Distribution of P1- and P2-purinoceptors in the guinea-pig and frog heart.

1 The effects of adenyl compounds were examined on the guinea-pig and frog heart in terms of the P(1)/P(2)-purinoceptor hypothesis.2 The effects of two slowly degradable adenosine 5'-triphosphate (ATP) analogues; beta,gamma-methylene adenosine 5'-triphosphate (APPCP) and alpha,beta-methylene adenosine 5'-triphosphate (APCPP) were also examined.3 Adenosine, adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), ATP and APPCP produced inhibitory effects in guinea-pig atria. These inhibitory effects were antagonized competitively by theophylline and potentiated by dipyridamole. APCPP did not produce a similar inhibitory response.4 Guinea-pig ventricles were insensitive to adenyl compounds.5 ATP and ADP produced initial excitatory effects in frog atria which were followed by inhibitory effects. Adenosine and AMP produced inhibitory effects alone whereas APCPP produced excitatory effects only. The inhibitory effects were antagonized competitively by theophylline and potentiated by dipyridamole.6 ATP, ADP, APPCP and APCPP evoked excitatory responses in frog ventricles. These responses were not affected by theophylline or dipyridamole. Adenosine and AMP were inactive on frog ventricles.7 It is concluded that only P(1)-receptors are present in guinea-pig atria; that both P(1)- and P(2)-receptors are present in frog atria; and that only P(2)-receptors are present in frog ventricles. No evidence was found for the presence of either P(1)- or P(2)-purinoceptors in guinea-pig ventricles.