Animal Hormones

Learning Objectives

  1. Identify the major glands and body structures involved in hormone synthesis in vertebrates
  2. Recall the functions of selected hormones produced by select major glands
  3. Describe the hormone pathway in given examples, including blood glucose, hunger, metamorphosis, stress, and/or sex, and make predictions on how an animal would respond to given stimuli for each case
  4. Recognize instances of negative feedback loops, positive feedback loops, and crosstalk in the example hormone pathways

Vertebrate Endocrine Glands and Hormones

The information below was adapted from OpenStax Biology 37.5

Unlike plant hormones, animal hormones are often (though not always) produced in specialized hormone-synthesizing glands (shown below). The hormones are then secreted from the glands into the blood stream, where they are transported throughout the body. There are many glands and hormones in different animal species, and we will focus on just a small collection of them.

Locations of endocrine glands in the human body. Image credit: OpenStax Anatomy and Physiology.

In vertebrates, glands and hormones they produce include (note that the following list is not complete):

  • hypothalamus: integrates the endocrine and nervous systems; receives input from the body and other brain areas and initiates endocrine responses to environmental changes; synthesizes hormones which are stored in the posterior pituitary gland; also synthesizes and secretes regulatory hormones that control the endocrine cells in the anterior pituitary gland. Hormones produced include
    • growth-hormone releasing hormone: stimulates release of growth hormone (GH) from the anterior pituitary
    • corticotropin-releasing hormone: stimulates release of adrenocorticotropic hormone (ACTH) from the anterior pituitary
    • thyrotropin-releasing hormone: stimulates release of thyroid-stimulating hormone (TSH) from the anterior pituitary
    • gonadotropin-releasing hormone: stimulates release of follicle -stimulating hormone and luteinizing hormone from the anterior pituitary
    • antidiuretic hormone (vasopressin): promotes reabsorption of water by kidneys; stored in posterior pituitary
    • oxytocin: induces uterine contractions labor and milk release from mammary glands; stored in posterior pituitary
  • pituitary gland: the body’s master gland; located at the base of the brain and attached to the hypothalamus via a stalk called the pituitary stalk; has two distinct regions: the anterior portion of the pituitary gland is regulated by releasing or release-inhibiting hormones produced by the hypothalamus, and the posterior pituitary receives signals via neurosecretory cells to release hormones produced by the hypothalamus. Hormones produced (or secreted) by the gland include:
    • anterior pituitary: the following hormones are produced by the anterior pituitary and released in response to hormone signals from the hypothalamus
      • growth hormone: stimulates growth factors
      • adrenocorticotropic hormone (ACTH): simulates adrenal glands to secrete glucocorticoids such as cortisol
      • thyroid-stimulating hormone: stimulates thyroid gland to secrete thyroid hormones
      • follicle-stimulating hormone (FSH) and luteinizing hormone (LH): stimulates production of gametes and sex steroid hormones
      • prolactin: stimulates mammary gland growth and milk production
    • posterior pituitary: the following hormones are produced by the hypothalamus and stored in the posterior pituitary
      • antidiuretic hormone: promotes reabsorption of water by kidneys; stored in posterior pituitary
      • oxytocin: induces uterine contractions during labor and milk release from mammary glands during suckling; stored in posterior pituitary
  • thyroid gland: butterfly-shaped gland located in the neck; regulated by the hypothalamus-pituitary axis; produces hormones involved in regulating metabolism and growth:
    • thyroxine (T4) and triiodothyronine (T3): increase the basal metabolic rate, affect protein synthesis and other metabolic processes, help regulate long bone growth (synergy with growth hormone)
  • adrenal glands: two glands, each located on one kidney; consist of adrenal cortex (outer layer) and adrenal medulla (inner layer), which each produce different sets of hormones:
    • adrenal cortex:
      • mineralocorticoids, such as aldosterone: increases reabsorption of sodium by kidneys to regulate water balance
      • glucocorticoids, such as cortisol and related hormones: long-term stress response hormones that increase blood glucose levels by stimulating synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells; promote the release of fatty acids from adipose tissue
    • adrenal medulla:
      • epinephrine (adrenaline) and norepinephrine (noradrenaline): short-term stress response (“fight-or-flight”) hormones that increase heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels; accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue; release of epinephrine and norepinephrine is stimulated directly by neural impulses from the sympathetic nervous system
  • pancreas: located between the stomach and the proximal portion of the small intestine; regulates blood glucose levels via the hormones:
    • insulin: decreases blood glucose levels by promoting uptake of glucose by liver and muscle cells and conversion to glycogen (a sugar storage molecule)
    • glucagon: increases blood glucose levels by promoting breakdown of glycogen and release of glucose from the liver and muscle
  • gonads: produce sex steroid hormones that promote development of secondary sex characteristics and regulation of gonad function:
    • ovaries (in females):
      • estradiol: regulates development and maintenance of ovarian and menstrual cycles
      • progesterone: prepares uterus for pregnancy
    • testes (in males): regulates development and maintenance of sperm production

