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.L(max) and above; at L(max) it was 4.7 +/- 0.2 g/mm2 for the control group and 3.3 +/- 0.2 g/mm2 for the paced group (P < 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 transarcolemmal 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.