Fig. 4. (A,B) Responses to adenosine 5'-triphosphate (ATP) in the absence and presence of the ectonucleotidase inhibitor 6-N,N-diethyl-D-β,-dibromomethylene ATP (ARL67156, 100 µmol l^–1) in (A) prometamorphic tissues (N=8) and in (B) juveniles (N=9). A statistically significant difference between responses was obtained at 0.01 µmol l^–1 in prometamorphosis and between 1 and 10 µmol l^–1 in juveniles. (C,D) Responses to ATP in the absence and presence of the A₁ receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) in (C) prometamorphic tissues (1 µmol l^–1, N=9) and in (D) juveniles (3 µmol l^–1, N=8). A statistically significant difference between responses was obtained between 3 and 30 µmol l^–1 in prometamorphic tissues but not at all in juvenile tissues. Control value prior to administration of antagonist or enzyme blocker is denoted C1 and that prior to ATP administration as C2 on the x-axis (A–D). Statistical significance for ARL67156/DPCPX effects was calculated using Student's unpaired t-test for each concentration.