Atypical relaxation by scorpion venom in the lamb urethral smooth muscle involves both NO-dependent and -independent responses.

The sustained depolarisation induced by alpha-toxins from scorpion venom (20 microg/ml(-1)) was used to test the hypothesis that an endogenous, photo-sensitive, nitrocompound could act as a stable nitrergic transmitter in the sheep (lamb) urethra. Scorpion venom-treatment effectively abolished neurogenic responses to electrical field stimulation, but it did not modify the spontaneous urethral photorelaxation. On the other hand, scorpion venom induced an atypical relaxation in noradrenaline-contracted preparations, which could be reverted, but not prevented, by tetrodotoxin (TTX, 1 microM). However, after TTX-pretreatment, relaxations elicited by scorpion venom were significantly delayed and slowed down, and similar responses were obtained in the presence of ouabain (10 microM), low sodium medium, or after the inhibition of the NO-cGMP pathway. Although the involvement of K(+) and Cl(-) channels can be ruled out since both charybdotoxin (300 nM) and chlorotoxin (50 nM) did not elicit any urethral relaxation nor modified the scorpion venom-induced one. However, a slow Ca(2+) channel seems to be involved. GVIA omega-conotoxin (1 microM), but not MVIIC omega-conotoxin (1 microM), significantly inhibited both EFS- and scorpion venom-induced relaxations and almost abolished the partial relaxation that was resistant to NO synthase inhibition. On the other hand, the presence of L-cis-diltiazem (0.3 mM), a selective inhibitor of cyclic nucleotide gated channels (CNGCs), also delayed and slowed down relaxation induced by scorpion venom, as well as abolish its reversal by TTX. L-cis-diltiazem pre-treatment induced a progressive decay in urethral relaxation brought about by electrical field stimulation only when repetitive, long duration stimulation protocols were used. Taken together, our results do not support the hypothesis of the endogenous, photo-sensitive, urethral nitrocompound as reflecting a stable nitrergic transmitter instead of NO. However, they suggest the involvement of both a NO-cGMP-dependent and TTX-sensitive component and a NO-independent response, mediated by GVIA omega-conotoxin-sensitive Ca(2+) channels, in the neurogenic relaxation of the urethral muscle. In addition, the likely involvement of CNGCs as an additional component of the cGMP signalling mechanism is suggested.