Interaction between autoantibodies against the β 1-adrenoceptor and isoprenaline in enhancing L-type Ca 2+ current in rat ventricular myocytes

Abstract

Autoantibodies against the β 1-adrenoceptor (β 1-AAB) in the serum of patients with dilated cardiomyopathy (DCM) are associated with stimulatory effects at cardiac β 1-adrenoceptors. They enhance cardiomyocyte shortening and increase the amplitude of L-type Ca 2+ current, I Ca . However, in contrast to the unselective β-adrenoceptor agonist (–)-isoprenaline, β 1-AAB produce positive responses in a fraction of myocytes (responder cells) only and fail to do so in the remaining ones (non-responder cells). To understand this peculiar behaviour, the electrophysiological characteristics of I Ca in response to β 1-AAB and (–)-isoprenaline were investigated in responder and non-responder cells. The immunoglobulin G (IgG) fractions containing β 1-AAB (β 1-IgG) were obtained from patients with DCM undergoing immunoabsorption therapy. Only antibody preparations that tested positive in the neonatal rat cardiomyocyte bio-assay by enhancing beating rate were used for further experimentation. Calcium currents were measured with the standard patch clamp technique in adult rat ventricular myocytes. Less than half of all cells exposed to β 1-IgG or purified β 1-AAB were responder cells in which I Ca amplitude increased. I Ca increase by β 1-IgG or (–)-isoprenaline was reversed by addition of carbachol. Exposure to subtype-selective β-adrenoceptor blockers indicated that the effects of IgG were mediated via β 1-adrenoceptors. In responder cells, there were no differences between β 1-IgG- and (–)-isoprenaline-induced changes in current-voltage relationship of I Ca , in the time constants of fast inactivation, and in steady-state activation and steady-state inactivation curves. (–)-Isoprenaline (1 μM) effectively increased I Ca after wash-out of antibody in all cells including non-responder cells. However, when non-responder cells were challenged with (–)-isoprenaline in the presence of β 1-IgG, any further increase in I Ca was completely suppressed. Conversely, in responder cells, the cumulative concentration-response curves for (–)-isoprenaline on top of the autoantibodies reached the same maximum I Ca amplitude as in control cells. From these interactions we conclude that β 1-AAB not only may enhance I Ca via stimulation of β 1-adrenoceptors but also may inhibit β 1-adrenoceptor-mediated increase upon stimulation with catecholamines suggesting a receptor interaction distinct from that with (–)-isoprenaline.