Endocrine Metabolism I

Hormones are usually defined as messengers that are transported by the blood to distal target cells. Because they are released into the interstitial space and then into the blood, they are called “endocrine” secretions to distinguish them from those that are released into the external environment. Chemically, there are four types of hormones: (1) amino acid derived amines, (2) peptides, (3) proteins or glycoproteins, and (4) steroids or eicosanoids. The chapter discusses the feedback regulation of an endocrine system that involves both simple feedback loops (for example, insulin secretion regulated by plasma glucose levels) and complex feedback loops (for example, hypothalic, pituitary, thyroid in the secretion of thyroxine). Some of the feedback loops can be positive. The physiological response to a hormone is determined by the presence of a specific receptor at the target cell. The receptors may be located on the plasma cell membrane, in the cytosol, or in the nucleus. It is noted that hormone recognition and binding at their specific receptor binding site initiates the signal transduction amplification pathways that culminates in an appropriate biological response.