Abstract
Acclimatization is a process which occurs as a result of repeated mild increases in core
temperature, which allows an organism to carry out increased work in the heat due to
better heat dissipation (Moseley, 1997). In order to prevent the occurrence of heat
illness, it is necessary for individuals who perform work in hot, humid environments
to undergo acclimatization.
Exposure to different types of stimuli, such as heat exercise and oxidative stress,
results in the formation of proteins in the cells which are known as heat shock protein
(HSP) (Powers et al, 2001). The main function of HSP is to act as molecular
chaperones. Specifically, the 70 kDa HSP, known as Hsp70, play an important part in
cellular protection and adaptation during and following exposure to stressful events.
Two prominent members of the Hsp70 family are Hsp72, which is the inducible form
of Hsp70, and Hsp73, which is the constitutively synthesized form of the protein.
Acquisition of thermotolerance in vitro in Chinese hamster fibroblast cells is
accompanied by increased Hsp72 levels during a heating episode, indicating that
Hsp72 tends to target proteins which have been damaged by a stress situation more
than Hsp73 (Li and Werb, 1982).
A number of different hsp70 genes are found, which include hsp70-1, hsp70-2 (3¢utr
and pst I) and hsp70-hom (Milner and Campbell, 1990). Both the hsp70-1 and hsp70-2
genes encode the primary heat-induced Hsp70 protein as an identical protein product
(Hunt and Morimoto, 1985). Following heat shock, no increase in the hsp70-hom
mRNA levels is observed (Milner and Campbell, 1990).
The use of the Hsp70 protein and hsp70 gene polymorphisms as markers of
acclimatization were investigated by subjecting twenty-two individuals to exercise in a
hot, humid environment. These individuals were exposed to an initial heat stress,
where they participated in a step test at an external work rate of 70 W, followed by
participation in an acclimatization program which involved exercising at various
combinations of 35 W and 70 W of the external work rate. After acclimatization, the
individuals were exposed to a second heat stress, identical to the initial heat stress. The
Hsp70 levels both before and after acclimatization were determined in response to
heat stress in blood serum by means of the StressXpress ELISA Kit (Stressgen
Biotechnologies) and in human monocytes by means of Western blots, using a mouse
antiñHsp70 monoclonal primary antibody (Stressgen Biotechnologies) and a goat
antiñmouse IgG (H+L) peroxidase conjugated secondary antibody. The hsp70 gene
polymorphisms were determined by means of polymerase chain reaction (PCR), using
primers specific to the hsp70-1, hsp70-2 (3¢utr), pst I and hsp70-hom genes, so that the
genotype combinations for each individual were determined. Blood type was also
assessed.
It was found that the basal serum Hsp70 levels in individuals who exhibited the ability
to acclimatize decreased in response to the acclimatization program, which allowed
more Hsp70 to be induced in response to the second heat stress compared to the initial
heat stress. The individuals who were unable to acclimatize showed increased basal
serum Hsp70 levels in response to acclimatization, which prevented these individuals
from inducing high Hsp70 levels in response to the second heat stress. The Hsp70
induced in the monocytes during this program followed the same pattern in both the
individuals able to acclimatize and those who were unable to acclimatize, and can
therefore not be used as a marker of acclimatization.
For the female participants, the current menstrual phase of each woman had to be
taken into account, as this had an affect on the core temperature and therefore
influenced the division of the female participants into their respective groups. These
were the group of individuals who demonstrated the ability to acclimatize or the group
of individuals who were unable to acclimatize. The use of oral contraceptives also had
to be taken into account, as this too had an influence on the core temperature and
therefore also affected the division of the individuals into the group who demonstrated
the ability to acclimatize or those who were unable to acclimatize. Cyclic changes
during the menstrual cycle may have also changed the Hsp profile.
Regarding the hsp70 gene polymorphisms, the A/A, P2 P2 and A1 A1 genotype
combination was not present in any of the individuals who were unable to acclimatize,
however six of the individuals who showed the ability to acclimatize possessed this
genotype combination.
The level of induced Hsp70 levels present in the serum of individuals able to
acclimatize and the presence or absence of the A/A, P2 P2 and A1 A1 genotype
combination therefore have the potential to be used as markers of acclimatization.
Dr. M. Cronje