IP3-dependent nuclear Ca2+ signalling in the mammalian heart

Abstract

In cardiac myocytes the type-2 inositol 1,4,5-trisphosphate receptor (IP3R2) is the predominant isoform expressed. The IP3R2 channel is localized to the SR and to the nuclear envelope. We studied IP3-dependent nuclear Ca2+ signals ([Ca2+]Nuc) in permeabilized atrial myocytes and in isolated cardiac nuclei. In permeabilized myocytes IP3 (20 μM) and the more potent IP3R agonist adenophostin (5 μM) caused an elevation of [Ca2+]NUC. An IP3-dependent increase of [Ca2+]NUC was still observed after pretreatment with tetracaine to block Ca2+ release from ryanodine receptors (RyRs), and the effect of IP 3 was partially reversed or prevented by the IP3R blockers heparin and 2-APB. Isolated nuclei were superfused with an internal solution containing the Ca2+ indicator fluo-4 dextran. Exposure to IP3 (10 μM) and adenophostin (0.5 μM) increased [Ca2+]NUC by 25 and 27%, respectively. [Ca2+]NUC increased to higher levels than [Ca2+]Cyt immediately adjacent to the outer membrane of the nuclear envelope, suggesting that a significant portion of nuclear IP 3 receptors are facing the nucleoplasm. When nuclei were pretreated with heparin or 2-APB, IP 3 failed to increase [Ca2+]NUC. Isolated nuclei were also loaded with the membrane-permeant low-affinity Ca2+ probe fluo-5N AM which compartmentalized into the nuclear envelope. Exposure to IP3 and adenophostin resulted in a decrease of the fluo-5N signal that could be prevented by heparin. Stimulation of IP3R caused depletion of the nuclear Ca2+ stores by approximately 60% relative to the maximum depletion produced by the ionophores ionomycin and A23187. The fluo-5N fluorescence decrease was particularly pronounced in the nuclear periphery, suggesting that the nuclear envelope may represent the predominant nuclear Ca2+ store. The data indicate that IP 3 can elicit Ca2+ release from cardiac nuclei resulting in localized nuclear Ca2+ signals. © 2007 The Authors. Journal compilation © 2007 The Physiological Society.