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Departments of 1 Health and Kinesiology and 2 Medical Physiology, and Cardiovascular Research Institute, Texas A&M University, College Station, Texas 77843; 3 Department of Bone and Cartilage Biology, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania 19406; and 4 Department of Orthopaedic Surgery, Mayo Clinic, Rochester, Minnesota 55905
Bone loss occurs as a
consequence of exposure to microgravity. Using the
hindlimb-unloaded rat to model spaceflight, this study had as its
purpose to determine whether skeletal unloading and cephalic fluid
shifts alter bone blood flow. We hypothesized that perfusion would be
diminished in the hindlimb bones and increased in skeletal structures
of the forelimbs and head. Using radiolabeled microspheres, we measured
skeletal perfusion during control standing and after 10 min, 7 days,
and 28 days of hindlimb unloading (HU). Femoral and tibial perfusion
were reduced with 10 min of HU, and blood flow to the femoral shaft and
marrow were further diminished with 28 days of HU. Correspondingly, the
mass of femora (
11%, P < 0.05) and tibiae (6%,
P < 0.1) was lowered with 28 days of HU. In contrast,
blood flow to the skull, mandible, clavicle, and humerus was increased
with 10 min HU but returned to control levels with 7 days HU.
Mandibular (+10%, P < 0.05), clavicular (+18%,
P < 0.05), and humeral (+8%, P < 0.1) mass was increased with chronic HU. The data demonstrate that
simulated microgravity alters bone perfusion and that such alterations
correspond to unloading-induced changes in bone mass. These results
support the hypothesis that alterations in bone blood flow provide a
stimulus for bone remodeling during periods of microgravity.
bone blood flow; hindlimb unloading; vascular resistance
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