ASGSB 1998 Annual Meeting Abstracts

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EFFECTS OF 14-DAY HINDLIMB UNLOADING ON RAT CEREBRAL, SPLENIC, AND MESENTERIC ARTERIAL MORPHOLOGY.   M.K. Wilkerson¹, J.M. Delp², P.N. Colleran¹, and M.D. Delp¹. Departments of Health & Kinesiology, ¹Texas A&M University, College Station, TX, and ²Sam Houston State University, Huntsville, TX.

Hindlimb unloading (HU) of rats has been shown to cause a caudal shift in body fluids. As a result of this shift, the systemic blood pressure gradient is altered so that there is a higher arterial pressure in the head region while arterial pressure in the abdomen remains unchanged. We postulated that this caudal pressure change would alter the medial layer structure of small resistance arteries from 14-day HU male Sprague-Dawley rats. The middle cerebral artery (MCA) and mesenteric (MA) and splenic (SA) feed arteries were removed from control (C, n=8) and HU (n=7) animals. The vessels were cannulated on glass pipettes attached to a fluid reservoir and luminal pressure was set to 60 cm H₂O. The vessels were relaxed with 10^-4 M nitroprusside, fixed with paraformaldehyde, and embedded in paraffin. From vessel cross-sections (5 m thick), cross-sectional area (CSA) and thickness of the medial layer were determined using an image analysis system. No differences in medial layer CSA or thickness from MA and SA were detected between groups. However, in the MCA, both medial CSA (HU 17893 ± 2539 m; C 12904 ± 1433 m) and thickness (HU 33.9 ± 4.06 m; C 22.3 ± 3.19 m) were increased with HU, while no significant changes were detected for number of nuclei and medial outer perimeter. These findings demonstrate that the medial layer of MCA expands with 14-day HU. We suggest that the chronic increases in arterial pressure that occur caudally with HU elevate transmural pressure in the MCA and result in the hypertrophy of smooth muscle cells in the medial layer. This hypertrophy may be involved in the reduction of MCA blood flow velocity observed in humans following prolonged bed rest and space flight.

(Supported by NASA grants NAGW-4842 and NAG5-3754 and NSBRI grant NCC-9-58.)

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