Extracellular ATP plays a role in the maintenance of vascular tone ¹. It is released from perivascular nerves as a cotransmitter with noradrenaline (NA), and from endothelial cells in response to changes in blood flow and hypoxia. The role and distribution of ATP-activated P2 receptors (P2R) is characterised for many blood vessels but not in equine digital vessels. Haemodynamic disturbances leading to ischaemic and reperfusion injury of the equine digit are thought to be involved in the debilitating condition laminitis. This study aims to characterise ATP signalling in equine digital vessels, before investigating ATP signalling in laminitic digital vessels, where abnormal ATP signalling may play a role. Equine digital arteries (EDA) and veins (EDV) were collected from horses killed in an abattoir. Limbs were collected within 10 min of slaughter and iced Krebs solution was infused into the digital circulation to remove blood. Sections of isolated EDA and EDV were used in organ-bath experiments, whereby electric field stimulation (EFS) in the absence and presence of antagonists to P2R and α₁-adrenoceptors was examined. Concentration-response curves were constructed to NA and ATP on low and raised tone. Immunofluorescent localization of P2R subtypes were performed. Data are mean±s.e.m. On low tone preparations EFS (28 V, 0.3 ms, 4-32 Hz) induced vasoconstriction in EDA and EDV; in both cases this was abolished by tetrodotoxin (1 μM; n=6). In EDA, vasoconstriction was partially inhibited (by 28±9%) by the P2R antagonist suramin (10 μM; n=6), but not by the P2R antagonist PPADS (10 μM; n=6), and predominantly inhibited (by 78±12%) by phentolamine (10 μM; n=6). In EDV, vasoconstriction was partially inhibited by suramin (10 μM; n=6) and PPADS (10 μM; n=6) to a similar degree (by 43±18% and 39±7%, respectively), and to a lesser extent (by 18±6%) by phentolamine (10 μM; n=6). Exogenous NA and ATP mimicked EFS in EDA; immunostaining for P2X₁, P2X₂ and P2X₃ receptor subunits was seen in vascular smooth muscle (SM) and P2X₁, P2X₂ and P2X₃ subunits in endothelium. Denuding the EDA of endothelium did not alter EFS- or ATP-evoked vasoconstriction. Exogenous NA and ATP mimicked EFS in EDV; immunostaining for P2X₁ and P2X₇ receptor subunits was seen in vascular SM. ATP failed to induce vasodilation on raised tone preparations in either EDA or EDV. ATP and NA are cotransmitters in sympathetic nerves supplying the equine digital vasculature, NA being the dominant partner in arteries and ATP being the dominant partner in veins. ATP did not produce either endothelium-dependent or -independent vasodilatation; P2X₁, P2X₂ and/or P2X_1/2, or, P2X₁ receptors are likely to mediate vasoconstriction on EDA and EDV, respectively. We will now use this information to investigate whether ATP signalling is altered in the digital vasculature of horses with laminitis.