Effects of Beta-Adrenergic Stimulation on Rat Failing Cardiomyocytes

Abstract

Beta-adrenoceptors start the most powerful signaling pathway for the sympathetic control of the cardiac function. Human and animal models of heart failure (HF), one of the most important causes of mortality worldwide, show alterations of the sympathetic system. Number and function of beta-adrenoceptors, namely the beta-1 subtype, are impaired in HF, representing a major (mal)adaptive feature of myocardial remodeling. Moreover, HF is characterized by structural and functional remodeling of T-tubules (TT). We previously observed failure of action potential conduction in about 6% of HF TTs accompanied by profound alterations of local Ca2+ transient (Crocini et al. PNAS 2014). Here, we employ an ultrafast random access multi-photon (RAMP) microscope to assess the effect of beta-adrenoceptor activation on Ca2+ release in electrically coupled and uncoupled tubular elements. We find that HF cells exhibit a significantly higher Ca2+ sparks frequency compared to controls that are not affected by beta-receptor activation (10-7 M isoproterenol). Control cardiomyocytes treated with isoproterenol show instead a more than 3-fold increase of Ca2+ sparks frequency. To better examine this difference, we assess beta-adrenergic signaling role on Ca2+ release. We observe an accelerated Ca2+ rise exclusively in the vicinity of electrically coupled TT, while Ca2+ rise close to failing elements is unchanged in beta-adrenergic stimulated cells. This finding indicates a patchy response of HF cells to beta-adrenergic stimulation. On the contrary, isoproterenol reduces the beat-to-beat variability of Ca2+ release time to peak at every membrane site of HF cardiomyocytes. Simultaneous recording of action potential and Ca2+ transient allows us to probe the effect of beta-adrenergic stimulation at subcellular level, disclosing specific local effects that may help understanding the changes of beta-adrenergic signaling in HF.