Introduction: In humans, +Gz exposure immediately preceded by exposure to z
ero or -Gz can result in unexpected incapacitation ("pushpull" effect). Our
goals were to establish whether this phenomenon exists in rats and to eval
uate the importance of varying the duration of -Gz exposure on magnitude of
the push-pull effect on cerebral perfusion pressure. Methods: Eight consci
ous male rats were studied in the transition from + 5 Gr to +10 Cr imposed
by centrifugation. This was done with (push-pull or without (control) 2 s e
xposure to -5 Gz applied using a counterbalanced design. Seven isoflurane a
nesthetized rats were studied in the transition from 0Gz (+1Gy) to + 1Gz im
posed by tilting. This was done with (push-pu[ii or without (control) 0.5,
1, 3, or 9 a exposure to -1Gz imposed immediately prior to the transition a
pplied using a counterbalanced designed. Results: Exposure to 2 s of -5 Gz
significantly (p < 0.01) reduced carotid artery pressure in the 4(th) throu
gh a s of exposure to +10 Ct by an average of 15 mmHg compared with control
. In the tilt experiments, a push-pull effect was found with mild Cr exposu
re (+/-1Gz) with as little as 0.5 s -Gz exposure. Varying the head-down dwe
ll time did not alter the magnitude of the exaggerated hypotension induced
by "push-pull" (p = 0.90. Conclusions: We conclude that rats express a "pus
h-pull" effect similar to that observed in humans but that altering the dur
ation of exposure to -Gz does not influence the magnitude of the "push-puil
" effect.