BIOB34 Module 3 - Thermal Physiology Lecture Slides PDF

Overview of Thermal Physiology Thermal energy influences chemical interactions that affect macromolecular structure and biochemical reactions Thermal strategy Behavioral, biochemical, and physiological responses that ensure body temperature (TB) is within an acceptable limit Ambi...

Overview of Thermal Physiology Thermal energy influences chemical interactions that affect macromolecular structure and biochemical reactions Thermal strategy Behavioral, biochemical, and physiological responses that ensure body temperature (TB) is within an acceptable limit Ambient temperature (TA) Temperature of the animal’s surroundings Most important environmental influence on animal’s thermal strategy Animals must be able to survive thermal extremes and thermal change specific to their environment © K. Welch - Do Not Distribute 1  Thermal Strategies Two major thermal strategies: Tolerance Body temperature is allowed to vary with ambient temperature Regulation Body temperature does not vary with ambient temperature Both strategies have costs and benefits What is the strategy of your focal animal? (if you’ve identified one) Can you imagine any costs/benefits? © K. Welch - Do Not Distribute 2  Heat Fluxes Body temperature is a reflection of the thermal energy of the molecules in the body Thermal energy can move from the animal to the environment or vice versa Thermal energy moves “down” a temperature gradient There are many sources and sinks of thermal energy Total thermal energy ∆𝐻 ∆𝐻. ∆𝐻. ∆𝐻. ∆𝐻. ∆𝐻. © K. Welch - Do Not Distribute 3  Heat Fluxes Total thermal energy (a dynamic thing) ∆𝐻 ∆𝐻. ∆𝐻. ∆𝐻. ∆𝐻. ∆𝐻. © K. Welch - Do Not Distribute 4  Types of heat flux Convection Transfer of thermal energy between an object and an external medium that is moving Rate of heat exchange depends on: The thermal gradient The rate of flow of the fluid The conductivity of the fluid Radiation Emission of electromagnetic radiation The most important source of radiant heat is the sun Basking Darker colors enhance heat absorption © K. Welch - Do Not Distribute 5  Types of heat flux Evaporation Water molecules absorb thermal energy from a surface when making the transition from liquid to vapor Evaporative cooling Magnitude of heat loss depends on the volume of water and heat of vaporization Salt increases the heat of vaporization © K. Welch - Do Not Distribute 6  Types of heat flux Conduction Transfer of thermal energy from one object or fluid to another Heat flux (𝑄) Rate of heat transfer (from hotter to colder) What should the units for be? © K. Welch - Do Not Distribute 7 Fourier’s Law: quantifying Joseph Fourier thermal conduction (1768 – 1830), France ∆ 𝑄 𝑘𝐴 𝑄 = rate of heat flow (𝑊) 𝑘 = thermal conductivity ( ) ° ∆𝑇 = temperature gradient (in °𝐾; ∆𝑇 𝑇 𝑇) 𝑑 = distance of flux (𝑚) © K. Welch - Do Not Distribute 8  X-section of polar bear skin Fourier’s Law: quantifying Guard hairs thermal conduction Underfur Darkly pigmented skin ∆ 𝑄 𝑘𝐴 Blubber/fat layer 𝑄 = rate of heat flow (𝑊) 𝑘 = thermal conductivity ( ) ° ∆𝑇 = temperature gradient (in °𝐾; ∆𝑇 𝑇 𝑇) 𝑑 = distance of flux (𝑚) © K. Welch - Do Not Distribute 9  Conduction Thermal conductivity varies with the type of material and is affected by geometry Water has very high thermal conductivity © K. Welch - Do Not Distribute 10  Insulation Layer of material that reduces thermal exchange Internal insulation (under the skin) Blubber External insulation (on the body surface) Hair Feathers Air Water Effectiveness of insulation depends largely on its thickness © K. Welch - Do Not Distribute 11  Surface Area to Volume Ratio Influences all aspects of heat exchange Large animals exchange heat more slowly than small animals Bergmann’s rule Animals living in cold environments tend to be larger Allen’s rule Animals in colder climates have smaller extremities Behavioural adjustments Body posture can alter exposed surface area Huddling behavior reduces effective surface area © K. Welch - Do Not Distribute 12  Thermal Strategies Relative stability of body temperature Poikilotherm Variable body temperature Homeotherm Stable body temperature © K. Welch - Do Not Distribute 13  Thermal Strategies Source of thermal energy – Ectotherm Environment determines body temperature – Endotherm Animal generates internal heat to maintain body temperature Most animals best described by a combination of