RUMINANT NUTRITION SYMPOSIUM: Ruminant Production and Metabolic
Responses to Heat Stress
L. H. Baumgard and R. P. Rhoads
J ANIM SCI 2012, 90:1855-1865.
doi: 10.2527/jas.2011-4675 originally published online December 28, 2011
The online version of this article, along with updated information and services, is located on
the World Wide Web at:
http://www.journalofanimalscience.org/content/90/6/1855
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RUMINANT NUTRITION SYMPOSIUM:
Ruminant Production and Metabolic Responses to Heat Stress1,2
L. H. Baumgard*3 and R. P. Rhoads†
*Department of Animal Science, Iowa State University, Ames 50011; and †Department of Animal and Poultry Sciences,
Virginia Polytechnic Institute and State University, Blacksburg 24061
ABSTRACT: Heat stress compromises efficient animal production by marginalizing nutrition, management,
and genetic selection efforts to maximize performance
endpoints. Modifying farm infrastructure has yielded
modest success in mitigating heat stress-related losses,
yet poor production during the summer remains arguably
the costliest issue facing livestock producers. Reduced
output (e.g., milk yield and muscle growth) during heat
stress was traditionally thought to result from decreased
nutrient intake (i.e., a classic biological response shared
by all animals during environmental-induced hyperthermia). Our recent observations have begun to challenge
this belief and indicate heat-stressed animals employ
novel homeorhetic strategies to direct metabolic and
fuel selection priorities independently of nutrient intake
or energy balance. Alterations in systemic physiology
support a shift in carbohydrate metabolism, evident by
increased basal and stimulated circulating insulin concentrations. Perhaps most intriguing given the energetic
shortfall of the heat-stressed animal is the apparent lack of
basal adipose tissue mobilization coupled with a reduced
responsiveness to lipolytic stimuli. Thus, the heat stress
response markedly alters postabsorptive carbohydrate,
lipid, and protein metabolism independently of reduced
feed intake through coordinated changes in fuel supply
and utilization by multiple tissues. Interestingly, the systemic, cellular, and molecular changes appear conserved
amongst different species and physiological states.
Ultimately, these changes result in the reprioritization
of fuel selection during heat stress, which appears to be
primarily responsible for reduced ruminant animal productivity during the warm summer months.
Key words: heat stress, homeorhesis, lactation, metabolism
© 2012 American Society of Animal Science. All rights reserved.
INTRODUCTION
Agricultural animal productivity is maximized
within narrow environmental conditions. When the temperature is either below or above the respective threshold values, efficiency and thus profitability, are compro1Based on a presentation at the Ruminant Nutrition Symposium,
“Modulation of metabolism through nutrition and management,” at
the Joint Annual Meeting, July 10-14, 2011, New Orleans, LA with
publication sponsored by the American Animal Science Association
and the Journal of Animal Science.
2Projects and results described herein were supported by
National Research Initiative Competitive Grant no. 2008-3520618817 and Agriculture and Food Research Initiative Competitive
Grant no. 2010-65206-20644 from the USDA National Institute of
Food and Agriculture while the authors were on the faculty of the
University of Arizona.
3Corresponding author: [email protected]
Received September 7, 2011.
Accepted December 19, 2011.
J. Anim. Sci. 2012.90:1855–1865
doi:10.2527/jas2011-4675
mised because nutrients are diverted from productive
purposes to maintain a safe body temperature. Heat
stress negatively impacts a variety of productive traits
including milk yield, growth, reproduction, and carcass
traits. In addition, a heat load increases health care costs
and animals can even succumb to severe thermal stress,
especially lactating cows and animals without shade. A
2006 California heat-wave purportedly resulted in the
death of more than 30,000 dairy cows (CDFA, 2006),
and a recent heat-wave in Iowa killed at least 4,000 beef
cattle (Drovers Cattle Network, 2011) illustrating that
most geographical locales within the U.S. are susceptible to extreme and lethal heat. Thus, environmental
heat stress is a significant financial burden (i.e., approximately $900 million/yr for dairy, and > $300 million/yr
for both beef and swine in the U.S. alone; St. Pierre et
al., 2003; Pollman, 2010). Advances in environmental
management (i.e., cooling systems and barn construc-
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Baumgard and Rhoads
tion; Armstrong, 1994; Stowell et al., 2009) have alleviated some of the negative impacts thermal stress inflicts
on animal agriculture, but production still decreases during the summer months with some facets of production,
notably reproduction, exhibiting losses several months
after environmental conditions have cooled.
