Characterisation of the effects of the phospholipase c inhibitor U73122 on P2Y receptor-mediated contractions of the rat isolated pulmonary artery

P2Y receptor agonists induce vasodilation via endothelial P2Y receptors and vasoconstriction via smooth muscle P2Y receptors (Chootip et al., 2002). However, the intracellular signalling pathways by which vasoconstriction is induced are not well characterised. The aim of this study was to determine the role of phospholipase C (PLC) in nucleotide-evoked vasoconstriction of rat small intrapulmonary artery (SPA) using the PI-PLC inhibitor U73122. 5 mm rings of endothelium-denuded rat SPA were mounted under isometric conditions in 1 mL organ baths at 37C and a resting tension of 0.5 g. Contractions were elicited by addition of the P2Y receptor agonists UTP and UDP (300 lM), the FP receptor agonist PGF2a (10 lM) or KCl (40 mM) to the bath. UTP, UDP and PGF2a each evoked slowly developing contractions, which reached a peak within 5 min. Pre-incubation with U-73122 (1 lM and 3 lM) for 15 min had no effect on the resting tone of the arteries nor on the amplitude of the contractions evoked by UTP (n = 5), UDP (n = 5), PGF2a (n = 5) or KCl (n = 5). Increasing the concentration of U73122 to 10 lM and the preincubation period to 30 min, also left the responses to UTP, UDP and KCl unchanged (n = 6). In contrast, the contractions to PGF2a were inhibited significantly under these conditions (77 ± 6% of control, n = 6, P < 0.05). These results show that the PI-PLC inhibitor U73122 had no effect on P2Y receptormediated contractions of rat pulmonary artery under conditions that inhibited the responses induced by PGF2a. This calls into question the involvement of PLC in the P2Y receptor-mediated contraction of rat pulmonary artery and further studies are ongoing to determine the involvement of other intracellular signalling components