Abstract 642: The Nature and Potential Role of Cellular Calcium Handling Abnormalities in Bradycardia-related Acquired Long QT Syndrome
Background: Sustained bradycardia predisposes to long QT syndrome (LQTS) and Torsades de Pointes (TdP) in man. Rabbits with chronic AV block (CAVB) paced at slower than normal sinus rates show LQTS and TdP with downregulation of IKr and IKs. CAVB rabbit cardiomyocytes are hypercontractile, suggesting Ca2+ handling abnormalities. This study assessed mechanisms and consequences of Ca2+ handling changes in CAVB rabbits.
Methods: CAVB rabbits (37% formalin into AV node) were paced at 110 bpm × 1 wk, 90 bpm × 2 wks. Ventricular cardiomyocytes were isolated from CAVB and control (CTL) hearts. Action potentials (APs) were recorded with whole cell perforated patch. Ca2+ i was measured with Indo 1-AM. Western blot was used to quantify Ca2+ handling proteins.
Results: CAVB did not change resting potential or AP amplitude (APA), but slowed repolarization (eg. APD90 at 1 Hz: 258±10 CTL vs 350±13 ms CAVB; P<0.001). CAVB increased systolic Ca2+ i (2 Hz) from 373±31 (CTL) to 860±77 nM (CAVB, P<0.001) and accelerated Ca2+ transient decay (time constant 192±19 CTL, 139±12 ms CAVB, P<0.05). Sarcoplasmic reticulum (SR) Ca2+ load measured by caffeine (10 mM) puff-induced Ca2+ release increased from 274±17 (CTL) to 630±61 nM (CAVB, P<0.001). CAVB-induced Ca2+ abnormalities were associated with spontaneous and isoproterenol-induced Ca2+ release events after 3 Hz pacing (0.9±0.2 CTL vs 6.0±1.9 events/min CAVB, P<0.05). CAVB (but not CTL) cardiomyocytes showed both spontaneous and isoproterenol-induced early (EADs) and delayed (DADs) afterdepolarizations, which were suppressed by depleting SR Ca2+ stores with the SR Ca2+ uptake inhibitor thapsigargin (1 μM) or blocking SR Ca2+ release with ryanodine (10 μM). NCX, SERCA and phospholamban (PLB) expression was not changed by CAVB, but PLB phosphorylation was increased by 63% (P<0.01), enhancing SR Ca2+ uptake by removing PLB inhibition of SERCA.
CAVB-induced bradycardia causes cardiomyocyte Ca2+-loading by enhancing SR Ca2+-uptake via PLB-phosphorylation, which disinhibits SERCA, and by APD prolongation.
Ca2+ handling abnormalities contribute importantly to arrhythmogenic afterdepolarizations.
Our results for the first time directly implicate Ca2+-handling abnormalities in acquired TdP.