In transfected human embryonic kidney (HEK 293) cells the P2Y₁₄ receptor, which is activated by UDP-glucose, UDP-glucuronic acid, UDP-N-acetylglucosamine and UDP-galactose reportedly couples to pertussis toxin sensitive Gi/o proteins. However, the functional coupling of endogenously expressed P2Y₁₄ receptors to the inhibition of adenylyl cyclase activity has not been reported. The overall aim of this study was to investigate functional expression of the endogenously expressed P2Y₁₄ receptors. This was initially observed in two cell lines that reportedly express P2Y₁₄ receptor mRNA, namely SH-SY5Y neuroblastoma and U373 MG astrocytoma cells. In U373 MG cells, but not in SH-SY5Y cells, UDP-glucose inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin sensitive, concentration-dependent manner (pEC₅₀ = 4.5 ± 0.3). However, RT-PCR analysis did not detect P2Y₁₄ receptor mRNA expression in U373 MG and SH-SY5Y cell lines. Due to the ambiguity, the functional expression of the P2Y₁₄ receptor was then investigated in primary cells. Previous quantitative RT-PCR analysis has detected moderate levels of mRNA in spleen and lymphocytes, with high levels of mRNA also present in neutrophils. Therefore, a further aim of this study was to determine functional expression of the P2Y₁₄ receptor in enriched populations of murine spleen-derived T and B lymphocytes, and in human neutrophils. UDP-glucose, but not the related sugar nucleotides UDP-galactose, UDP-N-acetylglucosamine and UDP-glucuronic acid inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin sensitive manner, suggesting the involvement of Gi/o-proteins coupling to the adenylyl cyclase pathway. In T lymphocytes, UDP-glucose, UDP-galactose, UDP-N-acetylglucosamine and UDP-glucuronic acid have an anti-proliferative effect, as all four agonists inhibited anti-CD3 and IL-2 induced T lymphocyte proliferation. In human neutrophils, UDPglucose (100 μM) had no effect on calcium release, but did produce a modest increase in ERKl/2 phosphorylation, whereas the other sugar nucleotides had no effect on ERKl/2 activation. Finally, UDP-glucose and related sugar nucleotides had no significant effect on N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced elastase release from neutrophils. Further studies are required to investigate the effect on other neutrophil functions such as chemotaxis, cell adhesion, or oxidative burst. In summary, this study provides the first report describing the endogenous functional expression of the P2Y₁₄ receptor in murine-derived T lymphocytes and human neutrophils. These observations suggest that UDP-glucose and related sugar nucleotides, presumably via the P2Y₁₄ receptor, may play an important role in modulating immune function. UDP-glucose appears to have an anti-proliferative effect in T lymphocytes, but further studies are required in order to establish the functional role of the P2Y₁₄ receptor that is expressed in human neutrophils.