ATRIAL-NATRIURETIC-PEPTIDE INHIBITS THE PURINERGIC AND NOT THE ADRENERGIC COMPONENT OF ELECTRICALLY-INDUCED CONTRACTILE RESPONSES IN GUINEA-PIG VAS-DEFERENS

The effects of atrial natriuretic peptide (ANP) on the purinergic and adrenergic components of electrically induced contractile responses in the prostatic portion of the guinea pig vas deferens were studied. AN P (0.01-100 nM) was found to inhibit the two phases of the biphasic co ntractions induced by field electrical stimulation (300 pulses, 0.1 ms ec, 9 Hz). The effects of ANP at a concentration of 10 nM were examine d further. ANP had no effect on /-phenylephrine-induced or exogenous A TP-induced contractions in the presence of tetrodotoxin, indicating a prejunctional rather than postjunctional modulatory effect of ANP. Nei ther yohimbine, nor indomethacin, nor naloxone influenced the inhibito ry effect of ANP on either phase of the electrically induced contracti ons. However, the inhibitory effect of ANP on the first phase was vera pamil-sensitive and nitrendipine-insensitive, whereas the effect on th e second phase was sensitive to both verapamil and nitrendipine. Thus, at least two different calcium pools could be involved in the ANP inh ibitory effect. The inhibitory effect of ANP on the purinergic compone nt of the electrically induced contractile responses after prazosin re mained the same, whereas after alpha,beta-methylene adenosine-5'-triph osphate there was no effect of ANP on the residual adrenergic componen t. Therefore, ANP exerted an inhibitory effect on the purinergic and n ot on the adrenergic component of the electrically induced mechanical responses in the guinea pig vas deferens via prejunctional mechanisms which do not depend on alpha-2 adrenoceptors, opiate receptors or the production of prostaglandins and depend on different calcium pools.