Despite the important role of electro-mechanical alternans in cardiac arrhythmogenesis, its molecular origin is not well understood. In this paper we want to study alternans that can be associated with alternations in the cytosolic calcium transient. To this end we study particularly, how a malfunction of Ryanodine Receptor (RyR) recovery time from inactivation may induce beatto- beat alternations in intracellular calcium concentration and how this could affect the stimulus propagation in inhomogeneous tissue, whose cells exhibit different calcium dynamics.