Which of the following choices includes only first messenger signaling molecules?

• A. cAMP, IP\(_3\), and calcium ions
• B. GTP, ATP, and cGMP
• C. Protein kinase A, adenylyl cyclase, and phospholipase C
• D. Epinephrine, insulin, and acetylcholine

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of first messenger signaling molecules. First messengers are extracellular signaling molecules, such as hormones or neurotransmitters, that bind to receptors on the cell surface to initiate a signaling cascade. They are distinct from second messengers, which are intracellular signaling molecules.
2. Step 2: Analyze the given options to identify which molecules are first messengers. First messengers are typically proteins, peptides, or small molecules that act outside the cell, whereas second messengers (e.g., cAMP, IP\(_3\), calcium ions) are involved in intracellular signaling.
3. Step 3: Evaluate the first option: cAMP, IP\(_3\), and calcium ions. These are second messengers, not first messengers, as they function inside the cell to propagate the signal initiated by the first messenger.
4. Step 4: Evaluate the second option: GTP, ATP, and cGMP. These molecules are involved in energy transfer and intracellular signaling, but they are not first messengers.
5. Step 5: Evaluate the fourth option: Epinephrine, insulin, and acetylcholine. These are extracellular signaling molecules that bind to cell surface receptors, making them first messengers. This is the correct answer.