Page 80 - JSOM Fall 2023

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FIGURE 4 Respiratory Exchange Ratio and substrate concentrations diving but in agreement with cold-water immersion efforts.
following cold-water dives in military personnel. (A) Respiratory Thus, the disparity in adrenaline/noradrenaline response within
Exchange Ratio (RER) and (B) Oxygen consumption (VO ) were SCUBA dive data may be due to water temperature. Water tem-
2
measured following the first (Post 1) and second (Post 2) dives of 1
each day. (C) Plasma free fatty acid, (D) blood ketone, and (E) Blood perature in the current effort was 3.3°C as compared to 28°C.
glucose concentrations were measured before (Pre 1 and Pre 2) and Noradrenaline is largely responsible for peripheral vasocon-
after (Post 1 and Post 2) each dive on both diving days. (F) Lactate striction when skin temperature decreases below 35°C and
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concentrations were measured before (Pre) and after the first the vasoconstriction stimulus is maximal at 31°C. Skin tem-
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(Post 1) and second (Post 2) dive of the day. Each circle represents
one participant’s value and the bars represent the mean concentration perature reached an average of 27°C post-dive despite being
for each time point (n=5). thermally protected. Cold skin likely created a stimulus for ex-
tended secretion of noradrenaline to promote vasoconstriction,
thereby shunting blood from the periphery to preserve core
temperature, which remained stable pre- to post-dive.
The rise in catecholamine also serves to “prime” the metabolic
system in the event core temperature decreased and shivering
thermogenesis is evoked. Catecholamines stimulate fuel mobi-
lization, increasing both glycogenolysis and lipolysis. Post-dive
RER (>0.99) suggest carbohydrate utilization was increased
during the dives, and post-dive free-fatty acids were elevated
indicative of triglyceride breakdown from adipose stores sup-
portive of metabolic shifts. We recently reported a 53% in-
crease in metabolism during prolonged (6-hour) cold-water
diving with the increase in metabolic rate driven by an overall
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increase in fat oxidation. Thus, while duration of dive was
shorter in the current study, the increased mobilization of lip-
ids suggests priming of the metabolic system through adrener-
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gic stimulation of lipolysis if cold exposure were to persist.
Moreover, blood glucose levels independently induce SAM/
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SNS response ; however, in this effort, blood glucose levels
remained within the physiological range and likely did not
contribute to the rise in catecholamines.
Recent evidence suggests osteocalcin may regulate glucose ho-
meostasis and serve as a signal to implement metabolic adapta-
tions when in negative energy balance. 31,32 While both un-Ocn
*p < 0.05 vs. Pre; #p < 0.05 vs. Post 1; †p < 0.05 vs. respective value and c-Ocn were measured in this effort, only un-Ocn can act
on day 1; $p < 0.05 vs. Pre 2. as a hormone in blood, and it decreased in response to cold-
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water diving. c-Ocn showed no change. Interestingly, un-Ocn is
There are few data on the acute stress response to extreme suggestive to create a thermogenic environment in adipose tis-
cold-water submersion (diving) as described in this effort. To sue ; however, to our knowledge, there are no studies that have
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our knowledge, a handful of studies exist that examine the measured un-Ocn in response to cold-water exposure. Display-
impact of diving on the stress response. Data in moderately ing a similar down-regulation, leptin, an adipokine involved
cold water (15–19°C) collected in recreational SCUBA divers in metabolic regulation, significantly decreased, which concurs
show on a mixed change in cortisol (e.g., increase, decrease, or with previous reports of leptin suppression following cold ex-
no change) along with a suppression of norepinephrine. 1,2,14 In posure. 35,36 Cold-induced SAM/SNS down-regulation of leptin
concurrence with the current effort, Marlinge et al. reported secretion is thought to reduce metabolic rate and conserve
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a decrease in cortisol during cold-water SCUBA dives. Cou- energy to prepare an organism for starvation (hibernation).
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pled to a decrease in cortisol was a decrease in ACTH, sugges- Collectively, these data suggest that maintenance of homeosta-
tive of a down-regulation of the HPA axis. After 40 minutes sis during prolonged cold-water exposure involves a concert of
of cold-exposure (4°C), both ACTH and cortisol decreased hormones to effectively shunt blood from periphery to deeper
significantly compared to the acute rise noticed immediately tissue layers and adjust metabolism to preserve life.
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after stress induction. Thus, the significant decrease noted
herein could be a function of time in cold water. Alternatively, Thyroid hormones T4 and T3 are known to regulate metabo-
activation of the HPA axis has been shown to be inversely pro- lism and adaptive thermogenesis. Cold exposure activates
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portional to the level of physical fitness. Participants in this 5-deiodinase type 2 (Dio2), which converts T4 to T3. Dio2
effort were highly fit, and the level of activity was low, which expression is increased in response to increased concentrations
may have contributed to the attenuated HPA response. of noradrenaline when exposed to the cold, which has been
observed to increase T3 systemically. In the current effort, T3
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In parallel with HPA response is the more rapid SAM/SNS significantly increased by day 3 as compared to day 1, which is
pathway, which results in secretion of adrenaline and nor- indicative of an adaptive thermogenic response. While partici-
adrenaline. Both hormones significantly increased in response pants were native, they had not been training in the high north
to the dive and, as expected, recovered between dive days. This Arctic Circle region where temperatures were sub-zero, thus
increase is disparate to others that showed an attenuation of the need for an adaptive response. Further, in adipose tissue,
noradrenaline during short duration (15 minutes) warm-water T3 induces lipolysis of triacylglyceride resulting in increased
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78 | JSOM Volume 22, Edition 3 / Fall 2023