Transmembrane enzyme-linked receptors

Drugs may act on intracellular receptors, membrane receptors directly coupled to ion channels, receptors linked via coupling proteins to intracellular effectors, receptors influencing cGMP and nitric oxide signaling, receptors that function as enzymes or transporters, receptors that function as transmembrane enzymes, or receptors for cytokines. 

Phospholipase C, which occurs in different subtypes in the cell, is a key enzyme of phosphatide inositol metabohsm (for cleavage specificity, see Fig. 5.24). Two central signaling pathways regulate phosphohpase C activity of the cell in a positive way (Fig. 6.4). Phospholipases of type CP (PL-CP) are activated by G-proteins and are thus linked into signal pathways starting from G-protein-coupled receptors. Phosphohpases of type Y (PL-Cy), in contrast, are activated by transmembrane receptors with intrinsic or associated tyrosine kinase activity (see Chapter 8, Chapter 10). The nature of the extracellular stimuli activated by the two major reaction pathways is very diverse (see Fig 6.4), which is why the phosphohpase C activity of the cell is subject to multiple regulation. 

Examples of the actions of agonists can be taken from each of the two major molecular superfamilies. For the family of seven-transmembrane-region receptors linked to G proteins and enzymic second-messenger systems, the agonist would turn on the synthesis of second messenger to the greatest extent possible (i.e., the action... 

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