terms © K. Welch - Do Not Distribute 14  Temporal and Regional Endothermy Hypometabolic phase accompanied by a decrease in body temperature E.g. hibernation, torpor Metabolic energy that would normally be used for thermoregulation is saved Time course differs among animals and types of dormancy Torpor: usually less than a day Hibernation/estivation: longer © K. Welch - Do Not Distribute 15  Homeothermy is relative Circadian rhythm evident doi: 10.1080/23328940.2020.1743605 in: Metabolic rate Body temperature White-tailed antelope squirrel Ammospermophilus leucurus (housed indiv. in lab at 25°C.) © K. Welch - Do Not Distribute 16  Homeothermy is relative Animal breeding is big business Important to know when fertilization is possible Can be tracked in hormones costly Solution: Easier way of tracking fertility Metabolic rate and body temperature vary with reproduction cycle © K. Welch - Do Not Distribute 17  Homeothermy is relative Solution: Easier way of tracking fertility Metabolic rate and body temperature vary with reproduction cycle © K. Welch - Do Not Distribute 18 Some subjects are “willing” participants Natural Cycles “The only FDA Cleared birth control app” © K. Welch - Do Not Distribute 19 Body temperature varies with reproductive status in a large mammal © K. Welch - Do Not Distribute 20 Homeothermy is relative © K. Welch - Do Not Distribute 21 Homeothermy is relative Mean daily body temp range (°C) Unsurprisingly, there is a general relationship between body size and the magnitude of these daily fluctuations. Mean daily body temp (°C) © K. Welch - Do Not Distribute 22  Homeotherm response to temperature variation Consider treeshrews Native to Borneo Danielle Levesque Univ. of Maine Pygmy treeshrew Large treeshrew Tupaia minor Tupaia tana ~ 65 g ~ 250 g Figure 1. Thermoregulatory parameters of Tupaia minor measured during the rest phase of the activity cycle. Individuals were captured from the wild in Malaysian Borneo and kept overnight for measurements. The ambient temperature at which the physiological character of interest increased or decreased significantly, as calculated using segmented regression analysis, is represented by the solid vertical line, with the dashed lines showing the 95% confidence interval of the breakpoint analysis. Cdry = dry thermal conductance; Cwet = wet thermal conductance; EHL = evaporative heat loss; EWL = evaporative water loss; MHP = metabolic heat production; Tsub = subcutaneous body temperature. © K. Welch - Do Not Distribute 23  Thermal Zones of Homeotherms Thermoneutral zone Optimal range for physiological processes; metabolic rate is minimal Upper critical temperature (UCT) Metabolic rate increases as animal induces a physiological response to prevent overheating Lower critical temperature (LCT) – Metabolic rate increases to increase heat production Animals differ in the width of their thermoneutral zone, UCT, and LCT © K. Welch - Do Not Distribute 24 Thermal Zones of Homeotherms © K. Welch - Do Not Distribute 25 Homeotherm response to temperature variation Variation in TNZ breadth Pygmy treeshrew Danielle Levesque Large treeshrew Univ. of Maine Tupaia minor Tupaia tana ~ 65 g ~ 250 g © K. Welch - Do Not Distribute 26 Homeotherm response to temperature variation Pygmy treeshrew Danielle Levesque Large treeshrew Univ. of Maine Tupaia minor Tupaia tana ~ 65 g ~ 250 g Variation in TNZ breadth Figure 2. Width of the thermoneutral zone (TNZ) and the basal metabolic rate (BMR) of a number of mammal species. Treeshrews from Borneo (Tupaia minor and Tupaia tana) are represented by blue squares; other scandentians are represented by yellow triangles. The dashed lines are the regression lines determined using ordinary least squares, and the solid black lines are the regression lines determined using phylogenetic generalized least squares (pgls; see text). The gray lines show the 95% confidence intervals of the pgls analysis. All BMR data from Genoud et al. (2017) were used to calculate the confidence intervals, but only species less than 1 kg are shown for visual clarity. © K. Welch - Do Not Distribute 27  Thermal Tolerance of Poikilotherms Poikilotherms do not have a thermoneutral zone, UCT, or LCT Preferred temperature Ambient/body temperature for optimal physiological function Incipient lethal temperature Ambient temperature at which 50% of animals die Incipient upper lethal temperature (IULT) Incipient