Microdomain–specific localization of functional L-type calcium channels in atrial cardiomyocytes: novel concept of local regulation and remodelling in disease

Recently, novel concept of microdomain-specific regulation in cardiac cells have greatly extended our understanding of how specific subcellular localization impacts on channel function and regulation. Microdomain is a small region of cell membrane, which has a distinct structure, composition and function. It has been recognized that discrete clusters of different ion channels exist in the sarcolemma in different microdomains such as T-tubules, caveolae. This study addresses the hypothesis that distinct spatial compartmentalization of functional calcium channels in different intercellular microdomains are coupled with structural proteins and receptors and play an important role in unique Ca2+ signaling in atrial cardiomyocytes in health and pathology. Using several technical approaches (super-resolution scanning and whole-cell patch-clamp, confocal and electron microscopy), this study aims to investigate characteristics of subcellular micrdomains such as T-tubules and caveolae in atrial cardiomyocytes; and to answer the question whether in atrial cardiomyocytes functional L-type calcium channels (LTCCs) are specifically distributed within different microdomains and forming signalling complexes with receptors, that potentially causes a unique atrial cardiomyocyte Ca2+ signaling process. First, it was found that atrial cells could be characterised by heterogeneous T-tubule system the structure of which influenced by the cell size and atrial chamber localization. This study provides the first direct evidence for two distinct subpopulations of functional LTCCs in rat and human healthy atrial cardiomyocytes, with a micro-domain-specific regulation of their biophysical properties. In atrial cells, L-type calcium channels are equally distributed inside and outside of T-tubules, in contrast to ventricular cardiomyocytes where LTCCs are clustered in T-tubules (Bhargava, Lin et al. 2013). The population of LTCCs observed outside of T-tubules was associated with caveolae. LTCCs located in caveolae contribute essentially to atrial Ca2+ signaling, particularly in cardiomyocytes lacking the organized T-tubule network. Second, β1-adreneric stimulation, which increases single LTCC activity and antiadrenergic effect of adenosine on functional LTCCs were investigated in both microdomains in rat atrial cariomyocytes. Third, using animal model, heart failure was found to be associated with loss of T-tubule structure and decrease in single amplitude of T-tubular LTCCs localized in atrial cardiomyocytes. Fourth, human studies revealed, that chronic atrial fibrillation is associated with the loss of T-tubule structure and downregulation of the L-type calcium current with increased activity of single LTCCs localized in T-tubule microdomains and the loss channels outside of T-tubules. Decrease of calcium current was associated with the downregulation of gene expression. These results support the notion that functional L-type calcium channels are linked with structural components of cardiac membrane and undergo remodelling in association with loss of structures during pathology.