Inotropic Effect Of Ouabain Research Articles

Hypercontractile Cardiac Phenotype in Mice with Migraine-Associated Mutation in the Na+,K+-ATPase α2-Isoform.

Two α-isoforms of the Na+,K+-ATPase (α1 and α2) are expressed in the cardiovascular system, and it is unclear which isoform is the preferential regulator of contractility. Mice heterozygous for the familial hemiplegic migraine type 2 (FHM2) associated mutation in the α2-isoform (G301R; α2+/G301R mice) have decreased expression of cardiac α2-isoform but elevated expression of the α1-isoform. We aimed to investigate the contribution of the α2-isoform function to the cardiac phenotype of α2+/G301R hearts. We hypothesized that α2+/G301R hearts exhibit greater contractility due to reduced expression of cardiac α2-isoform. Variables for contractility and relaxation of isolated hearts were assessed in the Langendorff system without and in the presence of ouabain (1 µM). Atrial pacing was performed to investigate rate-dependent changes. The α2+/G301R hearts displayed greater contractility than WT hearts during sinus rhythm, which was rate-dependent. The inotropic effect of ouabain was more augmented in α2+/G301R hearts than in WT hearts during sinus rhythm and atrial pacing. In conclusion, cardiac contractility was greater in α2+/G301R hearts than in WT hearts under resting conditions. The inotropic effect of ouabain was rate-independent and enhanced in α2+/G301R hearts, which was associated with increased systolic work.

Comparative effects of ouabain on isolated papillary muscle from tree shrews, guinea-pigs and rats

Inotropic effects of ouabain were investigated in the isolated papillary muscle preparations of the tree shrew (Tupaia glis), guinea-pigs and rats. In the guinea-pig and shrew papillary muscles, ouabain at concentrations of 10(-8) to 3 x 10(-7) M caused a concentration-dependent positive inotropic response in a similar magnitude, while in the rat, ouabain at concentrations of 10(-7) to 3 x 10(-6) M elicited negative inotropic one. Either phentolamine or pindolol in a concentration sufficient to block the alpha- and beta-adrenoceptors did not antagonize the positive inotropic effect of ouabain in the papillary muscle preparation of the shrews.

Factors underlying the variable inotropic effect of ouabain on isolated rat atria. Changes in contractile frequency and adrenergic mechanisms.

Abstract The effects of concentrations of ouabain on the isometric developed tension (IDT) of isolated left auricles driven at 0·4, 0·8 and 3·3 Hz, were explored. With 0·4 or 0·8 Hz, increasing concentrations of ouabain (3·4 or 6·8 times 10−6; 1·0; 3·4 or 6·8 times 10−6M) elicited a decreasing enhancement of atrial contractile peak tension, whereas with 3·3 Hz the effect was augmented with concentration. Ouabain (3·4 times 10−6 M) enhanced comparably the IDT at all the driving rates, but at higher concentrations the effect varied with the frequency. The positive inotropic action of ouabain at 3·4 times 10−6 M on preparations driven at faster frequencies was antagonized by Sotalol (MJ-1999) and also after catecholamine depletion by in vivo reserpinization followed by addition of tyramine in vitro. Cocaine or U-0521 significantly enhanced the positive inotropic effect of ouabain (3·4 times 10−5 M) in atria stimulated at low rates. On the contrary, Sotalol, cocaine, U-0521 or catecholamine depletion did not alter the positive inotropic influence of ouabain at 3·4 times 10−6 M at any of the frequencies used. It is suggested that ouabain influenced contractile peak tension of rat isolated atria through two mechanisms: one, seen after higher concentrations, which appeared to be associated with adrenergic factors and dependent on the frequency of stimulation; the other, observable with the lower concentrations, which did not appear to have a direct relationship with adrenergic processes and which was independent of the frequency of stimulation.