The detrimental effects of environmental heat stress
on animal welfare and production will likely become
more of an issue in the future if the climate of the Earth
continues to warm as most predict (IPCC, 2007). In
addition, much of the human population growth is expected to occur in tropical and subtropical areas of the
globe (Roush, 1994). Consequently animal agriculture
in these warm areas will need to expand (Renaudeau
et al., 2008) to keep pace with the global appetite for
high quality protein. Furthermore, basal metabolic heat
production increases with enhanced production (i.e., enhanced milk yield; Spiers et al., 2004) and rapid lean
tissue accretion (Brown-Brandle et al., 2004); therefore,
selection criteria based upon traditional production traits
may increase the susceptibility of animals to thermal
stress. Thus, there is an urgent need to have a better understanding of how heat stress alters nutrient partitioning and ultimately nutrient utilization. Defining the biology and mechanisms of how heat stress jeopardizes
animal performance is critical in developing approaches
to ameliorate current production issues and is a prerequisite for generating future mitigating strategies (e.g.,
genetic, managerial, nutritional, and pharmaceutical) to
improve animal well-being and performance, ensure a
constant and annual supply of animal products for human consumption, and secure and enhance agriculture
profitability.
MILK YIELD
Milk synthesis appears to be particularly sensitive,
compared with animal growth, to thermal stress as decreased yields of up to 35 to 40% are not unusual (West,
2003). For a detailed description of how cows accumulate
and dissipate heat and how, when, and where cows become heat stressed, see articles by Berman (2003, 2004).
It was traditionally thought that lactating cows become
heat stressed when conditions exceed a temperature humidity index (THI) of 72 (Armstrong, 1994). However,
recent climate controlled experiments indicate milk yield
starts to decrease (Zimbleman et al., 2009) at a THI of
68 and is supported by field observations evaluating the
THI when cow standing time (i.e., a classic response to
a thermal load) increases (Cook et al., 2007). The lower
THI at which cows are thought to become heat-stressed
is consistent with the hypothesis that higher producing
cows are more susceptible to a thermal load.
The mechanistic basis for environmental-induced
hyperthermia milk yield losses likely involves multiple
systems. An altered endocrine profile including reciprocal changes in circulating anabolic and catabolic hormones (discussed below) certainly contributes (Collier et
al., 2005; Bernabucci et al., 2010; Baumgard and Rhoads,
2011). In addition, intracellular signaling pathways responsible for maintenance, productivity and survival purposes are markedly impacted by heat stress (Collier et al.,
2008). Recent evidence also indicates that the mammary
gland utilizes a regulatory negative feedback system to
reduce milk synthesis during acute heat stress (Silanikove
et al., 2009). The net result of the aforementioned changes, coupled with marked decreases in nutrient intake, is
a highly coordinated event to prioritize acclimation and
survivability. How much each altered system contributes
to reduced milk yield is currently unknown.
Heat-stressed animals reduce feed intake and this is
presumably a survival strategy as digesting, especially
in ruminants, and processing nutrients generates heat
(i.e., thermic effect of feed). It has traditionally been assumed that inadequate feed intake caused by the thermal load was responsible for decreased milk production
(Fuquay, 1981; Beede and Collier, 1986; Silanikove,
2000; West, 2003; DeShazer et al., 2009). However,
our recent experiment demonstrated disparate slopes in
feed intake and milk yield responses to a cyclical heat
load pattern (Shwartz et al., 2009). This led us to hypothesize that heat stress reduces milk synthesis by both
direct and indirect (i.e., via reduced feed intake) mechanisms (Baumgard and Rhoads, 2007). To examine this
theory, we designed a series of pair-feeding experiments
enabling us to evaluate thermal stress while eliminating the confounding effects of dissimilar nutrient intake.
Employing this type of approach is required to differentiate between direct and indirect effects (e.g., reduced intake) of environmentally-induced hyperthermia because
both heat-stressed and malnourished animals share common responses (e.g., reduced milk yield and growth).
