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

Positive inotropic effect of ouabain on isolated heart is accompanied by activation of signal pathways that link Na+/K+-ATPase to ERK1/2

Exposure of cultured rat cardiac myocytes to ouabain is known to cause the interaction of Na+/K+-ATPase with adjacent proteins, leading to activation of multiple signal transduction pathways, regulation of growth-related genes, and hypertrophy. The aim of this work was to determine if the proximal signaling events identified in cultured myocytes also occur in isolated intact hearts of rat and guinea pig in response to positive inotropic doses of ouabain. Langendorff rat heart preparations were exposed to 50 microM ouabain to produce positive inotropy without toxicity, and assayed for Src kinase, protein kinase C, and extracellular signal-regulated kinases 1 and 2 (ERK(1/2)). These kinases were rapidly activated by ouabain as in cultured cells. In isolated guinea pig hearts, 1 microM ouabain caused ERK(1/2) activation comparable to the effect of 50 microM ouabain in rat heart and consistent with the higher ouabain sensitivity of the contractility of guinea pig heart. These data show that the proximal ouabain-induced signal pathways previously noted in cultured cells are not artifacts of dispersion/culturing of myocytes, and are not the peculiar properties of the rat heart with its relatively low ouabain sensitivity. They also suggest that treatment with positive inotropic doses of cardiac glycosides is likely to be associated with changes in the cardiac phenotype.

Role of protein kinase C in the signal pathways that link Na+/K+-ATPase to ERK1/2

We have shown before that Na(+)/K(+)-ATPase acts as a signal transducer, through protein-protein interactions, in addition to being an ion pump. Interaction of ouabain with the enzyme of the intact cells causes activation of Src, transactivation of EGFR, and activation of the Ras/ERK1/2 cascade. To determine the role of protein kinase C (PKC) in this pathway, neonatal rat cardiac myocytes were exposed to ouabain and assayed for translocation/activation of PKC from cytosolic to particulate fractions. Ouabain caused rapid and sustained stimulation of this translocation, evidenced by the assay of Ca(2+)-dependent and Ca(2+)-independent PKC activities and by the immunoblot analysis of the alpha, delta, and epsilon isoforms of PKC. Dose-dependent stimulation of PKC translocation by ouabain (1-100 microm) was accompanied by no more than 50% inhibition of Na(+)/K(+)-ATPase and doubling of [Ca(2+)](i), changes that do not affect myocyte viability and are known to be associated with positive inotropic, but not toxic, effects of ouabain in rat cardiac ventricles. Ouabain-induced activation of ERK1/2 was blocked by PKC inhibitors calphostin C and chelerythrine. An inhibitor of phosphoinositide turnover in myocytes also antagonized ouabain-induced PKC translocation and ERK1/2 activation. These and previous findings indicate that ouabain-induced activation of PKC and Ras, each linked to Na(+)/K(+)-ATPase through Src/EGFR, are both required for the activation of ERK1/2. Ouabain-induced PKC translocation and ERK1/2 activation were dependent on the presence of Ca(2+) in the medium, suggesting that the signal-transducing and ion-pumping functions of Na(+)/K(+)-ATPase cooperate in activation of these protein kinases and the resulting regulation of contractility and growth of the cardiac myocyte.

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.

Postnatal changes in the inotropic effect of ouabain on the rat heart ventricle

The concentration-dependent effects of ouabain on the contractility of postnatally developing rat heart ventricles were studied. Ouabain caused a positive inotropic effect in right ventricular strips of neonatal rats up to the age of about 30 days but a negative inotropic effect in the adult cardiac tissue. When extracellular Ca concentration was lowered from 2.5 to 1.0 mmol/l and the rate of stimulation was simultaneously elevated from 0.2 to 1.0 Hz a clear positive inotropic effect was also generated in the adult rat heart. The positive inotropic effect of ouabain showed a biphasic developmental pattern: the contractile force first grew from birth to about 15 days of age but steeply declined near to the adult level during the 3rd postnatal week. The force response to ouabain occurred within two distinct dose-ranges. In the newborn only the high-dose (above 3 X 10(-6) mol/l) effect was seen but in rats older than 5 days a mixed low-dose (below 3 X 10(-6) mole/l)/high-dose effect was apparent. In both ranges the positive inotropic effect of ouabain seemed to be dependent on caffeine sensitive Ca store, perhaps the sarcoplasmic reticulum. It is suggested that during the 3rd postnatal week a transition from extracellular to intracellular Ca stores occurs in the rat heart, which is reflected as a changing inotropic effect of ouabain on the developing cardiac tissue.

Sodium load and high affinity ouabain binding in rat and guinea-pig cardiac tissue

An estimation of the actual Na/K-ATPase transport activity in intact cardiac cells was made by measuring the binding of [3H]-ouabain to rat and guinea-pig ventricular strips. At the low [3H]-ouabain concentration of 1 nM equilibrium binding was hardly obtained after an incubation time of five hours. Different procedures known to alter the sodium load of the cardiac preparations influenced [3H]-ouabain binding: the sodium ionophore monensin enhanced [3H]-ouabain binding, the local anaesthetic dibucaine and a reduction of external sodium ion concentration diminished [3H]-ouabain binding; [3H]-ouabain binding was similarly affected by these procedures in the rat and guinea-pig. Since [3H]-ouabain binding occurred predominantly at the high-affinity binding sites of rat myocardium under the applied experimental conditions, it was concluded that these binding sites represent Na/K-ATPase molecules involved in sodium ion transport.

Some hormonal effects on myocardial phosphate efflux

The effects of 23 agonists on the rates of cellular 32P efflux and lactate dehydrogenase (LDH) release were tested in a perfused rat heart preparation which had been prelabelled in vitro with [32P]Pi. Some 13 compounds produced detectable changes at high doses within 10 min, and in most cases a polyphasic response was observed. Six classes of compound gave rise to substantial effects, as follows. Catecholamines and glucagon produced a transient initial stimulation of Pi efflux, followed by a long-term inhibition of Pi transport and an increased rate of LDH release. These effects were clearly different from the response seen after treatment with dibutyryl cyclic AMP, which had a slower, stimulatory, effect on Pi output in doses which gave rise to a pronounced inotropic effect, and produced a marked increase in both coronary flow and LDH release. Carbachol also gave rise to a large transient stimulation of Pi efflux, which was followed by smaller sustained increase in Pi output without any obvious effect on LDH release. Dibutyryl cyclic GMP had no effect on Pi efflux or LDH release. Insulin decreased the rate of Pi efflux, although the loss rate partially recovered towards the control value after prolonged exposure to the hormone. Insulin had no obvious inotropic effects and produced no change in the rate of LDH release. Corticosteroids increased the rate of Pi efflux, although the loss rate partially declined towards the control value with prolonged exposure to the hormones. Corticosteroids produced a very slight inotropic response, and large doses sometimes increased the rate of LDH release from the tissue. Aldosterone slightly stimulated Pi output. A small, transient and somewhat variable stimulation of Pi efflux was observed with vasopressin and angiotensin, whereas tri-iodothyronine was slightly inhibitory, but adenosine, histamine, spermidine, des-Asp1-angiotensin, prolactin, parathyroid substances, calcitonin and somatostatin had no significant effects under our experimental conditions. Ouabain stimulated Pi efflux in doses that had no detectable inotropic effect. It is suggested that Pi efflux involves the electroneutral transport of NaH2PO4 across the cardiac plasmalemma and that many of the hormonal effects might be explained by changes in the intracellular [Na+] and pH in addition to changes in the intracellular [Pi].