaphragm than in cardiac tissue. The Ki value for ouabain inhibition of the cardiac Na+,K+-ATPase was also approximately ten times higher than for the diaphragm enzyme. Ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity, was inhibited by ouabain at a time when it produced its transient positive inotropic effect in diaphragm preparations. These results indicate that the lack of a distinct and steady positive inotropic effect of ouabain in diaphragm was due neither to the difference in the ouabain-Na+,K+-ATPase interaction between diaphragm and cardiac tissues nor the failure of sodium pump inhibition by ouabain in diaphragm.

Accumulation of [3H]-ouabain in functionally different canine cardiac tissues: differential Rb+ uptake

1 In order to understand tissue-specific differences in sensitivity to ouabain, the distribution of [3H]-ouabain was investigated in seven functionally different, canine cardiac tissues. 2 Specialized cardiac tissues (e.g., pacemaker or conducting tissues) accumulate much less ouabain than do the contractile tissues such as the left ventricle or the papillary muscle. 3 This study confirms a greater Rb+ uptake in the Purkinje fibre than in the contractile tissue. 4 The results disprove the hypothesis that a high accumulation of ouabain in the Purkinje fibre is responsible for the well-known sensitivity to ouabain in this tissue.

Relationship between the positive inotropic effect of ouabain and its inhibitory effects on Na+, K+ -ATPase and active transport of Rb+ in the dog heart

Two previous reports have indicated that in the dog if sustained positive inotropy without arrhythmias is induced by ouabain infusion, inhibition of myocardial Na+, K+-activated adenosine triphosphatase is not observed. Another previous study has shown that in similar experiments induction of sustained inotropy is accompanied by inhibition of the active uptake of Rb+. To resolve the apparent inconsistencies between these findings, we repeated the experiments and measured the inhibition of the enzyme and the inhibition of Rb+ uptake after the induction of sustained inotropy with ouabain. Utilizing three different ouabain dosage regimens in open-chested dogs, we obtained three different quasi-steady state plasma levels of ouabain. At the highest level, ouabain toxicity accompanied by enzyme inhibition and inhibition to RB+ uptake was observed. With the intermediate levels, positive inotropy without significant toxicity was also accompanied by enzyme inhibition and the inhibition of Rb+ uptake. At the lowest ouabain plasma levels, sustained positive inotropy without significant inhibition of either enzyme activity or +b+ uptake was obtained. The data indicate that in this preparation either a small degree of enzyme inhibition, which is difficult to detect is sufficient to produce pronounced positive inotropy, or that the inhibition of the enzyme is not required for the induction of positive inotropy.

Is ouabain-sensitive rubidium or potassium uptake a measure of sodium pump activity in isolated cardiac muscle?

(1) The significance of the specific (ouabain-sensitive) 86Rb+ or 42K+ uptake by cardiac muscle preparations which are not 'sodium-loaded' was studied. (2) In left atrial preparations of guinea-pig heart, resting 86Rb+ uptake was relatively low. It was markedly increased by electrical stimulation. This stimulated uptake was further enhanced by isoproterenol and inhibited by verapamil. (3) In rat atria, the resting 86Rb+ uptake was somewhat higher than in guinea-pig atria, and the increase in uptake caused by electrical stimulation was smaller. In guinea-pig right ventricular papillary muscle, the resting uptake was highest among those tissues studied, and the response to electrical stimulation was smallest. In the latter tissue, verapamil produced only a minimal inhibition of the specific 86Rb+ uptake. (4) The effect of the frequency of electrical stimulation of 86Rb+ uptake paralleled its influence on the force of contraction, suggesting the involvement of intracellular sodium in both events. (5) In both left atrial and right papillary muscle preparations of guinea-pig heart, specific 42K+ uptake observed with 5.8 mM K+ was relatively high, and was increased only slightly by electrical stimulation. This electrical stimulation, however, increased ouabain-induced inhibition of 42K+ uptake, suggesting that the stimulation increases the amount of Na+ available to the sodium pump. (6) When the K+ concentration was 1 mM, the resting 42K+ uptake was low, and could be enhanced by electrical stimulation. (7) Thus, in cardiac muscle preparations which are not sodium loaded, the specific 86Rb+ or 42K+ uptake can be used to estimate the rate of sodium influx, which is equivalent to the rate of sodium efflux under steady-state conditions, provided that neither Rb+ nor K+ is in excess compared to the Na+ available to the pump. If Rb+ or K+ is in excess, its specific uptake may not reflect changes in transmembrane Na+ movement.