Our experiments demonstrate that reduced feed intake
only explains approximately 35 to 50% of the decreased
milk yield during environmental-induced hyperthermia
(Figure 1; Rhoads et al., 2009; Wheelock et al., 2010;
Baumgard et al., 2011). Our results agree with previous
data (Bianca, 1965), and indicate that when overall heat
stress (i.e., extent and duration) exceeds a given threshold that is as of yet unidentified, the cumulative thermal
load disrupts the nutrient intake and the milk production
relationship and milk yield declines beyond expected
levels. A similar experimental approach has been utilized to evaluate growth and, similar to our data on lactation, reduced feed intake explained only about 50% of
the reduction in chick BW gain (Geraert et al., 1996),
but nutrient intake appears to explain a majority of the
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Metabolic responses to heat stress
Figure 1. Effects of heat stress (HS) or pair-feeding (PF) on A) DMI
and B) milk yield in lactating Holstein cows. Solid lines with diamonds
represent PF cows. Dashed lines with squares represent HS cows. The mean
value from d 1 to 9 of the thermal neutral ad libitum period (P1) was used as a
covariate and is represented by P1 on the X axis. The d 1 to 9 results are from
P2 when cows were exposed to HS or exposed to thermal neutral conditions
and PF with the HS cows. Adapted from Rhoads et al. (2009).
effects of heat stress on BW gain of pigs (Safranski et
al., 1997; Pearce et al., 2011) and cattle (O’Brien et al.,
2010). However, heat stress growth models are limited
with regard to energetics because they generally do not
take into account composition of BW gain. This pairfeeding design is critical in determining the extent to
which an indirect effect of heat stress, such as reduced
feed intake, plays a role in physiological or metabolic
adaptations or both, which occur during increased environmental temperatures.
Metabolic Adaptations to Reduced
Feed Intake
A prerequisite for understanding metabolic adaptations that occur with heat stress is the appreciation for
the physiological and metabolic adjustments lactating
and growing animals initiate during malnutrition. These
changes in postabsorptive nutrient partitioning occur to
support a dominant physiological state (i.e., milk and
skeletal muscle synthesis; Bauman and Currie, 1980)
and one well described homeorhetic strategy is the “glucose sparing” effect that lactating animals utilize when
on an inadequate-plane of nutrition.
Early-lactation dairy cattle enter a unique physiological state during which they are unable to consume enough
1857
nutrients to meet maintenance and milk production costs,
and animals typically enter into negative energy balance
(NEBAL; Drackley, 1999). Negative energy balance is
associated with a variety of metabolic changes that are
implemented to support the dominant physiological condition of lactation (Bauman and Currie, 1980). Marked
alterations in both carbohydrate and lipid metabolism
ensure partitioning of dietary and tissue-derived nutrients
toward the mammary gland and, not surprisingly, many
of these changes are mediated by endogenous ST that
naturally increases during periods of NEBAL (Bauman
and Currie, 1980). During NEBAL, ST promotes NEFA
export from adipose tissue by accentuating the lipolytic
response to β-adrenergic signals and by inhibiting insulinmediated lipogenesis and glucose utilization (Bauman and
Vernon, 1993). This reduction in systemic insulin sensitivity is coupled with a decrease in circulating blood insulin
concentrations (Rhoads et al., 2004). The reduction in
insulin action allows for adipose lipolysis and NEFA mobilization (Bauman and Currie, 1980). Increased circulating NEFA are typical in “transitioning” and malnourished
cows and represent, along with NEFA-derived ketones, a
significant source of energy and precursors for milk fat
synthesis for cows in NEBAL. The severity of calculated
NEBAL is positively associated with circulating NEFA
concentrations (Bauman et al., 1988; Dunshea et al.,
1990), and it is generally thought that there is a linear relationship (i.e., concentration-dependent process) between
NEFA delivery, tissue NEFA uptake, and NEFA oxidation
(Armstrong et al., 1961). The magnitude of NEBAL and,
thus, lipid mobilization explains, in large part, why cows
can lose considerable amounts (i.e., > 50 kg) of BW during early lactation.
