Which of the following purinergic nucleotide receptors is a ligand-gated ion channel activated by ATP?

The purinergic nucleotide receptor that is classified as a ligand-gated ion channel and activated by ATP is P2X1. These receptors are part of a family that responds to extracellular ATP, leading to the opening of ion channels. When ATP binds to P2X receptors, including P2X1, it triggers a conformational change that allows ions, such as sodium and calcium, to flow through the channel, resulting in rapid physiological responses.

In contrast, the P2Y receptors (like P2Y1, P2Y12, and P2Y13) are G protein-coupled receptors (GPCRs) that do not form ion channels directly. Upon activation by ATP or other nucleotides, they initiate intracellular signaling cascades through G proteins, but they do not serve as direct pathways for ion flux across the cell membrane. This distinction is key when categorizing purinergic receptors and helps clarify why P2X1, as a ligand-gated ion channel, is the correct answer in this context.