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Previously, acyl derivatives of CoA were shown to antagonise human native and recombinant P2Y1 purine receptors. The main aim of this thesis was to study the effect of these endogenous nucleotide derivatives at endogenous P2Y1 receptors in blood vessels. Using isometric tension recordings, CoA, acetyl-CoA and palmitoyl-CoA (PaCoA) appeared to show selectivity for P2Y1 receptors (over P2Y2 and adenosine receptors) in the rat isolated thoracic aorta, with PaCoA being the most potent of the CoA derivatives used. In porcine isolated mesenteric arteries (PMA) and porcine isolated coronary arteries (PCA), isometric tension recordings indicated that ADP mediated endothelium-dependent and endothelium-independent relaxations, respectively. Relaxations in PMA were blocked by the P2Y1 receptor antagonist MRS2500 and PaCoA, whilst these were ineffective against ADP relaxations in the PCA. A FlexStation was used to monitor calcium responses in native HEK293 cells expressing P2Y1 and P2Y2 receptors using ADP and UTP, respectively. Responses to UTP were not significantly altered in the presence of PaCoA. In contrast, ADP-evoked responses were significantly inhibited in the presence of either MRS2500 or PaCoA. These data raise the possibility of an endogenous selective antagonism of P2Y1 receptors via CoA compounds, irrespective of species or cellular environment. Nicotinamide adenine dinucleotide (NAD) is an intracellular nucleotide which has been identified as an agonist at P2Y1, P2Y11, P2X and adenosine receptors. NAD evoked endothelium-independent concentration-dependent contractions of the pre-contracted PMA, which were unaltered in the presence of PaCoA. In contrast, αβ-methylene ATP (a desensitizing P2X receptor agonist) significantly reduced these responses suggesting the involvement of P2X1-like receptors. In both RTA and PCA, NAD evoked endothelium-independent concentration-dependent relaxations of the pre-contracted vessels, which were attenuated by SCH58261, but not PaCoA, which suggests the involvement of smooth muscle A2A receptors. These results together emphasise the possibility of a tissue and receptor-specific role of NAD as an endogenous extracellular nucleotide in purinergic signalling.