Does resting vasomotor tone impact +Gz tolerance?

Abstract: The ability of an individual to withstand elevated head-to-toe gravitoinertial (+Gz) forces is determined by the capacity of their body to maintain sufficient head-level arterial pressure. Recent studies have shown a relationship between resting blood-vessel stiffness and an individual’s +Gz-tolerance, although the mechanisms behind this relationship are unclear. The aim of this project is to determine whether or not +Gz-tolerance is affected by a change inresting vasomotor tone. To evaluate this relationship, seven participants were asked to complete a +Gz-tolerance protocol using a human-use centrifugeon two different occasions. On both visits, gradual onset rate (0.1 G.s−1)and rapid onset rate (3.5 G.s−1) tests were done to evaluate the participants+Gz-tolerance. On one of the two visits, prior to the +Gz-tolerance testing,participants performed a 20-min cycle intervention to induce postexercisehypotension, with the aim of temporarily reducing participants’ resting bloodpressure and vasomotor tone. The cycling intervention was successful atinducing postexercise hypotension, as mean arterial pressure was significantlylower on the cycling visit (P<0.05). +Gz-tolerance was significantly lower(P<0.05) on the cycling visit compared with the non-cycling visit for both theGOR and ROR tests (absolute difference of 0.5 G and 0.25 G, respectively).The effect of the type of test on +Gz-tolerance was not influenced by the effectof the cycling intervention (P>0.05). Being the most documented mechanismlinked to postexercise hypotension, sustained vasodilation was assumed tohave occurred. This would have increased distensibility of the affected vessels,explaining the decrease in +Gz-tolerance. The decrease in +Gz-tolerance wassimilar for both tests, indicating that the baroreflex was not affected by thecycling intervention. Assuming that vasodilation occurred, this study showedthat a decrease in resting vasomotor tone decreased +Gz-tolerance, indicatingthe importance of this variable in the relationship between resting blood-vesselstiffness and an individual’s +Gz-tolerance.