Vascular Muscle Series Elastic Element Stiffness during Isometric Contraction

Series elastic element stiffness was studied during isometric muscle contraction in cylindrical segments of dog common carotid artery. These vessel segments were mounted in vitro and restored to in situ length. Vessel diameter was constantly monitored with a linear displacement transducer. The arteries were subjected to isometric contraction by adding norepinephrine to the bath and elevating the transmural pressure enough to maintain constant vessel diameter. Small, rapid quick release-quick stretch cycles were performed repeatedly during isometric contractions to evaluate the stiffness of the undamped series elastic element. After contraction was complete, repeated quick release-quick stretch cycles were also performed during inactivation of the muscle caused by the administration of potassium cyanide. At strains up to the peak in the length-active tension curve, series elastic element stiffness was a linear function of stress thoughout both isometric contraction and isometric relaxation for both quick releases and quick stretches. Vessels excited at strains less than that associated with the peak in the length-tension curve (100 mm Hg) exhibited similar stress-series elastic element stiffness slopes, but vessels excited at larger strains exhibited decreased or negative stress-series elastic stiffness slopes. Studies on potassium cyanide-poisoned vessels failed to indicate any alteration in the quick release properties of the connective tissue at large strains. The concomitant decline in the stress-series elastic element stiffness slope and the active isometric stress at large strains suggests (1) that the series elastic element undergoes mechanical yielding at high stresses or (2) that the series elastic element is an integral part of the force-generating apparatus.