Beta-Blockers

Like all nervous system transmitter chemicals and many hormones, norepinephrine and epinephrine exert

their effects by interacting with proteins on the target cell's outer surface. Scientists refer to the ones on which epinephrine and norepinephrine act as adrenergic receptors, and group them into two major classes. These classes are formally known as a- and b-adrenergic receptors. However, many medical articles use the short forms "alpha receptors" and "beta receptors," respectively.

The most fundamental distinction between alpha and beta receptors is their response or lack of response to specific synthetic chemicals. They also respond differently to their natural stimuli: alpha receptors are more responsive to norepinephrine than to epinephrine, while beta receptors respond equally to both.

Some cell types carry both alpha and beta receptors, while others carry only one, or neither. The two classes of receptors often have opposite effects. This allows the body to "fine-tune" its response by varying the relative amounts of circulating epinephrine and locally released norepinephrine in different tissues. In the circulatory system, however, both alpha and beta receptors raise blood pressure. Nevertheless, they do so in different ways: alpha receptors by constricting the blood vessels, beta receptors by increasing the force and rate of the heartbeat.