Purinergic signaling — Burnstock's framework, vindicated over 50+ years — comprises ATP-binding P2 receptors and adenosine-binding P1 receptors. This pathway covers P2 (ATP / ADP / UTP / UDP). The adenosine P1 family lives in adenosine receptor signaling. P2 splits into: (1) P2X (ionotropic — ATP-gated trimeric cation channels P2X1–7, fast ms-scale Ca²⁺/Na⁺ influx; P2X7 is the slow pore-forming subtype on macrophages → NLRP3 inflammasome trigger, cross-link pyroptosis gasdermin); (2) P2Y (metabotropic GPCRs — P2Y1/2/4/6/11/12/13/14; Gq/Gi/Gs coupling varies). Sources of extracellular nucleotides: regulated release (synaptic vesicles, taste cells, autocrine via Cx43 / pannexin-1 hemichannels), passive release (cell stress, ischemia, injury — ATP as DAMP). Hydrolysis cascade: ATP → ADP → AMP → adenosine via CD39 (ENTPD1) + CD73 (NT5E) ecto-enzymes — links P2 to P1. Therapeutic landscape — best-developed at platelet P2Y12: clopidogrel + prasugrel (irreversible prodrugs requiring CYP activation), ticagrelor (reversible), cangrelor (IV reversible) are antiplatelet mainstays for ACS, PCI, stent thrombosis prevention. P2X7 antagonists trialed for inflammatory disease — programs largely abandoned. IV adenosine for SVT (transient AV block) and pharmacologic stress imaging via P1. Suramin = non-selective P2 antagonist (historical anti-trypanosomal). Cross-links: platelet aggregation (P2Y12 antiplatelet pharmacology), adenosine receptor signaling (P1 sister family), pyroptosis gasdermin (P2X7 → NLRP3).
Organ Systems
cardiovascular
nervous
immune-hematologic
digestive
Pathway Steps
cellular ATP (regulated release or stress) → extracellular ATP / ADP / UTP / UDP — via synaptic vesicles, pannexin-1 / Cx43 hemichannels, or passive release from damaged cells (DAMP). ATP is not only an intracellular energy currency but an extracellular signal: it is released by regulated exocytosis, through pannexin/connexin channels, or by leakage from damaged cells. Extracellular ATP/ADP/UTP/UDP thus act as an activity/“danger” signal — high in injury, inflammation, and at synapses.
extracellular ATP → P2X1–7 (ionotropic) Ca²⁺/Na⁺ influx — via fast ms-scale channels; P2X7 is the slow pore-forming subtype. P2X receptors (P2X1-7) are ATP-gated ion channels — fast, ionotropic signaling that admits Ca²⁺ and Na⁺ to depolarize the cell. Being directly ATP-gated, they mediate rapid responses such as sensory (pain) signaling and smooth-muscle contraction, distinct from the slower GPCR arm.
extracellular ATP / ADP / UTP / UDP → P2Y1/2/4/6/11/12/13/14 (metabotropic GPCRs) — via Gq/Gi/Gs coupling differs by subtype; P2Y12 on platelets is Gi-coupled. P2Y receptors are metabotropic GPCRs activated by ATP, ADP, UTP, or UDP, signaling through Gq or Gi to second messengers. This slower, amplified arm covers platelet aggregation, vascular tone, and secretion — and its nucleotide selectivity across subtypes gives subtype-specific pharmacology.
ATP → ADP — via CD39 (ENTPD1) ecto-nucleotidase — rate-limiting in many tissues. Extracellular ATP is rapidly hydrolyzed to ADP by ectonucleotidases (CD39/NTPDases). This both terminates ATP signaling and generates ADP — itself an agonist at P2Y receptors (notably platelet P2Y12) — so the enzymatic cascade actively reshapes which receptors are engaged over time.
AMP → adenosine — via CD73 (NT5E) — joins P2 + P1 systems (cross-link). Further dephosphorylation by CD73 converts AMP to adenosine, switching signaling from the pro-inflammatory P2 (ATP) system to the largely anti-inflammatory P1 (adenosine receptor) system. This CD39-CD73 relay is a key immunoregulatory checkpoint linking purinergic to adenosine signaling.
P2X7 sustained activation (macrophage) → NLRP3 inflammasome priming + pore formation — via ATP as DAMP triggers IL-1β maturation; cross-link to pyroptosis. Sustained P2X7 activation by the high ATP at sites of cell death is a major danger signal in macrophages: it provides “signal 2” for NLRP3 inflammasome activation and, with prolonged stimulation, forms a large membrane pore. P2X7 thus couples extracellular ATP to IL-1β-driven inflammation — a drug target.
P2Y12 antagonist (Rx) → platelet aggregation inhibition — via antiplatelet pharmacology — ACS / PCI / stent thrombosis prevention. The platelet ADP receptor P2Y12 is the target of major antiplatelet drugs (clopidogrel, prasugrel, ticagrelor): blocking it inhibits ADP-amplified platelet aggregation. This is among the most clinically important purinergic interventions, central to preventing arterial thrombosis after stenting and ACS.