Electrophysiological properties and mechanism of aging for the susceptibility of left atrium to arrhythmogenesis in rabbits

Abstract

To explore the electrophysiological properties and mechanism of aging for the susceptibility of left atrium (LA) to arrhythmogenesis in rabbits. A total of 50 male New Zealand rabbits were divided into young (6 months, n = 25) and aged (26 months, n = 25) groups. According to comparisons before and after isoproterenol (ISO) dosing, young rabbits were further divided into young control and medication groups while aged ones into aged control and medication groups. Electrophysiological study was performed on perfused isolated left atrium and conventional microelectrodes were used for recording transmenbrane action potentials (tAP), resting membrane potential (RMP), amplitude of APs (APA), maximum upstroke velocity (Vmax), APs duration at 20%, at 50% or at 90% repolarization (APD20, APD50, APD90), atrial effective refractory periods (AERP) and atrial arrhythmia inducibility before and after ISO dosing. L-type calcium current (ICa-L) was measured via whole-cell patch clamp technique. And fluorescent intensity of intracellular Ca²⁺ was detected with Flup-3/AM loading by laser scanning confocal microscope in enzymatically dissociated single LA myocytes before and after dosing. Compared with major parameters of tAP in young control group, APDs and AERP significantly increased and APA, Vmax, RMP and AERP/APD90 obviously decreased in aged control group (all P < 0.05). Otherwise, the conduction of tAP was rapid and delayed afterdepolarizations (DADs) were induced in both young and aged medication groups after ISO dosing. However, as compared with major parameters of young medication group, APDs dramatically declined while APA, Vmax, RMP and DAD amplitude significantly increased. And triggered activity was induced in aged medication group (all P < 0.05). With voltage clamp model, ICa-L significantly declined in aged control group versus young control group. The peak current density of ICa-L dramatically decreased ((6.3 ± 0.8) pA/pF in aged control group vs (9.5 ± 1.3) pA/pF in young control group, P < 0.01) under 10 mV clamp voltage. And current-voltage (I-V) curve of ICa-L was greatly upgraded in aged control group on the top of all I-V curves without changing direction. The current density of ICa-L increased much more in aged medication group than that in young medication group ((15.9 ± 3.1) vs (11.3 ± 2.6) pA/pF, P < 0.01). And I-V curve of ICa-L shifted the bottom of all I-V curves in aged medication group. Fluorescent intensities of intracellular free Ca²⁺ became completely attenuated ((437.9 ± 21.7) µmol/L in aged control group vs (778.3 ± 19.5) µmol/L in young control group, P < 0.01). Under the effect of ISO administration, fluorescent intensities of intracellular free Ca²⁺ significantly increased in both young and aged medication groups. But [Ca²⁺]i were altered more in aged medication group than that in young medication group ((1 635.6 ± 51.6) vs (1 008.7 ± 34.5) µmol/L, P < 0.05). The electrophysiological characteristics are significantly altered with electrical remodeling before and after ISO dosing in aging LA in rabbits. And it may contribute to the vulnerability and pathophysiological basis for reentry and atrial arrhythmogenesis. The electrophysiological mechanism is probably attributed to the alterations of ICa-L and [Ca²⁺]i before and after ISO perfusion in aging LA myocytes.