Environmental Physiology of Livestock (Collier/Environmental Physiology of Livestock) || Regulation of Acclimation to Environmental Stress

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    Chapter 4

    Regulation of Acclimation toEnvironmental StressKajal Sankar Roy and Robert J. Collier

    What Is Acclimation?

    A variety of environmental factors such as ambient temperature, solar radiation, relativehumidity, and wind speed are known to have direct and indirect effects on domestic animals(Collier et al., 1982, 2004, 2005). The direct effects involve impacts of the environment onthermoregulation, the endocrine system, metabolism, production, and reproduction. Indirecteffects include impacts of the environment on food and water availability, pest and pathogenpopulations, and resistance of the immune system to immunologic challenges. Animals havedeveloped copingmechanisms tominimize the impact of these environmental stressors on theirbiological systems. These responses are broadly described as acclimation, acclimatization, andadaptation. Acclimation is the coordinated phenotypic response developed by the animalto a specic stressor in the environment (Fregley, 1996), while acclimatization refers tothe coordinated response to several individual stressors simultaneously (e.g., temperature,humidity, and photoperiod; Bligh, 1976). In general, there is hardly ever a case in the naturalenvironment where only one environmental variable changes. Thus, in the vast majority ofcases the animal is undergoing acclimatization to the changing environment. Acclimation andacclimatization involve phenotypic and not genotypic change, and the acclimation responseswill decay if the stress is removed. The overall impact of acclimation and acclimatization is toimprove the tness of the animal in the environment. In many cases the acclimation response isinduced by sudden environmental change. In other examples the acclimation response is drivenby changes in photoperiod or other environmental cues such as the lunar cycle, which permit theanimal to anticipate the coming change in the environment leading to seasonal acclimationadjustments in insulation (coat thickness, fat deposition), feed intake, or reproductive activityin advance of the actual environmental change. However, in every case, the process is drivenby the endocrine system and is homeorhetic. Homeorhesis is dened as the coordination of

    Environmental Physiology of Livestock, First Edition. Edited by R. J. Collier and J. L Collier.C 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

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    50 Environmental Physiology of Livestock

    metabolism to support a specic physiologic state (Bauman and Currie, 1984). In this casethe specic physiologic state is the acclimatized animal.

    Adaptation: Bos indicus versus Bos taurus

    If environmental stressors are present for prolonged periods of time (e.g., years) the metabolicand physiologic adjustment can become xed genetically and we refer to this state as theadapted state of animal. Examples are Bos indicus breeds of cattle, which have evolved underconditions of high temperature and humidity and display a number of genetic differences thatendow them with improved thermotolerance, compared to Bos taurus breeds of cattle, whichevolved under temperate weather conditions.

    Bos indicus cattle have greater thermoregulatory capability than Bos taurus. As pointedout by Hansen (2004), Bos indicus cattle produce less heat, have increased capacity to loseheat toward the environment, or a combination of both. This suggests that low metabolic ratesresulting from reduced growth rates and milk yields of many Indicus breeds constitute a majorcontributing factor to thermotolerance. The basal metabolic rate of B. indicus is in fact lowerthan that for B. taurus (Finch, 1985). The physiological and cellular basis for this differencehas not been identied. One possibility for improved heat loss in B. indicus is that the density ofarteriovenous anastomoses is higher in B. indicus. Since these structures have lower resistanceto ow than vascular passages involving capillary networks, they facilitate increased bloodow to the skin during heat stress (Hales et al., 1978).

    The vascularity and degree of insulation of the skin and quality of the hair coat (hair andskin coat color, thickness and density of hair bers) also contribute to the effectiveness ofheat loss in cattle (Gebremedhin et al., 2008, 2010). All of these are affected by breed andcontribute to well-known genotype environment effects.

