The measurement and dynamic implications of thin filament lengths in heart muscle.

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Abstract
The lengths of the thin filaments in amphibian and mammalian cardiac muscle were determined from electron micrographs of serial transverse sections. Thin filament lengths in frog atrial trabeculae range from 0.8 to grater than 1.3 .mu.m, with a maximum error of 0.14-0.15 .mu.m. In rat atrial tissue the span is from 0.6 to more than 1.1 .mu.m, papillary muscle the breadth of the distribution is from 0.9 to greater than 1.1 .mu.m. Double overlap of thin filaments should exist over a wide range of sarcomere lengths. Thin filaments from opposite halves of a sarcomere accommodate each other by flexing up to an angle of about 2.degree. and moving from the trigonal position among the thick filaments to the center of the region between 2 thick filaments and probably contributes to the internal resistance to shortening in the muscle. Except for variation in thin filament lengths the over-all morphology of the cardiac sarcomere is similar to skeletal muscle. Thick filaments in heart muscle are uniform in length and their profiles change along their lengths. They are generally round in the M band, triangular adjacent to the M band, round again in the overlap region and either round or triangular near the tapered tips. The M bridges in rat cardiac tissue link adjacent thick filaments to form a symmetric hexagonal array. In the frog atrium the M bridge connections are incomplete and often form isolated triangular clusters. Computed sarcomere length-developed tension curves were calculated using the thin filament length distributions and assumptions basic to the sliding filament theory of muscle contraction. The curves for atrial tissue have plateau regions approximately as wide as the 1/2 .mu.m variation in thin filament length. Work done against the internal loads during systole may be stored as potential energy and released during diastole to produce sarcomeric re-extension.