Submitted on December 17, 2006
Accepted on July 26, 2007
Serotonin increases L-type Ca2+ current and SR Ca2+ content through 5-HT4 receptors in failing rat ventricular cardiomyocytes
Jon Arne Kro Birkeland1*, Fredrik Swift2, Nils Tovsrud3, Ulla H Enger3, Per Kristian Lunde1, Eirik Qvigstad4, Finn Olav Levy5, Ole M. Sejersted3, and Ivar Sjaastad3
1 Institute for Experimental Medical Research, University of Oslo, Oslo, Norway
2 Institute for Experimental Medical Research, University of Oslo, Oslo, Oslo, Norway
3 Institute For Experimental Medical Research, University of Oslo, Oslo, Norway
4 Department of Pharmacology, University of Oslo, Oslo, Norway
5 Dapartment of Pharmacology, University of Oslo, Oslo, Norway
* To whom correspondence should be addressed. E-mail: j.a.birkeland{at}medisin.uio.no.
Rats with congestive heart failure (CHF) develop ventricular inotropic responsiveness to serotonin (5-HT), mediated through 5-HT2A and 5-HT4 receptors. Human ventricle is similarly responsive to 5-HT through 5-HT4 receptors. We studied isolated ventricular cardiomyocytes to clarify the effects of 5-HT on intracellular Ca2+ handling. Left ventricular cardiomyocytes were isolated from male Wistar rats 6 weeks after induction of post-infarction CHF. Contractile function and Ca2+ transients were measured in field stimulated cardiomyocytes, and L-type Ca2+-current (ICa,L) and sarcoplasmatic reticulum (SR) Ca2+ content in voltage clamped cells. Protein phosphorylation was measured by Western blotting or phosphoprotein gel staining. 5-HT4 and 5-HT2A receptor stimulation induced a positive inotropic response of 33% and 18% (both p<0.05), and also increased the Ca2+ transient (44% and 6%, respectively, both p<0.05). ICa,L and SR Ca2+ content increased only after 5-HT4 receptor stimulation (57% and 65%, both p<0.05). Phospholamban serine16 (PLB-Ser16) and troponin I phosphorylation increased by 26% and 13% after 5-HT4 receptor stimulation (p<0.05). 5-HT2A receptor stimulation increased the action potential duration, did not significantly change the phosphorylation of PLB-Ser16 or troponin I, but increased myosin light chain (MLC2) phosphorylation. In conclusion, the positive inotropic response to 5-HT4 stimulation results from increased ICa,L and increased phosphorylation of PLB-Ser16 which increases the SR Ca2+ content. 5-HT4 stimulation is thus, like 
-adrenoceptor stimulation, possibly energetically unfavorable in CHF. 5-HT2A receptor stimulation, previously studied in acute CHF, induces a positive inotropic response also in chronic CHF, probably mediated by MLC2 phosphorylation.
