Abstract
The most accepted animal model for simulation of the physiological and morphological consequences of microgravity on the cardiovascular system is one of head-down hindlimb unloading. Experimental conditions surrounding this model include not only head-down tilting of rats, but also social and restraint stresses that have their own influences on cardiovascular system function. Here, we studied levels of spontaneous locomotor activity, blood pressure, and heart rate during 14 days under the following experimental conditions: cage control, social isolation in standard rat housing, social isolation in special cages for hindlimb unloading, horizontal attachment (restraint), and head-down hindlimb unloading. General activity and hemodynamic parameters were continuously monitored in conscious rats by telemetry. Heart rate and blood pressure were both evaluated during treadmill running to reveal cardiovascular deconditioning development as a result of unloading. The main findings of our work are that: social isolation and restraint induced persistent physical inactivity, while unloading in rats resulted in initial inactivity followed by normalization and increased locomotion after one week. Moreover, 14 days of hindlimb unloading showed significant elevation of blood pressure and slight elevation of heart rate. Hemodynamic changes in isolated and restrained rats largely reproduced the trends observed during unloading. Finally, we detected no augmentation of tachycardia during moderate exercise in rats after 14 days of unloading. Thus, we concluded that both social isolation and restraint, as an integral part of the model conditions, contribute essentially to cardiovascular reactions during head-down hindlimb unloading, compared to the little changes in the hydrostatic gradient.
Highlights
It is well established that exposure to microgravity results in dramatic reduction of general movement and loading in astronauts, which leads to hypokinesia and hypodynamia
We evaluated spontaneous locomotor activity (SLA), blood pressure, and heart rate changes that occurred as a result of 14 days of social isolation, restraint, and hindlimb unloading in rats
Restraint did not change the profile of spontaneous locomotor activity compared to the isolated group, and attached animals showed a very low level of activity throughout a 24 h period, which we considered as a depressive-like period
Summary
It is well established that exposure to microgravity results in dramatic reduction of general movement and loading in astronauts, which leads to hypokinesia and hypodynamia. There is a demand for animal ground models for the simulation of physiological and morphological consequences of space flight. Various modifications of hindlimb unloading and head-down tilt models in rats were described by Morey [1], which are predominantly used in the field. These models induce muscle atrophy and changes in bone structure as physiological consequences observed in humans after space flight or bed-rest [2,3,4,5,6,7,8,9]. Heterogeneity can be attributed to differences in animal tilting techniques, angle of tilt, suspension duration, blood pressure measurement protocols, recovery period duration after surgery, and control group choice [15,16,17]
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