Baroreflex control of heart rate at rest and during exercise in microgravity conditions

One of the most important features of prolonged weightlessness is a progressive impairment of muscular function. Physiological modifications related to exposure to microgravity, e.g. altered blood volume distribution, impaired myocardial function, and a decrease in stroke volume, in addition to the alterations in muscular function, contribute to the reduced physical exercise capacity commonly reported after space flight. The interplay between muscular, hemodynamic and body fluid modifications occurring in microgravity could alter also the neural mechanisms of cardiovascular regulation both in resting conditions and during exercise in comparison to those that are normally operating on the ground. Indeed, an impairment in arterial baroreflex control of heart rate has been reported in the resting state during spaceflight. Moreover, cardiovascular and autonomic responses to exercise have been reported to be either impaired or preserved after simulated or actual microgravity conditions. Differences in results might depend on different experimental settings: the majority of studies on exercise in microgravity have been conducted using handgrips as a convenient model of muscular exercise. However, microgravity-induced muscular impairment involves mainly the antigravity muscles, i.e. leg muscles, which are those primarily involved in the reduced exercise capacity and hence are more representative of the marked changes in the musculoskeletal system induced by microgravity. This brief review aims to summarize current knowledge on baroreflex control of heart rate in resting and exercise conditions in microgravity.