Differing sensitivities of Purkinje fibers and myocardium to inhibition of monovalent cation transport by digitalis

The extent of inhibition of monovalent cation active transport in Purkinje fibers and myocardium in response to toxic and inotropic doses of digitalis were studied in the dog to elucidate the factors underlying the different relative sensitivities of these tissues to the toxic arrhythmogenic effects of digitalis. Monovalent cation transport inhibition was assessed by measuring uptake of the K(+) analog Rb(+) in samples of myocardium and Purkinje fibers after in vitro ouabain exposure and after acute or chronic administration of digoxin in vivo. The active uptake of Rb+ was determined as the difference between total uptake and uptake in the presence of 1.0 mM ouabain. Mean active uptake of Rb(+) by Purkinje fibers from control hearts was 1.62+/-0.11 (SEM) nmol/mg wet wt per 15 min, significantly greater than the value of 0.49+/-0.05 for myocardium (P < 0.001). Concentration-effect curves for inhibition of monovalent cation active transport by in vitro exposure to graded concentrations of ouabain showed that the concentration for half-maximal inhibition of monovalent cation transport, IC(50), for Purkinje fiber transport was 0.4 muM, significantly less than the value of 1.4 muM for myocardial samples. Dogs were given toxic doses of digoxin (0.3 mg/kg i.v.). At onset of sustained ventricular tachycardia, they were killed and monovalent cation transport measured in myocardial and Purkinje fiber samples. Active Rb(+) uptake was inhibited by 44% in myocardial samples, whereas a significantly greater inhibition of 76% was noted in Purkinje fibers (P < 0.01). Similar data were obtained for both transmural myocardial biopsy samples and subendocardial trabecular samples obtained from regions adjacent to Purkinje fibers. Another group of dogs received a nontoxic dose of 0.02 mg/kg i.v. daily for 6 d. Myocardium showed a 17% reduction in Rb(+) active uptake compared to control animals receiving no drug, whereas Purkinje fiber transport was reduced in these dogs to a significantly greater extent averaging 44% (P < 0.001). Thus, both toxic and inotropic (non-toxic) doses of digoxin inhibited monovalent cation transport in Purkinje fibers to a greater extent than in myocardium. This difference in apparent sensitivity of monovalent cation transport to digoxin may contribute to the previously reported tendency of digitalis toxic arrhythmias to arise in Purkinje fibers.

Possible mechanisms for inotropic actions of vanadate in isolated guinea pig and rat heart preparations

The mechanism of inotropic actions of vanadate studied in isolated, electrically stimulated atrial and ventricular muscle preparations of rat or guinea-pig heart. Vanadate produced a negative inotropic effect in guinea-pig left atrial preparations associated with a marked shortening of the action potential plateau. In guinea-pig papillary muscle, or rat atrial or ventricular muscle preparations, vanadate produced a positive inotropic effect, which was not affected by either propranolol, phentolamine or metiamide. The positive inotropic effect was observed when action potential duration was either increased or decreased. Inotropic concentrations of vanadate failed to significantly alter the ouabain-sensitive 86Rb+-uptake, an estimate of sodium pump activity, or tissue concentration of cyclic AMP in electrically stimulated preparations. In partially depolarized rat atrial preparations in which fast sodium channels were inactivated in the presence of a high concentration of K+ (22 mmol/l), vanadate restored electrical activity (calcium-dependent action potentials) and the contraction, similar to isoproterenol. This action of vanadate was abolished by Mn2+, a slow channel inhibitor, but not by tetrodotoxin. The characteristic of vanadate- and isoproterenol-restored preparations, however, were substantially different. Moreover, vanadate failed to restore the contraction or action potential in partially depolarized guinea pig atrial preparations unlike isoproterenol. These results indicate that vanadate may either enhance or inhibit slow channels in cardiac muscle.

