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Sarcomere length 'orders' relaxation in cardiac muscle.

Some microscopic aspects of relaxation were studied in thin, intact heart muscles isolated from the right ventricles of rats. Mean sarcomere length (SL) was observed, and changes in SL distribution inferred by light diffraction from 100 to 300 μm regions of the muscle. Direct feedback or programmed stretch and release allowed suitable control of regional SL.

When the muscle was isometric, regional sarcomere shortening and lengthening appeared uniform and synchronous throughout a preparation's undamaged, fully striated 1 to 2 mm midlength. When a preparation's cross-sectional area varied, asynchronies in regional relaxation behavior were seen. However, direct control of regional SL produced contractions similar in shape and duration but varying in strength with the amount of muscle in any region. Uniformity during shortening and its enhancement during late relaxation agreed well with the observed SL force-velocity behavior.

There was most often no direct evidence of increased dispersion when the SL was controlled directly, and contractile tension often fell completely without slowing. Whether measured as (1) the fall of tension at constant SL, or (2) the decrease of sarcomere shortening after a load release, the relative speed of relaxation always slowed at longer SL (2.0<SL<2.4 μm). The length-induced slowing of relaxation sometimes revealed a late, abrupt fall in tension followed by an exponential decay. Neither SL nor activation substantially influenced the shape of the rising phase of contraction when SL was held constant.

These results indicate a specific well ordered influence of SL on relaxation dynamics. Like the increment of developed tension, the slowing of relaxation at longer SL may reflect an intrinsic property of the contractile lattice, and this feature may promote contractile uniformity in living heart muscle.