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1 Department of Medical Physiology, College of Medicine, Cardiovascular Research Institute Division of Lymphatic Biology, Texas A&M University System Health Science Center, College Station, Texas, USA
2 Department of Health and Kinesiology, Texas A&M University, College Station, Texas, USA
* To whom correspondence should be addressed. E-mail: gashev{at}tamu.edu.
During space flight the normal head-to-foot hydrostatic pressure gradients are eliminated, and body fluids shift toward the head, resulting in a diminished fluid volume in the legs and increased fluid volume in the head, neck and upper extremities. Lymphatic function is important in the maintenance of normal tissue fluid volume but it is not clear how microgravity influences lymphatic pumping. We performed a detailed evaluation of the influence of simulated microgravity on lymphatic diameter, wall thickness, elastance, tone and other measures of phasic contractility in isolated lymphatics. Head-down tail suspended (HDT) rats were used to simulate the effects of microgravity. Animals were exposed to HDT for two weeks, after which data were collected and compared to control non-HDT group. Lymphatics from four regional lymphatic beds (thoracic duct, cervical, mesenteric and femoral lymphatics) were isolated, cannulated and pressurized. Input and output pressures were adjusted to apply a range of transmural pressures and flows to the lymphatics. Simulated microgravity caused a potent inhibition of pressure/stretch stimu-lated pumping in all four groups of lymphatics. The greatest inhibition was found in cer-vical lymphatics. These findings presumably are correlated to the cephalic fluid shifts, which occur in HDT rats as well as those observed during space flight. Flow-dependent pump inhibition was increased after HDT, especially in the thoracic duct. Mesenteric lymphatics were less strongly influenced by HDT, which may support the idea that lymph hydrodynamic conditions in the mesenteric lymphatic during HDT are not dramatically altered.
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