Avian hormones and the control of metabolism

Introduction Hormones are generally defined as substances released into the bloodstream by specialised tissues, known as endocrine glands, and which interact with distant target tissues where they exert their effect. The general awareness of hormones as regulatory molecules dates from the early part of this century. More recently other types of regulatory molecule have been discovered that are released from one type of cell to diffuse through the intercellular fluid and exert their effects on other types of cell. Because of their different transport route these are referred to as paracrine secretions. A third group of substances are released by one type of cell and exert their effects on the same cell type. These are referred to as autocrine secretions. Endocrine secretions are, therefore, just one type of a more general intercellular messenger. This chapter is largely restricted to the role of endocrine secretions, as defined above, in controlling metabolism (topics covered in Chapters 2 to 6). Paracrine secretions are discussed in connection with morphogenesis in Chapter 12, paracrine and autocrine secretions in connection with oncogenesis in Chapter 11, and steroid hormones in Chapter 10. Current understanding of the mechanism of action of hormones suggests that there are two general mechanisms operative. In the first, the hormone or first messenger (the intercellular messenger) interacts with receptors on the cell surface of the target tissue, activating the release of the second messenger (intracellular mesenger), which in turn interacts with various intracellular enzymes to modulate intracellular reactions.