CHARACTERIZATION OF THE CONTRACTION TO 5-HT IN THE CANINE COLON LONGITUDINAL MUSCLE

1. Although conscious dogs have often been used for colonic motility s tudies with 5-hydroxytryptamine (5-HT), the effects of 5-HT on the iso lated colon have not been thoroughly characterized yet. The current st udy was undertaken to characterize the response to 5-HT of the canine isolated colon longitudinal muscle. 2 Longitudinal strips of canine mi dcolon deprived of (sub)mucosa were prepared for isotonic measurement. 5-HT induced contractions from 3 nM onwards, which were not affected by selective inhibition of 5-HT re-uptake, monoamine oxidase or blocka de of a-adrenoceptors. Tetrodotoxin (0.3 mu M) did not affect the resp onses to 5-HT, suggesting that smooth muscle 5-HT receptors are involv ed. The selective 5-HT4 receptor antagonist SE 204070 (10 nM) slightly enhanced contractions to 5-HT and therefore it was included in the or gan bath solution in all further experiments. The 5-HT1 and 5-HT2 rece ptor antagonist methysergide (0.1 mu M) depressed the curve to 5-HT, b ut the selective 5-HT3 receptor antagonist granisetron (0.3 mu M) had no effect. 3 Besides 5-HT, alpha-methyl-5-HT (alpha-Me-5-HT), 5-methox ytryptamine (5-MeOT), 2-methyl-5-HT (2-Me5-HT) and 5-carboxamidotrypta mine (5-CT) also induced contractions, with the following rank order o f potency (pEC(50) values in parentheses): 5-HT (6.9) = alpha-methyl-5 -HT (6.9) > 2-Me-5-HT (5.8) = 5-MeOT (5.7) = 5-CT (5.6), indicative of 5-HT2 receptor involvement. alpha-Me-5-HT produced a bell-shaped curv e, which was not affected by alpha-adrenoceptor blockade. 5-HT, 5-MeOT , 2-Me-5-HT and 5-CT produced a monophasic concentration-response curv e, consistent with an interaction with a single receptor site. 8-Hydro xy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and tryptamine only induc ed contractions at a concentration exceeding 1 mu M. 4 The selective 5 -HT2B receptor antagonist SE 204741 (0.3 mu M) did not affect the curv e to 5-HT. Ketanserin, cisapride and spiroxatrine behaved as competiti ve antagonists with pK(b) values of, respectively, 8.4, 8.1 and 6.7. S piroxatrine (1 mu M) shifted the curve to 5-MeOT rightward yielding an apparent pA(2) of 7.1. Other antagonists at 5-HT2A receptors also sur mountably inhibited the contractions to 5-HT (apparent pA(2) value in parentheses): mesulergine (8.2), cinanserin (8.2), yohimbine (6.2) and mianserin (8.6). However, as well as a rightward shift, methiothepin (8.3), pizotifen (8.6) and spiperone (8.8) also caused a depression of the curve, indicative of 'pseudo-irreversible' antagonism. Taken toge ther, the above mentioned affinity estimates most closely corresponded to literature affinity values for 5-HT2A receptors. 5 It was conclude d that 5-HT induces contractions of the canine midcolon longitudinal m uscle primarily by stimulation of smooth muscle 5-HT2A receptors. The presence of inhibitory 5-HT4 receptors cannot be ruled out.