Viscoelastic properties of canine pulmonary arteries

Ring segments of extralobar and intralobar canine pulmonary arteries were used in vitro to study their frequency-dependent viscoelastic properties. Measurements of force in response to sinusoidal perturbations of length were made over the frequency range of 0.002-10 Hz. These measurements were made under three conditions: 1) passive smooth muscle at a physiological value of wall circumference (passive), 2) active smooth muscle at the same circumference (active), and 3) passive smooth muscle at a higher force (circumference) approximating the active value (high strain). Most of the frequency dependence of the dynamic modulus occurred over the range of 0.002-0.1 Hz. Variations in dynamic modulus for frequencies between 0.1 and 10 Hz were smaller in comparison. Larger variations in dynamic modulus were found 1) under conditions of active smooth muscle than under passive or high strain conditions, 2) at high strain compared with low strain (passive) conditions, and 3) for the intralobar compared with extralobar arteries under equivalent conditions. Values of modulus ratio were computed by dividing dynamic modulus by a low-frequency value (f = 0.002 Hz). Values of modulus ratio were similar at high and low wall strain under passive conditions and were higher for intralobar than for extralobar arteries. Values of modulus ratio with active muscle were larger than for passive conditions, but no differences were found between extralobar and intralobar arteries. The "amount" of viscoelasticity correlated with the amount of muscle at the two sites but not with the force-generating capacity of the muscle, so that viscoelasticity and force development may represent different manifestations of muscle properties.