Cyclic AMP Acts as a Second Messenger for a Number of Regulatory Molecules

Epinephrine is only one of many hormones, growth factors, and other regulatory molecules that act by changing the intracellular [cAMP] and thus the activity of PKA (Table 12-4). For example, glucagon binds to its receptors in the plasma membrane of adipocytes, activating (via a Gs protein) adenylyl cyclase. PKA, stimulated by the resulting rise in [cAMP], phosphorylates and activates two proteins critical to the conversion of stored fat to fatty acids (perilipin and hormone-sensitive tri-acylglycerol lipase; see Fig. 17-3), leading to the mobilization of fatty acids. Similarly, the peptide hormone ACTH (adrenocorticotropic hormone, also called corticotropin), produced by the anterior pituitary, binds to specific receptors in the adrenal cortex, activating adenylyl cyclase and raising the intracellular [cAMP]. PKA then phosphorylates and activates several of the enzymes required for the synthesis of cortisol and other steroid hormones. The catalytic subunit of PKA can also move into the nucleus, where it phosphorylates a protein that alters the expression of specific genes.

Some hormones act by inhibiting adenylyl cyclase, lowering cAMP levels, and suppressing protein phos-phorylation. For example, the binding of somatostatin to its receptor leads to activation of an inhibitory G protein, or Gi, structurally homologous to Gs, that inhibits adenylyl cyclase and lowers [cAMP]. Somatostatin therefore counterbalances the effects of glucagon. In adipose tissue, prostaglandin E1 (PGE 1; see Fig. 10-18b) inhibits adenylyl cyclase, thus lowering [cAMP] and slowing the

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Essentials of Human Physiology

Essentials of Human Physiology

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