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First published online March 28, 2008
Journal of Experimental Biology 211, 1270-1280 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012005

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Changes in the control of gastric motor activity during metamorphosis in the amphibian Xenopus laevis, with special emphasis on purinergic mechanisms

Monika Sundqvist^* and Susanne Holmgren

Department of Zoophysiology, Göteborg University, SE 405 30 Göteborg, Sweden

^* Author for correspondence (e-mail: monika.sundqvist{at}astrazeneca.com)

Accepted 7 February 2008

The stomach of the amphibian Xenopus laevis is subject to extensive remodelling during metamorphosis. We investigated the changes in gastric activity control during this period using in vitro circular smooth muscle preparations mounted in organ baths. The nitric oxide synthase inhibitor L-NAME increased mean force in metamorphic and juvenile frogs but not in prometamorphic tadpoles. Serotonin (5-HT) relaxed stomach muscle prior to metamorphosis but elicited a biphasic response in juveniles consisting of contraction at low concentrations and relaxation at high concentrations. The effects of 5-HT were blocked by methysergide. In the prometamorphic tadpole, ATP elicited relaxation that was blocked by the ectonucleotidase inhibitor ARL67156 and the adenosine A₁ receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), suggesting adenosine as the mediator. Exogenous adenosine and the A₁ receptor agonist N⁶-cyclopentyladenosine (CPA) induced relaxation at all stages. After metamorphosis, the potency of ATP decreased and neither DPCPX nor ARL67156 could block ATP-induced relaxation. Uridine 5'-triphospate (UTP) induced relaxation prior to metamorphosis, but caused contraction of muscle strips from metamorphosing tadpoles. Single doses of UTP blocked phasic contractions in juveniles in a tetrodotoxin (TTX)-sensitive manner while the simultaneous increase in muscle tension was TTX insensitive. The P2X₁/P2X₃ receptor agonist -β-MeATP elicited pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive contractions at all stages investigated. These results indicate the development of an inhibitory nitrergic tonus during metamorphosis and a 5-HT receptor involved in muscle contraction. Also, the development of UTP receptors mediating increased tension and neural UTP receptors decreasing contraction frequency in juveniles is indicated. An adenosine A₁-like receptor mediating relaxation and a P2X-like receptor mediating contraction is demonstrated at all stages.

Key words: metamorphosis, development, amphibian, Xenopus laevis, 5-HT, nitric oxide, adenosine, UTP, ATP, purinergic