49. | Principles of Surgery, Companion Handbook

Principles of Surgery Companion Handbook

FIGURE 1-1 Three major classes of membrane receptors for hormones and neurotransmitters. Receptor Kinases: Mediators such as insulin bind receptors that activate the tyrosine kinase pathway that leads to phosphorylation of proteins. G Protein-Coupled Receptors: Some hormones, such as the peptides, neurotransmitters, and prostaglandins, bind to receptors (R) coupled to guanine nucleotide-binding or G proteins (G). The G proteins in turn activate effectors (E), which may be enzymes such as adenylate cyclase. G proteins coupled to adenylate cyclase increase cAMP. If G protein is coupled to phospholipase C, the active second messenger products are inositol triphospate (IP₃) and diacylglycerol (DAG). IP₃ stimulates the release of free calcium from the endoplasmic reticulum. The free calcium then binds to calmodulin to activate a specific phosphorylase kinase. DAG (not shown) remains in the membrane, where it activates protein kinase C, which opens a membrane channel for calcium entry. This activity, resulting from the initial activation of G proteins, may be coupled with the activity of Ligand-Gated Ion Channels. (From: Habener JF: Genetic control of hormone formation, in Wilson JD, Foster DW: Williams Textbook of Endocrinology, 8th ed. Philadelphia, WB Saunders, 1992, chap 4, with permission.)