Evidence for participation of catecholamines in cardiac action of ouabain. Release of catecholamines

Isoform-specific stimulation of cardiac Na/K pumps by nanomolar concentrations of glycosides

It is well-known that micromolar to millimolar concentrations of cardiac glycosides inhibit Na/K pump activity, however, some early reports suggested nanomolar concentrations of these glycosides stimulate activity. These early reports were based on indirect measurements in multicellular preparations, hence, there was some uncertainty whether ion accumulation/depletion rather than pump stimulation caused the observations. Here, we utilize the whole-cell patch-clamp technique on isolated cardiac myocytes to directly measure Na/K pump current (I(P)) in conditions that minimize the possibility of ion accumulation/depletion causing the observed effects. In guinea pig ventricular myocytes, nanomolar concentrations of dihydro-ouabain (DHO) caused an outward current that appeared to be due to stimulation of I(P) because of the following: (1) it was absent in 0 mM [K(+)](o), as was I(P); (2) it was absent in 0 mM [Na(+)](i), as was I(P); (3) at reduced [Na(+)](i), the outward current was reduced in proportion to the reduction in I(P); (4) it was eliminated by intracellular vanadate, as was I(P). Our previous work suggested guinea pig ventricular myocytes coexpress the alpha(1)- and alpha(2)-isoforms of the Na/K pumps. The stimulation of I(P) appears to be through stimulation of the high glycoside affinity alpha(2)-isoform and not the alpha(1)-isoform because of the following: (1) regulatory signals that specifically increased activity of the alpha(2)-isoform increased the amplitude of the stimulation; (2) regulatory signals that specifically altered the activity of the alpha(1)-isoform did not affect the stimulation; (3) changes in [K(+)](o) that affected activity of the alpha(1)-isoform, but not the alpha(2)-isoform, did not affect the stimulation; (4) myocytes from one group of guinea pigs expressed the alpha(1)-isoform but not the alpha(2)-isoform, and these myocytes did not show the stimulation. At 10 nM DHO, total I(P) increased by 35 +/- 10% (mean +/- SD, n = 18). If one accepts the hypothesis that this increase is due to stimulation of just the alpha(2)-isoform, then activity of the alpha(2)-isoform increased by 107 +/- 30%. In the guinea pig myocytes, nanomolar ouabain as well as DHO stimulated the alpha(2)-isoform, but both the stimulatory and inhibitory concentrations of ouabain were approximately 10-fold lower than those for DHO. Stimulation of I(P) by nanomolar DHO was observed in canine atrial and ventricular myocytes, which express the alpha(1)- and alpha(3)-isoforms of the Na/K pumps, suggesting the other high glycoside affinity isoform (the alpha(3)-isoform) also was stimulated by nanomolar concentrations of DHO. Human atrial and ventricular myocytes express all three isoforms, but isoform affinity for glycosides is too similar to separate their activity. Nevertheless, nanomolar DHO caused a stimulation of I(P) that was very similar to that seen in other species. Thus, in all species studied, nanomolar DHO caused stimulation of I(P), and where the contributions of the high glycoside affinity alpha(2)- and alpha(3)-isoforms could be separated from that of the alpha(1)-isoform, it was only the high glycoside affinity isoform that was stimulated. These observations support early reports that nanomolar concentrations of glycosides stimulate Na/K pump activity, and suggest a novel mechanism of isoform-specific regulation of I(P) in heart by nanomolar concentrations of endogenous ouabain-like molecules.

