Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intracellular and Intercellular Communication
Characteristics of the Endocrine System
The Target Gland Concept

The Human Body contains about 200 types of differentiated Cells. Only a few of them produce Hormones, yet all 75 trillion cells in the human body serve as targets for one or more of the 50 known hormones. A hormone may target a single tissue or multiple Tissues. According to the classical definition, a target tissue is one in which a hormone elicits a specific biochemical or physiological response. For example, The Thyroid Gland is a specific target gland for TSH; under METABOLISM/18.html">The Influence of TSH, the number and size of thyroid acinar cells increase, and The rate of all stages of thyroid hormone Biosynthesis accelerates. In contrast, Insulin acts on many tissues: in Muscle, it increases glucose uptake and oxidation; in adipose tissue, it stimulates Lipogenesis; in The Liver and lymphocytes, it promotes Amino Acid Transport; and in the liver and muscle, it enhances Protein Synthesis, and so on. Later, with the characterization of Cell surface and intracellular receptors, THE CONCEPT OF a "target tissue" was extended to all tissues where a given hormone binds to its receptor, regardless of whether a classical biochemical or physiological tissue response to the hormone has been identified (e.g., insulin binding to endothelial cells). This definition is also inadequate, yet it remains useful because it implies that certain hormonal effects may still be undiscovered.

The overall response of a target tissue to a hormone is determined by a variety of factors. First and foremost is the local concentration of the hormone near the target tissue, which depends on: 1) the rate of hormone Synthesis and Secretion; 2) the anatomical proximity of the target tissue to the hormone source; 3) the association and dissociation constants of the hormone with a specific plasma carrier protein, if present; 4) the rate of conversion of an inactive or weakly active hormone form into the active one; and 5) the rate of hormone clearance from the Blood through degradation or excretion, primarily performed by the liver and Kidneys. The actual tissue response is determined by: 1) the relative activity and/or occupancy state of specific Hormone Receptors on The Plasma Membrane or within the Cell Cytoplasm or Nucleus; and 2) the state of cellular sensitization or desensitization, which depends on post-receptor mechanisms. A change in any of these parameters can affect the hormone's action on a given target tissue, a factor that must be taken into account when evaluating classical concepts of feedback-mediated hormonal regulation.