The hormones produced and/or stored by the pituitary gland are summarized here:

Modification of work by OpenStax College – Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148147

This video provides a nice overview of the glands and hormones of the vertebrate endocrine system:

Hormonal Regulation of Body Processes in Animals

The information below was adapted from OpenStax Biology 37.3

Hormones have a wide range of effects and modulate many different body processes. The key regulatory processes that will be examined here are those affecting blood glucose, hunger, metamorphosis, stress, and sex. We will primarily focus on these processes in vertebrates, but will also consider invertebrates in some cases.

Blood Glucose

Glucose is the primary energy source for most animal cells, and it is distributed throughout the body via the blood stream. The ideal, or target, blood glucose concentration is about 90 mg/100 mL of blood, which equates to about 1 tsp of glucose per 6 quarts of blood. After a meal, carbohydrates are broken down during digestion and absorbed into the blood stream. The amount present following a meal is typically more than what the body needs at that moment, and so the extra glucose must be removed and stored for later use. The opposite phenomenon occurs following a period of fasting. Insulin and glucagon are the two hormones primarily responsible for maintaining appropriate blood glucose levels.

Insulin is produced by the beta cells of the pancreas, which are stimulated to release insulin as blood glucose levels rise (for example, after a meal is consumed). Insulin lowers blood glucose levels through several processes:

  • enhances the rate of glucose uptake and utilization by target cells, which use glucose for ATP production
  • stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use
  • increases glucose transport into certain cells, such as muscle cells and the liver
  • stimulates the conversion of glucose to fat in adipocytes and the synthesis of proteins.

These actions together cause cause blood glucose concentrations to fall, called a hypoglycemic ‘low sugar’ effect, which inhibits further insulin release from beta cells through a negative feedback loop.

When blood glucose levels decline below normal levels, for example between meals or when glucose is utilized rapidly during exercise, the hormone glucagon is released from the alpha cells of the pancreas. Glucagon raises blood glucose levels, eliciting what is called a hyperglycemic effect through several mechanisms:

  • stimulates the breakdown and release of glucose from glycogen in skeletal muscle cells and liver cells
  • stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose
  • stimulates adipose cells to release fatty acids into the blood

Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. These actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising blood glucose levels inhibit further glucagon release by the pancreas via a negative feedback mechanism. In this way, insulin and glucagon work together to maintain homeostatic glucose levels, as shown in below.

Insulin and glucagon regulate blood glucose levels. When blood glucose levels fall, the pancreas secretes the hormone glucagon. Glucagon causes the liver to break down glycogen, releasing glucose into the blood. As a result, blood glucose levels rise. In response to high glucose levels, the pancreas releases insulin. In response to insulin, target cells take up glucose, and the liver converts glucose to glycogen. As a result, blood glucose levels fall. Image credit: OpenStax Biology

Uncontrolled blood glucose levels can lead to different types of serious medical conditions:

  • Impaired insulin function can lead to a condition called diabetes mellitus. Diabetes mellitus can be caused by low levels of insulin production by the beta cells of the pancreas, or by reduced sensitivity of tissue cells to insulin. Either of these situations prevents glucose from being absorbed by cells, causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult for the kidneys to recover all the glucose from nascent urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced; this may result in dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system.
  • Oversecretion of insulin can cause hypoglycemia, low blood glucose levels, which causes insufficient glucose availability to cells, often leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.

This animation describes diabetes and the roles of insulin and the pancreas in blood glucose regulation:

Hunger Management

The immediate form of energy for most animal cells is glucose, and extra glucose is stored as glycogen which is readily broken down into glucose when needed. Longer term reserves of energy are stored as fats, in cells called adipocytes.  Too little fat means there may no be enough energy reserves in times when food is less available, and will cause an animal to feel hungry; however, too much fat is generally unhealthy and is likely to cause an animal to feel satisfied. The hormone leptin helps maintain an appropriate amount of fat reserves in the body.

Rather than being secreted from a specialized gland, leptin is produced by adipocytes in proportion to their number and size. More and larger adiopcytes means more leptin; fewer and smaller adipocytes means less leptin. Leptin levels are detected by sensors in the hypothalamus. High lepin levels suppress appetite and speed up metabolism, while low levels of leptin stimulate hunger and slow down metabolism, resulting in a negative feedback loop. These activities are mediated through signaling from the hypothalamus-pituitary axis to the thyroid, which plays a major role in regulating metabolic function.

