Structural evidence for perinuclear calcium microdomains in cardiac myocytes

Abstract

At each heartbeat, cardiac myocytes are activated by a cytoplasmic Ca2+ transient in great part due to Ca2+ release from the sarcoplasmic reticulum via ryanodine receptors (RyRs) clustered within calcium release units (peripheral couplings/dyads). A Ca2+ transient also occurs in the nucleoplasm, following the cytoplasmic transient with some delay. Under conditions where the InsP3 production is stimulated, these Ca2+ transients are regulated actively, presumably by an additional release of Ca2+ via InsP3 receptors (InsP3Rs). This raises the question whether InsP3Rs are appropriately located for this effect and whether sources of InsP3 and Ca2+ are available for their activation. We have defined the structural basis for InsP3R activity at the nucleus, using immunolabeling for confocal microscopy and freeze-drying/shadowing, T tubule “staining” and thin sectioning for electron microscopy. By these means we establish the presence of InsP3R at the outer nuclear envelope and show a close spatial relationship between the nuclear envelope, T tubules (a likely source of InsP3) and dyads (the known source of Ca2+). The frequency, distribution and distance from the nucleus of T tubules and dyads appropriately establish local perinuclear Ca2+ microdomains in cardiac myocytes.