Regulation of Release

Hormone release is controlled by three general mechanisms:

1. Secretion can be regulated by another endocrine gland. As explained above, the anterior pituitary gland releases tropic hormones that control the release of other hormones.
2. Secretion can be regulated by some other substance in the blood. Insulin secretion is regulated by the amount of glucose in the blood.
3. Secretion can be regulated by the nervous system. Epinephrine release from the adrenal gland is controlled by the autonomic nervous system.

Regardless of the mechanism of control, hormone release is typically regulated through negative feedback loops (Module 1: Bio 264). For example, if blood glucose levels increase, insulin secretion will increase which will lower the levels of glucose in the blood. As glucose levels return toward normal insulin secretion will decrease.

Transport: Endocrine hormones are transported to their target tissues via the blood plasma. Since the plasma is mostly water this presents a problem for some hormones. Hydrophilic hormones easily dissolve in the plasma and pose no major problem for transport. Hydrophobic hormones, on the other hand, cannot dissolve in the plasma and so this presents a challenge for their transport. These hormones must bind to plasma proteins produced by the liver in order to be transported. These "carrier" proteins have hydrophobic cores that shield the hormones from the aqueous plasma. However, even for these hydrophobic hormones a small fraction does circulate free in the plasma. We distinguish between the two forms as "bound" and "free" hormones. It is the ratio of bound to free that determines the overall potency of the hormone. The free hormone is the biologically active form and as the levels of free hormone drop, more bound hormones will be released from the transport proteins to maintain the equilibrium ratio of bound to free.

Lifespan of a hormone: The only way to turn off an endocrine response is to remove the hormone from the circulation. There are several mechanisms for removing the hormones. These include: removal by the kidneys, removal by the liver, enzymatic destruction of the hormone, and re-uptake and recycling of the hormone. Steroid hormones, for example, are usually removed from the blood by the liver while protein hormones often end up in the urine. We express the lifespan of a hormone in the blood as its half-life (T ½). Recall that one half-life is the amount of time necessary to remove 50% of the hormone from the circulation. Half-lives for hormones range from a few minutes to several days. The concentration in the blood of hormones with short half-lives tend to fluctuate markedly while the concentrations of hormones with longer half-lives tend to be more constant. Since hormones bound to carrier molecules are shielded from the mechanisms that remove them from the blood their half-lives tend to be longer and their concentrations do not fluctuate as rapidly.

The concentrations of hormones in the blood are typically very low, ranging from 10^-11 to 10^-9 moles/Liter. Thus, hormone receptors must have very high affinities for their particular hormone. The affinity of a hormone for its receptor is a measure of how easily and strongly it binds to the receptor. Hormone affinities are expressed as the dissociation constant (K_D) for the hormone. The units of the dissociation constants are molar units (M) and correspond to the concentration of the hormone required to bind exactly one-half of available receptors. The lower the K_D number the higher the affinity of the receptor to the hormone.

There are many different hormones in the body, and making an accurate count is nearly impossible as new hormones are discovered every year. However, there are general groupings that can help distinguish the characteristics of the vast arrays of hormones. One common method of classifying hormones is based on their chemical structure, this type of classification results in three main classes of hormones: peptide/protein hormones, steroid hormones, and amino-acid derived hormones. The hormones within each class have similar functional properties. The table below describes the characteristics of each class of hormones based on the 5 properties defined below.

1. Synthesis: How is the hormone synthesized? It may be produced on demand or stored for later release.

2. Mode of release: Is the hormone released from vesicles through exocytosis or simply produced and allowed to diffuse out of the cell.

3. Transport: How is the hormone transported in the blood? It may circulate free or it may be bound to carrier proteins.

4. Half-life: How long does the hormone circulate in the blood before being broken down. It may be broken down quickly or it may stay in circulation for hours or even days.

5. Receptor: Which kind of receptors does it interact with (See Table below)

Classes of Hormones

As the name suggests, peptide/protein hormones are produced from amino acids and range in size from three amino acids to hundreds of amino acids. This is the most diverse and abundant type of hormone group and is made by tissues located throughout the body. Steroid hormones are always derived from cholesterol (and therefore hydrophobic) and are made by only a few organs, specifically the gonads and adrenal cortex. Hormones categorized as amino-acid derived are created from the amino acid tyrosine (catecholamines and thyroid hormones) or tryptophan (melatonin and serotonin). The tables below list the major endocrine glands, the hormones they produce and the major actions of the hormones.

Major Endocrine Glands and the Hormones They Secrete