We studied the mRNA expression and function of 5-hydroxytryptamine (5-HT) receptors as well as their signal transduction in right atrial tissue from patients undergoing cardiac surgery and right ventricular tissue from human donor hearts. In isolated, electrically driven strips from human right atrium, 5-HT exerted concentration-dependent positive inotropic effects (EC(50) value = 0.10 +/- 0.01 microM) and hastened relaxation (positive lusitropic effect). The 5-HT(4) receptor antagonists SB203186 or GR125487 antagonised these effects. 5-HT (2 microM) increased the content of cyclic adenosine monophosphate (cAMP) from 6.86 +/- 1.36 to 19.1 +/- 2.45 pmol/mg protein (n = 6, p < 0.05) but did not alter the tissue content of inositol-1,4,5-trisphosphate (IP(3)). With reverse transcription polymerase chain reaction, mRNAs coding for the 5-HT(4) receptor splice variants 5-HT(4(a)), 5-HT(4(b)) and 5-HT(4(c)) were detected in human right atrium and right ventricle. 5-HT(2A) mRNA only was measurable in human atrium. Expression level of total 5-HT(4) receptor mRNA in the right ventricle amounted to 41% (n = 5-8) of that in the right atrium. 5-HT (2 microM) increased the atrial phosphorylation states of phospholamban to 168% at serine-16 and to 150% at threonine-17 (n = 4; p < 0.05) and of the inhibitory subunit of troponin to 150% (n = 6; p < 0.05). In conclusion, the positive inotropic and lusitropic effects of 5-HT in electrically driven human right atria are mediated via 5-HT(4) receptors. These effects are accompanied by and probably due to an increase in cAMP content and the subsequent elevation of the phosphorylation state of Ca(2+) regulatory proteins.