    The actual rate of heat loss via sweating depends not only upon the extrusion of water at theskin surface but also upon the evaporation of that water. It has been observed that evaporativeheat loss rates were less affected by humidity for Indicus cows than for Holstein and BrownSwiss cows. For example, studies showed that the sweating rate in Indicus cattle exposedto heat stress was unaffected by humidity of the surrounding air while the sweating rate ofShorthorn cattle was reduced as humidity increased (Finch, 1985). This result was interpretedas reecting the greater trapping of humidied air in the dense hair coat of the Shorthorns.There is no evidence that respiratory capacity for heat loss is superior for Indicus cattle. Theproportion of evaporative heat lost via respiration was roughly similar for Indicus, Holstein,Jersey, and Brown Swiss (Kibler and Brody, 1952). During heat stress, evaporative heat loss viarespiration rate can be greater for European breeds and this occurrence also probably reectsthe greater engagement of heat loss mechanisms for the less-adapted breeds.

    There is a general belief that the appendages of B. indicus cattle contribute to their superiorthermoregulatory ability, as the appendages increase the surface area per unit body weight ascompared to B. taurus. The actual importance of these anatomical features is not likely to becrucial for thermoregulation because surgical removal of the dewlap or hump of Red Sindhibulls did not have a measurable impact on thermoregulatory ability (McDowell et al., 1958).Additionally, differences in regulation of rectal temperature in response to heat stress wereobserved between Jersey and Red Sindhi Jersey even though surface area per unit bodyweight or metabolic body weight was similar between the two genotypes.

    Heat stress has less severe effects on semen quality of Indicus bulls than it does on bulls ofEuropean breeds, and this phenomenon reects not only adaptations that affect whole-body

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    Regulation of Acclimation to Environmental Stress 51

    thermoregulation but also specic adaptations that enhance the local cooling of blood enteringthe testis. A study by Brito et al. (2004) demonstrated that the anatomical features of thetesticular thermoregulatory systemdiffered betweenNelore, crossbred (5/8 or 5/16CharolaisZebu) and Angus bulls. For example, the ratio of testicular artery length to testicular volumewas greatest for Nelore bulls, intermediate for crossbred bulls, and least for Angus bulls. Inaddition, the testicular artery wall thickness and the distance between arterialvenous bloodin the testicular vascular cone were least in Nelore, intermediate in crossbreds, and greatestin Angus. These anatomical differences were related to differences in testicular intra-arterialtemperature, which was lowest in Nelore, intermediate in crossbreds, and highest in Angus(Brito et al., 2004).

    When animals are adapted the physiologic differences between them and non-adaptedanimals do not disappear when the environment changes. This is not the case in acclimationwhere differences do disappear if the stress is removed. However, it is becoming clear that thesame systems that are involved in acclimatization are the systems that endow animals withthermotolerance or adaptation to heat. Therefore, obtaining a better understanding of the genenetworks involved in response to environmental stress will also lead us to those pathwayswhich offer promise to improve thermotolerance.

    Acclimatization requires several days to weeks to fully develop, there is a hormonal linkbetween the central nervous system and the effector cell types involved, and the effect ofthe hormonal change is to alter the responsiveness of the effector cells to environmentalchange (Bligh, 1976). These key features are hallmarks of a homeorhetic process in whichmetabolism of multiple tissues and organs is coordinated to support the new acclimatizedstate as contrasted to a homeostatic process (Bauman and Currie, 1984; Collier et al., 2004),and where regulation is occurring around a set point. We then need to consider the stages ofacclimation, the hormones that are driving acclimation, and what changes are occurring ineffector tissues to accomplish development of the acclimatized state.

    What Are the Stages of Acclimation?

    Acclimatization is generally considered to occur in two stages: acute or short term and chronicor long term (Johnson and Vanjonack, 1976; Horowitz, 2002; Garrett et al., 2009). Theacute phase includes a shock response at the cellular level (Carper et al., 1987; Sonna,2002) and homeostatic endocrine, physiological, and metabolic responses at the systemiclevel. The chronic or long-term phase results in acclimation to the stressor and involves thereprogramming of gene expression and metabolism (Horowitz, 2002; Collier et al., 2006).

    In agricultural animals there is generally a loss in productivity as animals progress throughthe acute phase and some or even all of this productivity is restored as animals undergoacclimation to the stress.

    What Is Involved in the Systemic Response?

