The use of neurally released agonist in the measurement of antagonism at alpha-adrenoceptors.

Graded contractile responses of the rat vas deferens were obtained to trains containing varying numbers of pulses delivered transmurally at 500 Hz, allowing construction of pulse-response curves. In the presence of postjunctional adrenoceptor blocking drugs, these curves were displaced to the right in a parallel manner, allowing the estimation of pA2 values for the drugs. Activity of drugs at prejunctional adrenoceptors was estimated from the reduction of inhibition of the response to a second pulse delivered 3 s after trains containing different numbers of pulses. It is suggested that the technique is more physiological than others used to determine the antagonistic activity of drugs at adrenoceptors.