Abstract Body: Perivascular adipose tissue (PVAT) is being recognized as an important arterial tunica. Stress relaxation, a gradual reduction in stress over time while under a constant strain, is considered a property of smooth muscle (ex. intestine, uterus, bladder). While aortic PVAT (aPVAT) contains a microvasculature, it does not contain organized smooth muscle and isolated PVAT stress relaxes to a greater extent than the vessel itself. This raised the idea that other tunicas lacking smooth muscle may also stress relax. Here we hypothesize that appropriately oriented and functional smooth muscle was not necessary for stress relaxation to occur. To test this, we used histochemical staining and isometric contractility of thoracic aorta from the male Dahl salt-sensitive (SS) rat. Some rings of aorta were separated into adventitia (no muscle), media (muscle), and PVAT (no muscle), and responses of these isolates compared to the intact aortic ring (muscle + integration of tunicas). Masson Trichrome (MT) and Verhoeff Van Gieson (VVG) staining validated the dissection of the different tunicas with minimal contamination of cells from other tunicas. Stress relaxation was measured after increasing, cumulative amounts of passive tension (6 grams total) was applied in the presence or absence of Ca²⁺, essential for smooth muscle contraction. Each step was followed by challenge with a maximum concentration of the a₁ adrenergic agonist phenylephrine (PE, 10^-5 M).
Stress relaxation occurred in all tunicas at each passive tension, whether or not Ca²⁺ was present (figures). In fact, loss of Ca²⁺ improved stress relaxation in adventitia and PVAT. Importantly, in the presence of Ca²⁺, PE contraction was observed at each passive tension in tissues containing smooth muscle: media (min, 250 mg PT: 26.67 +/- 8.51 mg; max, 6000 mg PT: 589.59 +/- 114.53 mg) and aorta + PVAT (min, 250 mg PT: 562.61 +/- 238.45 mg; max, 6000 mg PT: 1322.91 +/- 352.73mg). Little to no measurable contraction was recorded in the adventitia and aPVAT. There was little to no measurable contraction in tunicas in the absence of Ca²⁺. Taken together, these data suggest that all tunicas of the rat thoracic aorta stress relax and can do so without the presence of organized smooth muscle. Of all tunicas, PVAT demonstrated the greatest ability to stress relax. Collectively, these findings force reconsideration of individual responsibilities of arterial tunicas, all of which participate in the (dys)function of an artery.