ABSTRACT
Hormone receptor affinity and number are not static. Genetics, age, cell cycle stage, degree of differentiation, and the physiologic or pathophysiologic state are important variables. For example, ionic balance and temperature can affect receptor concentration and affinity, as can antibodies against the receptor or the concentration of homologous and heterologous hormones. The number of functional receptors is modulated up or down regularly, permitting target cells to respond optimally to small changes in hormone concentrations. Prolonged hormone exposure usually results in reduced target cell functional expression, since cells become desensitized. Down-regulated receptors can 1) be destroyed after endocytosis, 2) be internalized and remain stored in intracellular vesicles, 3) remain on the cell surface but bind hormones inappropriately, or 4) form receptor-hormone complexes that induce sub-optimal responses. Conversely, in developmental conditions, the first contact of hormones with their target cells usually results in receptor up-regulation, with accelerated binding and functional expression within target cells.
