Yumiko Wainai

Autoimmunity Against the Second Extracellular Loop of β1-Adrenergic Receptors Induces β-Adrenergic Receptor Desensitization and Myocardial Hypertrophy In Vivo

Abstract— Although immunoapheresis removing autoantibodies against the second extracellular domain of &bgr;1-adrenergic receptors (ARs) improves cardiac function in patients with cardiomyopathy, the underlying mechanisms have not been defined. We examined the role of autoimmunity against the domain in the development of cardiac dysfunction in vivo. Japanese white rabbits were immunized with a synthetic peptide corresponding to the second extracellular loop of &bgr;1-AR once a month with (&bgr;+biso rabbits, n=10) or without (&bgr; rabbits, n=13) bisoprolol treatment (2 mg/kg per day). Control rabbits received vehicle without bisoprolol treatment (n=13). Autoantibodies of IgG isotype against the domain were persistently detected in &bgr; and &bgr;+biso rabbits. Purified IgG from sera of &bgr; and &bgr;+biso rabbits increased cAMP production in a rabbit cardiac membrane preparation, which was blocked by bisoprolol. At 3 months, &bgr;-AR uncoupling with increased G protein-coupled receptor kinase 5 (GRK5) expression was found in &bgr; rabbits. At 6 months, left ventricular hypertrophy was noted with hemodynamic derangements in &bgr; rabbits. This was accompanied by decreased &bgr;1-AR density and increased inhibitory G protein and GRK5 expression, which were related to marked decrease in membrane cAMP production. These changes in &bgr; rabbits at 6 months were prevented in &bgr;+biso rabbits. There was no difference in the plasma norepinephrine concentration in the 3 groups over the observation period. Thus, autoimmunity against the second extracellular loop of &bgr;1-ARs induced profound &bgr;-AR desensitization and myocardial hypertrophy in vivo, associated with cardiac dysfunction. Sustained sympathomimetic-like actions of autoantibodies against the domain may be partly responsible for these changes.

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

BACKGROUND Myocardial Na+,K+-ATPase activities are decreased in congestive heart failure because of an increase in plasma norepinephrine levels, but it is difficult to monitor the activities in the clinical setting. METHODS AND RESULTS This study investigated whether erythrocyte Na+,K+-ATPase activity can reflect myocardial enzyme activity and whether isoform-specific alterations occur in the presence of catecholamine. Na+,K+-ATPase activity was measured by the colorimetric method by using the left ventricular myocardium and erythrocytes prepared from eight rabbits given norepinephrine for 7 days and from eight control rabbits that received saline. The protein levels of total catalytic subunit and alpha1- or alpha3-isoform of Na+,K+-ATPase were determined by Western blot analysis. Na+,K+-ATPase activity was lower in both myocardium and erythrocytes from norepinephrine-treated rabbits than control rabbits (P < .01 and P < .01, respectively). There was a close correlation in Na+,K+-ATPase activity between myocardium and erythrocytes (r = .963). Total catalytic subunit protein level was lower in myocardium from norepinephrine-treated rabbits than control rabbits, but the alpha1-isoform level was similar between the two groups. The alpha3-isoform level was lower in norepinephrine-treated rabbits than control rabbits. In erythrocytes, alpha1-isoform was lower in norepinephrine-treated rabbits than control rabbits. CONCLUSIONS Na+,K+-ATPase activity in myocardium could be reflected in erythrocyte membrane, although there was a difference in isoform-specific regulation between the two.

Beta-blockade prevents ventricular failure following aortic regurgitation in rabbits

This study investigated the effects of chronic beta-blockade on the pathophysiology of heart failure following induction of aortic regurgitation (AR). Nine rabbits with AR were administered propranolol continuously for 7 days (AR+P), 8 rabbits with AR received vehicle for the same period (AR+C), and 7 rabbits underwent sham operation. Cardiac output was lower and the left-ventricular end-diastolic pressure was higher in AR+C than in sham-operated rabbits, but there was no difference in the right-ventricular end-diastolic pressure between the two groups. Down-regulation of beta-adrenoceptors was observed in the left ventricle, but not in the right ventricle. All of these variables were reversed in AR+P. In left-ventricular failure produced by AR, (1) the augmentation of adrenergic drive occurred selectively in the left ventricle, and (2) propranolol blunted adrenergic drive and played a protective role against myocardial damage.

Atrial natriuretic peptide secretion in rabbits with aortic regurgitation

SummaryWe investigated whether or not the extent of secretion of atrial natriuretic peptide (ANP) was altered during the chronic course of aortic regurgitation (AR) in rabbits. AR was induced by aortic valve perforation in 20 rabbits. Left ventricular end-diastolic pressure (LVEDP) was used as an index of the severity of heart failure. LVEDP and plasma immunoreactive (IR)-ANP were measured before and at 15 min, 1 week, and 4 weeks after induction of AR. In each period a correlation between LVEDP and plasma IR-ANP was observed, and the coefficients and covariances did not vary throughout the experiment. We conclude, therefore, that a chronic change in ANP secretion does not develop during the first 4 weeks after induction of AR in rabbits.