One hormone, many responses

Epinephrine, the fight-or-flight hormone, reaches nearly every tissue through the blood. Yet the heart speeds up, the airways relax, and the pupils widen: each cell type answers the same hormone in its own way. The explanation lies in which proteins each cell makes.

Epinephrine around the body

Endocrine cells release epinephrine into the blood during danger and also during exercise. From there it coordinates the whole body’s preparation for intense physical activity.

Caffeine strengthens some of these effects. In skeletal muscle, epinephrine works by raising the level of cAMP, a small signaling molecule inside the cell. An enzyme called phosphodiesterase normally clears cAMP away by converting it to AMP once the hormone is gone, and caffeine inhibits phosphodiesterase.

With the cleanup enzyme blocked, cAMP lingers. Epinephrine’s effects last longer, and even a small amount of epinephrine has a bigger effect. That helps explain the alert, energized feeling coffee and energy drinks can give you.

Same genes, different proteins

Your heart cells and lung cells carry the same complete set of genes, called the genome. But cells don’t use all of their genes: heart cells switch on some genes that lung cells keep off, and the reverse.

Turning different genes on in different cell types is differential gene regulation. It gives each cell type its own set of proteins, its proteome.

Five ways cell types differ

The proteins a cell makes decide how it answers a signal. A few reminders before the details: a G-protein-coupled receptor (GPCR) passes its signal to a G protein inside the cell, a ligand-gated ion channel is a receptor that opens to let ions through, and a receptor with a lower Kd binds its signal more tightly. Cell types can differ at five points:

Check yourselfSkeletal muscle cells and heart muscle cells both have receptors for acetylcholine, yet they respond to it differently. Why?

They make different kinds of acetylcholine receptor. Skeletal muscle cells have a ligand-gated ion channel, while heart muscle cells have a GPCR, so acetylcholine activates a different pathway in each.

Check yourselfWhy can caffeine make a small amount of epinephrine act like a larger amount?

Caffeine inhibits phosphodiesterase, the enzyme that breaks down cAMP. The cAMP made in response to epinephrine persists longer, so the pathway stays active and the response is stronger and longer lasting.