Noradrenaline-induced desensitization in cultured heart cells as a model for the defects of the adenylate cyclase system in severe heart failure

Cultivation of rat heart muscle cells for up to 5 days in the presence of 10(-6) mol/l (-)noradrenaline leads to a desensitization of the cells to inotropic stimulation and cAMP formation at different levels of the cAMP-system: The decrease in the responsiveness to the beta-adrenoceptor agonist isoprenaline quantitatively parallels the down-regulation of beta 1-adrenoceptors. The impairment of the positive inotropic effect of the phosphodiesterase-inhibitor IBMX is associated with a diminished basal cAMP-formation but an unchanged phosphodiesterase-activity in noradrenaline-treated cells. This decrease in basal cAMP-accumulation as well as the attenuation of the forskolin-stimulated cAMP-formation indicate that a defect in the adenylate cyclase system beyond the receptor must be involved in noradrenaline-induced desensitization. In contrast to the diminished effectiveness of cAMP-increasing agents the positive inotropic effect of ouabain and the alpha-adrenoceptor-mediated positive inotropic effect of phenylephrine are unaltered in noradrenaline-treated cells. It is concluded from these results that in addition to the down-regulation of beta 1-adrenoceptors a defect on the post-receptor level of adenylate cyclase occurs in noradrenaline-induced desensitization not affecting mechanisms beyond the catalytic subunit of adenylate cyclase. As these findings are very similar to those observed in isolated preparations from severely failing human hearts, the presumption is confirmed that noradrenaline-induced desensitization might constitute an important etiological factor in the subsensitivity of failing human hearts to inotropic stimulation.

Myocardial region (right or left ventricle) and aetiology of heart failure can influence the inotropic effect of ouabain in failing human myocardium.

To investigate whether the inotropic effect of ouabain in failing human myocardium varies according to the heart chamber tested (right or left ventricle) or the aetiology of the heart disease, i.e. ischaemic or idiopathic. The inotropic effect of ouabain was measured, as the percentage change in baseline tension, in myocardial strips isolated from right (RV; n=21) and left ventricles (LV; n=21) of hearts explanted from patients with idiopathic (IDC; n=11) and ischaemic cardiomyopathy (CAD; n=10). Concentration-effect curves obtained with ouabain (0.05-1.6 micromol l-1 ) were analysed using the Emax sigmoidal model, and the following parameters were calculated: Emax, EC50, n and EC10 (threshold concentration). The influence of ventricular chamber and heart failure aetiology on these parameters was evaluated by means of a two-way anova. Age and baseline haemodynamic parameters did not differ between IDC and CAD patients. Baseline strip contractility was highly variable (range: 0.48-10.0 mN), but neither ventricular chamber nor aetiology could explain such variability. A two-way anova showed that EC10 was greater in CAD than in IDC preparations (0.097+/-0.013 micromol l-1 vs 0.059+/-0. 009 micromol l-1; 95% C.I. for difference 0.043, 0.071) and Emax was lower in RV than in LV (121+/-21% vs 250+/-38%; 95% C.I. -221, -36), while EC50 and n were not significantly different between groups. The inotropic effect of ouabain in human myocardium may vary according to aetiology of heart failure and the ventricle being tested. Although our results do not support the hypothesis of increased sensitivity to cardiac glycosides in CAD patients, they may explain the diminished effect observed in patients with RV failure.

Small doses of canrenone block the effects of ouabain on the mechanical activity of the heart and vessels of the rat.

Canrenone has been described as an antihypertensive drug that blocks endogenous ouabain effects in volume-dependent hypertensive models. Considering that some canrenone metabolites may be putative mutagenic factors, therapeutic dose reduction might be advantageous if the blockade of ouabain effects is maintained. In this study, the effects of low doses or concentrations of canrenone were investigated in rats by using isolated papillary muscles, Langendorff-perfused hearts, perfused rat-tail vascular bed, and anesthetized animals. Canrenone (0.5, 1, 2, and 5 mg/ml) produced a dose-dependent negative inotropic effect in papillary muscles contracting isometrically and blocked the positive inotropic effect produced by 660 microM ouabain. In Langendorff-perfused hearts beating spontaneously, a low concentration of canrenone (10 microg/ml) increased the isovolumic systolic pressure obtained at several diastolic pressures. Higher concentrations of canrenone (20, 30 microg/ml) brought the isovolumic pressure toward control values, and 100 microg/ml canrenone produced an isovolumic pressure reduction. In these preparations, 20 microg/ml canrenone reduced significantly the positive inotropic effects of 100 microM ouabain. Investigating the vascular smooth muscle reactivity to phenylephrine (PE; 0.5, 1, and 2 microg bolus injections) in the perfused rat-tail vascular bed, it was observed that canrenone blocked completely the enhancement of PE pressor effect produced by 1-h treatment with 100 microM ouabain. Similar results were obtained with the arterial blood pressure reactivity to PE in anesthetized rats. In these animals, canrenone (1 mg/kg) blocked the sensitizing effect of 18 microg/kg ouabain on PE reactivity. In conclusion, results presented here suggest that canrenone may block ouabain effects at very low concentrations. It blocked myocardial positive inotropic effects of ouabain on both papillary muscle and perfused hearts, and the sensitization of PE pressor effects. The results also suggest that canrenone at very small doses might be used to reduce arterial blood pressure in hypertensive conditions accompanied by increased ouabain plasma levels as the main therapeutic procedure or as an adjunct treatment to prevent ouabain sensitizing effects on pressor responses.

