I determined the dynamic mechanical properties of single relaxed cardiac fibers from the Dungeness crab Cancer magister. Single fibers were mechanically isolated, chemically skinned and subjected to small-amplitude sinusoidal length perturbations over a wide range of strain rates and sarcomere lengths to characterize their viscoelastic behavior. The observed mechanical properties, together with transcardiac pressure recordings and ultrastructural measurements, were related to the overall function of the heart. Single fibers, often longer than 1 mm, could be mechanically dissected from the heart of Cancer magister. They typically ranged from 20 to 100 micrometre in diameter and were surrounded by a 100- 400 nm thick extracellular matrix. In situ, under normal physiological loads, the heart of Cancer magister generated transcardiac pressures of about 1000 Pa and beat at 1 Hz, while the sarcomere lengths of fibers changed by 10 % from about 4.0 to 4.4 micrometre during contractions. The total stiffness of all fibers increased from approximately 0.01 MPa to 1 MPa in the sarcomere length range from 3.8 to 6.0 micrometre and increased two- to threefold with a rise in strain rate from 0.01 to 5 rad s-1. In the physiological range of sarcomere length (4.0-4.4 micrometre) and strain rate (0.5-1.2 rad s- 1), single cardiac fibers behaved viscoelastically, with average values for the relative energy dissipation ranging from 0.5 to 0.7. The volume fraction of the extracellular matrix correlated positively with the stiffness of single cardiac fibers. On the basis of these results, I propose a dual role for the viscoelastic behavior of Cancer magister cardiac fibers: (1) the viscous energy dissipation confers dynamic mechanical stability at the level of the single fiber, and (2) the storage and return of elastic strain energy saves energy at the level of the whole heart.

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