Independence of the positive inotropic effect of ouabain from the inhibition of the heart Na+/K+ pump

Isolated left atria from guinea pigs were stimulated at 3.3 Hz and bathed at 30 degrees C in Tyrode's solution containing 6 mM KCl. After equilibration, this solution was replaced by a low-K solution or by Tyrode's solution containing ouabain or dihydroouabain. These treatments evoked an increase in the contractility of the atria. The time to peak increase was about 30 min, and the inotropic effect was sustained for at least 40 min. After 30 min, 42K was added to the bathing solution in order to estimate the activity of the Na+/K+ pump. A linear relationship was observed between the degree of inhibition of the Na+/K+ pump and the increase in systolic tension. The regression line was the same for low-K solutions and dihydroouabain but not for ouabain. For a given degree of inhibition of the pump, ouabain evoked a higher increase in contractility. These findings indicate that inhibition of the Na+/K+ pump can be the only mechanism responsible for the positive inotropic effect of dihydroouabain but cannot be the sole mechanism for that of ouabain.

The cardioactive properties of SC4453, a digoxin analogue with a C17 beta-pyridazine ring

SC4453 is a digoxin analogue with a pyridazine instead of a lactone ring in C17 beta. SC4453 was compared with digoxin with respect to effect on contractility and on activity of the sodium pump in guinea-pig isolated left atria stimulated at 3.3 Hz. The two glycosides stimulated the sodium pump at low concentrations and inhibited at high concentrations. At the time to peak inotropic effect, for a similar inhibition of the Na pump, the increase in systolic tension was higher with SC4453 than with digoxin, whereas the increase in diastolic tension was similar for both. These observations confirm that the inhibition of the Na pump is not the only mechanism responsible for the positive inotropic effect of cardiac glycosides.

Ouabain receptor binding of hydroxyprogesterone derivatives

1 A specific and sensitive radioreceptor assay ahs been devised which is based on high affinity, saturable binding of 9 nM [3H]-ouabain to the total particulate fraction isolated from dog heart. Ouabain and other cardiac glycosides, including the aglycones, were about equipotent in their ability to displace [3H]-ouabain from its receptor, the IC50s ranging from 10 to 30 nM. 2 The only other substances found to compete significantly in the assay were derivatives of hydroxyprogesterone having a 17 alpha-acetate substituent: chlormadinone acetate, megestrol acetate, cyproterone acetate and medroxyprogesterone acetate, with IC50s of 2, 7.4, 9 and 21 microM, respectively. Prednisolone-3,20-bisguanyl-hydrazone, reported to have inotropic activity, gave an IC50 of 6.4 microM. Cyproterone-17 alpha-OH was less active (IC50 90 microM) than cyproterone-17 alpha-acetate. 3 A large number of peptide and protein hormones, steroid hormones and their metabolites, amines, and drugs were inactive.

Differential effect of potassium on the action of digoxin and digoxigenin in guinea-pig heart

The effect of potassium on the binding of digoxin or digoxigenin to isolated Na+, K+-ATPase was compared with that of potassium on the positive inotropic action of the agents in guinea-pig hearts. The binding of digoxigenin to the enzyme in vitro was reduced to a greater extent by potassium than was the binding of digotoxin. The digoxigenin-induced increase in the force of contraction of left atrial preparations estimated at steady state was reduced at higher potassium concentrations. Potassium had a lesser effect when digoxin was used as the inotropic agent. In contrast, potassium concentrations. Potassium had a lesser effect when digoxin was used as the ininotropic agent. In contrast, potassium reduced the rate of development and also the rate of loss of the positive inotropic action of digoxin observed with left atrial and Langendorff preparations, respectively, to a greater extent than those of digoxigenin. The loss of the positive inotropic effect was more rapid with digoxigenin than with digoxin at each KCl concentration. These data support the contention that the extent of the interaction of digitalis with Na+,K+-ATPase determines the degree of the positive inotropic effect.

Sodium influx rate and ouabain-sensitive rubidium uptake in isolated guinea pig atria

