Hemodynamic characteristics of extracellular UTP in the perfused rat liver

Nucleotide-mediated relaxation in guinea-pig aorta: selective inhibition by MRS2179

The vasodilatory effects of nucleotides in the guinea-pig thoracic aorta were examined to determine the relationship between molecular expression and function of P2Y receptors. In aortic rings precontracted with norepinephrine, vasodilatory responses to purine nucleotides exhibited a rank-order of potency of 2-methylthio-ATP>ADP>ATP. Responses to UTP, but not UDP suggested a functional role for P2Y4 but not P2Y6 receptors. Aortic endothelial cells express at least four P2Y receptors; P2Y1, P2Y2, P2Y4 and P2Y6. In primary culture, these cells exhibit desensitizing transient calcium responses characteristic of P2Y1, P2Y2 and P2Y4, but not P2Y6 receptors. UDP had no effect on endothelial cell calcium. The pyrimidinergic receptor agonist UTP is capable of eliciting robust vasodilation in aortic rings and causing calcium responses in cultured guineapig aortic endothelial cells. These responses are equivalent to the maximum responses observed to ATP and ADP. Measurement of intracellular calcium release in response to ATP and 2-methylthio-ATP were similar, however only the 2-methylthio-ATP response was sensitive to the P2Y1 antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179). In aortic rings, vasodilatory responses to 2-methylthio-ATP, ATP and ADP were all blocked by pre-incubation of tissues with MRS2179. MRS2179 pretreatment had no effect of the ability of UTP to cause relaxation of norepinephrine responses in aortic rings or the ability of UTP to cause calcium release in aortic endothelial cells. We demonstrate robust effects of purine and pyrimidine nucleotides in guineapig aorta and provide functional and biochemical evidence that MRS2179 is a selective P2Y1 antagonist.

Volume-sensitive purinergic signaling in human hepatocytes

BACKGROUND/AIMS

Purinergic signaling potentially contributes to many liver functions. Therefore, the purpose of these studies was to characterize adenosine 5'-triphosphate (ATP) release from human hepatocytes, and to determine the role of extracellular ATP in the autocrine regulation of Cl- permeability and cell volume homeostasis.

METHODS

Release of ATP (luciferase-luciferin assay), Cl- currents (whole-cell patch clamp), and cell volume (Coulter Multisizer) were measured in human hepatocytes within 12 h of isolation.

RESULTS

Hepatocyte swelling increased bioluminescence from basal values of 11.21+/-0.45 to 178.29+/-44.49 and 492.15+/-89.41 arbitrary light units following 20 and 40% buffer dilutions, respectively (p<0.001), representing an increase in extracellular ATP from approximately 10 to >300 nM. Whole-cell Cl- currents activated during exposure to hypotonic buffer (15% less mosmol, 126.34+/-36.49 pA/pF) and ATP (10 microM, 71.92+/-15.48 pA/pF) exhibited outward rectification, time-dependent inactivation at depolarizing potentials, and sensitivity to the anion channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). Removal of extracellular ATP (apyrase) prevented volume-sensitive current activation. Exposure to hypotonic buffer (30% less mosmol) increased mean relative volume to 1.092+/-0.004 by 2.5 min, and volume recovery (1.019+/-0.002 by 30 min) was abolished by NPPB, apyrase, and the P2 receptor antagonist suramin.

CONCLUSIONS

These findings indicate that human hepatocytes exhibit constitutive and volume-dependent ATP release, which is a critical determinant of membrane Cl- permeability and cell volume regulation. ATP release may represent an extracellular signaling pathway that couples the cellular hydration state to important hepatic functions.