Fluid shift vs. body size: changes of hematological parameters and body fluid volumes in hindlimb-unloaded mice, rats and rabbits

Abstract

Cardiovascular system is adapted to gravity, and reactions to its vanishing in space are presumably dependent on body size. Dependency of hematological parameters and body fluids reaction to simulated microgravity have never been studied as an allometric function before. Thus we estimated RBC, blood and extracellular fluid volumes in hindlimb-unloaded (HLU) or control (ATT) mice, rats and rabbits. RBC decrease was found to be size-independent, and the allometric dependency for red blood loss in HLU and ATT animals shared a common power (−0.054±0.008) but differrent Y0 (8.66±0.40 and 10.73±0.49 correspondingly, p<0.05). Blood volume in HLU animals was unchanged compared to controls disregarding the body size. The allometric dependencies for interstitial fluid volume in HLU and ATT mice shared Y0 (1.02±0.09) but had different powers N (0.708±0.017 and 0.648±0.016 correspondingly, p<0.05), indicating that interstitial fluid volume increase during hindlimb unloading is more pronounced in larger animals. Our data underscore the importance of size-independent mechanisms of cardiovascular adaptation to weightlessness. Despite use of mice hampers application of a straightforward translational approach, this species is useful for gravitational biology as a tool to investigate size-independent mechanisms of mammalian adaptation to microgravity.