The fade of the purinergic neurogenic contraction of the guinea-pig vas deferens: analysis of possible mechanisms.

The purinergic response of the guinea-pig vas deferens to long trains of pulses at high frequency consists of an initial twitch followed by a much lower plateau. Mechanical, neurochemical and electrophysiological techniques were used to examine the reason for the fade. Mechanical measurements. In tissues stimulated by trains of 180 pulses/10 Hz and treated with prazosin to suppress the noradrenergic contraction component, the response to alpha, beta-methylene ATP and to exogenous ATP was as high during the secondary plateau of the purinergic neurogenic contraction as it was outside electrical stimulation periods; the response to 50 pulses/100 Hz was also unchanged during the low plateau. The plateau was not increased by reactive blue 2,8-(p-sulphophenyl)theophylline, propranolol or capsaicin. Neurochemical measurements. In tissues preincubated with [3H]-noradrenaline, electrical stimulation elicited an overflow of tritium and of ATP. In the absence of drugs as well as in the presence of prazosin and suramin to suppress contractions, the overflow of tritium per pulse decreased slightly in the course of trains of 90 pulses/10 Hz; the overflow of ATP per pulse decreased to a greater extent on average, but the decrease was not statistically significant. In the presence of prazosin and nifedipine, also to suppress contractions, the overflow of tritium per pulse again decreased slightly in the course of trains of 105 pulses/10 Hz, but the overflow of ATP per pulse if anything tended to increase. Electrophysiological measurements.(ABSTRACT TRUNCATED AT 250 WORDS)