Serotonin increases L-type Ca2+ current and SR Ca2+ content through 5-HT4 receptors in failing rat ventricular cardiomyocytes

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 wk after induction of postinfarction CHF. Contractile function and Ca2+ transients were measured in field-stimulated cardiomyocytes, and L-type Ca2+ current (I-Ca,I-L) and sarcoplasmic reticulum ( SR) Ca2+ content were measured 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). I-Ca,I-L and SR Ca2+ content increased only after 5-HT4-receptor stimulation (57 and 65%; both P < 0.05). Phospholamban serine(16) (PLB-Ser(16)) 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 and did not significantly change the phosphorylation of PLB-Ser(16) or troponin I, but it increased myosin light chain 2 (MLC2) phosphorylation. In conclusion, the positive inotropic response to 5-HT4 stimulation results from increased ICa, L and increased phosphorylation of PLB-Ser(16), which increases the SR Ca2+ content. 5-HT4 stimulation is thus, like beta-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.