Prostaglandins and CGRP release from cardiac sensory nerves

(1) The possible influence of prostaglandins (PG) E1 and I2 as well as ischaemia, ouabain and bradykinin on the outflow of calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-like immunoreactivity (LI) from the guinea-pig heart was studied in vitro. (2) Exposure to PGE1 (10(-5) M), but not PGI2 (10(-5) M), induced an increased outflow, suggesting release of CGRP-LI. PGE1 simultaneously increased the contractile force and heart rate while no effects were observed on perfusate volume or outflow of NPY-LI. PGI2 had no effect on contractile parameters or coronary flow. In separate experiments on capsaicin-pretreated animals, the stimulatory effects of PGE1 on heart rate and contractile force remained unchanged while no increased CGRP-LI outflow was detectable. (3) Ouabain, bradykinin and reperfusion after total stop-flow ischaemia was associated with an indomethacin-resistant increase in perfusate levels of CGRP-LI but not of NPY-LI. While ouabain markedly increased the contractile force, exposure to bradykinin or ischaemia did not induce any clear-cut changes in contractile force or heart rate. (4) Capsaicin-exposure evoked a markedly increased outflow of CGRP-LI but not of NPY-LI in combination with an increase in heart rate and a decrease in contractile force. Repeated administration of capsaicin induced tachyphylaxis. The stimulatory effects of capsaicin on CGRP-LI outflow and heart rate, but not the negative inotropic effect, did not occur in capsaicin-pretreated animals. (5) It is concluded that PGE1, but not PGI2, can activate cardiac capsaicin-sensitive fibres as revealed by increased outflow of CGRP-LI.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of buffer magnesium on positive inotropic agents in guinea pig cardiac muscle

Experiments examined effects of extracellular Mg2+ concentration (Mgo2+) on dose-dependent actions of strophanthidin, norepinephrine, Bay K-8644 and extracellular Ca2+ (Cao2+) in electrically stimulated atrial and ventricular muscle isolated from guinea pig heart. Mgo2+ itself elicited a concentration-dependent negative inotropic effect. Elevation of Mgo2+ between 0.6 and 12 mM increased the concentration of strophanthidin necessary to produce its toxic effects without affecting the maximum developed tension prior to toxicity. Similarly, Mgo2+ did not alter the maximum contractile force elicited by cumulative addition of norepinephrine, Bay K-8644 or Cao2+, but increased their ED50 values. These data suggest that interactions between Mgo2+ and the four positive inotropic agents were not mediated by effects on receptor binding or Na+,K+-ATPase, but rather by alterations at one or more steps involved in excitation-contraction coupling.

Effects of ouabain on 86Rb-uptake, 3H-5-HT-uptake and aggregation by 5-HT and ADP in human platelets

In the search of sensitive models for actions of digitalis-like substances on intact cells or tissues, the effects of ouabain on human platelets were investigated. In a concentration-dependent manner ouabain 10(-8)-10(-4) M inhibited Na+-K+-ATPase activity measured as uptake of 86Rubidium (86Rb), with about 90% inhibition of the total uptake at ouabain greater than or equal to 10(-6) M. An almost identical concentration-effect curve was found for platelet uptake of 3H-serotonin (3H-5-HT). The platelet shape change reaction to exogenous 5-HT (1 X 10(-6) M) was suppressed by ouabain (10(-8)-10(-4) M) in a concentration-dependent manner, but with no clear maximum effect within the range tested. Aggregation induced by adenosine-di-phosphate (ADP 2 X 10(-6) M) was enhanced by ouabain 10(-8)-10(-6) M. At the highest concentration tested the rate of aggregation was increased by 31% and the change in light transmission by 54%. At low concentrations (less than 10(-9) M) of ouabain, there was a tendency towards increased aggregation as well as increased uptake of 86Rb, which may be a parallel to observations of positive inotropic effects of low concentration of glycosides, which do not inhibit Na+-K+-ATPase. The results show that human platelets can be used as a model tissue for studying effects of cardiac glycosides. This suggests that it may be useful for further investigations of the biological effects of agents with a similar effect profile, e.g. endogenous digitalis-like substances.