Inotropic effect of ouabain in hypertrophied rat heart

The contribution of the heterogeneous digitalis receptors to the inotropic effect of ouabain was studied in hypertrophied rat hearts (aortic stenosis) by using isolated Langendorff heart preparations. Development and washout of the biological effects as well as the dose/response curves revealed two inotropic components of high and low drug sensitivity. Maximal inotropy was observed with 100 microM ouabain in both control and hypertrophied rat hearts. The high-sensitivity component accounted for only one-third of the response in control hearts but for two-thirds in hypertrophied hearts. The respective apparent affinities (10-20 nM and 10-20 microM) of the two inotropic components found in isolated hearts were similar to those of the high- and low-affinity Na+,K(+)-ATPase activities detected in isolated cardiac sarcolemma. Onset and reversion of the pharmacological effects of ouabain occurred at respective rates that were similar to those of the association and dissociation of ouabain to the Na+,K(+)-ATPase level. In hypertrophied heart, the high- and low-sensitivity components (or receptors) reacted seven- and threefold, respectively, more slowly than the corresponding sites in normal hearts. These alterations in inotropic responsiveness and properties of the digitalis receptors in cardiac hypertrophy suggest that new regulations should be taken into account in the adaptation to pressure overload.

Inotropic Effect of Meperidine

Meperidine increases developed force in isolated rat atria. We initiated this study to determine if this positive inotropic effect was attenuated by commonly used receptor- and ion channel blockers. Neither naloxone (opioid blocker), phentolamine (alpha-adrenoceptor blocker), propranolol (beta-adrenoceptor blocker), polaramine (H1-receptor blocker), ranitidine (H2-receptor blocker), verapamil (calcium-channel blocker), nor lidocaine (fast sodium-channel blocker) attenuated the positive inotropic effect of meperidine. However, after lidocaine pretreatment meperidine increased contractile force by 57% (27%-80%) (median and 95% confidence interval), which is significantly more (P less than 0.001) than the 21% (13%-35%) increase seen after pretreatment with saline. After Na,K-pump inhibition by ouabain, meperidine caused no further increase in contractility, but the atria still responded to isoproterenol with an increase in developed force. Conversely, meperidine prevented the positive inotropic effect of ouabain. These findings suggest that the positive inotropic effect of meperidine is mediated by an increase in intracellular sodium activity and not by regulation of the slow inward calcium channel.

Metabolic Requirement for the Inotropic Effects of Digitalis and BAY K 8644

The positive inotropic effects of ouabain and of BAY K 8644 are not apparent in rabbit atria suspended in substrate free medium or in a medium containing 5 mM pyruvate. Addition of glucose in graded concentrations (1-11 mM) during continued exposure of the preparations to the inotropic agents yields graded inotropic effects. The possible involvement of the glycolytic pathway to the development of the inotropic effect of ouabain and BAY K 8644 was tested by using inhibitors of glycolysis that act at two different steps. Iodoacetic acid completely blocks the inotropic effect of ouabain in atria at 0.1 mM and papillary muscles at 0.05 mM. The inotropic effect of BAY K 8644 was blocked partially in atria and papillary muscles. Iodoacetic acid did not change the inotropic effects of isoproterenol and Ca2+. Addition of 1 mM fluoride did not affect significantly the inotropic effect of either ouabain or BAY K 8644 in atria but partially blocked the effect of BAY K 8644 in papillary muscles. The response of atria to ouabain was not changed significantly when glyceraldehyde (10 mM) was substituted for glucose. We suggest that glycolytic ATP may be important for the full inotropic effect of ouabain and BAY K 8644.