lower lethal temperature (ILLT) Range of tolerance Range of ambient temperatures between IULT and ILLT © K. Welch - Do Not Distribute 28  Thermal Tolerance of Animals Eurytherm Can tolerate a wide range of ambient temperatures Stenotherm Can tolerate only a narrow range of ambient temperatures Eurytherms can occupy a greater number of thermal niches than stenotherms © K. Welch - Do Not Distribute 29  Temporal and Regional Endothermy Regional heterotherms – Body temperature varies in regions of the body Billfish heater organs near eyes © K. Welch - Do Not Distribute 30  Thermogenesis by Ion Pumping Ion gradients degrade for two main reasons Membrane proteins use electrochemical energy to drive transport and biosynthesis Ions leak across membranes Ions must be continually pumped Ion-pumping membrane proteins produce heat E.g. Billfish heater organs (modified muscles that don’t contract) Plasma membranes of endotherms are leakier than those of ectotherms Increased thermogenesis due to ion pumping © K. Welch - Do Not Distribute 31  Temporal and Regional Endothermy Mackerel Tuna Regional heterotherms – Body temperature varies in regions of the body Billfish heater organs near eyes Tuna retain heat in red muscle © K. Welch - Do Not Distribute 32  Temporal and Regional Endothermy Regional heterotherms – Body temperature varies in regions of the body Billfish heater organs near eyes Tuna retain heat in red muscle © K. Welch - Do Not Distribute 33  Temporal and Regional Endothermy Regional heterotherms – Body temperature varies in regions of the body Billfish with heater organs near eyes Tuna retain heat in red muscle The thorax of some flying insects © K. Welch - Do Not Distribute 34  Heat Production in Insects Prior to Flight Mechanisms: Carbohydrate metabolism in flight muscles Futile cycling: 2 opposing enzymes are activated simultaneously Antagonistic flight muscles contract simultaneously Energy is expended and heat is produced without movement Not uncoordinated, like in shivering Wing movement Frequency and orientation of the wings are controlled to avoid generating lift – wing buzzing © K. Welch - Do Not Distribute 35 Temperature effects: Biochemistry and Physiology Proteins and lipids are affected by temperature over the normal range encountered by animals Hydrogen bonds and van der Walls forces are disrupted by high temperature Hydrophobic interactions are stabilized at high temperatures © K. Welch - Do Not Distribute 36  Macromolecule Structure Proteins and lipids are affected by temperature over the normal range encountered by animals Hydrogen bonds and van der Walls forces are disrupted by high temperature Hydrophobic interactions are stabilized at high temperatures **Show JOVE video on membrane fluidity** © K. Welch - Do Not Distribute 37  Membrane Fluidity Membrane fluidity is affected by temperature Low temperatures cause membrane lipids to solidify High temperatures increase membrane fluidity Changes in membrane fluidity affect protein movement and membrane function Increased protein movement with increased fluidity © K. Welch - Do Not Distribute 38  Homeoviscous Adaptation Homeoviscous adaptation Maintain membrane fluidity at different temperatures by changing membrane lipids, cholesterol content Mechanisms of homeoviscous adaptation Fatty acid chain length Shorter chains increase fluidity Saturation More double bonds increase fluidity © K. Welch - Do Not Distribute 39  Homeoviscous Adaptation Mechanisms of homeoviscous adaptation, cont. Phospholipid classes Phosphatidylcholine (PC): decrease fluidity Phosphatidylethanolamine (PE): increase fluidity Cholesterol content Prevents solidifying when the membrane is cooled © K. Welch - Do Not Distribute 40  Thermal adaptation, acclimation, acclimatization Ectotherms remodel tissues in response to long-term changes in temperature Quantitative strategy More metabolic “machinery” For example, increase the number of muscle mito- chondria in low temp Qualitative strategy Alter the type of metabolic “machinery” For example, different myosin isoforms in winter and summer © K. Welch - Do Not Distribute 41  Cold Adaptation Psychrotrophs Animals that thrive at low temperatures Protein “breathing” Changes in 3-D shape during the catalytic cycle Enzymes do not breathe well at low temperatures because weak bonds are strengthened Decreased enzyme efficiency Psychrotrophs possess cold adapted enzymes