A comparison of the pharmacological properties of guinea-pig and human recombinant 5-HT4 receptors

Background and purpose: 5-HT4 receptor agonists are used therapeutically to treat disorders of reduced gastrointestinal motility. Since such compounds are evaluated in guinea- pigs, we cloned, expressed and pharmacologically characterized the guinea- pig 5-HT4 and human 5- HT4(b) splice variant, which share 95% homology. The functional properties of guinea- pig 5- HT4(b) receptors were compared with native receptors in guinea- pig colon. Experimental approach: Membrane radioligand binding and whole cell cAMP accumulation assays were used to determine the affinities, potencies and intrinsic activities ( IA). Contraction of the guinea- pig distal colon longitudinal muscle myenteric plexus preparation ( LMMP) was monitored to evaluate functional activity. Key results: pK(i) values for guinea- pig and human recombinant receptors, and guinea- pig striatum 5- HT4 receptors, were in agreement, as were the potency and IA values for guinea- pig and human 5- HT4 receptors expressed at a similar density (similar to 0.2 pmol mg(-1) protein). Tegaserod was a potent ( pEC(50) = 8.4 and 8.7, respectively), full agonist at both guinea- pig and human 5- HT4 receptors. In contrast, in the LMMP preparation, tegaserod was a potent, partial agonist ( pEC(50) = 8.2; IA 66%). Conclusions and implications: Close agreement between the pharmacological properties of guinea- pig and human 5- HT4 receptors support the use of guinea- pig model systems for the identification of 5- HT4 receptor therapeutics. However, the mechanisms underlying the different agonist properties of tegaserod in recombinant and isolated tissue preparations, and the extent to which these impact the clinical efficacy of tegaserod as a prokinetic agent, remain to be determined.