Diastolic time in congestive heart failure

Diastolic perfusion time is an important determinant of coronary blood flow and subendocardial perfusion. It has been proposed that subendocardial ischemia may exacerbate and perpetuate left ventricular dysfunction in congestive heart failure. Diastolic perfusion time in relation to heart rate was analyzed in 29 digitalized (group 1) and 12 nondigitalized patients (group 2) with heart failure and in 58 normal control subjects. In group 1 there was a strong negative exponential correlation (r = -0.85) and in group 2 a strong negative logarithmic correlation (r = -0.95) between heart rate and diastolic time; both regressions differed significantly from normal control. There was a 9% increase of diastolic time at a heart rate of 60 bpm in group 1 and a 7% increase in group 2 (both p less than 0.05) compared with normal subjects. The curves intersected the regression line of normal subjects at a heart rate of 98 bpm in group 1 and 93 bpm in group 2. At 120 bpm there was a 10% decrease in diastolic time for both groups with heart failure (both p less than 0.05). Changes in diastolic perfusion time relative to heart rate are more pronounced in congestive heart failure such that at faster heart rates this relationship may further impede subendocardial blood flow.

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.

Digitalis and the autonomic nervous system

Digitalis produces many of its effects in intact animals and human beings by modifying the properties of the autonomic nervous system. The parasympathetic limb of the autonomic nervous system is most sensitive to these effects of digitalis, and its properties are significantly altered with therapeutic concentrations of the drug. These actions are particularly important in mediating the electrophysiologic effects of digitalis. With toxic concentrations of digitalis, stimulation of sympathetic nerve activity may also occur. This latter action may be involved in the arrhythmogenic effects of digitalis. These effects of digitalis on the autonomic nervous system play a major role in determining the pharmacodynamic actions of the drug in patients. The effects of digitalis on the autonomic nervous system also provide a setting for important interactions with other drugs that modify the properties of the sympathetic and parasympathetic nervous systems.

Relation between intracellular sodium and twitch tension in sheep cardiac Purkinje strands exposed to cardiac glycosides

Changes in intracellular sodium ion activity (aiNa) produced by several cardiac glycosides were correlated with twitch tension in sheep cardiac Purkinje strands. Simultaneous measurements of aiNa and twitch tension were obtained through the use of Na-sensitive intracellular microelectrodes (ETH 227) in Purkinje preparations stimulated at a frequency of 1 Hz. All concentrations of ouabain, acetylstrophanthidin, and actodigin that were tested caused an increase in aiNa immediately before, or coincident with, a positive inotropic effect. No fall in aiNa was observed at any positive inotropic concentration of digitalis in these beating fibers. In all cases, the onset and washout of the positive inotropic effect were paralleled by the rise and fall in aiNa, respectively. No dissociation between changes in aiNa and twitch tension occurred at any concentration of any of the agents used. The relation between changes in aiNa and twitch tension was linear with 1 mM increase in aiNa producing about a 100% increase in the twitch magnitude. Propranolol did not significantly alter this relationship. The increase in aiNa with digitalis was also associated with a reduction in the maximum depolarization rate of the action potential, presumably as a consequence of a reduction in the transmembrane Na electrochemical gradient. These results indicate that the positive inotropic action of digitalis in sheep Purkinje strands is always associated with a rise in aiNa secondary to inhibition of the Na pump. This increase in aiNa could increase calcium available for contraction via the Na-Ca equilibrium exchange process. In addition, the increase in aiNa reduces Vmax, as a consequence of decreasing the electrochemical gradient for Na.

