Constriction of Arteries at High Transmural Pressures

Perfused, isolated segments of the central ear artery of the rabbit were made to constrict at different inside pressures by an infusion of epinephrine. Inside pressures were altered by changing the outflow resistance. External work done, at a near constant inside pressure and a fixed artery length, by the contracting arterial muscle was obtained from pressure-(outside radius)² curves of constricted and fully relaxed arteries. Work done reached a maximum of 1,553±97 ergs/cm length (±SE) at a transmural pressure of 135±4 mm Hg when the epinephrine concentration was 50 ng/ml of perfusion solution. A decrease in epinephrine concentration to 12.5 ng/ml lowered the pressure at which a maximum work of 438±58 ergs/cm length was done to 113±4.8 mm Hg. Arteries first constricted at low inside pressures by epinephrine (50ng/ml) withstood inside pressures 106±6 mm Hg greater than the maximum work pressure before constriction was overcome. The stress at the external elastic membrane, when the arteries were fully relaxed at the maximum work pressure, was 1,627±71 gwt/cm². These observations may be explained by an interaction, with positive feedback, between changes in radius to wall thickness ratio and changes in arterial muscle fiber length.