Postabsorptive carbohydrate metabolism is also
markedly altered by NEBAL, mediated mostly by reduced insulin action. During either early lactation or
inadequate nutrient intake, glucose is partitioned toward the mammary gland, and glucose’s contribution
as a fuel source to extramammary tissues is decreased
(Bell, 1995). The early lactation or NEBAL-induced
hypoglycemia accentuates catecholamine’s adipose lipolytic effectiveness (Clutter et al., 1981). This is a key
“glucose sparing” mechanism because increased NEFA
concentrations decrease skeletal muscle glucose uptake
and oxidation; this is often referred to as the “Randle
Effect” (Randle, 1998). The fact that insulin simultaneously orchestrates both carbohydrate and lipid metabolism explains why there is a reciprocal relationship between glucose and NEFA oxidation. Ultimately, these
are homeorhetic adaptations to maximize milk synthesis
at the expense of tissue accretion (Bauman and Currie,
1980). A thermal neutral cow in NEBAL could be considered “metabolically flexible” because extramammary
tissues can depend upon alternative fuels (i.e., NEFA
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Baumgard and Rhoads
and ketones) to spare glucose, which can be utilized by
the mammary gland to copiously produce milk.
Maintenance Costs during Heat Stress
Heat stress is thought to increase maintenance
requirements in rodents (Collins et al., 1980), poultry (Yahav, 2007), sheep (Whittow and Findlay, 1968;
Ames et al., 1971), and cattle (McDowell et al., 1969;
Morrison, 1983; Beede and Collier, 1986; Huber, 1996).
The enhanced energy expenditure during heat stress is
believed to originate from panting, sweating, and because the Van’t Hoff-Arrhenius equation predicts greater
chemical reaction rates (Kleiber, 1961; Fuquay, 1981).
Although it is difficult to quantify, maintenance costs
in lactating dairy cattle are estimated to increase by as
much as 25% during heat stress (NRC, 1989), and some
suggest it may be greater than 30% (Fox and Tylutki,
1998). However, because heat-stressed animals typically
have a reduction in thyroid hormones (Sano et al., 1983;
Johnson et al., 1988; Prunier et al., 1997; Garriga et al.,
2006), actual oxygen consumption (Hales, 1973) and
heat production may in fact decrease (Collin et al., 2001;
Brown-Brandle et al., 2004; Huynh et al., 2005). It appears a quadratic relationship between environment and
bioenergetics exists, where maintenance costs and total
body expenditure decline with mild heat stress but rapidly increase during severe heat stress as suggested by
others (Kleiber, 1961; Yunianto et al., 1997). Adaptation
also appears to influence energy expenditure during heat
stress because metabolic rate may increase during acute
but decrease during chronic heat stress (Bianca, 1965).
Clearly, a better understanding of the means and magnitude to which heat stress alters both basal metabolic
rates and maintenance costs are needed.
POSTABSORPTIVE CHANGES DURING
HEAT STRESS
Lipid Metabolism
Some production data indicate that heat stress alters
metabolism differently than would be expected based
upon calculated whole body energy balance. For example, heat-stressed sows (Prunier et al., 1997) and heifers (Ronchi et al., 1999) in negative energy balance do
not lose as much BW and body condition, respectively,
as do their pair-fed thermal neutral counterparts. In addition, carcass data indicate that rodents (Schmidt and
Widdowson, 1967), chickens (Geraert et al., 1996), and
pigs (Heath, 1983) have increased lipid retention when
reared in heat stress conditions. We and others have
demonstrated that basal plasma NEFA concentrations
(a product of adipose lipolysis and mobilization) are
Figure 2. Effects of period of thermal challenge on the NEFA response
(area under the curve) to the epinephrine challenge in lactating Holstein cows.
During Period 1, all cows were housed in thermal neutral conditions and allowed to eat ad libitum. During Period 2, cows were exposed to heat stress
(HS) or exposed to thermal neutral conditions and pair-fed (PF) with the HS
cows. Adapted from Baumgard et al. (2011).
