Heterotrimeric guanine nucleotide-binding proteins (G proteins) usually function in signal transduction cascades by modulating the concentration of intracellular messengers. Intracellular messengers include all of the following EXCEPT

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Epinephrine is not an intracellular messenger, but rather an extracellular ligand that binds to a receptor on the cell surface, leading to activation of G proteins and subsequent production of intracellular messengers like 3',5'-adenosine monophosphate, calcium ion, 1,2-diacylglycerol, and inositol 1,4,5-trisphosphate.

Heterotrimeric Guanine Nucleotide-Binding Proteins (G Proteins) Usually Function in Signal