Isoform-specific alterations in cardiac and erythrocyte Na+,K+-ATPase activity induced by norepinephrine

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Antigen-specific effects of autoantibodies against sarcolemmal Na–K-ATPase pump in immunized cardiomyopathic rabbits

OBJECTIVES

We examine antigen-specific actions of autoantibodies directed against sarcolemmal Na-K-ATPase.

BACKGROUND

Autoantibodies against some receptors or pumps were detected in patients with dilated cardiomyopathy. Although immunoglobulin adsorption therapy improved cardiac function in such patients, direct pathogenic effects of autoantibodies remain to be proven.

METHODS

Japanese white rabbits were immunized once a month with purified Na-K-ATPase (NKA rabbits, n=10) or a synthetic peptide corresponding to the second extracellular loop of beta1-adrenergic receptors (beta rabbits, n=10), respectively. Control rabbits (n=10) received vehicle in the same manner.

RESULTS

At 6 months, cardiac hypertrophy along with increased left ventricular end-diastolic pressure was observed in both NKA and beta rabbits, and inhibitory G protein level increased in both NKA and beta rabbits. Histological findings showed similar myocyte hypertrophy and interstitial fibrosis in both rabbits. Enzymatic activities of Na-K-ATPase were lower in NKA rabbits than in other groups. Immunoblotting showed that alpha3-isoform of Na-K-ATPase was selectively reduced in myocardium from NKA rabbits.

CONCLUSIONS

Our present findings suggested that isoform-specific alterations of myocardial Na-K-ATPase activity were induced by immunizing rabbits. This was not secondary change due to cardiac hypertrophy. Thus, autoantibodies against sarcolemmal Na-K-ATPase have antigen-specific effect on the heart in vivo.

Differential effects of etomoxir treatment on cardiac Na+-K+ ATPase subunits in diabetic rats

Etomoxir, an inhibitor of mitochondrial carnitine palmitoyltransferase-1, is known to attenuate the changes in myosin isoforms and sarcoplasmic reticular function that occur in diabetic rat hearts. In the present study, we tested the hypothesis that etomoxir also prevents the diabetes-induced depression of sarcolemmal (SL) Na+-K+ATPase activity by differentially affecting its alpha and beta-subunit levels. Streptozotocin-induced diabetes was associated with a decreased in alpha2-, alpha3-subunit levels, whereas the alpha1-and beta1-subunits were unchanged. Treatment of diabetic rats for 4 weeks with etomoxir (8 mg/kg/day) increased the alpha1-subunit levels, but failed to prevent the decrease in alpha2- and alpha3-subunit levels. In euglycemic control rats, etomoxir increased the alpha1-subunit protein level per g heart weight, but did not alter the alpha2-, alpha3- and beta1-subunit levels. The large decrease in Na+-K+ ATPase activity per g heart weight in diabetic rats was prevented by etomoxir, which suggests that the increased alpha1-subunit levels seen with this drug compensated for the decreased alpha2- and alpha3-subunit levels. The SL yield was also increased by etomoxir in euglycemic rats in proportion to the higher alpha1-subunit level, which resulted in an unchanged alpha1-content when expressed per mg SL protein; however, the alpha2- and beta1-subunit levels were reduced (p < 0.05). The depressed alpha2- and beta3 subunit levels of diabetic rats were associated with reduced mRNA abundance. However, no increase in alpha1-subunit mRNA abundance was seen in the etomoxir treated rats, which suggests that possibly post-transcriptional mechanisms are occurring in these hearts.

Erythrocyte Na+, K+-ATPase activity in patients with congestive heart failure

In order to determine if the Na+, K+-ATPase activity in erythrocyte membranes is altered in congestive heart failure, and to examine its clinical significance with respect to other clinical variables, erythrocyte Na+, K+-ATPase activity was measured in 51 patients with left ventricular ejection fractions <40% (coronary artery disease, n=26; dilated cardiomyopathy, n=25) and 24 control patients. Na+, K+-ATPase activity was lower in both coronary artery disease and dilated cardiomyopathy groups than control group even in the absence of digitalis use. There was a significant inverse correlation between Na+, K+-ATPase activity and plasma norepinephrine. The presence of non-sustained ventricular tachycardia was associated with a lower Na+, K+-ATPase activity in both groups with congestive heart failure without digitalis use than those without ventricular tachycardia. Plasma norepinephrine was higher in patients with non-sustained ventricular tachycardia than those without in the coronary artery disease group, but not in the dilated cardiomyopathy group. Na+, K+-ATPase activity may be helpful in predicting electrophysiologic instability in patients with heart failure.