typically reduced in heat-stressed rodents, pigs (Pearce
et al., 2011), sheep, and cattle, despite marked reductions in feed intake (Sano et al., 1983; Ronchi et al.,
1999; Shwartz et al., 2009), and especially when compared with pair-fed thermal neutral controls (Rhoads
et al., 2009; Wheelock et al., 2010). Furthermore, we
have recently demonstrated that heat-stressed cows
have a blunted (compared with pair-fed thermal neutral
controls) NEFA response to an epinephrine challenge
(Figure 2; Baumgard et al., 2011). These observations
agree with rodent data indicating heat stress reduces in
vivo lipolytic rates and in vitro lipolytic enzyme activity
(Torlinska et al., 1987). Moreover, heat stress increases
adipose tissue lipoprotein lipase (Christon, 1988), indicating adipose tissue of hyperthermic animals has an increased capacity to liberate fatty acids from circulating
triglycerides for storage. The changes in carcass composition, blood lipid profiles, and lipolytic capacity are
surprising because heat stress causes a well-described
increase in stress and catabolic hormones (i.e., epinephrine, cortisol, and glucagon; Bianca, 1965; Beede and
Collier, 1986). What is more, the blunted lipolytic capacity of adipose tissue is especially unusual as heatstressed cows are severely energy-restricted (i.e., 30 to
40%), in a calculated NEBAL (approximately −5 Mcal;
Moore et al., 2005; Shwartz et al., 2009; Wheelock et al.,
2010) and lose considerable BW (i.e., >40 kg; Rhoads
et al., 2009; Wheelock et al., 2010); all traits typically
associated with increased circulating NEFA concentrations (Bauman et al., 1988; Dunshea et al., 1990).
Carbohydrate Metabolism
Evidence from many nonruminant species indicates
that carbohydrate metabolism is altered during heat
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Metabolic responses to heat stress
stress. For example, acute heat stress was first reported
to cause hypoglycemia in cats, which was originally hypothesized to be one of the reasons for reduced worker/
laborer productivity during warm summer months (Lee
and Scott, 1916). In addition, heat-stressed human athletes consistently have increased hepatic glucose production and whole body enhanced carbohydrate oxidation at the expense of lipids (Fink et al., 1975; Febbraio,
2001; Jentjens et al., 2002). Furthermore, hepatic glucose production typically decreases after ingesting carbohydrates; however, exogenous sugars are unable to
blunt heat stress-induced liver glucose output (Angus et
al., 2001). The increased hepatic glucose output originates from increased glycogenolysis (Febbraio, 2001)
and increased gluconeogenesis (Collins et al., 1980).
Gene expression of hepatic pyruvate carboxylase, a ratelimiting enzyme controlling lactate and alanine entry
into the gluconeogenic pathway, is increased during heat
stress in multiple animal models (O’Brien et al., 2008;
Wheelock et al., 2008; White et al., 2009; Rhoads et al.,
2011), and, in hyperthermic rodents, the contribution
of lactate to gluconeogenesis increases (Collins et al.,
1980; Hall et al., 1980). Interestingly, numerous studies
indicate that plasma lactate concentrations increase during exercise in the heat (Rowell et al., 1968; Fink et al.,
1975; Angus et al., 2001), during porcine malignant hyperthermia (Hall et al., 1980), in heat-stressed growing
steers (Elsasser et al., 2009), and in heated melanoma
cells (Streffer, 1988), presumably stemming from skeletal muscle efflux. Surprisingly, the increased muscle
lactate production and efflux is not the result of reduced
muscle blood or oxygen flow (Yaspelkis et al., 1993).
Whether the increased blood lactate originates from anaerobic or from aerobic glycolysis is currently unknown.
Irrespective, these studies appear to indicate peripheral
tissues increase glycolysis and the Cori cycle may be
an important mechanism to maintain glucose and energy
homeostasis during heat stress.
Our recent environmental physiology experiments
in lactating dairy cows indicated that heat-stressed
animals secreted approximately 200 to 400 g/d less
milk lactose than did pair-fed thermal neutral controls
(Rhoads et al., 2009; Wheelock et al., 2010). The quantity of secreted lactose is generally equivalent to a similar amount (on a molar basis) of secreted glucose, but it
is unclear whether the liver produces 200 to 400 g less
glucose or whether extramammary tissues utilize glucose at an increased rate. We have generated 2 lines of
evidence indicating the latter is occurring. First, heatstressed cows dispose of exogenous glucose quicker
after a glucose tolerance test (Wheelock et al., 2010).
Second, despite reports suggesting the liver becomes
partially dysfunctional during heat stress (Ronchi et al.,
1999; Willis et al., 2000; Ramnath et al., 2008), we have
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Figure 3. Effects of period of thermal challenge on whole animal
glucose rate of appearance in lactating Holstein cows. During Period 1, all
cows were housed in thermal neutral conditions and allowed to eat ad libitum. During Period 2, cows were exposed to heat stress (HS) or exposed to
thermal neutral conditions and pair-fed (PF) with the HS cows. Adapted from
Baumgard et al. (2011).
shown using stable isotopes that whole-body glucose
production (presumably primarily from hepatic tissue)
does not differ between heat-stressed and pair-fed thermal neutral controls (Figure 3; Baumgard et al., 2011).
