Hormones and Receptors

Question 1

What is the pathway of synthesis of peptide/protein hormones?

Answer 1

The secretion of peptide and protein hormones follows the classical pathway for secretion of protein from cells. After synthesis as a pre-prohormone on ribosomes from their respective mRNAs, the hormone is targeted to the rough endoplasmic reticulum. Here the pre-prohormone is cleaved and the prohormone is transported to the Golgi apparatus where it is further processed and packaged into secretory vesicles.

Question 2

Name four tyrosine derivative hormones

Answer 2

Epinephrine Norepinephrine Dopamine Thyroxine

Question 3

Name six steroid hormones

Answer 3

Testosterone Cortisol Estrogen Aldosterone Vitamin D Progesterone

Question 4

Name five peptide hormones

Answer 4

Oxytocin Vasopressin (ADH) Angiotensin Thyrotropin Releasing Hormone Gonadotropin Releasing Hormone

Question 5

Name six protein hormones

Answer 5

Insulin Glucagon Growth hormone ACTH Prolactin Thyroid Stimulating Hormone

Question 6

Name three hormones that regulate water and mineral metabolism

Answer 6

Vitamin D Aldosterone Vasopressin

Question 7

Name three hormones that regulate energy metabolism

Answer 7

Insulin Glucagon Cortisol

Question 8

Name three hormones that regulate reproduction

Answer 8

Estrogen Testosterone Progesterone

Question 9

Name three hormones that regulate growth

Answer 9

Growth hormone Testosterone Estrogen

Question 10

How do peptide and protein hormones reach their targets, what are the exceptions, and what regulates their half-lives?

Answer 10

Most peptide and protein hormones freely diffuse in the blood to their target organ as free hormones. The exceptions are Growth Hormone, Prolactin, and Insulin-like Growth Factor. The half-life of these proteins is limited by the many proteases in the blood stream.

Question 11

How are the carbons in steroid hormones numbered?

Answer 11

Question 12

How are steroid hormone synthesized and released into the blood stream?

Answer 12

Steroid hormones are synthesized from cholesterol precursors. They are strongly hydrophobic and diffuse freely through cell membranes. They are not stored in vesicles.

Question 13

What are the consequences of steroid hormone’s structure on blood transport and half-life?

Answer 13

The hydrophobicity of steroids requires them to be transported by carrier proteins in the blood. In the blood, steroids exist in equilibrium between the bound and free forms and typically less than 5% of the steroid is present in the free state at any one time. However, steroids are only active in their free form, thus the total body content of a steroid may not be informative. The bound form essentially serves as a reserve of the steroid. Steroids also persist in the blood longer than peptide/protein hormones, with halflives on the order of hours to days.

Question 14

What are two methods for measuring hormone levels and how do they work?

Answer 14

Bioassays:

Bioassays measure hormone activity and in this case hormone function is measured by using an exogenous system e.g. cell lines, to measure hormone activity.

Immunoassays:

Radio-immunoassays (RIA) and enzyme linked immunosorbent assays (ELISA) measure antibody binding to a specific region of the hormone. They might not be useful if an abnormal form of the hormone is being secreted by the patient.

Question 15

What sort of effects can peptide, protein, and tyrosine derivative hormones have?

Answer 15

Activation of catecholamine, protein and peptide hormones can have rapid consequences, like increased cytosolic calcium, exocytosis, phosphorylation of enzymes and ion channels. In addition, they can have effects that are slower and involve changes in gene expression.

Question 16

Where do peptide and protein hormones bind to cause their effects?

Answer 16

When protein and peptide hormones (as well as some of the hormones derived from tyrosine, like epinephrine and norepinephrine) reach their target, they bind to specific receptors on the plasma membrane of the target cells.

Question 17

Where do tyrosine derivative hormones bind and what effects can they induce?

Answer 17

Receptors for epinephrine and norepinephrine belong to the family of G-protein coupled receptors. Binding of the hormone to these receptors results in changes in the levels of intracellular second messengers like cAMP, diacylglycerol and inositol phosphates.

Question 18

What type of receptors do growth hormone and prolactin bind to and what effects do they produce?

Answer 18

Hormones like growth hormone and prolactin have receptors that belong to the JAK/STAT family of receptors. Activation of these receptors results in coupling and activation of a tyrosine kinase (Janus kinase or JAK), which then causes the phosphorylation of a group of proteins called signal transducers and activators of transcription (STATs).

Question 19

What type of receptors do insulin and IGF-1 bind to and what effects can they induce?

Answer 19

Receptors for hormones like Insulin and IGF-1 belong to a large family of protein tyrosine kinase receptors. In this case the receptors themselves are tyrosine kinases that can be activated upon hormone binding.

Question 20

What receptors do steroid hormones bind to and what effects can they induce?

Answer 20

Unlike the peptide hormones, steroid receptors are nuclear in their location. Once steroid hormones reach their target cells, they enter the cell and bind to their receptors in the cytosol or the nucleus. The receptor-hormone complexes then bind to specific hormone responsive elements (HRE) and activate transcription of specific genes.

Question 21

How are hormone levels typically regulated in the body?

Answer 21

Hormone secretion often occurs in a pulsatile fashion and the levels of many hormones are regulated by diurnal variation and thus show characteristic circadian rhythms. Levels of various hormones, metabolites, and minerals in the body are also very tightly regulated around a specific set point. This is achieved mainly by feedback loops. There are two classes of these feedback loops. One where the hormone level is the regulated variable and the other where the plasma concentration of a metabolite or a mineral acts as the regulated variable. Nearly all biological feedback systems are negative feedback loops.

Question 22

Where are positive feedback loops found in endocrine signaling?

Answer 22

Purely positive feedback loops are rare in biology because a positive loop will lead to an unstable and often cataclysmic process. The only way a positive feedback loop can be terminated is by the exhaustion of the hormone or by an explosive event such as ovulation. An example of a positive feedback loop in endocrinology is the production of oxytocin during the birthing process. The stimulus for oxytocin secretion is the dilation of the uterine cervix. This dilation causes the release of further oxytocin thus creating a positive feedback loop that is terminated by the expulsion of the fetus and relaxation of the uterine muscles.

Question 23

What is the feedback pathway of T3 and T4 and what regulates this feedback?

Answer 23

The hormone level is the regulated variable in the Thyroid hormone feedback loop.

Question 24

What is the feedback pathway of insulin and glucagon and what regulates this feedback?

Answer 24

Blood glucose is the regulated variable in this pathway