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Question:

Molecular biologists studying signal transduction apply an agent to human cells that activates G-protein- dependent phospholipase C.  Which of the following intracellular substances is most likely to increase immediately after exposure to this agent?

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Explanation:

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A variety of hormone receptors are known to exert their intracellular effects via the phosphoinositol system.  Examples include α1-adrenergic, M1 and M3 cholinergic, V1 (vasopressin), H1 (histamine), oxytocin, angiotensin II, TRH, and GnRH receptors.  This signal transduction pathway proceeds through the following steps:

  1. Binding of a ligand to its cell surface receptor causes the exchange of GDP for GTP on the α-subunit of a Gq-protein associated with the receptor.  The activated α-subunit undergoes a conformational change and exposes a phospholipase C (PLC) activating site.

  2. After activation, PLC hydrolyzes phosphatidyl inositol bisphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3).

  3. DAG is able to directly stimulate protein kinase C (PKC), but the major activator of PKC is increased intracellular Ca2+ that occurs due to IP3 mediated-release of intracellular Ca2+ stores from the endoplasmic reticulum.  PKC is the major effector molecule in this pathway; it directly modulates the activity of other proteins via phosphorylation.

(Choices B and C)  Intracellular cAMP and cGMP concentrations increase during activation of adenylate or guanylate cyclase second messenger systems, respectively.  Levels can also increase following cyclic nucleotide phosphodiesterase inhibition, as seen on exposure to sildenafil, which selectively inhibits cGMP phosphodiesterease and results in smooth muscle relaxation in blood vessels.

(Choice D)  Intracellular Cl- concentration increases slightly after inhibitory neurotransmitters (eg, GABA, glycine) act on the neuron to increase Cl- membrane conductance (hyperpolarization).

(Choice E)  The intracellular concentration of mRNA increases during cellular states of elevated protein synthesis (eg, during cell division).

(Choice F)  Nitric oxide (NO) is a paracrine signaling molecule with a lifetime of a few seconds.  It can freely cross cell membranes and functions as a critical component of endothelium-mediated vasodilation.  NO is synthesized from arginine and O2 by the enzyme NO-synthase.

Educational objective:
The phosphoinositol second messenger system begins with ligand-receptor binding and Gq-protein activation leading to activation of phospholipase C (PLC).  PLC then hydrolyzes phosphatidyl inositol bisphosphate and forms diacylglycerol and inositol triphosphate (IP3).  Finally, IP3 activates protein kinase C via an increase in intracellular Ca2+.