Moreover, we recently reported that heat stress causes
altered hepatic gluconeogenic gene expression, perhaps
associated with different precursor supply (Rhoads et al.,
2011), as noted above. As a consequence, with regard
to hepatic glucose output, it appears that ruminant liver
remains functional and that glucose is preferentially utilized for processes other than milk synthesis (ostensibly
via insulin-responsive tissues) during heat stress.
Protein Metabolism
Heat stress also affects postabsorptive protein
metabolism, and this is illustrated by changes in the
quantity of carcass lean tissue in a variety of species
(Schmidt and Widdowson, 1967; Close et al., 1971; Lu
et al., 2007). Muscle protein synthesizing machinery
and RNA/DNA synthesis capacity are reduced by environmental hyperthermia (Streffer, 1982), and it appears
that similar effects occur with regard to mammary α and
β casein synthesis (Bernabucci et al., 2002). It also appears that skeletal muscle catabolism is increased in a
variety of species during heat stress because we and others have reported increased plasma markers of muscle
breakdown (Bianca, 1965; Hall, 1980; Marder et al.,
1990; Yunianto et al., 1997; Wheelock et al., 2010).
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Baumgard and Rhoads
EFFECTS OF HEAT STRESS ON ASPECTS
OF THE ENDOCRINE SYSTEM
Intracellular Energetics
Due to the mass of skeletal muscle and the fact
that it is an energetically expensive tissue to maintain,
small changes in its fuel efficiency can have large impacts on feed efficiency. The pyruvate dehydrogenase
(PDH) complex controls glucose flux through the tricarboxylic acid cycle cycle and is responsible for the
irreversible conversion of pyruvate to acetyl-CoA. The
PDH complex is primarily regulated via covalent modification by pyruvate dehydrogenase kinases (PDK) and
pyruvate dehydrogenase phosphatases (PDP), which
inactivate and active, respectively, PDH (Harris et al.,
2002). The activity of the PDK and PDP are themselves
regulated at the transcriptional level by intracellular energy status, metabolic intermediates (acetyl-CoA and
NADH), transcription factors, as well as by hormones
such as cortisol and insulin (Johnson and Denton, 2003;
Sugden and Holness, 2006). Cellular energy status (i.e.,
ATP to adenosine monophosphate ratio) is one of the
primary mechanisms for determining substrate utilization. Inactivating the PDH complex may be a glucosesparing mechanism, although reduced oxidative glucose
metabolism during heat stress may instead be the result
of events stemming from intracellular reactive oxygen
species (ROS). An attractive candidate known to shift
cellular metabolism toward glycolysis is hypoxia-inducible factor (HIF; Pouyssegur and Mechta-Grigoriou,
2006). Reports indicate HIF acts as a metabolic switch
for cellular adaptation to hypoxia by increasing PDK
expression and down regulating mitochondrial oxygen
consumption (Kim et al., 2006; Papandreou et al., 2006).
Although much research on HIF signaling has focused
on oxygen tension, there is growing understanding that
HIF is regulated by stressors, such as hyperthermia and
ROS (Jackson et al., 2006; Horowitz and Assadi, 2010).
In support of the previous reports, we consistently observe increased skeletal muscle PDK4 mRNA abundance during heat stress (O’Brien et al., 2008; Wheelock
et al., 2008). Moreover, our preliminary experiments
demonstrate a HIF-driven 5-fold increase in transcriptional activity during heat stress in primary satellite cells
(i.e., myocytes). Taken together, increased transcription
of PDK4 and the subsequent inactivation of the PDH
complex might serve as a mechanism to reduce substrate oxidation and mitochondrial ROS production and
prevent cellular damage during heat stress.
