Answer:
The true option is D: some G proteins inhibit the production of cyclic AMP.
Explanation:
A. False. GPCRs have evolved to recognize a diversity of extracellular physical and chemical signals, such as nucleotides, peptides, amines, Ca²⁺, and photons.
B. False. the cAMP is a second messenger present in different signaling cascades and provokes very different responses depending on the type of cell which it acts on, the substance that induces this cell and the receptor activated. Glucogen degradation and the stop of its synthesis in stressful situations are two examples of immediate responses mediated by the catalytic subunits of protein kinase A. Inhibition of glucogenesis and gastrin secretion by islets of Langerhans are two examples of late responses mediated by PKA.
C. False. The inositol phospholipid pathway (PIP₂), by hydrolysis, produces 2 second messengers: DAG and IP₃. These molecules activate different cascades. IP₃, for example, unites to receptors in the SER, which are ligand-regulated Ca²⁺ channels. This union allows the flow of Ca²⁺ to the cytosol. It forms an active complex with the protein calmodulin, and this complex binds to different proteins, including CaM kinases. These CaM kinases regulate gene expression phosphorylating transcription factors, like CREB.
D. True. Some hormones inhibit adenylate cyclase and diminish the levels of cAMP and suppress protein phosphorylation. For example, the union of somatostatin to its receptor activates an inhibitory G protein (Gi), which inhibits adenylate cyclase and lowers the concentration of cAMP.
E. False. The activity of the second messenger Ca²⁺, as well as the cAMP, can be restricted in space; after the response provoked by its liberation, Ca²⁺ is eliminated before it can diffuse to distant parts of the cell.
