Identification of P1 and P2 purinoceptors in the aorta of the lizard (Agama sp.).

In the isolated Agama lizard aorta, acetylcholine (ACh; 3 nM-100 microM), noradrenaline (NA; 30 nM-0.3 mM), adrenaline (Adr; 30 nM-300 microM), adenosine 5'-triphosphate (ATP; 30 nM-1 mM), alpha,beta-methylene ATP (alpha,beta-meATP; 10 nM-10 microM), beta,gamma-methylene ATP (beta,gamma-meATP; 0.1-300 microM), 2-methylthio ATP (2-meSATP; 30 nM-30 microM) and high concentrations of uridine triphosphate (UTP; 1 microM-1 mM), all produced constriction. The P2 receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 30 microM), suramin (0.1 mM) and Reactive blue 2 (30 microM) all raised vascular tone and could not be utilized and the antagonist 2'-O-(trinitrophenyl) ATP (TNP-ATP; 0.1 microM) had no effect on responses to the ATP analogues. alpha,beta-MeATP (3 microMx3) desensitised responses to alpha,beta-meATP (10 microM) and beta,gamma-meATP (0.3 mM), but not to ATP (0.3 mM) or 2-meSATP (30 microM). On pre-constricted aorta (EC50 concentration of either ACh or Adr), adenosine (1 microM-1 mM), the A1-selective agonist N6-cyclopentyl adenosine (CPA; 1-300 microM) [but not the A2- and A3-selective agonists CGS 21680 and IB-MECA respectively (both up to 30 microM)] and sodium nitroprusside (10 nM-100 microM) produced vasodilatation. Adenosine vasodilatation was antagonised by 8-p-sulfophenyl-theophylline (8-pSPT; 30 microM) but not by N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.1 mM). ATP (up to 0.3 mM), 2-meSATP (up to 10 microM) and UTP (up to 1 mM) were not vasodilators. In summary, A1 receptors mediating relaxation and excitatory P2X1 receptors were identified in the smooth muscle of the lizard aorta. However, in contrast to mammalian aorta, P2Y receptors on endothelial cells mediating vasodilatation via nitric oxide do not appear to be present.