We studied the role of K+ channels and Na+,K+-ATPase in the presynaptic inhibitory effects of prostaglandin E1 (PGE1) and PGE2 on the adrenergic responses of human vas deferens. Furthermore, we determined the effects of increasing extracellular K+ concentrations ([K+]o) and inhibition of Na+,K+- ATPase on neurogenic and norepinephrine-induced contractile responses. Ring segments of the epididymal part of the vas deferens were taken from 45 elective vasectomies and mounted in organ baths for isometric recording of tension. The neuromodulatory effects of PGEs were tested in the presence of K+ channel blockers. PGE1 and PGE2 (10_8 to 10_6 M) induced inhibition of adrenergic contractions. The presence of tetraethylammonium (10_3 M), charybdotoxin (10_7 M), or iberiotoxin(10_7 M), prevented the inhibitory effects of PGE1 and PGE2 on the adrenergic contraction. Botx glibenclamide (10_5 M) and apamin (10_6 M) failed to antagonize PGE1 and PGE2 effects. Raising the [K+]o from 15.8 mM to 25.8 mM caused inhibition of the neurogenic contractions. Ouabain at a concentration insufficient to alter the resting tension (10_6 M) increased contractions induced by electrical stimulation but did not alter the contractions to norepinephrine. The inhibition of neurogenic responses induced PGE1, PGE2 and increased extracellular concentration of K+ was almost completely prevented by ouabain (10_6 M). The results demonstrate that PGE1 and PGE2 inhibit adrenergic responses by a prejunctional mechanism that involves the activation of large-conductance Ca2+- activated K+ channels and Na+,K+-ATPase