The effects of the ATP affinity label periodate-oxidized ATP (ATP-2',3'-dialdehyde; P-ATP) on contractile responses of the guinea pig vas deferens to ATP was characterized and compared to the effects of the specific P2x-purinoceptor photoaffinity label antagonist, arylazido aminopropionyl ATP (ANAPP3). After incubation of vas deferens with 10(-2) M P-ATP for 5 min, the second phase of biphasic contractions to ATP was inhibited selectively. The inhibitory effect of P-ATP was specific for ATP, and resulted from affinity labeling in that it was long-lasting, was not reversed by washing, was related in magnitude to exposure period and was attenuated by ATP present during the incubation. In contrast to P-ATP, ANAPP3 inhibited selectively the first phase of ATP-induced responses. P-ATP and ANAPP3 together inhibited both phases of response. P-ATP inhibited selectively the second phase of responses to 5'-substituted ATP analogs which develop a prolonged second phase [e.g., adenosine 5'-O-(3-thiotriphosphate) and adenosine tetraphosphate] or an abbreviated second phase (e.g., beta,gamma-methylene ATP, beta,gamma-imido ATP and alpha,beta-methylene ATP). The greater the duration of the second phase, the more pronounced was the inhibitory effect. Two distinct and dissociable mechanisms mediate the biphasic response to ATP: the initial phase involves ANAPP3-sensitive P2x-purinoceptors, whereas the second is blocked by P-ATP and appears to involve cleavage of the phosphate chain. An additional effect observed as a result of P-ATP-treatment was potentiation of the first phase of contraction to ATP. This novel and irreversible effect may arise from the inhibition of degradative ecto-phosphohydrolase activity.