Blockade of human and porcine myocardial 5-HT4 receptors by SB 203186.

We investigated the blockade of the positive inotropic effects of 5-hydroxytryptamine (5-HT) by SB 203 186 (piperidinoethyl-indole-3-carboxylate hydrochloride) and its affinity for 5-HT4 receptors of human right atrium and piglet left atrium. We also compared the blocking effects of SB 203 186 against 5-HT-evoked tachycardia in anaesthetised adult Yucatan minipigs as well as new-born Camborough piglets. SB 203 186 caused competitive antagonism of the positive inotropic effects of 5-HT in electrically paced atrial preparations of man (pKB = 8.9) and piglet (pKB = 8.5) at concentrations (up to 0.3 micromol/l) which were devoid of depressant or stimulant effects. The affinity of SB 203 186 for atrial 5-HT4 receptors was 30-160 times higher than that of tropisetron. 5-HT caused tachycardia with similar potency and efficacy in Yucatan minipigs and new-born Camborough piglets. SB 203 186 (0.1-3 mg/kg, i.v.) surmountably antagonised 5-HT-evoked tachycardia in anaesthetised Yucatan minipigs or new-born Camborough piglets with similar potency. The blocking potency of SB 203 186 in Yucatan minipigs was 17 times higher than that of tropisetron. Intraduodenally administered SB 203 186 (0.3-3 mg/kg) to new-born Camborough piglets produced blockade of 5-HT-evoked tachycardia which was maximal after 20 min and lasted for more than 3 h with 0.3 mg/kg. The antagonism produced by the SB 203 186 administration in new-born Camborough piglets was dose-related and threefold greater through the intravenous route than through the intraduodenal route. We conclude that SB 203 186 is an antagonist with nanomolar affinity for both human and porcine atrial 5-HT4 receptor. The in vivo results demonstrate that the sinoatrial 5-HT4 receptors function is similar in new-born Camborough piglets and adult Yucatan minipigs. Both porcine breeds are valid models for human atrial 5-HT4 receptors as demonstrated with the antagonist SB 203 186.