Inotropic Interactions of Dichloroacetate with Amrinone and Ouabain in Isolated Hearts from Endotoxin-Shocked Rats

The inotropic effects of ouabain and amrinone singly and in combination with dichloroacetate (DCA) were assessed using isolated working perfused hearts from endotoxin-shocked (LD50/6 h) rats, with glucose and free fatty acids as substrates. Amrinone (2.7 X 10(-4) M) and ouabain (10(-5) M) alone improved myocardial mechanical performance from 25 to 75%, depending on the preload. Amrinone was more effective than ouabain at all left atrial filling pressures tested. DCA enhanced the inotropic effect of both compounds, improving myocardial work greater than 65% in combination with ouabain (at low filling pressures) to 125% with amrinone (at high filling pressures). Glucose oxidation rose two- to threefold when DCA was present with either drug. Ouabain was without effect on pyruvate dehydrogenase (PDH) activity; however, when present with DCA, PDH activity increased fourfold. Amrinone alone augmented PDH activity 2.5 times as compared with controls, and 4.5 times when combined with DCA. All three compounds individually elevated myocardial ATP levels, but in contrast to the inotropic effects, when used in combination caused no further increase in ATP. Myocardial cyclic AMP (cyclic AMP) levels were augmented five times control values in the presence of amrinone. The simultaneous presence of DCA and amrinone did not further augment the myocardial concentration of cAMP. DCA alone enhanced the myocardial oxidation of glucose in isolated myocardial cells from endotoxin-shocked animals. In contrast, ouabain and amrinone did not affect cellular glucose oxidation. These data indicate that the provision of DCA can enhance the inotropic effect of amrinone and ouabain on the isolated working heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Respective involvements of high- and low- affinity digitalis receptors in the inotropic response of isolated rat heart to ouabain

High- and low-affinity digitalis receptors coexist in rat cardiac sarcolemma. In this study, their relative involvement in the inotropic effect of ouabain was evaluated on an isolated Langendorff rat heart preparation working under isovolumic conditions at a low external calcium concentration (0.25 mM). This involvement was estimated according to both the development of the inotropic response to ouabain (10 −8–10 −4 M) and the time course of the washing out the biological effect. In each phenomenon considered, and whatever the index of inotropy chosen, the high-affinity digitalis receptor ( ec 50: 1–2 × 10 −8 M) contributed to 25–40% of the maximal inotropy (evoked by 10 −4 m ouabain). Low-affinity receptors ( ec 50: 1–2 × 10 −5 M) acounted for 60–75%. These apparent affinities were identical to those previously determined in sarcolemma isolated from rat heart perfused with 0.25 mM Ca 2+. The biphasic effect of ouabain was related to both the inhibition of high- and low-sensitivity Na +, K +-ATPase forms and the corresponding number of ouabain-binding sites occupied. These results support the concept that the Na +, K +-ATPase highly sensitive to ouabain as revealed by lowering calcium is in the in vivo manifestation of the high-sensitivity inotropic component.

Additive positive inotropic effects of milrinone, ouabain and calcium in diseased human ventricular myocardium.

The interactions of milrinone, ouabain and calcium on force of contraction in isolated, contracting human papillary muscle strips were measured. Milrinone (EC50, 8 X 10(-5)M) increased force of contraction maximally by 2.8 +/- 0.8 mN at 5 X 10(-4)M; significantly less than either ouabain (1 X 10(-7)M; 4.8 +/- 0.5 mN increase) or calcium (15 mM; 6.2 +/- 0.6 mN increase). A submaximal, but not a maximal, inotropic effect of ouabain could be increased by the addition of milrinone; in contrast, both ouabain and calcium increased the maximal inotropic effect of milrinone by 1.7 +/- 0.2 mN and 2.7 +/- 0.3 mN, respectively. The combined inotropic effect of milrinone with either ouabain of 4.2 +/- 0.3 mN or calcium of 5.6 +/- 0.4 mN was not different from that with calcium or ouabain alone. We conclude that further positive inotropic effects should be expected when digitalis is given to patients with congestive heart failure who are already optimally treated with milrinone.

Prolonged responsiveness to ouabain in hypertrophied rat heart: physiological and biochemical evidence