Somatotropic Axis
Somatotropin and IGF-I are 2 of the most potent
and well-characterized lactogenic hormones (Bauman
and Vernon, 1993). Normally, ST partitions nutrients
toward the mammary gland by decreasing nutrient uptake by extramammary tissues and stimulating hepatic
IGF-I synthesis and secretion. During NEBAL (i.e., during early lactation), the somatotropic axis uncouples
and hepatic IGF-I production decreases despite increased circulating ST concentrations (McGuire et al.,
1992). We originally hypothesized that NEBAL caused
by heat stress and early lactation differentially affects
the somatotropic axis. For example, although acute heat
stress increased ST concentrations in steers (Mitra and
Johnson, 1972), chronic heat-stressed cows (which are
presumably in NEBAL) had or tended to have reduced
ST concentrations (Mohammed and Johnson, 1985;
Igono et al., 1988; McGuire et al., 1991). To evaluate
this further, we analyzed basal ST pulsatility characteristics and pituitary responsiveness to a ST secretagoue
and reported no differences in either characteristic in
heat stress vs. pair-fed thermal neutral controls (Rhoads
et al., 2009). However, we did measure a modest reduction in circulating IGF-I, which may indicate that the
metabolic milieu favors uncoupling of the somatotropic
axis during heat stress (Rhoads et al., 2009). We investigated whether hepatic ST-responsiveness was altered
during heat stress by measuring ST receptor abundance
and signal transducer and activator of transcription
(STAT)-5 phosphorylation (Rhoads et al., 2010). Heat
stress, independent of reduced feed intake, decreased
hepatic ST receptor abundance although both heat stress
and malnutrition were sufficient to decrease STAT-5
phosphorylation. Consistent with reduced ST signalling
through STAT-5, hepatic IGF-I mRNA abundance was
less in heat-stressed animals. Thus, the reduced hepatic
ST responsiveness observed during heat stress appears
to involve mechanisms independent of reduced feed intake. The physiological significance of reduced hepatic
ST receptor abundance during heat stress is unclear at
this time but may serve to alter other ST-dependent hepatic processes, such as gluconeogenesis. Moreover, we
hypothesize that reduced IGF-I may be one mechanism
by which the liver and mammary tissues coordinate the
reduction in milk synthesis so nutrients (i.e., glucose)
can be utilized to maintain homeothermia.
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Metabolic responses to heat stress
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Figure 4. Effects of heat stress (HS) and pair-feeding (PF; in thermoneutral conditions) on the insulin response to a glucose tolerance test (GTT) in growing
Holstein bull calves. Adapted from O’Brien et al. (2010).
Insulin
Despite hallmarks traditionally associated with hypoinsulinemia, such as 1) marked reductions in feed intake, 2) calculated NEBAL, and 3) rapid BW loss (i.e.,
>40 kg), we have demonstrated that basal insulin values gradually increase in lactating heat-stressed cows
(Wheelock et al., 2010), and we have confirmed this in
growing heat-stressed calves (Figure 4; O’Brien et al.,
2010) and pigs (Pearce et al., 2011). Increased plasma insulin concentrations in our experiments agree with data
from another report on heat-stressed ruminants (Itoh et
al., 1998), a malignant hyperthermic pig model (Hall
et al., 1980), and heat-stressed rodents (Torlinska et al.,
1987). In addition, we have reported that heat-stressed
cows and calves have an increased insulin response to
a glucose tolerance test (O’Brien et al., 2010; Wheelock
et al., 2010), and this also agrees with other reports utilizing different insulin secreteagogues (Achmadi et al.,
1993; Itoh et al., 1998). At present, there is not a clear
explanation for the insulin increase during heat stress,
but may involve the key role of insulin in activating and
up-regulating heat shock proteins (Li et al., 2006). The
lack of a NEFA response during heat stress may allow
for the increase in circulating insulin as excessive NEFA
cause pancreas β-cell apoptosis (Roche et al., 2000).
Regardless of how, heat stress is one of the very few
nondiabetic models we are aware of where nutrient intake is markedly reduced, but basal and stimulated insulin concentrations are increased.
The increased insulin may be an essential part of
the heat stress adaptation mechanism. For example, diabetic humans are more susceptible to heat-related illness and death (Shuman, 1972; Semenza et al., 1999;
Kovats and Hajat, 2008). Similarly, diabetic rats have
an increased mortality rate when exposed to severe heat,
and exogenous insulin increases their survival time (Niu
et al., 2003). Furthermore, nonlethal heat stress ameliorates proxies of insulin insensitivity in diabetic rodents
(Kokura et al., 2007, 2010) or rodents fed high fat diets (Gupte et al., 2009). This is similar to reports indicating that thermal therapy (e.g., saunas and hot baths)
improves insulin sensitivity in humans (McCarty et al.,
2009). Consequently, it appears insulin or maintenance
of insulin action or both has a critical role in the ability
to respond and ultimately survive a heat load.
