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Background: Cellular electrical coupling is essential for normal propagation of the cardiac action potential, whereas reduced electrical coupling is associated with arrhythmias. Known cellular uncoupling agents have severe side effects on membrane ionic currents. We investigated the effect of carbenoxolone on cellular electrical coupling, membrane ionic currents, and atrial and ventricular conduction. Methods and Results: In isolated rabbit left ventricular and right atrial myocytes, carbenoxolone (50 μmol/l) had no effect on action potential characteristics. Calcium, potassium, and sodium currents remained unchanged. Dual current clamp experiments on poorly coupled cell pairs revealed a 21±3% decrease in coupling conductance by carbenoxolone (mean±S.E.M., n=4, p<0.05). High-density activation mapping was performed in intact rabbit atrium and ventricle during Langendorff perfusion of the heart. The amplitude of the Laplacian of the electrograms, a measure of coupling current in intact hearts, decreased from 1.45±0.66 to 0.75±0.51 μA/mm3 (mean±SD, n=32, p<0.05) after 15 min of carbenoxolone. Carbenoxolone reversibly decreased longitudinal and transversal conduction velocity from 66±15 to 49±16 cm/s and from 50±14 to 35±15 cm/s in ventricle, respectively (mean±SD, n=5, both p<0.05). In atrium, longitudinal and transversal conduction velocity decreased from 80±29 to 60±16 cm/s and from 49±10 to 38±10 cm/s (mean±SD, n=8, both p<0.05). Conclusions: Carbenoxolone-induced uncoupling causes atrial and ventricular conduction slowing without affecting cardiac membrane currents. Activation delay is larger in poorly coupled cells. [Copyright &y& Elsevier]

Objective: After-depolarization associated arrhythmias are frequently observed in heart failure and associated with spontaneous calcium release from sarcoplasmic reticulum (SR), calcium after-transients. We hypothesize that disturbed SR calcium handling underlies calcium after-transients in heart failure (HF). Methods: We measured the stimulation rate dependence (0.2–3 Hz) of diastolic calcium, calcium transient amplitude and SR calcium content in left ventricular myocytes isolated from hearts of rabbits with pressure and volume overload-induced HF and age-matched control animals. Cytosolic calcium was measured with indo-1. In some experiments, delayed after-depolarizations (DADs) were monitored with the voltage sensitive dye di-4-Annepps. SR calcium content was estimated from the response to rapid cooling (RC). After-transients were elicited in the presence of norepinephrine (100 nmol/l) after cessation of burst pacing. Results: With increasing stimulation rate (0.2–3.0 Hz): (1) steady state diastolic [Ca]i increased from 102 to 174 nmol/l in HF and from 44 to 103 nmol/l in control, (2) calcium transient amplitudes decreased from 310 to 254 nmol/l in HF and increased from 186 to 429 nmol/l in control, (3) SR calcium content decreased from 1.25 to 1.09 mmol/l in HF and increased from 1.51 to 2.48 mmol/l in control, (4) in HF and control, the end diastolic SR membrane calcium gradient decreased by about 30%; at any stimulation rate, the magnitude of gradient in HF was one-third of control, (5) systolic depletion of SR was 85% in HF and 60% in control. In HF, noradrenaline (100 nmol/l) increased SR calcium content and SR membrane gradient by 40% versus about 7% in control. Calcium after-transients were observed in 14 out of 18 HF rabbits, and none in eight control animals and were associated with DADs. Calcium after-transients were associated with a 35% decrease in SR calcium content. The frequency of occurrence of calcium after-transients was related to diastolic calcium. Conclusions: in HF, diastolic calcium is increased and both SR calcium content and SR membrane calcium gradient are decreased in a stimulation rate-dependent manner. In HF, β-adrenergic stimulation can partly restore the SR calcium content and SR membrane gradient at higher stimulation rates in a meta-stable condition; upon transition to low stimulation rates, the SR membrane can no longer maintain this high unbalanced SR calcium load at increased diastolic calcium, the magnitude of which is causally related to the occurrence of calcium after-transients. [Copyright &y& Elsevier]