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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

Acute hemodynamic responses in the head during microgravity induced by free drop in anesthetized rats

Department of Physiology, Gifu University School of Medicine, Gifu 500–8705, Japan

Submitted 11 November 2003 ; accepted in final form 30 January 2004

To examine acute hemodynamic responses to microgravity (µG) in the head, we measured carotid artery pressure (CAP) and jugular vein pressure (JVP) to calculate cephalic perfusion pressure (CPP = CAP – JVP) and recorded images of microvessels in the iris to evaluate capillary blood flow velocity (CBFV) and capillary diameter (CD) in anesthetized rats during 4.5 s of µG induced by free drop. Rats were placed in 30° head-up whole body-tilted (HU, n = 7) or horizontal (flat, n = 6) position. In the flat group, none of the measured variables was significantly affected by µG, whereas in the HU group, CAP, JVP, and CPP increased, respectively, by 23.4 ± 2.6, 1.3 ± 0.2, and 22.9 ± 3.1 mmHg, and CBFV and CD increased, respectively, by 33 ± 8 and 9 ± 3%, showing an increase in capillary blood flow. To further examine the mechanisms underlying these CAP and JVP increases, another experiment was performed in which CAP and JVP were measured in anesthetized rats (n = 6) during a postural change from HU to flat. In these animals, the change in JVP was similar to that observed during actual µG, but no change in CAP was seen, indicating that the JVP increase during actual µG is caused by disappearance of the gravitational pressure gradient in the head-to-foot axis, whereas the CAP increase is not. In conclusion, actual µG elicits an increase in CPP due to a greater increase in CAP than JVP, resulting in increased capillary blood flow. Although the increase in JVP is explained by the disappearance of gravitational pressure gradient in the head-to-foot axis as a result of µG, the larger increase in CAP is not.

fluid shift; carotid artery pressure; jugular vein pressure; capillary blood flow

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