title: An unusual excitatory action of adenosine on the ventricular muscle of the South African clawed toad (Xenopus laevis).
abstract: The effects of adenyl compounds and some of their analogues were examined on atrial and ventricular muscle of Xenopus laevis. In contrast to all heart muscle preparations studied previously, adenosine produced excitation of the ventricle of Xenopus. 2-Chloroadenosine, ATP, beta, gamma-methylene ATP (APPCP) and alpha, beta-methylene ATP (APCPP) also elicited excitation of Xenopus ventricles; the order of potency being 2-chloroadenosine greater than ATP greater than or equal to adenosine greater than APPCP = APCPP. The excitatory effects of 2-chloroadenosine, adenosine, ATP and APPCP were antagonised by 8-phenylthe-ophylline (8-PT). A small excitatoty response to ATP persisted in the presence of 8-PT. The effects of adenosine were not potentiated by dipyridamole. Propranolol antagonised the excitatory responses to adrenaline, phenylephrine, 5-hydroxytryptamine (5-HT) and dopamine. High concentrations of histamine were needed to produce even small excitatory responses. Neither propranolol nor phentolamine affected the responses to adenosine and ATP. Adenosine, ATP and APPCP elicited inhibition of Xenopus atria which was antagonised by 8-PT but was not potentiated by dipyridamole. APCPP, which is resistant to degradation, did not produce a similar inhibitory response. It is concluded that excitation, mediated largely by excitatory P1- and possibly by some P2-purinoceptors, occurs in the Xenopus ventricle and that there are inhibitory P1-purinoceptors in the Xenopus atrium.
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology*; D000818:Animals; D004558:Electric Stimulation; D006325:Heart Atria / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology; D014982:Xenopus laevis
chemical_list: D013806:Theophylline; C028322:8-phenyltheophylline; D000241:Adenosine