Comparison of the densities of 5-HT4 receptors, beta(1)- and beta(2)-adrenoceptors in human atrium: Functional implications

We measured in human atrium the density of 5-HT4 receptors, labelled with [I-125]-SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1, 4-benzodioxan-5-carboxylate), and compared it with the density of beta(1)- and beta(2)-adrenoceptors, labelled with (-)-[I-125]-cyanopindolol. [I-125]-SB 207710 (5-1200 pmol/l) labelled a small population of saturable binding sites (B-max approximate to 4 fmol/mg protein) with a pK(D) of 9.7 and with 5-HT4 receptor characteristics, as assessed with competing ligands. The density of atrial binding sites with 5-HT4 receptor characteristics was 10 and 5 times lower, respectively, than the density of beta(1)- and beta(2)-adrenoceptors. We suggest that the small 5-HT4 receptor population may in part explain why the positive inotropic effects of 5-HT are smaller than those of catecholamines mediated through beta(1)- and beta(2)-adrenoceptors.