21.6: Target Cell Response to Hormones
Hormones intricately bind to receptors on the surface or within target cells, initiating a cascade of cellular responses.
Notably, the cellular response can be regulated by altering the number of receptors expressed in the cell. For example, prolonged exposure to elevated hormone levels results in a gradual decline or down-regulation in the number of receptors for that specific hormone on the cell surface. Conversely, in response to low hormone levels, cells may use up-regulation, producing an increased quantity of a particular receptor to enhance cellular sensitivity.
The interplay between hormones can lead to diverse cellular responses. Certain hormones exhibit permissive interactions, where the presence of one hormone enables another to exert its effects synergistically. An example is the collaboration between epinephrine and thyroid hormones, where thyroid hormones facilitate the effective stimulation of lipolysis by epinephrine in target cells. Furthermore, hormones like follicle-stimulating hormone and testosterone demonstrate synergistic action, working in concert to elicit an amplified response, such as the normal production of sperm. On the contrary, some hormones engage in antagonistic relationships, resulting in opposing cellular responses. A classic illustration is the interaction between insulin and glucagon, where insulin stimulates a decrease in blood glucose levels while glucagon acts to increase them. This intricate network of interactions underscores the dynamic nature of hormonal regulation and its profound influence on cellular physiology.