Active transport and inotropic state in guinea pig left atrium

Although the positive inotropic effect of cardiac glycosides correlates well with inhibition of Na+ pump activity in many preparations, digitalis at low concentrations (10(-9) to 10(-8) M) may produce an apparent stimulation of monovalent cation transport in isolated intact myocardium or produce an inotropic effect that does not correlate with pump inhibition. Digitalis is known to modify tissue metabolism of endogenous neurotransmitters that may affect inotropic state, Na,K-ATPase activity, and K+ permeability. We examined the interactions of low concentrations of ouabain with adrenergic and cholinergic influences in isolated guinea pig left atria stimulated at 3.3 Hz in which inotropic state and monovalent cation transport (measured as 86Rb+ uptake) were assessed simultaneously. Ouabain (10(-9) M) stimulated Rb+ transport (+25%) without an inotropic response; the stimulatory effect on transport was abolished by propranolol or atropine pretreatment. In atria pretreated with atropine, 10(-8) M ouabain produced a small positive inotropic effect (+10%) without measurable associated Na+-K+ pump inhibition. This inotropic response was abolished in catecholamine-depleted atria. Ouabain (10(-7) M) always produced a positive inotropic response (about +25%) independent of catecholamine depletion, beta-adrenergic blockade, or muscarinic blockade, but Rb+ uptake inhibition was observed only in beta-adrenergically-blocked atria. In all preparations, ouabain concentrations greater than 10(-7) M caused an inotropic response associated with pump inhibition. At concentrations 3 X 10(-7) M and higher, mechanical toxicity was observed in all preparations except those pretreated with propranolol. Incubation with low concentrations of ouabain did not modify the inotropic response to isoproterenol. At concentrations of isoproterenol sufficient to stimulate Rb+ transport by 25%, there was a large (+80%) inotropic response. We conclude first, that, in guinea pig atria exposed to ouabain, the mechanism as well as the extent of inotropic response and of monovalent cation transport modification is concentration dependent, second, that at low concentrations (1-10 X 10(-9) M), in vitro inotropic and monovalent cation transport responses are in part mediated by an effect of ouabain on endogenous neurotransmitters; and third, that in this preparation at concentrations between 10(-9) and 10(-7) M ouabain, monovalent cation transport as measured by tissue 86Rb+ uptake does not correlate with inotropic response.

Stimulation of monovalent cation active transport by low concentrations of cardiac glycosides. Role of catecholamines

The stimulatory effect of low concentrations of ouabain on the Na-K pump in isolated guinea pig left atria was studied in vitro by assessing active transport of the K(+) analog Rb(+). Active transport of Rb(+) was stimulated 20+/-8% (SEM, P < 0.05) above control values by 3 nM ouabain, but was inhibited by concentrations >10 nM. Preincubation with the beta-adrenergic antagonist propranolol (1 muM) completely blocked stimulation of active transport of Rb(+) by 3 nM ouabain. Norepinephrine, 10 nM, increased Rb(+) active transport 29+/-10% (P < 0.02) above control values. The beta-adrenergic agonist l-isoproterenol, 10 nM, increased active transport of Rb(+) by 33+/-10% (P < 0.01) above control levels. This stimulatory effect was abolished if tissues were first exposed to propranolol. Tyramine (0.1 muM), a stimulator of endogenous catecholamine release, increased active transport of Rb(+) 26+/-12% (P < 0.05) above control values. Rb(+) active transport was not significantly changed when left atrial tissues were incubated with alpha-adrenergic agonists or antagonists. Ouabain stimulation of Rb(+) active transport was prevented by in vivo depletion of myocardial endogenous catecholamines by either reserpine or 6-hydroxydopamine. These findings indicated that in myocardial tissue, Na-K pump stimulation by low concentrations of ouabain is mediated at least in part through beta-adrenergic effects of endogenous catecholamines.

Effect of timolol on autonomic neural discharge associated with ouabain-induced arrhythmia

The purpose of this study was to determine the effect of the beta-blocking agent timolol on postganglionic cardiac and preganglionic splanchnic sympathetic and vagal neural discharge, ouabain-induced arrhythmia, heart rate and mean arterial blood pressure. Cats were anesthetized with alpha-chloralose, given atropine, and pretreated with timolol (5 mg/kg, i.v. infused at a rate of 0.5 mg/kg/min for 10 min). Bolus injections of ouabain (25 microgram/kg, i.v.) were given every 15 min until the animals died; the first injection was given 15 min after the end of the timolol infusion. When compared with cats (n = 13) receiving only ouabain, timolol (n = 11 cats) increased the time to ouabain-induced arrhythmia and death from 23 +/- 3 to 48 +/- 7 and 46 +/- 3 to 76 +/- 9 min, respectively (p less than 0.05). Both heat rate and mean arterial blood pressure had decreased from 137 +/- 4 to 104 +/- 6 beats/min and 133 +/- 6 to 103 +/- 7 mm Hg, respectively (P less than 0.05); ouabain did not reverse the decreases. Neural activity monitored from the vagus and from the postganglionic cardiac and preganglionic splanchnic sympathetic nerves was not significantly altered by the infusion of timolol. The administration of ouabain after timolol did not alter splanchnic nor vagal discharge. Most important was the observation that postganglionic cardiac sympathetic neural discharge exhibited both increases and decreases, i.e., a nonuniform neural discharge, at the time of ouabain-induced arrhythmia and that the ouabain-induced nonuniformity did not occur in the cats pretreated with timolol. Thus, the protective effect of timolol may be due, in part, to prevention of the nonuniform postganglionic cardiac sympathetic neural discharge and to the prevention of ouabain-induced increases in vagal discharge. The establishment of beta-blockade and a direct negative inotropic action may also have contributed to the antiarrhythmic action of timolol.

