Isotonic muscle and sarcomere shortening in rabbit right ventricular preparations

Cardiac muscle fibers are suspended within and attached to an elaborate connective tissue matrix that includes numerous compliant interconnections. Myocardial muscle fibers are not branched, but connect at small angles to each other to form a branched array. Therefore, fiber shortening occurs as a vector within a connective tissue framework and individual fiber work may exceed external muscle work. To evaluate the latter we measured isotonic muscle shortening simultaneous with sarcomere shortening. The hearts were obtained from rabbits (n = 4) anesthetized with intravenous pentobarbital sodium. We isolated right ventricular trabeculae or free wall papillary muscles in Krebs-Ringer's solution (2.5 mM Ca2+, 28 degrees C). Cross-sectional area was 0.038 +/- 0.003 mm2 (+/- SE throughout) and resting sarcomere length was 2.33 +/- 0.12 microns. Sarcomere length was measured with laser diffraction (He-Ne, lambda = 632.8 nm) during force clamps in single- and paired-stimulation twitches. Relative sarcomere shortening (delta SL) was isotonic sarcomere shortening divided by sarcomere length at the onset of isotonic shortening. Relative muscle shortening (delta ML) was isotonic muscle shortening divided by muscle length at zero load; the latter was estimated from the stress-strain relation of elastic recoil at the onset of load clamps. Average delta SL/delta ML at peak shortening was 3.38 +/- 0.16 and was independent of stimulus pattern, isotonic load, amount of shortening, time during a twitch or laser beam position along a muscle. Therefore, the ratio greater than 1 was neither a function of activation nor heterogeneous sarcomere length change.(ABSTRACT TRUNCATED AT 250 WORDS)