Angiotensin II (vascular smooth muscle) Catecholamines (a receptors) Gonadotropin-releasing hormone (GnRH) Growth hormone-releasing hormone (GHRH) Oxytocin
Thyroid-releasing hormone (TRH) Vasopressin (Vi receptor, vascular smooth muscle)
DAG, the other lipid second messenger, activates the enzyme protein kinase C (PKC), which then phos-phorylates a large number of proteins, leading to the cell's response (Figure 74-8). In addition to these effects, the lipid portion of DAG is arachidonic acid, which is the precursor for the prostaglandins and other local hormones that cause multiple effects in tissues throughout the body.
Another second messenger system operates in response to the entry of calcium into the cells. Calcium entry may be initiated by (1) changes in membrane potential that open calcium channels or (2) a hormone interacting with membrane receptors that open calcium channels.
On entering a cell, calcium ions bind with the protein calmodulin. This protein has four calcium sites, and when three or four of these sites have bound with calcium, the calmodulin changes its shape and initiates multiple effects inside the cell, including activation or inhibition of protein kinases. Activation of
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