The inotropic effect of ouabain on cardiac hypertrophy was evaluated on an isolated Langendorff rat heart preparation with performances registrated by means of an intraventricular balloon. These effects were compared with the drug action on the sarcolemma-bound Na+-K+-ATPase activity. On both normal and pressure-overload induced hypertrophied rat hearts (ventricular wt-to-body wt ratios of 2.1 and 3.3, respectively) the inotropic effect of ouabain (10(-9)-10(-4) M) was evaluated at 0.25 mM external Ca2+. Compared with normal hearts, the recovery of a normal contractile function after the inotropic response was significantly slower in hypertrophied hearts. This was valid with the two protocols applied. During a 30-min washout period, the inotropic response remained nearly unchanged in hypertrophied hearts, whereas it was almost completely reversed in control groups. Sarcolemmal vesicles from both heart groups exhibited high Na+-K+-ATPase activities (sp. act.: 105 +/- 16 mumol X h-1 X mg-1). In both normal and hypertrophied cardiac sarcolemmal preparations, the Na+-K+-ATPase was heterogeneous, with high- and low-sensitivity forms. Their relative proportion was two-to-one. In both heart groups, their respective apparent affinities for ouabain were similar (inhibitory concentration of 50% = 10(-8) and 10(-6) M, respectively). The release of ouabain from these two sites was measured, in washout experiments, by the rates of enzyme relief from inhibition. High- and low-sensitivity forms in hypertrophied heart preparations released ouabain at seven- and threefold lower rates, respectively, than the corresponding forms present in normal cardiac sarcolemmal vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat cardiac hypertrophy. The prolonged inotropic effect of ouabain in vivo is associated with a slower release of ouabain from the digitalis receptors in vitro

Rat cardiac hypertrophy. The prolonged inotropic effect of ouabain in vivo is associated with a slower release of ouabain from the digitalis receptors in vitro

Reversibility of inotropic effects of ouabain on canine atrial and ventricular tissues

The experiments reported here were designed to quantitate reversibility of ouabain-induced inotropy in atrial and ventricular trabeculae of the dog heart. At 30 degrees C, 1 Hz, 2 mM Ca2+ and 5.9 mM K+, the positive inotropic effects, measured after a 1 h exposure to a single priming concentration of 0.5 microM ouabain, decreased after the removal of the drug with a half time (t1/2) of washout of 9.52 +/- 2.72 h in atrial and 7.64 +/- 1.58 h in ventricular trabeculae. When toxicity occurred it was of three types: transient toxicity, toxicity resembling a 'fast washout,' and severe toxicity characterized by profound and persistent negative inotropy and contracture. Use of the classical glycoside bioassay (Hatcher's digitalis titration) during and after washout, provided a semiquantitative estimation of ouabain remaining in the trabeculae; after a 4 h washout of the drug, the trabeculae were challenged by the addition of one-half of the initial concentration of ouabain (0.25 microM). Trabeculae, which had responded to ouabain with only positive inotropic effects and no toxicity, still contained substantial concentrations of ouabain. Trabeculae which reacted clearly with toxicity to the priming dose of ouabain, showed a significant loss of contractile force and development of contracture. The fastest ouabain washout we observed was 3.1 h.

Non-additive positive inotropic effects of amrinone and ouabain on cat papillary muscles.

Amrinone has been shown to produce haemodynamic benefits in digitalis-treated patients. Since amrinone is a positive inotropic agent on isolated heart muscle, these benefits may mean that amrinone increases the maximal ouabain-induced increase in force of contraction, without causing toxicity. We have therefore measured, in cat right ventricular papillary muscles, the inotropic effects of ouabain, amrinone alone and amrinone with a maximally effective, non-toxic ouabain concentration (2 X 10(-7) M). Ouabain is much more potent than amrinone (EC50-values: ouabain, 8 X 10(-8) M, amrinone, 1-2.8 X 10(-3) M). The highest amrinone concentration used (6 X 10(-3) M) produced a significantly lower increase in force of contraction than ouabain (2 X 10(-7) M) in the same muscles. After ouabain (2 X 10(-7) M) produced a stable effect, no further increase in force of contraction was observed with any amrinone concentration. Sustained arrhythmias were observed in five of six muscles at 3 X 10(-3) M amrinone with ouabain (2 X 10(-7) M), but in only one of these muscles with amrinone 3 X 10(-3) M alone. Since the positive inotropic effects of amrinone are not additive with those from a maximally effective ouabain concentration, the haemodynamic benefits seen in patients are probably due to non-cardiac effects of amrinone such as vasodilatation.

