Neurogenic chloride secretion induced by scorpion venom and veratrine in rabbit colon

In earlier reconstitution experiments the venom of the scorpion Leiurus quinquestriatus, LQV, was shown to block Ca(2+)-activated high-conductance K+ channels from the basolateral cell membrane of rabbit colonocytes (Turnheim K, Costantin J, Chan S, Schultz SG (1989) J Membrane Biol 112:247-254). These LQV-sensitive K+ channels do not seem to be involved in active Na+ transport across rabbit colon, as absorptive Na+ fluxes were not significantly affected by serosal addition of LQV to isolated epithelia of rabbit descending colon. While Na+ absorption was not changed, LQV and veratrine caused electrogenic Cl- secretion in this tissue by a neural (tetrodotoxin sensitive) mechanism. The secretory effect of LQV was partly inhibited by atropine, suggesting the involvement of m-cholinoceptors, and by a VIP-antagonist. In contrast to the neurogenic secretion in the small intestine of guinea pig, rat and cat, 5-hydroxytryptamine (5-HT) does not seem to be involved in neurogenic secretion in rabbit colon, as 1) several 5-HT receptor antagonists did not inhibit the LQV effect with the exception of high concentrations of tropisetron, 2) exogenous 5-HT had no secretory effect, and 3) there was no significant release of 5-HT from the tissue during neurogenic secretion. The inhibitory effect of tropisetron on intestinal Cl- secretion seems to be unrelated to its property as a 5-HT3 receptor antagonist.