Diverse cell morphology and intracellular calcium dynamics in pulmonary vein cardiomyocytes

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.