Dissecting Tunica Responsibility in Arterial Stress Relaxation: Smooth Muscle Need Not Apply

Introduction: Stress relaxation is considered a property of smooth muscle. Rings of thoracic aorta with/without perivascular adipose tissue (PVAT) stress relax. However, rings of isolated PVAT, containing no organized smooth muscle, also stress relax. We hypothesize that smooth muscle is not necessary in the individual tunicas of the aorta for stress relaxation to occur. Methods: Histochemical staining and isometric contractility were performed on whole or tunicas of the thoracic aorta from the male and female Dahl salt-sensitive rats on normal diet. Masson trichrome and Verhoeff Van Gieson staining validated the isolation of the different tunicas. Stress relaxation was measured after cumulative amounts of passive tension were applied in the presence or absence of extracellular Ca²⁺, followed by challenge with the a₁ adrenergic agonist phenylephrine (PE, 10⁻⁵ M). Results: Stress relaxation occurred in all tunicas at each passive tension regardless of Ca²⁺. PE-induced contraction was observed in tissues containing smooth muscle (media, whole vessel) but not in the adventitia and aPVAT with Ca²⁺. No contraction was observed with no Ca²⁺. Conclusions: All tunicas of the rat thoracic aorta stress relax, doing so without dependence on smooth muscle or Ca²⁺. PVAT demonstrated the greatest ability to stress relax.