Hindlimb unweighting affects rat vascular capacitance function

Microgravity is associated with an impaired stroke volume and, therefore, c ardiac output response to orthostatic stress. We hypothesized that a decrea sed venous filling pressure due to increased venous compliance may be an im portant contributing factor in this response. We used a constant flow, cons tant right atrial pressure cardiopulmonary bypass procedure to measure tota l systemic vascular compliance (C-T), arterial compliance (C-A), and venous compliance (C-V) in seven control and seven 21-day hindlimb unweighted (HL U) rats. These compliance values were calculated under baseline conditions and during an infusion of 0.2 mug.kg(-1).min(-1) norepinephrine (NE). The c hange in reservoir volume, which reflects changes in unstressed vascular vo lume (DeltaV(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 < 0.05). Infusion of NE decreased C-T and C-V by similar to 20% in both HLU and control rats (P < 0.01). C-A was also significantly decreased in b oth groups of rats by NE (P < 0.01), but values of C-A were similar between HLU and control rats both at baseline and during the NE infusion. Addition ally, the NE-induced DeltaV(0) was attenuated by 53% in HLU rats compared w ith control rats (P < 0.05). The larger C-V and attenuated DeltaV(0) in HLU rats could contribute to a decreased filling pressure during orthostasis a nd thus may partially underlie the mechanism leading to the exaggerated fal l in stroke volume and cardiac output seen in astronauts during an orthosta tic stress after exposure to microgravity.