Myocardial depression produced by sustained tachycardia in rabbits

Much recent attention has been focused on the tachycardia-induced heart failure model. We hypothesized that sustained tachycardia would lead to myocardial depression in rabbits, as it does in dogs and swine. We evaluated the passive and active length-tension relations and postrest contraction behavior in right ventricular papillary muscles from 22 New Zealand White rabbits, 11 controls, and 11 subjected to ventricular pacing at a rate of 400 beats/min for 29.4 +/- 10.6 days. Studies were performed in oxygenated buffer at 22 degrees C. Active tension was significantly reduced at muscle lengths of 0.95.Lmax and above; at Lmax it was 4.7 +/- 0.2 g/mm2 for the control group and 3.3 +/- 0.2 g/mm2 for the paced group (P less than 0.005). Both groups showed increased force development when the concentration of calcium in the buffer was increased. There were no differences between the groups in the passive length-tension relations. Of note, postrest contraction data showed that the second postrest beat was smaller for the paced animals for rest intervals up to 2 min, suggesting that beat-to-beat trans-sarcolemmal calcium handling may differ from normal in this model. We conclude that sustained tachycardia will lead to myocardial depression in rabbits; the extension of this model to a small animal species may offer new ways to explore its causative mechanisms.