Interaction between autoantibodies against the beta1-adrenoceptor and isoprenaline in enhancing L-type Ca2+ current in rat ventricular myocytes.

Autoantibodies against the beta1-adrenoceptor (beta1-AAB) in the serum of patients with dilated cardiomyopathy (DCM) are associated with stimulatory effects at cardiac beta1-adrenoceptors. They enhance cardiomyocyte shortening and increase the amplitude of L-type Ca2+ current, ICa. However, in contrast to the unselective beta-adrenoceptor agonist (-)-isoprenaline, beta1-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 ICa in response to beta1-AAB and (-)-isoprenaline were investigated in responder and non-responder cells. The immunoglobulin G (IgG) fractions containing beta1-AAB (beta1-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 beta1-IgG or purified beta1-AAB were responder cells in which ICa amplitude increased. ICa increase by beta1-IgG or (-)-isoprenaline was reversed by addition of carbachol. Exposure to subtype-selective beta-adrenoceptor blockers indicated that the effects of IgG were mediated via beta1-adrenoceptors. In responder cells, there were no differences between beta1-IgG- and (-)-isoprenaline-induced changes in current-voltage relationship of ICa, in the time constants of fast inactivation, and in steady-state activation and steady-state inactivation curves. (-)-Isoprenaline (1 microM) effectively increased ICa 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 beta1-IgG, any further increase in ICa was completely suppressed. Conversely, in responder cells, the cumulative concentration-response curves for (-)-isoprenaline on top of the autoantibodies reached the same maximum ICa amplitude as in control cells. From these interactions we conclude that beta1-AAB not only may enhance ICa via stimulation of beta1-adrenoceptors but also may inhibit beta1-adrenoceptor-mediated increase upon stimulation with catecholamines suggesting a receptor interaction distinct from that with (-)-isoprenaline.