Fewer weak bonds Enzymes breathe (jiggle) more easily at low temperature But cold-adapted enzymes are more vulnerable to temperature- dependent unfolding © K. Welch - Do Not Distribute 42  Strategies for Surviving Freezing Temperatures Freeze-tolerance Animals can allow their tissues to freeze in a controlled, safer way Freeze-avoidance Animals use behavioral and physiological mechanisms to prevent ice crystal formation Snow flea © K. Welch - Do Not Distribute 43  Strategies for Surviving Freezing Temperatures  Supercooling  In the absence of a nucleator, water can remain liquid below 0°C (lowest is –40°C)  Ice crystal formation needs a trigger  Either a cluster of water molecules or a macromolecule that acts as a nucleator  Deleterious effects of ice crystal formation  Points and edges can pierce membranes, if crystals grow large enough  Crystal growth removes surrounding water  Osmolarity increases © K. Welch - Do Not Distribute 44  Freeze-Avoidance Solutes depress the freezing point of a liquid (colligative property of water) As osmolarity increases, freezing point decreases Antifreeze macromolecules Proteins or glycoproteins that depress the freezing point by noncolligative actions Disrupt ice crystal formation by binding to small ice crystal and preventing growth © K. Welch - Do Not Distribute 45  Freeze-Tolerance Two mechanisms of freeze-tolerance Produce nucleators outside of the cell Control the location and kinetics of ice crystal growth Extracellular fluid freezes, but intracellular fluid remains liquid Produce intracellular solutes to counter the movement of water © K. Welch - Do Not Distribute 46  Maintaining a Constant Body Temperature Endothermy intertwined with high metabolic rate High metabolic rate causes  heat production Thermogenesis Advantages of high body temperature  growth, development, digestion, biosynthesis Endothermy requires ability to regulate Thermogenesis Heat exchange with environment Both endotherms and ectotherms produce metabolic heat Only endotherms have the ability to retain enough heat to elevate body temperature above environmental temperature Endotherms possess futile cycles Metabolic reactions whose sole purpose is to produce heat © K. Welch - Do Not Distribute 47  Regulation of Body Temperature Coordination of multiple physiological systems Internal Thermostat Mammals Information from central and peripheral thermal sensors is integrated in the hypothalamus Hypothalamus sends signals to the body to alter rates of heat production and dissipation Birds Thermostat is located in the spinal cord © K. Welch - Do Not Distribute 48  Shivering Thermogenesis Unique to birds and mammals Uncoordinated myofiber contraction that results in no coordinated net muscle work Works for short periods of time Muscles rapidly depleted of nutrients and become exhausted Has costs Prevents the animal from using locomotory muscle for foraging or predator avoidance © K. Welch - Do Not Distribute 49  Shivering Thermogenesis Patterns of fuel use during shivering (as function of intensity) Mirror patterns of fuel use in muscle with exercise intensity Muscle glycogen use dominates at high intensities. © K. Welch - Do Not Distribute 50  Brown Adipose Tissue (BAT) Used for nonshivering thermogenesis Found in small mammals and newborns that live in cold environments Located near the back and shoulder region Differs from white adipocytes Higher levels of mitochondria Produces the protein UCP1 (Uncoupling Protein 1) UCP uncouples the mitochondrial electron transport system and proton pumping from ATP synthesis (leads to futile cycling of protons) High rate of fatty acid oxidation Energy is released as heat © K. Welch - Do Not Distribute 51  Brown Adipose Tissue (BAT) Activated by sympathetic nervous input Activates fatty acid beta oxidation And upregulates UCP translation/transcription This, then, activates UCP activity There is something wrong with this figure. Can you spot it? © K. Welch - Do Not Distribute 52  Brown Adipose Tissue (BAT) ETC received reducing energy Pumps protons, building/maintaining proton motive force UCP lets H+ move down gradient So does ATP Synthase Unlike ATP Synthase, UCP’s moving of H+ does not power ATP synthesis Futile cycle © K. Welch - Do Not Distribute 53 Brown Adipose Tissue (BAT) Oops!! © K. Welch - Do Not Distribute 54 Ground squirrel orbital fat depot: thermogenic but no UCP1? Similar