Nucleoplasmic [Ca] Transients Alterations and Perinuclear Ca Stores Remodeling after Pressure Overload-Induced Hypertrophy in Adult Cardiac Myocytes

Abstract

Nucleoplasmic calcium concentration ([Ca]) in cardiac myocytes (CMs) regulates exitation-transcription coupling and is involved in remodelling processes. Perinuclear Ca stores contribute to the regulation of nucleoplasmic [Ca] transients (CaTs). We thus characterized alterations in perinuclear Ca stores and nucleoplasmic CaTs after pressure overload-induced hypertrophy in adult CMs.Pressure overload was induced by transverse aortic constriction (TAC) in adult wild-type mice. Sham-operated mice served as controls. Ventricular CMs were isolated 1 and 7 weeks after TAC/Sham. Perinuclear Ca stores were visualized using confocal imaging and staining with Mag-Fluo-4/AM (10μM). CaTs were recorded in electrically-stimulated CMs loaded with Fluo-4/AM (8μM).In Sham CMs, staining of perinuclear Ca stores revealed a nuclear envelope and tubular structures transversing the nucleus. Rapid application of caffeine (20 mM, n=7) reversibly abolished Mag-Fluo-4 fluorescence. Fluorescence recovery after depletion was identical in nuclear envelope and tubular structures. A significant increase in number of tubules per nucleus was observed during physiological growth (1week: 4.2±0.2; 7weeks: 4.7±0.2 (n=90)). Nuclear dimensions as well as cyto- and nucleoplasmic CaTs remained unaltered. In TAC CMs, the number of tubules progressively decreased (1week; 4.3±0.2; 7weeks: 3.4±0.2 (n=90)), whereas length and width of nuclei increased (1week: 13.3±0.3 and 4.9±0.2μm; 7weeks: 17.8±0.7 and 5.5±0.2μm). One week after TAC changes in the kinetics and amplitude of CaTs were found in the nucleus (TTP: 127±2 vs 194±5ms; RT50: 360±7 vs 380±5ms; amplitude: 491±19 vs 376±28nM; Sham vs TAC (n=15); all P<0.05). Seven weeks after TAC similar changes also occurred in the cytoplasm.Perinuclear Ca stores and nucleoplasmic CaTs undergo significant changes during hypertrophy progression, which appear to precede changes in cytoplasmic Ca regulation. These results raise the possibility that altered nucleoplasmic [Ca] may contribute to the development and/or progression of hypertrophy.