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Past Research:
Heritability of
the mammalian dive response:
The heart and brain are organs
that are vitally dependent upon a continuous supply of oxygen.
Interruption of oxygen supply for only a short time, as occurs during
sleep apnea, stroke or heart disease, can cause permanent damage or
death. The mammalian dive response is a conserved physiological trait
that arises from natural selection, reflecting genotypic adaptations
to prolonged apnea or asphyxia. The response is observed in both
aquatic, and semi-aquatic (rat and musk rat) mammals, preventing
permanent damage of heart and brain during prolonged apnea. A similar,
although less prominent, type of response also occurs in man, making
studies on appropriate animal models particularly relevant to humans.
Previous studies have shown a
great variability in the human dive response and individual factors
such as age and diving experience have been suggested to modify the
bradycardia. It is known that that several factors induce or modify
these cardiovascular responses, such as arterial hypoxia and
hypercapnia, cessation of respiratory movements, and face immersion.
Even though the physiological mechanism of this response has been
thoroughly investigated, no study has investigated the genetic basis
of this physiological trait. However, recent research have shown that
the response is repeatable within the same subject over years
suggesting that the trait is heritable.
In voluntarily diving rats and
humans, the dive response has been shown to maintain perfusion to the
heart and brain while reducing blood perfusion to viscera and muscle,
and to reduce cardiac work, thus reducing overall oxygen consumption.
This enables breath-hold diving mammals to remain submerged for as
long as 120 min without apparent damage to the heart and brain. Thus,
from a clinical perspective, a better understanding of the molecular
mechanism of the dive response may provide novel strategies for
treatments of various pathological oxygen depriving states. At the
same time, this study will reveal the genotypic adaptations necessary
for prolonged apnea and asphyxia in a mammalian animal model.
We hypothesized that variation
in dive response phenotypes would be present in standard inbred rat
strains. We further believed that these differences could be exploited
in the identification of the genes responsible for the molecular basis
of the dive response. Simple genetic traits represent only a small
portion of the total genetic contribution to human health problems.
Most traits with great impact on human health are complex, involving
many genes that can interact with one another and with environmental
factors, making the prediction of disease state for a given genotype a
difficult task. For this reason, we used inbred rats as a model system
to study the complex genetics underlying the highly conserved
mammalian dive response. Working with inbred animals assured a reduced
genetic variability within strains, i.e. all animals within a single
strain are homozygous at most loci. Therefore, any differences in the
quantitative trait between strains facilitated isolation of the
genetic basis of the dive response. Rats were an excellent animal
model as inbred strains are readily available, they are semi-aquatic
animals that are easily trained and possess a strong dive response
during forced as well as voluntary dives.
Respiratory muscle training:
The percentage improvement in aerobic
fitness is inversely related to the initial fitness level of the
person. The respiratory system has traditionally
been viewed to be capable of meeting the substantial demands for
ventilation and gas exchange and the cardiopulmonary interactions
imposed by short-term maximum exercise or long-term endurance
exercise. However, the respiratory musculature can account for up to
10-15% of the total VO2 during strenuous
exercise in moderately and highly fit subjects and may
restrict endurance capacity. For this reason it has been suggested
that respiratory muscle training (RMT) can improve the endurance and
strength of the respiratory muscles in healthy subjects and it was
concluded most well controlled and rigorously designed studies have
shown that RMT has a beneficial influence upon exercise performance.
During diving and especially in the course of military operations, the
aerobic fitness could mean the difference between success or failure.
Thus, the aerobic fitness during underwater operations is important.
For this reason it was suggested that divers may benefit from RMT
training and a previous study has shown that 30 min per day for 5 days
per week of either respiratory isocapnic hyperpnea increased the
endurance capacity as compared to control subjects .
In addition, the same study showed that pressure threshold training
against a spring loaded pressure of 50 cmH2O significantly
increased endurance capacity by as much as 66%.
The purpose of this study was to
examine the effect of a modified RMT training schedule (3 sessions per
week and 30 min per session) would significantly increase the
underwater endurance swimming ability similar to that previously shown
during 5 sessions per week.
Fasting related metabolic and cardiovascular changes in king
penguins:
The use of heart rate (fH)
to estimate metabolic rate in the field has recently received
considerable attention and is based on the relationship between rate
of oxygen consumption () and
fH
as formulated in the Fick equation for convection of O2 in
the cardiovascular system. Unfortunately, it cannot always be assumed
that the relationship between
fH
and for a species remains the same under all conditions. The
relationship may be affected by several factors such as gender, type
and level of activity, physiological state (fasting, breeding,
molting), and seasonal changes.
For this reason we wanted to determine if the relationship
between and
fH
in king penguins (Aptenodytes
patagonicus) were affected by extended (up to 30 days) fasting in
water and air. Initially, five male king penguins were exercised
at various speeds on repeated occasions during a fasting period of
24-31 days. In addition,
and fH were measured in the
same animals during rest in cold air and water (4°
C). Resting and fH and
during exercise decreased with fasting. There
was a significant relationship between
and fH (r2 = 0.56),
and it was concluded that there was a significant change in theand fH relationship with
fasting during exercise. However, there was no change in the ƒH/ relationship in penguins at rest in water (13).
