Why Adrenaline Fades in a Minute and Cortisol Lingers: How Hormones Deliver Messages

Steroid hormones such as cortisol last much longer in the blood than amino acid-derived ones. Epinephrine, derived from an amino acid, has a half-life of approximately one minute, the time required for half of the hormone to be degraded. Cortisol, derived from a lipid, has a half-life of approximately 60 to 90 minutes. Because lipid-derived hormones are not water-soluble, they travel bound to a transport protein, which extends their half-life.

Two families, three classes

A hormone travels the whole body in the blood but acts only on target cells that have a receptor for it. Once it binds, a chain of events leads to the cell's response, and those responses drive reproduction, growth, metabolism, fluid and electrolyte balance, sleep and more. By structure, hormones fall into two groups: those derived from amino acids, which are amines, peptides and proteins, and those derived from lipids, the steroids.

Because steroids do not dissolve in water, and blood is water-based, they must ride in the blood bound to a transport protein. That bulkier form is what extends their half-life well beyond that of amino acid hormones.

Amine, peptide, protein and steroid hormone structure (Figure 17.3, OpenStax, CC BY 4.0).

Two ways in

A receptor is a protein, inside the cell or in its membrane, that recognizes molecules of a specific shape and responds only to the hormones it knows. The same receptor can sit on cells in different tissues and trigger different responses, so the response depends on the target cell as well as the hormone.

Inside the cell. Steroid hormones can diffuse through the lipid membrane. The hormone binds a receptor in the cytosol or nucleus, and the pair moves to the chromatin and binds to DNA, which makes the cell transcribe a gene into mRNA and make a protein. Thyroid hormones, which are lipid-soluble, cross the membrane by a carrier that needs energy and sodium, and bind receptors already attached to DNA.

Binding of lipid-soluble hormones (Figure 17.4, OpenStax, CC BY 4.0).

At the surface. Water-soluble hormones cannot cross the membrane, so they bind a receptor on its outer surface; the hormone is the "first messenger" and a "second messenger" carries the signal inside. The commonest second messenger is cyclic AMP (cAMP). The receptor activates a G protein, which activates an enzyme, adenylyl cyclase, that turns ATP into cAMP; cAMP activates protein kinases, which start a cascade of phosphorylation across many proteins. Thousands of signaling events can begin at once from a very low hormone concentration, but the signal is short because the enzyme phosphodiesterase quickly deactivates cAMP. Calcitonin, glucagon and thyroid-stimulating hormone use cAMP. Some G proteins lower cAMP instead: when somatostatin binds in the pituitary, the level of cAMP falls and growth hormone secretion is inhibited. An alternative system uses calcium ions as the second messenger, via the enzyme phospholipase C, which splits a phospholipid into DAG and IP3; IP3 frees calcium from storage in the cell. Angiotensin II, which narrows blood vessels, and growth hormone-releasing hormone use it.

Binding of water-soluble hormones (Figure 17.5, OpenStax, CC BY 4.0).

What changes the response

A cell exposed to a lot of a hormone can reduce its receptors (downregulation) and become less reactive; with a chronically low level, it adds receptors (upregulation) and becomes more sensitive. Hormones also interact in three common ways. In a permissive effect, one hormone lets another act: a dietary iodine deficiency, which hits thyroid hormones, can affect reproductive development. In a synergistic effect, two hormones with similar effects amplify each other, as FSH and estrogens are both needed for eggs to mature. In an antagonistic effect they oppose each other: insulin stores glucose as glycogen in the liver and lowers blood glucose, while glucagon breaks glycogen down and raises it.

Keeping levels in check

Most hormone secretion is governed by feedback. In a positive feedback loop, a hormone's release leads to more of it: oxytocin during childbirth makes the uterus contract, which stretches the cervix, which signals the pituitary to release more oxytocin, until the birth. The more common negative feedback shuts secretion off once levels are adequate. In the adrenal glucocorticoid loop, the hypothalamus and pituitary signal the adrenals to release glucocorticoids, and when the blood level rises they cut their signaling back.

A negative feedback loop for glucocorticoid hormones (Figure 17.6, OpenStax, CC BY 4.0).

Glands respond to three kinds of stimulus. Humoral stimuli are changes in blood chemicals: high blood osmolarity makes the hypothalamus release ADH, so the kidneys reabsorb water, and high glucose releases insulin. Hormonal stimuli arise when one gland's hormone triggers another, often through the hypothalamus. Neural stimuli include danger, which makes sympathetic neurons tell the adrenals to release norepinephrine and epinephrine, which widen blood vessels, speed heart and breathing rates and suppress the digestive and immune systems.

When something mimics a hormone

Bisphenol A (BPA), used in hard plastics and epoxy resins, including can linings, food containers and baby bottles, is thought to be an endocrine disruptor. It mimics the effects of estrogens and has the opposite effect of androgens. The FDA says traditional toxicology studies support the safety of low exposure, but newer studies raised concern about the brain, behavior and prostate in fetuses, infants and young children. Canada and the European Union banned BPA in baby products; some experts caution that some studies may be flawed. The FDA advises avoiding bottles with recycling code 3 or 7 and not microwaving food in any plastic.

Sources

• J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble and Peter DeSaix, Anatomy and Physiology, section 17.2, Hormones, OpenStax (Rice University), under CC BY 4.0. Changed here: condensed and rewritten in hubnx's own words, with the tables, learning objectives and some figures left out; rewritten in hubnx's own words.