Visualization of biphasic Ca2+diffusion from cytosol to nucleus in contracting adult rat cardiac myocytes with an ultra-fast confocal imaging system

Abstract

In contracting cardiac myocytes, the rapid changes in cytosolic and nuclear Ca2+make it difficult to determine whether the nuclear Ca2+transient is caused by diffusion from the cytosol or by Ca2+release channels on the inner nuclear membrane, or both. The propagation mechanism in the nucleoplasm also remains unknown. We have developed an ultra-fast Nipkow confocal imaging system able to acquire two-dimensional images at ~4 ms/full frame speed and employed it to analyze Ca2+waves and the dynamics of the cytosolic and nuclear Ca2+transients after electrical stimulation of cardiac myocytes. The pattern of nuclear Ca2+upon stimulation was well described by a mathematical model of Ca2+diffusion across the nuclear envelope. No evidence of Ca2+release from perinuclear Ca2+stores was obtained. The Ca2+diffusion constant appeared to change during contraction, with essentially free diffusion of Ca2+through nuclear pore complexes at low cytosolic Ca2+and partially restricted diffusion at high cytosolic Ca2+. The Ca2+in the nucleoplasm propagated by diffusion and no Ca2+release phenomena were seen in the nucleus.