It has recently been shown in epithelial cells that the ATP-gated ion channel P2X7R is in part, associated with caveolae and colocalized with caveolin-1. In the present study of the mouse heart, we show for the first time, using immunohistochemistry and cryoimmunoelectron microscopy, that P2X7R is expressed in atrial cardiomyocytes and in cardiac microvascular endothelial cells, but not in the ventricle cardiomyocytes. Furthermore, biochemical data indicate the presence of two forms of P2X7R, the classical glycosylated 80 kDa isoform and a protein with the molecular weight of 56 kDa, in both cardiomyocytes and endothelial cells of the mouse heart. The functionality of both proteins in heart cells is still unclear. In cardiac tissue homogenates derived from caveolin-1 deficient mice (cav-1 ^-/-), an increase of the P2Xrx7 mRNA and P2X7R protein (80 kDa) was found, particularly in atrial samples. In addition, P2rx7 ^-/- mice showed enhanced protein levels of caveolin-1 in their atrial tissues. Although the details of cellular mechanisms that underlie the relationship between caveolin-1 and P2X7R in atrial cardiomyocytes and the electrophysiological consequences of the increased P2X7R expression in atrial cells of cav-1 ^-/- mice remain to be elucidated, the cardiomyopathy detectable in cav-1 ^-/- mice is possibly related to a disturbed crosstalk between P2X7R and caveolin-1 in different heart cell populations.