Ageing causes a progressive loss of L-type calcium current and a depression of the SR calcium content linked to lower SERCA2 and calsequestrin-2 expression in human atrial myocytes

Abstract

Purpose: Ageing is associated with an increased incidence of atrial fibrillation and this arrhythmia has been associated to alterations in intracellular calcium handling. The purpose of this study was to test the hypothesis that ageing per se alters calcium handling in human atrial myocytes.Methods: Whole membrane currents were measured in the perforated patch configuration in human atrial myocytes from 74 patients free of atrial fibrillation and with normal left atrial size. Patients were categorized as young (<55 years, n=21), middle aged (between 55 and 75 years, n=42), and old (>75 years, n=11). Protein expression was determined with Western blot technique in right atrial samples from 7 young and 7 old patients.Results: The expression of the alpha-subunit of the L-type calcium channel was lower in old patients, and statistical analysis showed that aging progressively and significantly (p<0.01) reduced the L-type calcium current (ICa) amplitude, even when the effects of age, sex, ACE inhibitors, beta-blockers, angiotensin receptor blockers, calcium antagonists, and left ventricular ejection fraction were taken into account. ICa decreased from 2.4±0.3 pA/pF in the young to 1.2±0.3 pA/pF in the old patient group (p<0.01). The current-voltage relationship was not affected by age but fast ICa inactivation was significantly slower in old patients. The tau for ICa inactivation was 20.9±1.9 ms in old vs. 14.5±0.9 ms in young patients (p<0.01). Similarly the tau for slow ICa inactivation was 120±12 ms in old vs. 73±3 ms in young patients (p<0.001). The caffeine releasable sarcoplasmic reticulum (SR) calcium content also decreased with age (from 10.1±0.8 amol/pF in young to 7.3±0.7 amol/pF in old patients, p<0.05). This was accompanied by a significant decrease in the expression of both SERCA2 and calsequestrin-2 protein levels. By contrast, age did not affect the frequency of spontaneous SR calcium release induced transient inward currents (1.4±0.3 in young vs. 1.1±0.8 events/min in old patients, p=0.5). Moreover, age did not affect the ability of myocytes to maintain a uniform beat-to-beat response when the stimulation frequency was increased.Conclusions: Ageing is associated with depression of L-type calcium channel expression and current amplitude as well as a reduction of the SR calcium content linked to lower SERCA2 and calsequestrin-2 expression in human atrial myocytes. These alterations may blunt atrial contraction and relaxation.