Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca transients is modulated by preload.

Preload and afterload dictate the dynamics of the cyclical work-loop contraction that the heart undergoes in vivo. Cellular Ca dynamics drive contraction, but the effects of afterload alone on the Ca transient are inconclusive. To our knowledge, no study has investigated whether the putative afterload dependence of the Ca transient is preload dependent. This study is designed to provide the first insight into the Ca handling of cardiac trabeculae undergoing work-loop contractions, with the aim to examine whether the conflicting afterload dependency of the Ca transient can be accounted for by considering preload under isometric and physiological work-loop contractions. Thus, we subjected ex vivo rat right-ventricular trabeculae, loaded with the fluorescent dye Fura-2, to work-loop contractions over a wide range of afterloads at two preloads while measuring stress, length changes, and Ca transients. Work-loop control was implemented with a real-time Windkessel model to mimic the contraction patterns of the heart in vivo. We extracted a range of metrics from the measured steady-state twitch stress and Ca transients, including the amplitudes, time courses, rates of rise, and integrals. Results show that parameters of stress were afterload and preload dependent. In contrast, the parameters associated with Ca transients displayed a mixed dependence on afterload and preload. Most notably, its time course was afterload dependent, an effect augmented at the greater preload. This study reveals that the afterload dependence of cardiac Ca transients is modulated by preload, which brings the study of Ca transients during isometric contractions into question when aiming to understand physiological Ca handling. This study is the first examination of Ca handling in trabeculae undergoing work-loop contractions. These data reveal that reducing preload diminishes the influence of afterload on the decay phase of the cardiac Ca transient. This is significant as it reconciles inconsistencies in the literature regarding the influence of external loads on cardiac Ca handling. Furthermore, these findings highlight discrepancies between Ca handling during isometric and work-loop contractions in cardiac trabeculae operating at their optimal length.