DYNAMIC-MECHANICAL PROPERTIES OF PASSIVE SINGLE CARDIAC FIBERS FROM THE CRAB CANCER-MAGISTER

I determined the dynamic mechanical properties of single relaxed cardi ac fibers from the Dungeness crab Cancer magister. Single fibers were mechanically isolated, chemically skinned and subjected to small-ampli tude sinusoidal length perturbations over a wide range of strain rates and sarcomere lengths to characterize their viscoelastic behavior. Th e observed mechanical properties, together with transcardiac pressure recordings and ultrastructural measurements, were related to the overa ll function of the heart. Single fibers, often longer than 1 mm, could be mechanically dissected from the heart of Cancer magister. They typ ically ranged from 20 to 100 mum in diameter and were surrounded by a 100-400 nm thick extracellular matrix. In situ, under normal physiolog ical loads, the heart of Cancer magister generated transcardiac pressu res 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 mum during contractions. The total stiffness of all fibers increased from approximately 0.01 MP a to 1 MPa in the sarcomere length range from 3.8 to 6.0 mum and incre ased 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 mum) and s train rate (0.5-1.2 rad s-1), single cardiac fibers behaved viscoelast ically, with average values for the relative energy dissipation rangin g from 0.5 to 0.7. The volume fraction of the extracellular matrix cor related positively with the stiffness of single cardiac fibers. On the basis of these results, I propose a dual role for the viscoelastic be havior of Cancer magister cardiac fibers: (1) the viscous energy dissi pation 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.