Load-dependent induction of apoptosis in multicellular myocardial preparations

Increased mechanical load has been proposed as an inductor of apoptosis, but it is unknown whether this can occur in the range of pre- and afterloads that prevail in the beating heart. We investigated apoptosis in cultured rabbit multicellular myocardial preparations over several days. Muscles contracted in absence of pre- and afterload (unloaded isotonic), in absence of preload but in presence of afterload (unloaded isometric), or in presence of both (loaded isometric). After up to 48 h of continuous contractions, apoptosis was assessed by TdT-mediated nick-end labeling (TUNEL) assay and DNA ladder analysis. In muscles that contracted loaded isometric, apoptosis was detected after 6-24 h. After 48 h, apoptosis was most prominent in this group, reflected by a high level of DNA ladder intensity (DLI; 27.8 +/- 11.5), whereas Bcl-xL (on RNA level) was significantly downregulated, and Fas remained unchanged. In unloaded isometric preparations, apoptosis was significantly less (6.9 +/- 5.9 DLI) and very similar to those contracting unloaded isotonic (6.1 +/- 5.1 DLI). We conclude that load-dependent apoptosis can occur at sarcomere lengths achievable in vivo and may mainly result from increased preload.