EFFECT OF OUABAIN ON MYOCARDIAL MECHANICAL FUNCTION AND SODIUM PUMP IN THE PREMATURE NEWBORN RABBIT

Effects of ouabain (0.75, 1, 1.5, 2.5, or 5 × 10−6M) on mechanical function and sodium pump were studied in the isolated arterially perfused fetal and newborn rabbit heart. Measurements of myocardial 86Rb+ active uptake was used as a marker of the sodium pump activity. The inotropic effect of ouabain in the fetus was not significantly different from that in the newborn at all ouabain concentrations. 2.5 × 10−6M ouabain caused mechanical toxicity in the fetus but not in the newborn. In the fetal muscle, perfusion with low calcium (0.5 mM) solution reduced mechanical toxicity of ouabain. High extracellular calcium (30 mM) per se caused mechanical toxicity in the fetus, but not in the newborn. Ouabain infusion decreased tissue potassium content, and this decrease in the fetus was not significantly different from the newborn. The ouabain inhibition of Rb+ uptake was similar in the two age groups. These data indicate that mechanical toxicity in the fetus is observed at lower ouabain concentrations than in the newborn. This difference in mechanical toxicity may not be explained by the age-related difference in sodium pump activity. The greater calcium toxicity in the fetus may be the reason for the increased ouabain toxicity in this age group.

Ouabain effect on myocardial mechanical function and sodium pump in the fetus.

Effects of ouabain (0.75, 1, 1.5, 2.5 or 5 X 10(-6) M) on the mechanical function and sodium pump were studied in the isolated arterially perfused fetal and newborn rabbit heart. Measurement of myocardial 86Rb+ active uptake was used as a marker of the sodium pump activity. The inotropic effect of ouabain in the fetus was not significantly different from that in the newborn; 2.5 X 10(-6) M ouabain caused mechanical toxicity (decrease in +dT/dtmax and increase in resting tension) in the fetus but not in the newborn. After ouabain infusion, both the inhibition of Rb+ uptake and the significant decrease in tissue potassium content were similar in the fetus and the newborn. In the fetal muscle perfused with low calcium (0.5 mM) solution, mechanical toxicity of ouabain was significantly less than in the control (1.5 mM Ca2+) solutions. High extracellular calcium (30 mM) per se caused mechanical toxicity in the fetus but not in the newborn. These data indicate that, in the isolated arterially perfused heart preparation, mechanical toxicity in the fetus is observed at lower ouabain concentrations than in the newborn. This difference in mechanical toxicity may not be explained by the age-related difference in sodium pump activity. The greater calcium toxicity in the fetus may be the reason for the increased ouabain toxicity in this age group.

Analysis of the inotropic action of ouabain in rat ventricles: two apparent ouabain inotropic responses.

Two distinct inotropic effects of ouabain were observed in 68 of 82 right ventricular strips of rats with ED50s of 0.5 and 20 microM referred to as "low-dose" and "high-dose" effects, respectively. The other 14 strips showed monophasic dose-response curves, with an apparent ED50 of 0.3 microM; the inotropic response to ouabain or isoproterenol developed by these atypical strips did not exceed 50% over control. In the strips showing the biphasic inotropic response curves, the proportion of the "low-dose" effect varied from 6 to 89% of the maximum response. After treatment with 160 microM ouabain, followed by 60 min of washout, the maximum developed tension in response to ouabain was unchanged, but the "low-dose" effect was abolished or dramatically reduced. In those cases with a remaining "low-dose" response, the response was only 12% or less of the maximum and the ED50 was shifted from 0.3 to about 3 microM ouabain by the washout. The atypical strips, which showed only the "low-dose" effects during the first ouabain exposure, revealed after washout a consistent "high-dose" effect with an ED50 of about 12 microM ouabain. The data show that the two inotropic responses exist in all ventricular strips and that in some strips, after ouabain washout, a residue of the "low-dose" effect remains.

The inotropic effect of ouabain and its antagonism by dihydroouabain in rat isolated atria and ventricles in relation to specific binding sites.

The inotropic effect of ouabain has been studied in rat ventricles and atria. The concentration-effect curve of ouabain may be fitted by a model assuming the existence of two saturable components. The component with the higher sensitivity to ouabain accounted for 30% of the maximal increase in systolic tension in ventricles and for only 5% in atria. Increase in diastolic tension was only apparent at ouabain concentrations required to observe the low sensitivity component. [3H]-ouabain binding has been examined in microsomes prepared from atria and ventricles. High and low affinity binding sites have been observed. The ratio of high and low affinity ouabain binding sites was 4 fold lower in microsomes from rat atria than from rat ventricles. This could account for the difference in the response of these two tissues to the inotropic action of ouabain. In ventricular strips the high sensitivity component was much less apparent in the presence of dihydroouabain than with ouabain. When ventricular strips were preincubated in the presence of dihydroouabain 3 microM, the increase in systolic tension evoked by ouabain 1 microM was significantly reduced. Cumulative concentration-effect curve studies showed dihydroouabain antagonism to the high sensitivity component.