    The systemic response to environmental stress is driven by two systems: (1) the central nervoussystem (CNS) and (2) peripheral nervous system and endocrine components (Charmandariet al., 2005). The central component is comprised of nuclei in the hypothalamus and thebrainstem, which release corticotropin-releasing hormone (CRH) and arginine vasopressin(AVP). The peripheral components of the stress system include the pituitary-adrenal axis,

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    the efferent sympathetic-adrenomedullary system, and components of the parasympatheticsystem (Habib et al., 2001). However, relative to environmental stress and acclimation, theinitial phases of the response involve receptor systems at the periphery that drive autonomicand endocrine responses to the changing environment (e.g., skin thermoreceptors andphotoreceptors in the retina).

    Sweating and panting are two of the primary autonomic responses exhibited by animalsunder heat stress. Sweating results in increased evaporative heat loss from the skin surface,whereas in panting, sensible heat from the body core is used to heat the water vapor and expelheat in the form of vaporized moisture from the lungs. However, these responses are likelydriven more by surface temperatures than core body temperatures. As shown in Figures 4.1and 4.2, evaporative heat loss from skin and the respiratory tract is highly correlated with skintemperature. In fact, skin temperature is more highly correlated with these parameters thancore temperature suggesting that thermal receptors in the skin initiate the autonomic systemicresponse to thermal stress. Another potential route of information ow from the surface to thewhole system would be via secreted heat shock protein (HSP) released from skin epitheliumduring heat stress, whichwould act as an alarm system to assist inmobilizing the acute responseto thermal shock. An examination of the relationship between skin temperature and expressionof the gene for inducible heat shock protein 70 (see Fig. 4.3) revealed that gene expression isincreased several-fold as skin temperature approaches 35C, which is below body temperaturebut represents the upper limit of the thermoneutral zone of cattle. Berman (2005) estimated thatthe stress response system in cattle would be activated at effective temperatures at and above35C. Previously, it has been demonstrated that evaporative heat loss and rectal temperaturerise dramatically above an effective environmental temperature of 35C (see gs. 4.1 and 4.2;Collier et al., 2008). It is now apparent that the heat shock response in bovine skin epithelialtissue is activated at effective environmental temperature of 35C as well. Activation of theheat shock response in cells in many cases leads to secretions of HSPs into the extracellularspace and plasma (Ireland et al., 2007).

    25.030.0

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    Bre

    aths

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    r = 0.73, P < 0.0001

    Surface Temperature (C)Figure 4.1. Correlation of left side surface temperature and respiration rate in Holstein cows in asemi-arid environment.

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    Regulation of Acclimation to Environmental Stress 53

    y = 1.1665x2 64.166x + 894.35R2 = 0.4348

    y = 4.2976x 71.289R2 = 0.0368

    < 35C > 35C

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    Surface Temperature (C)

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    Figure 4.2. Relationship between infrared coat surface temperature and evaporative heat loss (EVHL)in Holstein dairy cows. Open circles and regression correlation (R2 = 0.4348) denote EVHL below35C. Closed circles and regression correlation (R2 = 0.0358) denote EVHL above 35C. Slopes of tworegressions differ, P < 0.001.

    Figure 4.3. Relationship between skin temperature and fold increases in heat shock protein 70 geneexpression in cattle skin.

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    54 Environmental Physiology of Livestock

    Recently, secreted heat shock protein was identied in plasma of cattle. Kristenson andcolleagues (2004) in Australia have demonstrated that secreted HSP concentrations rise inplasma when the effective environmental temperature exceeds 35C (Gaughan and Bonner,2009). Thus, activation of the heat shock response in cells also leads to secretion of HSPs intothe extracellular space and plasma (Ireland et al., 2007). It has been hypothesized that secretedheat shock protein acts as an alarm signal for the immune system and several measures ofinnate immunity are increased following increases in secreted heat shock protein in blood(Fleshner and Johnson, 2005). Secreted heat shock protein has also been shown to improvesurvival of neural cells subjected to environmental and metabolic stressors (Tytell, 2005;Guzhova et al., 2001).

    Thus, the acute response in cattle is initially driven by thermal receptors in skin thatactivate the CNS and subsequently, the endocrine system and the peripheral components of theautonomic system. This response is augmented by secreted HSPs that rapidly rise in plasmaand are believed to provide protective effects to a variety of cell types as well as activatingthe innate immune system. At a skin surface temperature of 35C the respirat...

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