One of the greatest biologic advances was the discovery of the chemical transmission of information in living organisms. Endocrine glands communicate with each other and with other tissues by means of hormones. For example, the anterior pituitary makes, stores, and releases as necessary the hormone ACTH into the blood stream. Cortical cells of the adrenal gland detect the circulating ACTH and respond by releasing the hormone hydrocortisone into the blood stream. Motor nerve ends communicate in a similar manner with the effector cells that they innervate and control. In these chemical information transmission systems, there are three essential parts:(1) the biosynthesis, storage, and release of the transmitter or hormone,(2) the environment into which the transmitter is released in order to get to the effector cells, and (3) the receiving portion of the effec-tor, the receptor that detects the transmitter and initiates some characteristic response.

For the autonomic nervous system this process was foreseen by Elliott and Dale at the turn of the century. In the 1920s the chemical substance acetylcholine was shown by the Nobel laureate Otto Loewe to be one of the motor nervous system transmitters. At first, epinephrine was thought to be the adrenergic transmitter; yet it did not quite fit. Euler proposed norepinephrine as the transmitter. Although he also won the Nobel Prize, some of his ideas required modifi-cation. The present essay is concerned in general with the adrenergic transmission system, and in particular with the adrenergic receptive mecha-