Little is known about the purinergic regulation of intestinal motor activity in amphibians. Purinergic control of intestinal motility is subject to changes during development in mammals. The aim of this study was to investigate purinergic control of intestinal smooth muscle in the amphibian Xenopus laevis and explore possible changes in this system during the developmental phase of metamorphosis. Effects of purinergic compounds on mean force and contraction frequency in intestinal circular muscle strips from prometamorphic, metamorphic, and juvenile animals were investigated. Before metamorphosis, low concentrations of ATP reduced motor activity, whereas the effects were reversed at higher concentrations. ATP-induced relaxation was not inhibited by the P2-receptor antagonist pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) but was blocked by the ecto-nucleotidase inhibitor 6- N, N-diethyl-d-β,γ-dibromomethylene ATP ( ARL67256 ), indicating that an ATP-derived metabolite mediated the relaxation response at this stage. Adenosine induced relaxation before, during, and after metamorphosis, which was blocked by the A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). The stable ATP-analog adenosine 5′-[γ-thio]-triphosphate (ATPγS) and 2-methylthioATP (2-MeSATP) elicited contractions in the circular muscle strips in prometamorphic tadpoles. However, in juvenile froglets, 2-MeSATP caused relaxation, as did ATPγS at low concentrations. The P2Y11/P2X1-receptor antagonist NF157 antagonized the ATPγS-induced relaxation. The P2X-preferring agonist α-β-methyleneadenosine 5′-triphosphate (α-β-MeATP) evoked PPADS-sensitive increases in mean force at all stages investigated. This study demonstrates the existence of an adenosine A1-like receptor mediating relaxation and a P2X-like receptor mediating contraction in the X. laevis gut before, during, and after metamorphosis. Furthermore, the development of a P2Y11-like receptor-mediated relaxation during metamorphosis is shown.
Toxic effects of carbendazim andn-butyl isocyanate, metabolites of the fungicide benomyl, on early development in the African clawed frog,Xenopus laevis
