Effects of adenosine 5'-triphosphate, uridine 5'-triphosphate, adenosine 5'-tetraphosphate and diadenosine polyphosphates in guinea-pig taenia caeci and rat colon muscularis mucosae.

The functional effects of adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP), adenosine 5'-tetraphosphate (AP4) and the diadenosine polyphosphates P1,P3-diadenosine triphosphate (Ap3A), P1,P4-diadenosine tetraphosphate (Ap4A) and P1,P5-diadenosine pentaphosphate (Ap5A) were studied in two isolated smooth muscle preparations thought to contain P2Y (P2Y1) receptors, the guinea-pig taenia caeci (which relaxes to ATP) and the rat colon muscularis mucosae (which contracts to ATP). In addition, the breakdown of these compounds by the rat colon muscularis mucosae was investigated by high pressure liquid chromatography. In the guinea-pig taenia caeci all the purine nucleotides caused relaxation with a potency order of Ap3A=Ap4A> ATP>AP4=Ap5A, and these relaxations were antagonised by suramin with apparent pA2 values in the region of 5, consistent with activation of a P2Y1 receptor. In the rat colon muscularis mucosae the nucleotides caused contraction with a potency order of Ap3A = Ap4A>ATP=AP4 =Ap5A >UTP. However, while suramin (100 microM) inhibited responses to ATP and UTP at all concentrations of agonist, it only inhibited contractions induced by the higher concentrations of AP4, Ap3A and Ap4A and had little effect on contractions induced by Ap5A. A higher concentration of suramin (1 mM) enhanced contractions induced by ATP but greatly inhibited those induced by UTP and had no effect on responses to the other agonists. The A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10 nM) had no effect on responses to ATP or UTP but inhibited responses to Ap3A, Ap4A, Ap5A and AP4. A combination of suramin (1 mM) and DPCPX (10 nM) almost abolished responses to all the agonists. ATP and UTP were rapidly degraded by the rat colon muscularis mucosae while AP4, Ap3A, Ap4A and Ap5A were degraded more slowly, and the major product detected after breakdown of the purine nucleotides was inosine rather than adenosine. The breakdown of all the nucleotides was inhibited by suramin (1 mM), although this inhibition did not achieve statistical significance in the case of ATP. These results show that while the diadenosine polyphosphates appear to act as P2 agonists in the taenia caeci, in the rat colon muscularis mucosae their major action is via adenosine A1 receptors rather than via P2 receptors. In addition, although they are more stable than ATP or UTP, their action in this tissue is clearly affected by their degradation which complicates the effects of suramin.