In response to high levels of leptin, the hypothalamus releases thyrotropin-releasing hormone, signals to the anterior pituitary to release thyroid-stimulating hormone. The thyroid the releases thyroxine, also known as tetraiodothyronine or T4, and triiodothyronine, also known as T3. These hormones affect nearly every cell in the body except for the adult brain, uterus, testes, blood cells, and spleen. T3 and T4 activate genes involved in energy production and glucose oxidation, resulting in increased rates of metabolism and body heat production which together cause an increased rate of caloric usage. Low levels of leptin cause the opposite response, leading to a decreased metabolic rate to conserve energy.

This video describes how the thyroid manages metabolic processes:

Growth and Metamorphosis

In vertebrate species that undergo metamorphosis, such as amphibians, surges of T3 are responsible for initiating development of new structures, reorganization of  internal organ systems, and other processes that occur during metamorphosis. In insects, metamorphosis is controlled by a set of hormones that determine whether the animal grows into the next larval stage or changes into an adult as it gets larger. The corpus allatum, an endocrine gland in the brain, secretes a hormone called juvenile hormone during all larval stages, which maintains the larval status of the animal. As the larvae grows, another endocrine gland in the brain releases prothoracicotropic hormone, which signals to the prothoracic gland to release the hormone ecdysone. Ecdysone promotes either molting (shedding the exoskeleton) or metamorphosis, depending on the level of juvenile hormone. Ecdysone in combination with high juvenile hormone results in molting into the next larval stage; ecdysone in combination with low juvenile hormone results in metamorphosis into an adult.

Stress: Short vs Long Term Responses

One of the main functions of endocrine hormones is to ensure the body’s internal environment remains stable (homeostasis). Stressors are stimuli that disrupt homeostasis. Some stressors require immediate attention and activate the short term, “fight-or-flight” stress response, which stimulates an increase in energy levels through increased blood glucose levels. This prepares the body for physical activity that may be required to respond to stress: to either fight for survival or to flee from danger. The fight-or-flight response exists in some form in all vertebrates.

In contrast, some stresses, such as illness or injury, can last for a long time. Glycogen reserves, which provide energy in the short-term response to stress, are exhausted after several hours and cannot meet long-term energy needs. If glycogen reserves were the only energy source available, neural functioning could not be maintained once the reserves became depleted due to the nervous system’s high requirement for glucose. In this situation, the body has evolved a response to counter long-term stress through the actions of the glucocorticoids, which ensure that long-term energy requirements can be met. The glucocorticoids mobilize lipid and protein reserves, stimulate gluconeogenesis, conserve glucose for use by neural tissue, and stimulate the conservation of salts and water.

The sympathetic nervous system regulates the stress response via the hypothalamus. Stressful stimuli cause the hypothalamus to signal the adrenal medulla (which mediates short-term stress responses) via nerve impulses, and the adrenal cortex, which mediates long-term stress responses, via the hormone adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary.

Short-term Stress Response

When presented with a stressful situation, the body responds by calling for the release of hormones that provide a burst of energy. The hormones epinephrine (also known as adrenaline) and norepinephrine (also known as noradrenaline) are released by the adrenal medulla. These two hormones prepare the body for a burst of energy in the following ways:

  • cause glycogen to be broken down into glucose and released from liver and muscle cells
  • increase blood pressure
  • increase breathing rate
  • increase metabolic rate
  • change blood flow patterns, leading to increased blood flow to skeletal muscles, heart, and brain; and decreased blood flow to digestive system, skin, and kidneys

Watch this Discovery Channel animation describing the flight-or-flight response.

Long-term Stress Response

Long-term stress response differs substantially from short-term stress response. The body cannot sustain the bursts of energy mediated by epinephrine and norepinephrine for long times. Instead, other hormones come into play. In a long-term stress response, the hypothalamus triggers the release of ACTH from the anterior pituitary gland. The adrenal cortex is stimulated by ACTH to release steroid hormones called corticosteroids. The two main corticosteroids are glucocorticoids such as cortisol, and mineralocorticoids such as aldosterone. These hormones mediate the long-term stress response in the following ways:

  • glucocorticoids:
    • promote breakdown of fat into fatty acids in the adipose tissue and release into bloodstream for ATP production
    • stimulate glucose synthesis from fats and proteins to increase blood glucose levels
    • inhibit immune function to conserve energy
  • mineralcorticoids:
    • promote retention of sodium ions and water by kidneys
    • increase blood pressure and volume (via sodium/water retention)

Coticosteriods are under control of a negative feedback loop (illustrated below), which can become mis-regulated in cases of chronic long-term stress.

Diagram of physiologic negative feedback loop for glucocorticoids. Image credit: DRosenbach [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

In contrast to chronic long-term stress, the video below discusses some of the circumstances where stress can be good for you: