Gravity-induced loss of consciousness (G-LOC) is known to have occurre
d in pilots since the early 1920's. Most of the research in this area
has shown that G-LOC occurs due to a decrease in cerebral blood pressu
re and a concomitant reduction in brain perfusion. Since a reduction i
n cerebral blood flow can cause transient hypoxia, it is important to
study the cerebral metabolism during high +Gz exposure. One component
of these studies should include measurements of substrate availability
and degradative products. In the present study, adult baboons were gi
ven multiple high +Gz exposures (2 to 6) using the Armstrong Laborator
y human centrifuge. Venous blood was collected by an automatic syringe
withdrawal pump before, during and after centrifuge exposures. The co
ncentration of blood gases, glucose and lactate tended to decrease dur
ing the centrifuge exposure followed by an increase after the run. Tot
al creatine kinase activity in serum was not significantly altered. Th
ese results suggest that during +Gz exposure, anaerobic glycolysis is
stimulated resulting in elevated lactate production due to a reduction
in cerebral blood flow (CBF). The elevated tissue lactate is released
into the central circulation upon resumption of normal CBF (after the
termination of centrifuge run). Therefore, the observed decrease in l
actate concentration during the run may result from a lag in the relea
se of tissue lactate into the blood due to a reduction in CBF. It is s
peculated that at high +6 Gz, G-LOC may occur as a protective response
to reduce the brain metabolic rate, to maintain energy levels and to
prevent severe cellular acidosis. However, further research is needed
to establish the relationship between the brain's energy status and th
e onset of G-LOC.