1. Ouabain-sensitive 86Rb+ uptake by tissue preparations has been used as an estimate of Na+ pump activity. This uptake, however, may be a measure of the Na+ influx rate, rather than capacity of the Na+ pump, since intracellular Na+ concentration is a determinant of the active Na+/Rb+ exchange reaction under certain conditions. This aspect was examined by studying the effect of altered Na+ influx rate on ouabain-sensitive 86Rb+ uptake in atrial preparations of guinea pig hearts. 2. Electrical stimulation markedly enhanced ouabain-sensitive 86Rb+ uptake without affecting nonspecific, ouabain-insensitive uptake. Paired-pulse stimulation studies indicate that the stimulation-induced enhancement of 86Rb+ uptake is due to membrane depolarizations, and hence related to the rate of Na+ influx. 3. Alterations in the extracellular Ca2+ concentration failed to affect the 86Rb+ uptake indicating that the force of contraction does not influence 86Rb+ uptake. 4. Reduced Na+ influx by low extracellular Na+ concentration decreased 86Rb+ uptake, and an increased Na+ influx by a Na+-specific ionophore, monensin, enhanced 86Rb+ uptake in quiescent atria. 5. Grayanotoxins, agents that increase transmembrane Na+ influx, and high concentrations of monensin appear to have inhibitory effects on ouabain-sensitive 86Rb+ uptake in electrically stimulated and in quiescent atria. 6. Electrical stimulation or monensin enhanced ouabain binding to (Na+ + K+)-ATPase and also increased the potency of ouabain to inhibit 86Rb+ uptake indicating that the intracellular Na+ available to the Na+ pump is increased under these conditions. 7. The ouabain-sensitive 86Rb+ uptake in electrically stimulated atria was less sensitive to alterations in the extracellular Na+ concentration, temperature and monensin than that in quiescent atria. 8. These results indicate that the rate of Na+ influx is the primary determinant of ouabain-sensitive 86Rb+ uptake in isolated atria. Electrical stimulation most effectively increases the Na+ available to the Na+ pump system. The ouabain-sensitive 86Rb+ uptake by atrial preparations under electrical stimulation at a relatively high frequency seems to represent the maximal capacity of the Na+ pump in this tissue.

Ischemic-induced alterations in cardiac sensitivity to digitalis

Intravenous infusion of acetylstrophanthidin to 6 dogs, after a 60 min left anterior descending coronary artery occlusion, was associated with a 43.0 +/- 10.5% decrease in the dose of digitalis needed to produce ventricular arrhythmias as compared to the pre-ischemic dose (97.5 +/- 8.0 microgram/kg). Reperfusion of the ischemic region for 2 h after a 90 min occlusion resulted in a 54.4 +/- 6.7% decrease in the arrhythmogenic dose. Direct intracoronary infusions of digitalis into the ischemic region, after a 90 min coronary occlusion followed by 2 h of reperfusion, was associated with a 47.7 +/- 6.4% decrease in the dose of digitalis needed to produce arrhythmias. The pre-ischemic (control) arrhythmogenic dose of digitalis via the intracoronary infusion method was 1.5 +/- 0.3 microgram/kg (mean +/- S.E.M. of 7 dogs). Sodium pump activity, estimated from the ouabain-sensitive 86Rb uptake in sodium-loaded ventricular slices, was significantly higher in slices obtained from the ischemic regions (6.84 +/- 0.30 nmoles 86Rb/mg dry wt. (mean +/- S.E.M.), than from the non-ischemic regions (3.43 +/- 0.64 nmoles 86Rb/mg dry wt.). Sensitivity of the sodium pump activity to the inhibitory effect of ouabain also was increased in the ischemic regions as indicated by a shift in the log dose--response curve to the left. Thus, it appears that there is an increase in myocardial sensitivity to the toxic effect of digitalis after temporary ischemia and it appears to be related to an increase in the sensitivity of the Na+,K+-ATPase or sodium pump to the inhibitory effect of digitalis.

Saturable adenosine 5'-triphosphate-independent binding of [3H]-ouabain to brain and cardiac tissue in vitro

1. Several investigators have proposed that membrane Na+, K+-adenosine 5'-triphosphatase (Na+, K+-ATPase) is a mechanism for the transmembrane transport of cardiac glycosides, rather than the receptor for pharmacological actions of these agents. This implies that the glycosides bind to an intracellular constituent (receptor) other than Na+, K+-ATPase. 2. In search for such a receptor site, saturable ATP-independent [3H]-ouabain binding was studied in rat brain and dog and guinea-pig heart homogenates. The binding of the glucoside to this site results in a relatively unstable complex which is stabilized by K+ to a lesser extent than is the complex formed with the ATP-dependent binding to Na+, K+-ATPase. 3. The ATP-independent ouabain binding sites are more abundant in rat brain tissue than in cardiac tissue, and have a lower ouabain affinity compared to the binding sites on Na+, K+-ATPase. 4. These results do not support the contention that there are intracellular inotropic receptors for digitalis.

