Despite the important role of electro-mechanicalalternans in cardiac arrhythmogenesis, its molecularorigin is not well understood. In this paper we want tostudy alternans that can be associated with alternationsin the cytosolic calcium transient. To this end we studyparticularly, how a malfunction of Ryanodine Receptor(RyR) recovery time from inactivation may induce beatto-beat alternations in intracellular calciumconcentration and how this could affect the stimuluspropagation in inhomogeneous tissue, whose cells exhibitdifferent calcium dynamics.