Cells are typically exposed to many different types of signaling molecules. What determines the response of the cell? In part, that depends on the types of receptor present on the surface of the cell. These receptors determine which signals the cell is able to respond to. The response of the cell also depends on the set of proteins that is found in it, as different cell types have different sets of intracellular proteins and signaling pathways. As a result, the same signaling molecule can have different effects in different types of cells.

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In the case of the heart, activated PKA leads to the opening of calcium channels that are present in heart muscle cells. The resulting influx of calcium ions results in shorter intervals between muscle contractions and thus a faster heart rate. As long as adrenaline is bound to its receptor, the heart rate remains rapid. This increase in the heart rate in turn results in increased blood flow to the brain and skeletal muscles to deal with the stress that set off the signal in the first place.

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This example is typical of signaling through G protein-coupled receptors. These receptors tend to activate downstream enzymes or, in some cases, open ion channels. Because they often modify proteins that are already synthesized in the cell, their effects tend to be rapid, short-lived, and easily reversible, as we will see next.