morphology, composition to BAT Possibly thermogenic © K. Welch - Do Not Distribute 55 Ground squirrel orbital fat depot: thermogenic but no UCP1? Similar morphology, composition to BAT Possibly thermogenic But no UCP1/3 in tissue Thermogenic mechanism unknown? Comparison of the orbital lipid depot with brown adipose tissue. (A) Proton density fat fraction (PDFF) magnetic resonance image (MRI) of a ground squirrel and CryoViz image of thorax. Example (left) of an MRI slice from a hibernating ground squirrel. Areas highlighted in red indicate location of PDFF values between 30–70%, expected values for BAT. The CryoViz image (centre) shows the orbital lipid depot corresponding precisely with the position of the tissue used for MRI analysis (right) and shows a close visual resemblance. (B) Immunoblots of uncoupling protein 1 (UCP1) and uncoupling protein 3 (UCP3) from various tissues of thirteen-lined ground squirrels. Numbers indicate different individuals from which the orbital lipid depot or skeletal muscle was sampled. (C) Thermal images of a squirrel during an induced arousal. Arrowhead indicates approximate location of the eye and the arrow indicates approximate location of the ear. Time stamps in the bottom left corner of images in C indicate elapsed time after removal from home cage. © K. Welch - Do Not Distribute 56  Torpor: a spectrum of behaviours Anusha Shankar Daily changes in body Formerly PDF at Cornell U New Asst. Prof. at U. of Hyderabad temperature in various animals “Deep” torpor is identifiable But what about lots of examples of “shallow” torpor? A schematic depiction of body temperature (colored lines) relative to ambient temperature (black dashed line) at night: in sleep, shallow torpor and deep torpor. (A) A normothermic individual, with minimal circadian reductions in night-time body temperature (e.g. humans). (B) An individual that starts the night normothermic, then transitions into deep torpor, where body temperature drops with ambient temperature, minimizing the difference between minimum body temperature and ambient temperature (e.g. hummingbirds). (C) An individual that starts the night normothermic, then uses ‘shallow’ torpor, potentially because the species has a very high minimum body temperature of only 4–5°C below normothermic levels (e.g. some pigeon species). This use of shallow torpor can show a variety of patterns, either stabilizing or oscillating up and down (alternative pink dashed lines). (D) An individual that uses a combination of normothermy, and shallow and deep torpor, at times regulating its body temperature above its minimum torpid body temperature. Here, we investigate the presence of such a heterothermy spectrum in hummingbirds. © K. Welch - Do Not Distribute 57  Torpor: a spectrum of behaviours Anusha Shankar Shallow torpor can still save energy – Formerly PDF at Cornell U New Asst. Prof. at U. of Hyderabad via reduced metabolic rate A regulated, moderate temperature Doesn’t strongly track ambient temp. Contrast with deep torpor, which does track ambient Go as low as ambient allows? A regulated state: still will defend min Tb Duration of torpor, more than depth, predicts energy savings Predicted model fit from the linear mixed effects model of maximum surface temperature (eye temperature) as a function of ambient temperature, colored by category. As we predicted, deep torpor had a steep slope and low intercept, while shallow torpor and normothermy had similar shallow slopes and high intercepts (see Tables S3 and S4 for regression coefficients). The dashed line is an identity line to show where surface temperature would equal ambient temperature. BCHU, black-chinned hummingbird; BLUH, blue-throated mountain-gem; RIHU, Rivoli's hummingbird. © K. Welch - Do Not Distribute 58 Torpor: for more than just energy emergencies Normally, given how vulnerable is leaves them, we assume hummingbirds avoid using torpor unless necessary E.g. if they end day without Erich Eberts Deniz Kaya sufficient fat to stay normothermic PhD graduate of Welch Lab Graduate of UTM MSc program in Biomedical Communications through the night. © K. Welch - Do Not Distribute 59 Torpor: for more than just energy emergencies Normally, given how vulnerable is leaves them, we assume hummingbirds avoid using torpor unless necessary E.g. reach night without sufficient fat to stay normothermic through the night. © K. Welch - Do Not Distribute 60