Spatio-Temporal Properties of IP3 Receptor-Mediated Ca Release in Cardiac Myocytes

Abstract

In cardiac myocytes cytosolic Ca increase and subsequent cell contraction are mainly determined by ryanodine receptor (RyR) mediated Ca release. However atrial cells are also equipped with IP3 receptors (IP3Rs), a second type of Ca release channels. We investigated IP3R-mediated Ca release events and their subcellular spatio-temporal properties in single membrane-permeabilized rabbit atrial myocytes. Local Ca release events were detected by confocal microscopy (fluo-4, longitudinal line scan mode). Local IP3R-mediated Ca release events were evoked by exposure to inositol-1,4,5-triphosphate (IP3) in the presence of tetracaine to inhibit RyR-mediated Ca spark activity, and could be inhibited by the IP3R blocker 2-APB. We classified IP3R-mediated Ca release events as subsarcolemmal, perinuclear (events 5μm) or nuclear. Nuclear events were considered membrane associated when occurring at a distance <1μm from the nuclear envelope or membranes of the nucleoplasmic reticulum. With tetracaine the Ca spark frequency decreased to 0.7±0.3 (from 8.8±1.2 in control), whereas the frequency of events detected after subsequent addition of IP3 increased to 2.9±1.3 (events/100μm/s). Subsequent exposure to 2-APB reduced the frequency to 1.8±0.8. Compared to Ca sparks, local Ca release events in the presence of IP3 were lower in amplitude, prolonged in duration and had a slower rise time. The increase in frequency of local Ca release events was particularly pronounced in the perinucler regions compared to the cytosol or the subsarcolemmal space. Furthermore, IP3-mediated Ca release events could also be detected within the nucleus. These nuclear events occurred at a higher incidence at locations that were closely associated with membrane structures of the nuclear envelope or the nucleoplasmic reticulum. In conclusion, in atrial myocytes IP3R-mediated Ca release is spatially inhomogeneous and preferentially occurs in the nuclear and perinuclear regions.