Effects of rapid digitalization on total and regional myocardial performance in patients with coronary artery disease

In order to evaluate the effects of rapid digitalization on LV volumes, ejection fraction, and asynergy, 21 patients without heart failure were studied with a combination of hemodynamic and angiographic techniques before and after administration of intravenous ouabain (0.007 mg./Kg.). Seven patients had no CAD and served as normal (control) subjects (Group I), while 14 patients had extensive coronary disease (Group II). All pre-ouabain parameters were within the normal limits in Group I. After ouabain infusion, all indices of LV contractility: dP/dt, VCF, and ejection fraction rose significantly in the normal group, while LV filling pressure and end-diastolic volume remained unchanged. The baseline hemodynamic and volumetric values for Group II patients corresponded closely to their normal (Group I) counterparts, and exhibited similar changes after ouabain administration. Eight patients in Group II also had regional disorders of LV contractility, delineated by 23 abnormal hemiaxes of shortening. After ouabain, 15 out of 23 asynergic segments (65 per cent) improved, seven remained unchanged, and one worsened. It is therefore concluded that rapid digitalization not only enhances LV performance in normal subjects and in patients with CAD, but can also markedly reduce the extent of LV asynergy.

Neuronal norepinephrine as mediator for ouabain-induced smooth muscle contraction

With some latency, ouabain (10(-6)--10(-5) M) induced a long-lasting contractile response of the isolated vas deferens of the guinea-pig. The ouabain-induced contraction was potentiated by pretreatment with tropolone, whereas it was prevented by pretreatment with cocaine, bretylium, reserpine or phentolamine. The response was unaffected byatropine, methysergide and tetrodotoxin. Ouabain, perfused into the isolated tissue, enhanced dose-dependently the release of norepinephrine into the medium. However, this enhancement of norepinephrine release was prevented by reserpine or by removal of Ca from the medium. These findings support the hypothesis that the ouabain-induced contraction of vas deferens might be due to norepinephrine released from the adrenergic nerve ending granules through a Ca2+-dependent process.

Evidence for participation of catecholamines in cardiac action of ouabain: positive chronotropic effect

1 The shortening of cycle length (=positive chronotropic effect) by ouabain produced in isolated spontaneously beating atria of the guinea-pig was analyzed.2 The action of ouabain was dose-dependent; threshold response was seen at 1 x 10(-7) M, and maximal response occurred at 4 x 10(-7) M. The half-time of the ouabain effect was about 20 minutes.3 The positive chronotropic effect of ouabain was reduced to 40% by beta-adrenoceptor blockade (3.3 x 10(-9) M propranolol) or by reserpine-depletion of catecholamines. Incubation of reserpine-treated atria with noradrenaline partially restored the action of ouabain.4 The effect of ouabain was greatly dependent upon the calcium concentration. The optimal calcium level was 2.5 x 10(-3) M. Calcium and ouabain acted synergistically.5 Increasing calcium concentrations inhibited the positive chronotropic effect of noradrenaline in a manner similar to increasing ouabain concentrations.6 A hypothesis is proposed which explains the chronotropic effect of ouabain on the basis of two mechanisms: (1) increase of the catecholamine concentration affecting the pacemaker; (2) mobilization of calcium, i.e. increase of the biologically effective intracellular calcium level.