BACKGROUND: Ventricular arrhythmias are a major cause of sudden death in patients with heart failure and hypertrophy. The dog with chronic complete atrioventricular block (CAVB) has biventricular hypertrophy and ventricular arrhythmias and is a useful model to study underlying cellular mechanisms. We investigated whether changes in Ca(2+) homeostasis are part of the contractile adaptation to CAVB and might contribute to arrhythmogenesis. METHODS AND RESUTLS: In enzymatically isolated myocytes, cell shortening, Ca(2+) release from the sarcoplasmic reticulum (SR), and SR Ca(2+) content were enhanced at low stimulation frequencies. Ca(2+) influx through L-type Ca(2+) channels was unchanged, but Ca(2+) influx via the Na/Ca exchanger was increased and contributed to Ca(2+) loading of the SR. Inward Na/Ca exchange currents were also larger. Changes in Ca(2+) fluxes were less pronounced in the right versus left ventricle. CONCLUSIONS: Enhanced Na/Ca exchange activity may improve contractile adaptation to CAVB but at the same time facilitate arrhythmias by (1) increasing the propensity to Ca(2+) overload, (2) providing more inward current leading to (nonhomogeneous) action potential prolongation, and (3) enhancing (arrhythmogenic) currents during spontaneous Ca(2+) release.