Effect of adenosine 5'triphosphate (ATP) on the isolated rectum of the rainbow lizard Agama agama

ATP but not adenosine contracts lizard isolated rectal muscles. Theophylline, quinidine, nifedipine and lack of Ca2+ inhibited ATP contractions, but dipyridamole, indomethacin, atropine, D-tubocurarine, phentolamine, L-propranolol, cimetidine, mepyramine and methysergide did not significantly modify the contractions. ATP inhibited Ca2+-induced contractions in Na+-free, high K+ media. In normal Ringer, it relaxed carbachol-contracted muscles and inhibited KCl contractions. The results suggest that ATP contractions are dependent upon external Ca2+, and are probably associated with extracellular Ca+ influx. The contractions appear to have occurred independently of receptor activation and may have been mediated by a depolarising action of the nucleotide. The relaxant effect of ATP possibly occurred as a consequence of inhibition of Ca2+ influx into carbachol or K+-depolarised muscles.