Myosin MgADP Release Rate Decreases at Longer Sarcomere Length to Prolong Myosin Attachment in Skinned Rat Myocardial Strips

Abstract

Cardiac contractility increases as sarcomere length (SL) increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. Myosin's capacity to generate force is Ca2+-regulated by thin-filament proteins and sarcomere length, which dictates the number of potential actin-myosin cross-bridge interactions. One mechanism underlying greater cardiac contractility at longer SL could involve longer myosin attachment duration (ton). To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure ton as [MgATP] varied (0.05-5 mM) at 1.9 and 2.2 µm SL. From this ton-MgATP relationship, we calculated cross-bridge MgADP release rate (k-ADP) and MgATP binding rate (k+ATP). As MgATP increased ton decreased hyperbolically for both SL, but ton was roughly 50% longer for 2.2 vs. 1.9 µm SL at each [MgATP] (25±3 vs. 16±1 ms at 5 mM MgATP, 17° C, p<0.05). These ton differences arose from slower k-ADP at 2.2 µm SL (42±3 vs. 74±8 s−1, p<0.001), as MgATP binding rates did not differ with SL (281±56 vs. 327±93 mM−1 s−1). Absolute tension values were greater at 2.2 vs. 1.9 µm SL for relaxed (4.4±0.7 vs. 0.8±0.2 kPa at pCa 8.0, p<0.001) and maximally activated (20.0±1.4 vs. 14.2±1.6 kPa at pCa 4.8, p<0.001) conditions, and the force-pCa relationship was more sensitive to Ca2+ at 2.2 µm SL (pCa50=5.45±0.01 vs. 5.36±0.01, p<0.05). These increased tension values suggest that cross-bridges may bear greater loads at longer SL, which diminishes MgADP release to prolong ton and amplify cooperative cross-bridge contributions to thin filament activation. Therefore, load-dependent rates of the actomyosin cross-bridge cycle may vary with SL to contribute, in part, to the Frank-Starling relationship in the heart.