Length-Tension Relationships Are Similar Between Cardiac Myocytes And Skeletal Muscle Fibers

Abstract

According to the Frank-Starling relationship increased ventricular volume increases ventricular output, which helps match peripheral energy supply with demand. The cellular basis for this relationship is, in large part, the myofilament length-tension relationship. The length-tension relationship in maximally activated skinned preparations is relatively shallow and similar between cardiac myocytes and skeletal muscle fibers. During twitch activations the length tension relationship becomes steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare myofilament length-tension relationships between cardiac myocytes, fast-twitch and slow twitch skeletal muscle fibers during half-maximal calcium activations. For these experiments, passive and active sarcomere length-tension relationships were characterized in skinned rat left ventricular cardiac myocytes, fast-twitch and slow-twitch skeletal muscle fibers. For active length-tension relationships, cells were mounted between a force transducer and motor and activated to yield 50-65% maximal force after which isometric force was measured over a range of ascending limb sarcomere lengths monitored by IonOptix SarLen system. Passive tension was considerably greater in cardiac myocytes (3.5 ± 2.5 kN/m2 at 2.30 ± 0.04μm) compared to both fast-twitch (1.1 ± 0.8 kN/m2 at 2.70 ± 0.07μm) and slow-twitch (1.4 ± 0.5 kN/m2 at 2.70 ± 0.07μm) muscle fibers. Active myofilament length-tension relationships were considerably shallower in slow-twitch fibers compared to fast-twitch fibers. Interestingly, cardiac myocytes exhibited two distinct populations of length-tension relationships, one nearly identical to fast-twitch fibers and the other similar to slow-twitch fibers. These results indicate that cardiac myocytes exhibit length-tension relationships very similar to skeletal muscle fibers. The finding of two populations of cardiac myocyte length-tension relationships suggests a mode for adjusting Frank-Starling relationships following acute and/or chronic ventricular adaptations.