EFFECT OF TITYUS-SERRULATUS SCORPION-VENOM ON THE RABBIT ISOLATED CORPUS CAVERNOSUM AND THE INVOLVEMENT OF NANC NITRERGIC NERVE-FIBERS

1 The effect of Tityus serrulatus scorpion venom and its toxin compone nts on the rabbit isolated corpus cavernosum was investigated by use o f a bioassay cascade. 2 Tityus serrulatus venom (3-100 mu g), acetylch oline (ACh; 0.3-30 nmol) and glyceryl trinitrate (GTN; 0.5-10 nmol) do se-dependently relaxed rabbit isolated corpus cavernosum preparations precontracted with noradrenaline (3 mu M). The selective soluble guany late cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3, alquinoxalin-1-one] (ODQ; 30 mu m) increased the basal tone of the rabbit isolated corpus cavernosum and abolished the relaxations induced by the agents mentio ned above. Methylene blue (30 am) also inhibited the relaxations induc ed by Tityus serrulatus venom but, in contrast to ODQ, the inhibition was irreversible. 3 The non-selective NO synthase (NOS) N-omega-nitro- L-arginine methyl ester (L-NAME; 10 mu M) and N-G-iminoethyl-L-ornithi ne (L-NIO; 30 mu M) also increased the tone of the rabbit isolated cor pus cavernosum and markedly reduced both Auckland Tityus serrulatus ve nom-induced relaxations without affecting those evoked by GTN. The inh ibitory effect was reversed by infusion of L-arginine (300 mu M), but not D-arginine (300 mu M). The neuronal NOS inhibitor 1-(2-trifluorome thylphenyl) imidazole (TRIM, 100 mu M) did not affect either the tone of the rabbit isolated corpus cavernosum or the relaxations induced by ACh, bradykinin (Bk), Tityus serrulatus venom and GTN. TRIM was appro ximately 1,000 times less potent than L-NAME in inhibiting rabbit cere bellar NOS in vitro, as measured by the conversion of [H-3]-L-arginine to [H-3]-L-citrulline. 4 The protease inhibitor aprotinin (Trasylol; 10 mu g ml(-1)) and the bradykinin Ba receptor antagonist Hoe 140 (D-A rg-[Hyp(3),Thi(5),D-Tic(7), Oic(8)]-BK; 50 nM) did not affect the rabb it isolated corpus cavernosum relaxations induced by Tityus serrulatus venom. The ATP-dependent K+ channel antagonist glibenclamide (10 mu M ) and the Ca2+-activated K+ channel antagonists apamin (0.1 mu M) and charybdotoxin (0.1 mu M) also failed to affect the venom-induced relax ations. Similarly, the Kt channel blocker tetraethylammonium (TEA; 10 mu M) had no effect on the venom-induced relaxations. 5 Capsaicin (3 a nd 10 nmol) relaxed the rabbit isolated corpus cavernosum in a dose-de pendent and non-tachyphylactic manner. Ruthenium red (30 mu M), an inh ibitor of capsaicin-induced responses, markedly reduced the relaxation s caused by capsaicin, but failed to affect those induced by Tityus se rrulatus venom. L-NAME (10 mu M) had no effect on the capsaicin-induce d relaxations of the rabbit isolated corpus cavernosum. 6 The sodium c hannel blocker tetrodotoxin (TTX; 1 mu M) abolished the relaxations of the rabbit isolated corpus cavernosum induced by Tityus serrulatus ve nom without affecting those evoked by capsaicin, ACh and GTN. Tetrodot oxin (1 mu M) also promptly reversed the response to the venom when in fused during the relaxation phase. 7 The bioassay cascade of the toxin components purified from Tityus serrulatus venom revealed that only f ractions X, XI and XII caused dose-dependent relaxations of the rabbit isolated corpus cavernosum and these were markedly reduced by either TTX (1 mu M) or L-NAME (10 mu M). 8 Our results indicate that Tityus s errulatus scorpion venom (and the active fractions X, XI and XII) rela xes rabbit corpus cavernosum via the release of NO. This release is sp ecifically triggered by the activation of capsaicin-insensitive cavern osal non-adrenergic non-cholinergic (NANC) fibres, that may possibly b e nitrergic neurones. Tityus serrulatus venom may therefore provide an important tool for understanding further the mechanism of NANC nitrer gic nerve activation.