Purinergic receptors in human placenta: Evidence for functionally active P2X₄, P2X₇, P2Y₂, and P2Y₆

Appropriate regulation of ion transport by the human placental syncytiotrophoblast is important for fetal growth throughout pregnancy. In nonplacental tissues, ion transport can be modulated by extracellular nucleotides that raise intracellular calcium ([Ca²⁺]_i) via activation of purinergic receptors. We tested the hypothesis that purinergic receptors are expressed by human placental cytotrophoblast cells and that their activation by extracellular nucleotides modulates ion (K⁺) efflux and [Ca²⁺]_i. P2X/P2Y receptor agonists 5-bromouridine 5′-triphosphate (5-BrUTP), ADP, ATP, 2′,3′-O-(4-benzoyl- benzoyl)adenosine 5′-triphosphate (BzATP), and UTP stimulated ⁸⁶Rb (K⁺ tracer) efflux from cultured cytotrophoblast cells at early (mononuclear) or later (multinucleate syncytiotrophoblast-like) stages of differentiation, with ATP and UTP particularly potent. 2-Methylthioadenosine 5′-triphosphate (2-MeS-ATP), and UDP elevated ⁸⁶Rb efflux only from multinucleated cells. All agonists caused a significant peak and plateau increase in [Ca²⁺]_i, although the magnitude of responses was variable. The effect of BzATP, UTP, and UDP in multinucleated cells was unaffected, and that of ATP partially inhibited, by removal of extracellular Ca²⁺, implicating P2Y receptor activation. mRNA encoding P2X₁, P2X₂, P2X₄, and P2X ₇ and P2Y₁, P2Y₂, P2Y₄, P2Y ₆, and P2Y₁₁ were identified in mono- and multinucleated cells, whereas P2X₃ and P2X₅ mRNA were absent from all samples. Western blot analysis revealed P2X₄, P2X₇, P2Y₂, and P2Y₆ protein in cytotrophoblast cells, but P2Y₄ was not detected. On the basis of published agonist selectivity, the data indicate the presence of functionally active P2X₄, P2X ₇, P2Y₂, and P2Y₆ receptors in cytotrophoblast cells. We propose that activation of these receptors, and subsequent elevation of [Ca²⁺]_i, modulates syncytiotrophoblast homeostasis and/or maternofetal ion exchange in response to extracellular nucleotides.