In addition, the data showed that resting
during
fasting in air decreased with an allometric mass exponent of 2.02
(13). We therefore hypothesized that fasting would elicit a similar
change in resting of penguins in water. Therefore resting
was measured in air and water in ten male king
penguins before (Pre, 0-2 days after returning from the sea) and after
(Post) an average fasting duration of 14.2
±
2.3 days (mean
±
1 SD, range 10-19 days) in air and water. The Pre- and Post-fasting
body masses were 13.8
±
1.2 kg and 11.0
±
0.6 kg (n = 10), respectively. There was no difference in air
temperature (P > 0.1, 1-way ANOVA) or in water temperature (P > 0.2,
t-test) between experiments and the mean temperatures in air and water
were 14.2
±
2.3° C and 8.5
±
0.6° C, respectively. After fasting, the resting
was 74% higher in water than in air (air: 86.0
±
8.6 ml O2
·
min-1;
water: 149.5
±
40.7 ml O2
·
min-1,
P < 0.01, Mann-Whitney), which is similar to other studies (Culik et
al., 1996, J. Exp. Biol. 199: 973-983). However, after returning from
the sea, there was no difference in resting
between air and water (air: 117.7
±
19.3 ml O2
·
min-1
water: 122.2
±
27.3 ml O2
·
min-1,
P > 0.6, t-test). Thus, the mass specific metabolic rates (ml O2
·
min-1
·
kg-1)
decreased in air and increased in water with fasting. This highlights
the problems of using mass specific metabolic rates in comparative
studies (17).
Hypoxia tolerance in humans:
Hypoxia is an environmental stressor that elicits
acute, acclimatory and genetic responses in humans. The hypoxic
ventilatory response (HVR), occurs over all three timescales and has
typically been studied in Andean and Himalayan highlanders compared
with each other or with Caucasian lowlanders. No studies have yet examined the hypoxia
response in African populations and our research aimed at investigate
population differences in this response in two distinct South African
populations; Caucasians and Xhosa. We initially developed a portable
breathing circuit that would enable us to measure HVR in the field (8). As
variation in the acute HVR
varies considerably between
and within
individuals
we initially
measured the within and between day variability of both HVR. Our study
showed that the HVR
is
a
reliable measure of physiological sensitivity to hypoxia
but repeated measures are necessary to deal with the high
intra-individual variability (14). There were no differences in HVR
between Caucasians and Xhosa, but the two populations showed
different ventilatory patterns in both hypoxia and normoxia (16). After controlling for
body size, the tidal volume was higher, the breathing frequency
smaller and the arterial O2 saturation lower in Xhosa as
compared to Caucasians (16).
Hydrogen biochemical decompression and diving research:
Underwater exploration involves stressful exposure
to elevated pressures, and the return to the normobaric environment
has been recognized as one of the most dangerous parts of diving due
to the risk of the so-called “bends,” or decompression sickness (DCS). DCS
is not only limited to diving, but is also a problem for caisson
workers and during aerospace flights, or any other exposure to
changing atmospheric pressures. This
research was performed under the supervision of Dr. Kayar, and we
utilized H2-metabolizing microbes to increase the washout
rate of the additional inert gas dissolved in the tissues after a
hyperbaric exposure to H2. We tested the hypothesis that
the increased washout would decrease the DCS incidence and make the
decompression phase of the hyperbaric exposure safer (4-7, 10).
Another area of research aimed at treating subjects already
suffering from DCS. Recompression therapy is the only viable DCS
treatment currently in use. The idea is to "re-compress" the subject
so that the inert gas bubbles that has formed and are blocking the
circulatory system dissolve and go into solution. An alternative would
be to increase the solubility of the inert gas by injecting
perfluorocarbon solutions, compounds with extremely high gas
solubility and that are inert to the human body (12).
Differential gene expression in hibernating squirrel hearts:
Hibernation is the key to winter survival for many small mammals
living in seasonally cold environments. Through a combination of
regulated metabolic rate depression, a resetting of the hypothalmic
set point for body temperature, and the consequent steep reduction in
body temperature (Tb) to near ambient, many small mammals
can lower their metabolic rate during hibernation to <2% of the
corresponding euthermic rate. As a result, the net energy savings
during the winter season (including the cost of periodic arousals) can
be as much as 88%, compared with
the costs of remaining euthermic
over the same time. The heart plays a vital role in hibernation for it
must continue to circulate blood throughout the entire course
hibernation although operating at a much lower body temperature and
higher peripheral resistance than during euthermia. Indeed, whereas
heart rate during hibernation may be only 1/30 or less of the
euthermic value, the force of myocardial contraction is actually
increased during torpor. Furthermore, although skeletal muscles showed
some disuse atrophy during hibernation, cardiac tissue mass actually
increased by 21 % and so did heart oxidative capacity as assessed by
citrate synthase activities. Hence, some reorganization of gene
expression to benefit heart function during hibernation should be
expected. Changes to heart protein products could define the
difference between the ready endurance of deep hypothermia by
hibernating mammals and the lethal consequences that equivalent
hypothermia exposure would have for most mammals, including man.
We
prepared a cDNA library from heart of hibernating golden-mantled
ground squirrels, Spermophilus lateralis. This library was
differentially screened to clone genes that were upregulated during
hibernation (1). Two differentially
expressed clones were found that were identified as the ventricular
isoform of myosin light chain 1 (MLC1v) and the mitochondrially-encoded
protein, subunit 2 of NADH-ubiquinone oxidoreductase (ND2).
Hibernation-induced up-regulation of MLC1v suggests that a
restructuring of myosin subunit composition could contribute to
changes in muscle contractility needed for hypothermic function
whereas changes in ND subunit composition may affect the function of
the electron transport chain during hibernation (1).
*Numbers refer to published papers on the
publication page.
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