P2Y receptors and kidney function

Abstract

Cellular release of nucleotides is of physiological importance to regulate and maintain cell function and integrity. Also in the tubular and collecting duct system of the kidney, nucleotides are released in response to changes in cell volume or luminal flow rate and act in a paracrine and autocrine way on basolateral and luminal P2Y receptors. Recent studies using gene knockout mice assigned a prominent role to G protein-coupled P2Y(2) receptors, which are activated by both ATP and UTP. The antidiuretic hormone, arginine-vasopressin (AVP), and possibly an increase in collecting duct cell volume induce ATP release. The subsequent activation of P2Y(2) receptors inhibits AVP-induced cAMP formation and water reabsorption, which stabilizes cell volume and facilitates water excretion. An increase in NaCl intake enhances luminal release of ATP and UTP in the aldosterone-sensitive distal nephron which by activating apical P2Y(2) receptors and phospholipase C lowers the open probability of the epithelial sodium channel ENaC, thereby facilitating sodium excretion. Thus, the renal ATP/UTP/P2Y(2) receptor system not only serves to preserve cell volume and integrity but is also regulated by stimuli that derive from body NaCl homeostasis. The system also inhibits ENaC activity during aldosterone escape, i.e. when sodium reabsorption via ENaC is inappropriately high. The P2Y(2) receptor tone inhibits the expression and activity of the Na-K-2Cl cotransporter NKCC2 in the thick ascending limb and mediates vasodilation. While the role of other P2Y receptors in the kidney is less clear, the ATP/UTP/P2Y(2) receptor system regulates NaCl and water homeostasis and blood pressure.

Figure 1. A proposed model for the regulation of vascular tone by P2Y receptors

Depicted are localizations and vascular responses to activation of P2Y₁ (activated by ATP, 2-methylthio ATP), P2Y₂ (ATP/UTP/INS45973) and P2Y₄ (ATP/UTP/INS45973) receptors. Activation of P2Y₁ receptors on endothelial cells induces endothelium-dependent vasodilation utilizing nitric oxide (NO) and endothelial derived hyperpolarization factor (EDHF). Activation of P2Y₄ receptors on endothelial cells lowers vascular tone through release of endothelial NO; in comparison, activation of P2Y₂ receptors on endothelial cells induces vasodilation independent of endothelial NO and may involve EDHF. Activation of smooth muscle P2Y₁ or P2Y₄ receptors results in vasoconstriction. Based on and modified from .

Figure 2. The inhibitory effect of ATP and dietary NaCl on ENaC open probability (P_O) in the aldosterone-sensitive distal nephron is mediated by P2Y₂ receptor activation

A) Continuous current traces from cell-attached patches on principal cells (containing ≥ two ENaC’s) before and after application of ATP (100 μM) in CNTs/CCDs harvested from WT and P2Y₂−/− mice fed a regular 0.32% Na⁺ diet. Patches clamped to −Vp = −60 mV and inward Li⁺ currents through ENaC are downward. Dashed lines indicate the respective current levels; c denotes the closed state. Summary graphs of ENaC Po changes in response to ATP are shown on the right. * P<0.05 vs WT. B) The suppression of ENaC P_O by dietary NaCl intake is absent in P2Y₂−/− mice. * P<0.05 vs. WT. C) Inhibition of local ATP signaling (hexokinase (+glucose) to degrade local ATP plus suramin to prevent P2 receptor activation) prevents regulation of ENaC P_O by dietary NaCl intake in WT mice. * P<0.05 vs. WT control. Based on and modified from .

Figure 3. A proposed model for the inhibition of ENaC open probability (P_O) by dietary NaCl in the aldosterone-sensitive distal nephron

Low dietary NaCl intake suppresses luminal release of ATP and UTP which may relate to increased aldosterone levels. This prevents activation of apical P2Y₂ receptors on principal cells (PC). As a consequence, the inner leaflet of the lipid bilayer contains a high concentration of negatively-charged phosphatidylinositol 4,5-bisphosphate (PIP₂), which binds to positively charged regions of the N-terminus of the β-subunit of ENaC, thereby maintaining the ENaC channel open and active. High dietary NaCl intake increases the luminal release of ATP and UTP, at least in part via a connexin30 (Cx30)-dependent mechanism in intercalated cells (IC). This activates apical P2Y₂ receptors and phospholipase C (PLC), which hydrolyzes and lowers the concentration of PIP₂. The resulting conformation change in the N-terminus of β-ENaC lowers ENaC P_O and activity, and increases Na⁺ excretion. See text for details. IP₃, inositol trisphosphate. Modified from .

Figure 4. A proposed model for cell volume-dependent ATP release and water transport inhibition by P2Y₂ receptor activation in IMCD cells

Arginine-vasopressin (AVP) activates vasopressin V₂ receptor (V₂R) to stimulate aquaporin-2 mediated water entry which increases cell volume. The latter increases basolateral and apical release of ATP (and potentially UTP?) by unknown mechanism. P2Y₂ receptor activation inhibits water reabsorption via multiple signaling pathways and stimulates volume regulatory K⁺ channels as shown in other cell types. These effects help to stabilize cell volume. In response to an acute water load, the relative hypotonic extracellular fluid may increase cell volume. The resulting release of nucleotides and activation of P2Y₂ receptor stabilizes cell volume and accelerates the excretion of free water before circulating AVP levels fall. AC, adenylyl cyclase; AQP2, -3, -4, aquaporin-2, -3, -4; COX1, cyclooxygenase 1; CREB, cAMP responsive element-binding protein; DAG, diacylglycerol; EP₃, PGE₂ E₃ receptor; ET-1, endothelin 1; ETB, endothelin B receptor; G_i, inhibitory G protein; Gq, G_s, stimulatory G protein; IP₃, inositol trisphosphate; P, phosphorylation; PGE₂, prostaglandin E₂; PLC, phospholipase C; PKA, protein kinase A; PKC, protein kinase C; UT-A1, urea transporter A1. Modified from .

Figure 5. Indirect evidence for a positive relationship between collecting duct cell volume and ATP release, and a role of P2Y₂ receptors in stabilizing cell volume

Quantitative urine collection was performed in P2Y₂−/− mice and littermate wild types (WT) in metabolic cages over 2 hours following acute vasopressin V₂ receptor inhibition (V₂R-I) or acute oral water loading (WL). (A) V₂R-I and WL both increased urinary flow rate but induced opposite effects on urinary ATP excretion. Urinary flow rate is not a good predictor of urinary ATP excretion. (B) V₂R-I is expected to reduce CD water uptake and thus CD cell volume. In contrast, the reduction in extracellular tonicity associated with WL may increase CD cell volume. If an increase in cell volume induces ATP release, which stabilizes cell volume via inhibition of water uptake through a P2Y₂ receptor-mediated mechanism, then ATP release is expected to be enhanced in P2Y₂−/− mice. This influence should be minimized by reducing water uptake by V₂R-I and maximized by water loading-induced cell swelling. Based on 23 and modified from .