The medial structure of three different branching orders of arteries in the rabbit ear was studied to assess the possible significance with regard to mechanical behavior determined with the in vitro myograph. The central ear artery, its main side branch, and a terminal branch were fixed while dilated in situ at 100 mmHg; this resulted in internal diameters of 960, 456, and 251 micron, respectively. The average ratio of medial thickness to internal diameter was 0.0293 for arteries in situ and 0.0487 for arterial rings in vitro. Measurements undertaken for the determination of smooth muscle (SM) stress included the percentage of SM in the media (74%) and the pitch of SM (0.89 degrees). These parameters were not significantly different in the three various-sized arteries or for the two myographs. Stereological estimates of SM cell volume, length, and diameter were based on counts of SM nuclei and their length. In evaluating the ring preparation of the vessel myographs, the damage due to the wires (20%) and cut edges (6.4%) was considered. The maximum active SM stress was approximately the same in the different-sized vessels with an average value of 3.14 X 10(5) N/m2. Since this stress is comparable to measurements made by others on different mechanical apparatus and consistent for the three arterial orders, these in vitro myographs should allow for valid comparison of arterial mechanical properties over the size range studied.
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