5HT4(a) and 5-HT4(b) receptors have nearly identical pharmacology and are both expressed in human atrium and ventricle - Bach, T; Syversveen, T; Kvingedal, A M; Krobert, K A; Brattelid, T; Kaumann, A J; Levy, F O (Naunyn Schmiedebergs Arch Pharmacol, 2001)