Hindlimb unweighting affects rat vascular capacitance function

1 Department of Biomedical Engineering and 2 Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205 Microgravity is associated with an impaired stroke volume and, therefore, cardiac output response to orthostatic stress. We hypothesized that a decreased venous filling pressure due to increased venous compliance may be an important contributing factor in this response. We used a constant flow, constant right atrial pressure cardiopulmonary bypass procedure to measure total systemic vascular compliance (C T ), arterial compliance (C A ), and venous compliance (C V ) in seven control and seven 21-day hindlimb unweighted (HLU) rats. These compliance values were calculated under baseline conditions and during an infusion of 0.2 µg · kg 1 · min 1 norepinephrine (NE). The change in reservoir volume, which reflects changes in unstressed vascular volume ( V 0 ) that occurred upon infusion of NE, was also measured. C T and C V were larger in HLU rats both at baseline and during the NE infusion ( P