Effects of lithium and thallous ions on sodium pump activity in the guinea-pig heart and their relationship to the positive inotropic action

It has recently been demonstrated that both Tl+ and Li+ produce concentration- and time-dependent positive inotropic effects in guinea-pig atrial preparations although Tl+ inhibits and Li+ stimulates isolated Na+,K+-ATPase in vitro. In order to elucidate the mechanism of the positive inotropic actions of these cations, the effects of Tl+ and Li+ on sodium pump activity were studied. Active 86Rb uptake in guinea-pig ventricular slices, an estimate of sodium pump activity, was highly sensitive to the inhibitory effect of the cardiac glycosides. Preincubation of slices with Tl+ caused a dose- and time-dependent inhibition of active 86Rb uptake. Similar concentration- and time-dependent inhibition of active 86Rb uptake was observed when Na+ in a Krebs-Henseleit solution was partially replaced with Li+. Lithium, however, stimulated a partially purified Na+,K+-ATPase in vitro. During heart slice incubation, Tl+ and Li+ accumulated in a time-dependent manner. This accumulation was not readily reversible when slices were transferred into Tl+- or Li+-free solutions. It appears that the inhibition of sodium pump activity is related to the positive inotropic action of these cations.

Reduction of the equilibrium binding of cardiac glycosides and related compounds to Na+,K+-ATPase as a possible mechanism for the potassium-induced reversal of their toxicity

The influence of potassium ions on the equilibrium state of the binding of cardiac glycosides and their derivatives to partially purified dog heart and rat brain enzyme preparations was studied in vitro. The addition of potassium to the incubation mixture containing enzyme preparation, 3H-ouabain, Na+, Mg2+ and ATP, at the time when the binding reaction is close to equilibrium, caused an immediate reduction of the bound drug concentration; the concentration apparently shifting toward a lower equilibrium state. The degree of the potassium-induced reduction in bound drug concentration was dependent on the potassium concentration and on the chemical structure of the compound. The binding of aglycones, pentacetyl-gitoxin and cassaine was affected to a greater extent than that of the glycosides. These data suggest that one of the mechanisms by which potassium antagonizes the toxic actions of digitalis on the heart is to reduce the drug binding to cardiac Na+,K+-ATPase.

Correlation between inhibition of (Na+, K+)-membrane-ATPase and positive inotropic activity of cardenolides in isolated papillary muscles of guinea pig

Concentrations of 17 cardenolides, cardenolide glucuronides and sulfates producing half-maximal inhibition of (Na+, K+)-membrane-ATPase from different organs and animal species were determined in vitro. In addition the concentrations that increased the contractility of guinea pig isolated papillary muscles to a particular level were investigated. Comparisons between ATPase-inhibiting and positive inotropic cardiac activities showed extensive parallelism: the correlation coefficients after log/log transformation were between 0.92 and 0.97. The same close correlations are found if dissociation constants of cardenolide receptor complexes and concentrations causing 86Rb-uptake inhibition in human erythrocytes are examined. The concentrations necessary for inhibition of (Na+, K+)-membrane-ATPase of the guinea pig heart and the concentrations required to achieve a defined positive inotropic effect in guinea pig papillary muscle showed a log/log correlation coefficient of 0.97 (P less than 0.001). In both tests the potencies covered more than three orders of magnitude. The results support Repke's hypothesis on the digitalis receptor.

Prednisolone-3, 20-bisguanylhydrazone: Na+, K+-ATPase inhibition and positive inotropic action

The relationship between two known actions of prednisolone-3, 20-bisguanylhydrazone (PBGH); Na+, K+-ATPase inhibition and positive inotropic effects, was investigated. In electrically driven left atrial preparations of guinea pig heart, the positive inotropic action of PBGH was not affected by beta-adrenergic or histamine antagonists. Pretreatment of animals with reserpine also failed to influence the positive inotropic action of PBGH. Inotropic concentrations of PBGH inhibited Na+, K+-ATPase and the ATP-dependent binding of (3/)-ouabain to Na+, K+- ATPase preparations in vitro. Additionally, ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity was inhibited when sodium-loaded ventricular slices were obtained from Langendorff preparations at the peak inotropic response to PBGH. Guinea pig heart was highly sensitive to PBGH to the positive inotropic action, the inhibition of Na+, K+ -ATPase and (3H)-ouabain binding, whereas rat, rabbit and dog heart were markedly less sensitive. These findings suggest that the mechanism of the positive inotropic action of PBGH resembles that of ouabain and probably involves Na+, K+ -ATPase inhibition, although the mode of interaction of these steroids with Na+, K+ -ATPase may be different.