Restricted Propagation of Cytoplasmic Ca2+ Oscillation into the Nucleus in Guinea Pig Cardiac Myocytes as Revealed by Rapid Scanning Confocal Microscopy and Indo-1

Abstract

Two-dimensional images of cytoplasmic and nuclear free Ca²⁺ movements in cardiac myocytes were obtained at 67-msec intervals using a Ca²⁺-sensitive fluorescence probe, indo-1, and a rapid scanning confocal laser microscope, Nikon RCM8000. Isolated guinea pig ventricular cells were loaded with indo-1 and stimulated at 0.5 Hz through patch pipettes. On stimulation, nuclear Ca²⁺ concentration ([Ca²⁺]) was observed to rise and fall following cytoplasmic [Ca²⁺] with an obvious delay. Application of isoproterenol significantly increased the peak [Ca²⁺] on stimulation in both the cytoplasm and nucleus with no substantial change in the basal [Ca²⁺]; the increase in peak [Ca²⁺]produced by application of isoproterenol was larger in the cytoplasm than in the nucleus. Under a low [Na⁺] condition, the basal [Ca²⁺] was increased from the control values in both the cytoplasm and nucleus; no difference in basal [Ca²⁺] was observed between the two regions. The increase in peak [Ca²⁺] by low [Na⁺] in the cytoplasm was significantly larger than that in the nucleus. When the cells were voltage clamped at 0 mV for 3 sec, no difference in the steady state [Ca²⁺] was observed between the cytoplasm and nucleus. Nuclear [Ca²⁺] was also observed to increase following a Ca²⁺ wave, a local increase in [Ca²⁺] propagating within the cytoplasm, with a delay. Thus, we demonstrated in isolated myocardial cells that cytoplasmic Ca²⁺ movements, although hampered by the nuclear envelope, are propagated into the nucleus, a mechanism through which factors affecting cytoplasmic Ca²⁺ may influence intranuclear events.