Pulmonary veins (PVs) contain cardiomyocytes with a complex cellular morphology and high arrhythmogenesis. Ca2+ regulation and Ca 2+ sparks play a pivotal role in the electrical activity of cardiomyocytes. The purpose of this study was to investigate whether the cell morphology can determine the PV electrical activity and Ca2+ homeostasis. Through confocal microscopy with fluo-3 Ca2+ fluorescence, Ca2+ sparks and Ca2+ transients were evaluated in isolated single rabbit left atria (LA) and PV cardiomyocytes according to the cell morphology (rod, rod-spindle and spindle/bifurcated). Twenty-two (20%) rod, 49 (43%) rod-spindle and 41 (37%) spindle/bifurcated cardiomyocytes were identified in the LA (n = 29) and PV (n = 83) cardiomyocytes. The PV cardiomyocytes with pacemaker activity had a higher incidence of spindle/bifurcated morphology than LA and PV cardiomyocytes without pacemaker activity. As compared to those in the rod or rod-spindle cardiomyocytes, spindle/bifurcated cardiomyocytes had a larger Ca2+ transient amplitude and higher frequency of the Ca2+ sparks with larger amplitude and longer duration. In contrast, rod-spindle and rod cardiomyocytes had similar Ca2+ transients and Ca2+ sparks. The cell length correlated well with the amplitude of the Ca 2+ transient and Ca2+ spark duration with a linear regression. In conclusion, cell morphology and cell length play a potential role in the Ca2+ homeostasis and Ca2+ spark. The large Ca 2+ transients and high frequency of Ca2+ sparks in spindle/bifurcated cardiomyocytes may cause a high arrhythmogenesis in the PV cardiomyocytes with pacemaker activity.
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