Acidic calcium stores in the heart provide a significant contribution to basal calcium transient amplitude and β-adrenergic responses in both atrial and ventricular myocytes1. In ventricular myocytes these acidic compartments, lysosomes, are positioned in close apposition to the sarcoplasmic reticulum and mitochondria, forming membrane contact sites which have the potential to act as signalling microdomains2. Atrial, but not ventricular, myocytes express a second small acidic organelle, the atrial granules (AG). AG are lysosome-related organelles which secrete ANP. They are known to be acidic and contain a high calcium content3 however their number relative to lysosomes and position with relation to other calcium signalling sites has not been fully explored. All animal experiments were performed in accordance with the United Kingdom Home Office Guide on the Operation of Animal (Scientific Procedures) Act of 1986. Guinea pig myocytes were isolated enzymatically and live cell imaging performed using spinning disk confocal microscopy. Human tissue samples (University of Freiburg) were collected from patients in sinus rhythm (SR) and atrial fibrillation (AF), fixed in Karnovsky fixative, embedded in Epon resin and imaged at the EMBL Heidelberg Electron Microscopy (EM) Core Facility. Staining of acidic organelles (LysoTracker) in freshly isolated guinea pig atrial and ventricular myocytes revealed fluorescent acidic puncta throughout the cytosol. In atrial myocytes there was an additional concentration of acidic organelles at the nuclear poles, consistent with published literature on the site of AG formation4. Inhibition of PAM (200 µM PBA), an essential component of AG membranes4, abolished not only staining at the nuclear poles but the majority of acidic puncta in atrial cells. Application of PBA to ventricular myocytes had no impact on LysoTracker staining. Consistent with our live cell results, EM of fixed human atrial tissue revealed the presence of both lysosomes and AG in atrial myocytes. In the SR sample, sarcomeres are regularly structured and surrounded by rows of mitochondria whilst in persistent AF, we observed myolytic areas. Qualitatively, both AG and lysosomes were significantly increased in samples from AF patients when compared to SR. Examples were seen of both lysosomes and AG in close apposition to the sarcoplasmic reticulum and mitochondria, as well as to each other. In the 1960s EM studies suggested that patients and dogs with atrial septal defects (AF is a common complication in these patients) have increased lysosome numbers5. Our imaging studies suggest that AG comprise a large percentage of acidic calcium stores within atrial cardiomyocytes and our preliminary EM studies in human samples indicate an increase in number during human atrial disease. Acidic calcium stores are known to play a significant role in normal atrial physiology1 and increase in number during atrial disease5. The strategic position of AG raises the question of whether they signal with other calcium containing organelles. It is an important next step to explore whether atrial granules contribute to physiological calcium signalling on top of their known hormone secretion role.