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

Abstract

The cardiovascular system is adapted to gravity, and reactions to the loss of gravity in space are presumably dependent on body size. The dependence of hematological parameters and body fluid volume on simulated microgravity have never been studied as an allometric function before. Thus, we estimated red blood cell (RBC), blood and extracellular fluid volume in hindlimb-unloaded (HLU) or control (attached) mice, rats and rabbits. RBC decrease was found to be size independent, and the allometric dependency for RBC loss in HLU and control animals shared a common power (-0.054±0.008) but a different coefficient (8.66±0.40 and 10.73±0.49, respectively,<0.05). Blood volume in HLU animals was unchanged compared with that of controls, disregarding body size. The allometric dependency of interstitial fluid volume in HLU and control mice shared (1.02±0.09) but had different powers (0.708±0.017 and 0.648±0.016, respectively, <0.05), indicating that the 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 the fact that the 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.

Characterization of mouse ocular response to a 35-day spaceflight mission: Evidence of blood-retinal barrier disruption and ocular adaptations

“…Similar cephalad fluid shifts have also been described in the hindlimb unloaded rat 30–32 , a ground-based rodent model to simulate a weightless environment. However, mice have a negligible hydrostatic pressure gradient on Earth owing to their small body size and do not show the same shifts in fluid volume during hindlimb unloading as that found in rats 33 . Additionally, during 30 days of spaceflight on the Bion-M1 satellite, mean arterial blood pressure in the aortic arch of mice is not altered 34 , and arteries in the head and hindlimbs of mice flown on the Bion-M1 and Space Shuttle do not show any structural remodeling to indicate chronic changes in fluid pressure associated with a cephalad fluid shift 26,27,35 .…”

Approaching Gravity as a Continuum Using the Rat Partial Weight-Bearing Model

“…The cardiovascular system is severely impacted during weightlessness [ 9 , 96 , 97 , 98 , 99 , 100 ], in particular, orthostatic intolerance appears to be common among astronauts upon returning to earth. Many studies have been conducted in the HU model [ 101 , 102 , 103 , 104 , 105 , 106 , 107 ], taking advantage of the 30 degree head-down-tilt designed to induce a cephalic fluid shift. Some of these HU models found hypovolemia, altered nitric oxide signaling, and altered sympathetic neurological activity [ 105 , 106 , 108 ], which may contribute to cardiovascular complications during spaceflight.…”