Chapter 5: Cell Signaling in Physiology

Cells communicate via specialized protein receptors that recognize specific messenger molecules, with the interaction governed by principles of specificity, affinity, saturation, and competitive binding. Agonists enhance receptor activity by mimicking natural messengers, while antagonists inhibit signaling by blocking ligand access, a distinction critical to understanding how drugs like decongestants and beta-blockers produce therapeutic effects. Cells dynamically regulate receptor sensitivity through upregulation and downregulation, adjusting expression levels and internalization rates in response to chronic changes in circulating messenger concentrations. The chapter distinguishes between two major pathways based on messenger solubility: lipid-soluble hormones such as steroids and thyroid hormone cross cellular membranes to activate intracellular nuclear receptors that function as transcription factors, directly modulating gene expression, while water-soluble messengers bind membrane-anchored receptors that launch intracellular signal transduction. Four principal receptor classes orchestrate these responses: ligand-gated ion channels provide direct electrical effects, receptor tyrosine kinases and JAK-associated receptors phosphorylate target proteins, and G-protein-coupled receptors activate downstream effectors including adenylyl cyclase and phospholipase C. This GPCR signaling generates diverse second messengers—cyclic adenosine monophosphate, cyclic guanosine monophosphate, inositol trisphosphate, diacylglycerol, and calcium ions—which activate protein kinases and calcium-binding regulatory proteins to amplify the original signal millions of fold. The chapter also addresses eicosanoids, lipid-derived inflammatory mediators synthesized from arachidonic acid via cyclooxygenase and lipoxygenase enzymes, whose inhibition by nonsteroidal anti-inflammatory drugs and glucocorticoids provides major therapeutic approaches. Signal termination through ligand degradation, receptor desensitization, and negative feedback prevents excessive cellular responses. A clinical case of pseudohypoparathyroidism demonstrates how mutations disrupting G-protein signaling cause hormonal resistance and metabolic dysfunction, illustrating the profound physiological consequences when cell signaling fails.