COORDINATED METABOLIC
CONSEQUENCES OF HEAT STRESS
Insulin is a potent regulator of both carbohydrate
and lipid metabolism and may play an important role
in mediating heat stress regulation of postabsorptive
nutrient partitioning. Insulin stimulates glucose uptake
via glucose transporter type 4 in responsive tissues (i.e.,
muscle and adipose tissue) and is likely responsible for
the heat-induced hypoglycemia frequently reported in
several animal models [i.e., rodents (Mitev et al., 2005),
poultry (Rahimi, 2005), cats (Lee and Scott, 1916), dogs
(Kanter, 1954), sheep (Achamadi et al., 1993), heifers
(Itoh et al., 1998), bulls (O’Brien et al., 2010), and cows
(Settivari et al., 2007; Wheelock et al., 2010)]. This occurs despite an increase in intestinal glucose absorptive
capacity (Garriga et al., 2006) and enhanced renal glucose reabsorptive ability (Ikari et al., 2005).
Insulin is also a potent antilipolytic hormone (Vernon,
1992) and may explain why heat-stressed animals do
not mobilize adipose tissue triglycerides. Limiting adipose tissue mobilization is the key step by which heatstressed animals are prevented from employing glucose
sparing mechanisms normally enlisted to maintain milk
or skeletal muscle synthesis during periods of temporary malnutrition. The lack of available NEFA to systemic tissues for oxidative purposes is coupled with
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1862
Baumgard and Rhoads
reduced volatile fatty acid (primarily acetate because
of decreased feed intake in ruminants) availability and,
thus, both glucose and AA oxidation may increase. The
efficiency of capturing ATP from amino acid oxidation
is low, meaning metabolic heat production is high (Berg
et al., 2007), so it is an unlikely fuel choice during hyperthermia. Therefore, it appears that glucose becomes
a favored fuel for heat-stressed animals (Streffer, 1988),
which is supported by the increase in respiratory quotient in hyperthermic humans (Febbraio, 2001).
The increase in skeletal muscle protein catabolism, as
mentioned previously, is peculiar given the role of insulin
in stimulating protein synthesis and preventing proteolysis (Allen, 1988). Heat stress is thought to increase membrane permeability allowing for cytostolic Ca+ leakage
that may increase protein sensitivity to heat stress (Roti
Roti, 2008). We theorize it is likely that breaking down
skeletal muscle may be a strategy to supply precursors
for gluconeogenesis, consistent with data of Rhoads et al.
2011, and acute phase proteins rather than for supplying
oxidative substrates, because of the inefficiency in capturing ATP from amino acid-derived substrates.
CONCLUSION
Clearly, the heat-stressed lactating cow initiates
a variety of postabsorptive metabolic changes that are,
in large part, independent of reduced feed intake and
whole-animal energy balance. Seemingly these changes
in nutrient partitioning are adaptive mechanisms employed to prioritize the maintenance of euthermia. The
primary difference between a thermal neutral and a heatstressed animal in a similar energetic state is the inability of the hyperthermic cow to employ “glucose sparing” mechanisms to homeorhetically prioritize milk and
meat synthesis. An inability to mobilize adipose tissue
reduces metabolic fuel options (i.e., NEFA and ketones),
and the animal becomes “metabolically inflexible” and
is forced to increase reliance on glucose. From an animal
agriculture standpoint, these survival strategies reduce
productivity and seriously jeopardize farm profitability.
Although science has characterized some changes
heat stress induces in postabsorptive metabolism, there
are a plethora of unknowns. For example, how heat
stress signals for an increase in circulating insulin is not
clear. We presume insulin prevents adipose tissue mobilization, but the mechanism is unknown, especially
during periods of enhanced catabolic signals and hypoglycemia. Defining the biology and mechanisms of how
heat stress jeopardizes animal health and performance is
critical in developing approaches to ameliorate current
production issues and is a prerequisite for generating future mitigation strategies to improve animal well-being
and performance (i.e., lactation, growth, and reproduction), thereby improving agricultural profitability.
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