Amy Twitchell and Faculty Mentor: Robert Hyldahl, Exercise Science

Introduction: Exercise has been known to improve mitochondrial function and increase its
content in muscle. However, those who suffer from certain diseases such as Chronic Obstructive
Pulmonary Disease (COPD) and Type II Diabetes often experience concomitant exercise
intolerance. In vitro research suggests that the application of a mild heat stress may be sufficient
to activate some of the same signaling proteins that become active during exercise, possibly
leading to mitochondrial biogenesis in skeletal muscle. However, this has not yet been confirmed
to occur in human skeletal muscle. Therefore, the purpose of this study was to investigate the
effect of heat therapy on mitochondrial biogenesis in human skeletal muscle. Our hypothesis
was that repeated heat stress would increase mitochondrial protein content in human skeletal
muscle.
Methodology: We used pulsed-shortwave diathermy to produce deep tissue heating within
human vastus lateralis muscle. We randomly assigned one leg to be heated (HEAT), and the
other leg to be control (CONTROL) in 20 (10 male, 10 female) volunteers who provided written
consent. We administered the heat therapy for 2 hours per day over 6 consecutive days. To
confirm that the pulsed-shortwave diathermy was actually heating to the same depth from which
we would be extracting human muscle, a temperature probe was inserted approximately 3.5 cm
into the vastus lateralis muscle during the first round of heating. Temperature was recorded every
five minutes during the 2-hour period. Muscle biopsies were taken from the vastus lateralis of the
quadriceps from both legs before and after the heat therapy program. We then tested these
muscle samples for signs of mitochondrial adaptation or biogenesis. This was done by
homogenizing the tissue and attaching immunomagnetic antibodies specific to each of the 5
mitochondrial proteins (Complex I, II, III, IV, V). Protein content was quantified using MAGPIX
eXPONENT software.
Results: Muscle temperature increased 3.9°C by 30 minutes, and remained elevated to the same
degree throughout the rest of the heating session. The recorded measurements confirmed that the
muscle was heated to levels observed during exercise (see Fig. 1).
In response to the heat therapy program, increases in mitochondrial protein content were
observed. Complex I and V expression increased within the heated muscle (p < 0.05). However,
no differences were observed in the expression of the other protein complexes (II, III, and IV)
(see Fig. 2).
Discussion: The increase in mitochondrial protein content serves as a sign of mitochondrial
adaptation resulting from the mild heat stress. Complex I is the first enzyme of the electron
transport chain (ETC), effectively transporting protons across the inner membrane through the
oxidation of NADH. This develops an electrochemical gradient which can be used for the
synthesis of ATP. Complex V is responsible for transporting protons back down the gradient in
order to create ATP from ADP and inorganic phosphate, allowing mitochondria to produce the
necessary energy for normal cell function. It is unclear why we did not observe increases in all
mitochondrial proteins. It is most likely the result of the timing of our biopsies (after only 6 days
of treatment) as mitochondrial biogenesis is an intricate process including fission (separation)
and fusion (joining). Our biopsies only represent a brief snapshot of what is occurring in the
muscle. In addition, it is possible that the diathermy treatment directed certain
transcriptional/translational pathways specific to complexes I and V, while leaving others
unaffected.
Conclusion: Repeated heat stress, through pulsed-short-wave diathermy, appears to be capable of
promoting beneficial mitochondrial adaptation in human skeletal muscle. The increased
mitochondrial protein content observed following the use of this modality suggest that mild heat
stress may provide some of the same benefits associated with exercise. Although there are other
health benefits associated with exercise that would not be gained through treatment with
diathermy, the possibility of essentially training a muscle without actually requiring a person to
exercise is an exciting discovery with important implications for all who might suffer from
diseases associated with exercise intolerance. It is my hope that this study can open the door for
future research regarding mild heat stress as a form of mitochondrial treatment strategy.