[LEC] Part 1 Endocrine System

What are the two types of glands?

Exocrine glands - deliver their products through ducts and their effects are extracellular

Endocrine glands - ductless, effects occur intracellularly. They affect the metabolism of cells

Endocrine glands secrete hormones directly into body fluids

What does the endocrine system do?

It helps regulate specific metabolic activities within specific cells or tissues, using specific chemical messengers called hormones

What are tropic hormones?

Anterior pituitary hormones whose target organs are other endocrine glands

Name some tropic hormones and detail them

FSH and LH - gonadotropins, whose target is the gonads

TSH - stimulates the growth of the thyroid gland and secretion of its hormones

ACTH - adrenocorticotropic hormone

• stimulates the growth of the adrenal cortex and secretion of its hormones

• induces fat catabolism in adipose tissue

• insulin secretion from the pancreas

Tropic hormones are stimulating hormones that cause other endocrine clands to secrete their hormones

Endocrine glands

Ductless glands of specialized epithelial tissues

Endocrine glands secrete hormones directly into body fluids

Hormones

Chemical messengers secreted into the bloodstream that affect metabolism of target cells which are usually located in another organ

• Potent steroid or non-steroid molecules that act on specific receptor sites (proteins) located on or in specific target cells or tissues

• Secreted by endocrine gland or cell

Endocrine glands secrete hormones directly into body fluids

What are the three classes of hormones?

Steroid hormones - s*x hormones or adrenal cortex hormones

HYDROPHOBIC

Monoamines - made from amino acids and retain one amino group

HYDROPHILIC

Peptide hormones - consist of chains of 3 to 200 or more amino acids

HYDROPHILIC

Steroid hormones

Either s*x hormones or adrenal cortex hormones made from cholesterol or one of its lipid derivatives

S*x hormones

• Testosterone

• Estrogen

• Progesterone

Adrenal cortex hormones

• Aldosterone

• Cortisol

Their mode of action is intracellular

What is the detailed mode of action of steroid hormones?

1. Hormone is escorted to target cells by specific transport protein

2. Hormone passes through the plasma membrane of a cell

3. Hormone combines with receptor protein within the cytoplasm or the nucleus

4. Hormone that was received then acts on the genes contained within the nucleus

5. Specific metabolic changes take place as a result of the activation or inhibition of the transcription of a gene for a metabolic enzyme or other protein

SUMMARY: A hormone passes through the plasma membrane of a cell and acts on the genes contained within the cell. The hormone determines whether or not that gene will be expressed, or whether or not the protein corresponding to that gene is created

Monoamines [Non-Steroid Hormones]

Made from amino acids

• Retain one amino group

• Typically enter the cell via a second messenger

Their mode of action is extracellular

Peptide hormones [Non-Steroid Hormones]

Consist of chains of 3 to 200 or more amino acids

• Typically enter the cell via a second messenger

Their mode of action is extracellular

What are the similarities between monoamines and peptide hormones?

Their modes of action are both extracellular, and they typically both enter the cell via a second messenger

What is the detailed mode of action of monoamines and peptide hormones?

1. Hormone is escorted to target cells by specific transport protein

2. Hormone combines with receptor site on target plasma membrane

3. Hormone-receptor complex activates adenylate cyclase

4. Adenylate cyclase converts ATP into cAMP

5. cAMP activates protein kinases (enzymes)

6. Protein kinases activate specific intracellular enzymes

7. Intracellular enzymes promote specific metabolic effects within the cell

SUMMARY: A hormone attaches to the surface of a plasma membrane but does NOT pass through. It activates a hormone-receptor complex that triggers the activation and transformation of several substances into other substances

Activation of adenylate cyclase → Adenylate cyclase converts ATP into cAMP → cAMP activates protein kinases → Protein kinases activate specific intracellular enzymes

What are important aspects to note about hormones?

• No hormone travels in the bloodstream alone

• No cell is exposed to only one hormone

• There are several hormones in the blood and tissue fluid at all times

• Most cells are not affected by the majority of present hormones because they have no receptors for them

• Most cells are sensitive to more than one hormone

What are the three kinds of interactive effects that hormones can have?

Synergistic effects - two or more hormones act together to produce an effect that is greater than if they were acting each on their own

Permissive effects - one hormone enhances the target organ’s response to a second hormone that is secreted later

Antagonistic effects - one hormone opposes the action of another

Negative feedback inhibition

The most common form of feedback

• Anterior pituitary gland stimulates and causes another endocrine gland to secrete its hormone

• The levels of the hormone released by the other endocrine gland feed back to the pituitary or the hypothalamus and inhibits further secretion from the pituitary hormone

Describe what occurs in the hypothalamo-pituitary-thyroid axis (a negative feedback loop)

1. The hypothalamus secretes thyrotropin-releasing hormone (TRH)

2. TRH stimulates the anterior pituitary gland to secrete thyroid-stimulating hormone (TSH)

3. TSH stimulates the thyroid gland to secrete thyroid hormone (TH)

4. TH stimulates the metabolism of most cells throughout the body

5. TH also inhibits the release of TSH by the pituitary

6. To a lesser extent, TH also inhibits the release of TRH by the hypothalamus

SUMMARY: Hypothalamus releases TRH → TRH causes TSH to release → TSH causes TH to release → TH stimulates metabolism of most cells in the body → TH also inhibits the release of more TSH and some TRH

Steps 5 and 6 showcase how this is a negative feedback loop, because its main purpose is to regulate levels of specific materials within the body, specifically all hormones listed (TRH, TSH, TH)

Describe what occurs during childbirth (a positive feedback loop)

1. Uterine stretching during childbirth sends a nerve signal to the brain that stimulates oxytocin (OT) release

2. OT stimulates uterine contractions, which push the infant downward

3. Stretching of the cervix causes more nerve signals to fire resulting in the release of more oxytocin

4. Feedback cycle continues until the infant is born

Prostaglandins

Tissue hormones derived from fatty acids

• Locally acting messenger molecules

• Differ from hormones because they are produced at several different locations throughout the human body

• Their target cells are present in their immediate vicinity; their destination is always close by

There are two types of prostaglandins, including paracrine signals and autocrine signals.

Paracrine - locally acting signals

Autocrine - signals that act on the same cell from which it was synthesized (effects are intracellular)

The secretion of hormones is primarily regulated by what type of feedback loop? Describe how this type of feedback loop occurs?

Hormone secretion is primarily regulated by negative feedback loops

Negative feedback loops regulate hormones by halting production of more hormones when levels are too high, and beginning production of more hormones when levels are too low; hormone levels oscillate around a set point

Describe two ways that the hypothalamus regulates hormone secretions within the body

1. Hypothalamus receives nerve impulse → hypothalamus stimulates the posterior pituitary gland → posterior pituitary gland releases specific hormones, oxytocin, and ADH

2. Hypothalamus unleashes releasing hormones to the anterior pituitary gland → anterior pituitary gland releases various hormones → various organs receive these hormones → various metabolic changes occur

What hormones are specific to being produced by the hypothalamus? What hormones are specific to being produced by the anterior pituitary gland?

Releasing hormones are a specific class of hormones only produced by the hypothalamus

Stimulating hormones are a specific class of hormones only produced by the anterior pituitary gland

Releasing hormones, which originate from the hypothalamus, stimulate the anterior pituitary gland