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

Modification of sarcolemmal enzymes by chagasic IgG and its effect on cardiac contractility

It has been shown that sera from chagasic patients contain an antibody which binds to beta-adrenoceptors of myocardium and modulates their activity. Chagasic IgG triggered a marked stimulation of myocardial contractility with an increase in intramyocardial cyclic AMP and inhibition of (Na+ + K+)-ATPase activity. Both the mechanical and enzymatic effects of the IgG could be prevented by beta-adrenoceptor blockade or after the absorption of chagasic IgG with turkey red blood cells. In contrast, guinea pig red blood cells were unable to remove the beta-reactivity of chagasic IgG. These findings suggest that the IgG from chagasic patients increases myocardial contractility by behaving as a beta-agonist. This effect is likely related to stimulation of the adenylate cyclase coupled to the cardiac beta-adrenoceptor.

The positive chronotropic effects of Bay K-8644 and calcium as influenced by temperature

The positive chronotropic effects of the dihydropyridine calcium agonist BAY K-8644 and calcium were studied in spontaneously beating right guinea-pig atria. Within the temperature range from 22 to 37 degrees C, the following observations were made: (1) The basal pacemaker frequency increased linearly with the ambient temperature; (2) the increase in pacemaker rate produced by a maximally effective concentration of BAY K-8644 was of the same magnitude as that produced by calcium; (3) the increases in pacemaker rate produced by BAY K-8644 and by calcium were directly proportional to the increase in temperature above 22 degrees C; and (4) at 37 degrees C, the apparent ED50 for the positive chronotropic effect of calcium was 2.3 X 10(-3) M and of BAY K-8644 was 0.7 X 10(-7) M. At temperatures below 22 degrees C, irregular pacemaking activity or complete arrest of pacemaking activity frequently occurred. The data are in strong support of the assumption that BAY K-8644 elicits its positive chronotropic effect by increasing the availability of calcium ions.

Endogenous ouabain-like compound increases heart muscle contractility

Cardiac glycosides such as digoxin or ouabain have long been known to influence the strength of contraction of cardiac muscle. Although the mechanism of action of these compounds remains unknown, all the proposed modes of action are based on initial binding to specific membrane receptors which are part of the (Na+ + K+)ATPase complex. These receptors, well characterized and defined, suggest the existence of an endogenous substance capable of binding to them, in analogy with endogenous opiates, discovered long after morphine and its receptors. As glycosides affect (Na+ + K+)ATPase activity, an endogenous substance may be a regulator of this important enzyme. Indeed, the search for endogenous regulators of the (Na+ + K+)ATPase or ouabain-like compounds (OLC) has recently intensified. These compounds, extracted and partially purified from mammalian brain, heart, blood and urine, and from toad skin and plasma, have been shown to inhibit 3H-ouabain binding and (Na+ + K+)ATPase activity. We report here that in addition to these effects, the OLC, highly purified from toad skin and sheep brain, increases the force of contraction of frog and guinea pig atrium.

Chagasic sera alter the effects of ouabain on isolated rat atria. Participation of adrenergic mechanisms

The effects of chagasic sera, containing an antibody (EVI antibody) which reacts with the plasma membrane of working myocardial cells, on "toxic" and "non-toxic" actions of ouabain upon isolated self beating or paced rat atria suspended in different media, were explored. Although ouabain produced a dose-dependent positive inotropic influence on atria suspended in Krebs-Ringer-bicarbonate (KRB) and in KRB plus normal human serum (KRB + NHS) it did not elicit any significant positive inotropic effect on atria beating in KRB plus EVI positive human chagasic serum (EVI(+)S). Additionally, EVI(+)S dose-response curves of classical signs of digitalis cardiac toxicity shifted to the left. The threshold concentration of ouabain required to elicit the onset of "toxic" effects was higher in control preparations (kept in KRB or KRB + NHS) than in EVI(+)S exposed preparations. (-)-Propranolol attenuated the overall toxic action of ouabain in EVI(+)S and facilitated its positive inotropic influence. In control media, the beta-adrenoceptor blocker failed to modify either the "non-toxic" or the "toxic" effect of ouabain. On the other hand, with control atria, subthreshold exogenous norepinephrine inhibited the positive inotropism of ouabain. The data suggest that an adrenergic mechanism is involved in the action of ouabain on cardiac tissue immersed in an EVI(+)S-containing solution. The foregoing results may explain the severe "toxic" effects observed with cardioactive glycosides when they are used in patients with Chagas' heart disease, even at low doses.

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

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.

Effect of arrhythmogenic doses of deslanoside on the uptake of monoamines in brain tissue and in cardiac tissue

The purpose of our study was to determine whether a toxic arrhythmogenic dose of digitalis administered to an in vivo preparation would affect the neuronal uptake of norepinephrine, serotonin and dopamine in brain tissue and norepinephrine in cardiac tissue. This was investigated by intoxicating anesthetized cats with deslanoside, removing cardiac and brain tissue at the onset of ventricular fibrillation, and examining the ability of brain tissue to accumulate [3H]-NE, [3H]-T-HT and [3H]-DA and cardiac tissue to accumulate [3H]-NE. It was found that deslanoside inhibited uptake of [3H]-NE into the left ventricle and [3H]5-HT into the area postrema. These selective effects may reflect greater blood flow to these regions or different sensitivities of the transport mechanisms for these amines. This inhibition of uptake into both left ventricular tissue and area postrema may contribute to some of the cardiovascular and emetic effects seen with digitalis drugs.

Effect of taurine on responses to noradrenaline, acetylcholine and ouabain in isolated auricles from digitalized guinea pigs

Responses of the isolated heart from the guinea pig treated chronically with digitoxin until the T wave had disappeared were studied. The development of the abnormal ECG pattern was abolished by a simultaneous injection of taurine and digitoxin. Taurine also inhibited the change by digitoxin of the responses to acetylcholine and ouabain but not to noradrenaline. The causal relationship between the inhibitory effect of taurine and prevention of the development of arrhythmias remains obscure.