Is pump stimulation associated with positive inotropy of the heart?

Characterization of the stimulation of neuronal Na(+), K(+)-ATPase activity by low concentrations of ouabain

Low concentrations (< 10(?7) M) of ouabain stimulate the activity of Na(+), K(+)-ATPase in whole homogenates of rat brain. The magnitude of this stimulation varies from 5 to 70%. The concentrations of ouabain which induces maximal stimulation is also highly variable and ranges between 10(?9) to 10(?7) M. The ouabain stimulation disappears following 1:50 dilution and 2 h preincubation or freezing and thawing of the membranes or their treatment with deoxycholate. "Aging" of a preparation of ATPase also results in loss of its ability to be stimulated by ouabain but ouabain inhibition is preserved. No stimulation of enzyme activity by ouabain is observed in rat brain microsomal fraction. The ?-adrenergic blocker propranolol does not inhibit the ouabain induced stimulation of ATPase activity. It is suggested that the stimulation of Na(+), K(+)-ATPase activity by low concentrations of cardiac glycosides if a result of either the displacement of an endogenous ouabain-like compound from the enzyme or an indirect effect by changing membrane surrounding environment of the Na(+), K(+)-ATPase.

Low concentrations of ouabain stimulate Na/Ca exchange in neurons

1. The effects of low concentrations of ouabain on 22Na efflux, 86Rb influx, 45Ca uptake and cyclic AMP levels were studied in snail ganglia. Ouabain, at concentrations below that which inhibits the Na-K pump as monitored by 86Rb influx, activated "reverse mode" Na/Ca exchange, as indicated by an increased 22Na efflux and 45Ca influx. 2. With electrophysiologic recordings ouabain, in the presence of K(+)-free saline to block Na/K transport, caused a membrane hyperpolarization. These concentrations of ouabain also caused elevation of intracellular cyclic AMP levels. 3. We suggest that the ouabain-induced stimulation of Na efflux is due to a stimulation of reverse Na/Ca exchange. since Na/Ca exchange is electrogenic, these observations are most consistent with ouabain stimulation of Na/Ca exchange in a reversed direction (intracellular Na for extracellular Ca). 4. The effect on Na/Ca exchange may be secondary to a rise in intracellular cyclic AMP.

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.

Calcium and the mechanism of action of digitalis

A critical review is made of the mechanism by which digitalis increases the force of contraction of heart muscle. First, it is concluded that the initial step is always an inhibition of the sodium pump, and that the postulated stimulation of the pump by low digitalis concentrations is, possibly, not a real phenomenon. Secondly, the major theories that try to explain the inotropic effect of digitalis are analyzed, and it is tentatively concluded that the effect occurs because Na increases close to the inner side of the plasma membrane, and this decreases Ca efflux through the Na-Ca "exchange" mechanism. An internal Ca store, probably the sarcoplasmic reticulum, that competes with the plasma membrane for Ca, is then able to capture and, subsequently release, a larger fraction of the Ca mobilized during each transient. It is also concluded that the digitalis-induced larger Ca transients can be entirely explained because of a greater Ca "injection" into the cytoplasm during each beat, and not because of changes in resting pCa. A comprehensive model is presented that seems to explain in some detail both the inotropic and the toxic effect of digitalis.

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.

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.

Ouabain stimulation of noradrenaline transport in guinea pig heart

Cardiac glycosides such as ouabain inhibit Na+-dependent high-affinity noradrenaline uptake in sympathetic nerve terminals by interacting with (Na+ + K+)ATPase located in the neuronal membrane. It has been suggested that the augmentation of noradrenaline concentration at the neuromuscular junctions following the inhibition of its uptake may contribute to the ouabain-induced cardiac arrhythmia seen with toxic doses of the drug. Although it is generally accepted that the biochemical basis for the pharmacological response of cardiac glycosides must involve specific interaction of this class of drugs with the cardiac cell membrane Na+, K+-exchange pump, the precise molecular mechanism for the improvement of muscle performance by ouabain is controversial. Many investigators claim that cardiac glycosides increase myocardial force of contraction by inhibiting (Na+ + K+)ATPase. According to this hypothesis, such an inhibition might increase the intracellular concentration of Na+, which may augment the activity of the transmembrane Na+, Ca2+-exchange pump described by several workers. We have found that ouabain stimulates Na+-dependent transport of noradrenaline by guinea pig heart sympathetic nerve terminals.

The relationship between sodium pump activity and twitch tension in cardiac Purkinje fibres

1. Sheep cardiac Purkinje fibres were studied under voltage clamp conditions to examine the relationship between the Na pump rate and twitch tension.2. The Na pump activity was measured following a period of decreased Na pump rate and increased internal Na activity, Na(i), by examining the electrogenic Na pump current transient produced by reactivating the Na pump (see Eisner & Lederer, 1980).3. Various concentrations of Cs or Rb were used to reactivate the Na pump in the absence of extracellular K, K(o). Results from such experiments showed that these ;activator cations' produced a monotonic increase in Na pump rate with increasing concentration while producing monotonic decreases in steady-state twitch tension. A given concentration of Rb was more potent than the same concentration of Cs in its effects on both Na pump rate and tension. Nevertheless, varying [Rb](o) or [Cs](o) produced the same relationship between Na pump activity and tension.4. After the preparation was exposed to low K(o) (below 4 mM), the Na pump was reactivated with 10 mM-Rb(o) (0-K(o)). The area under the resulting electrogenic Na pump current transient gives a measure of the increase of [Na](i) that occurred when the preparation was exposed to the test solution compared to the steady-state Na(i) in 10 mM-Rb(o). Increasing the duration of exposure to low K(o), further augments the twitch tension achieved at the end of the test period and the area under the electrogenic Na pump current transient. Similarly, for equal periods of exposure, the lower the [K](o) in the test solution, the greater the increase of twitch tension at the end of the test period and the greater the area under the electrogenic Na pump current transient.5. The relationship between tension and the area under the electrogenic Na pump current transient is the same for a variable duration exposure to 0-K(o) or for a constant exposure to various low K(o). It is concluded that a rise in [Na](i) is the rate limiting step linking Na pump activity and twitch tension.6. In an experiment similar to the one described in (4), the fibre was exposed to a test solution containing a variable concentration of one of the activator cations of the Na pump for a fixed period. The effects of different cations were compared with the effects of a test solution containing no activator cation. Increasing the concentration of a particular activator cation from zero reduced the twitch tension augmentation in the test solution and also reduced the area under the electrogenic Na pump current transient on subsequently reactivating the Na pump with 10 mM-Rb(o). It is concluded that the ability of a test solution to reduce the area of the electrogenic Na pump current transient reflects the ability of the test solution to activate the Na pump. Equivalent concentrations of the activator cations that are required to activate the Na pump are: 2 mM-K(o) = 2 mM-Rb(o) = 6 mM-Cs(o) = 6 mM-NH(4o) = 22 mM-Li(o). Tl(o) was more effective than K(o). The order of these cations to reduce twitch tension is found to be the same as that to reduce the area under the electrogenic Na pump current transient.7. We conclude that the effects of the activator cations on twitch tension are determined by their effects on the Na pump and thereby on [Na](i).