21.3: Chemical Signaling in the Endocrine System
A signaling cascade is a series of events that facilitates the transmission of information within or between cells, culminating in a targeted response in the recipient cell. As chemical messengers, hormones are pivotal in initiating and modulating these intricate signaling cascades based on their solubility.
Lipid-soluble hormones, such as steroid hormones, demonstrate an intracellular action. These hormones traverse cell membranes due to their lipid nature. Once inside the target cell, they bind to intracellular receptors in the cytoplasm or nucleus. For instance, testosterone, a steroid hormone, diffuses across the cell membrane into the extracellular fluid and binds to transport proteins in the bloodstream. Upon reaching the target cell, it dissociates and attaches to an androgen receptor inside the cell. This hormone-receptor complex enters the nucleus, regulating gene expression by binding to specific DNA sequences called hormone response elements. This process influences mRNA transcription and subsequent protein translation, altering cell metabolism and proliferation.
Conversely, water-soluble hormones, like peptides and amines, follow an extracellular mechanism. These hormones bind to receptors on the cell surface, initiating a cascade of events. The binding triggers intracellular signaling pathways, often involving second messengers like cyclic AMP. This cascade leads to cellular responses such as enzyme activation, altered membrane permeability, or gene transcription. Notably, the signaling pathway amplifies, allowing a small hormone concentration to elicit a robust cellular response. In summary, the divergent mechanisms of hormone action highlight the intricacies of cellular signaling pathways, ultimately shaping the physiological responses crucial for maintaining homeostasis.
