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Downregulation of the alpha2-isoform causes attenuated control of NCX activity in CHF, reducing its capability to extrude Ca2+ from cardiomyocytes.

Abstract-Cardiac responsiveness to neurohumoral stimulation is altered in congestive heart failure (CHF). In chronic CHF, the left ventricle has become sensitive to serotonin because of appearance of G s -coupled 5-HT 4 receptors. Whether this also occurs in acute CHF is unknown. Serotonin responsiveness may develop gradually or represent an early response to the insult. Furthermore, serotonin receptor expression could vary with progression of the disease. Postinfarction CHF was induced in male Wistar rats by coronary artery ligation with nonligated sham-operated rats as control. Contractility was measured in left ventricular papillary muscles and mRNA quantified by real-time reverse-transcription PCR. Myosin light chain-2 phosphorylation was determined by charged gel electrophoresis and Western blotting. Ca 2ϩ transients in CHF were measured in field stimulated fluo-4-loaded cardiomyocytes. A novel 5-HT 2A receptor-mediated inotropic response was detected in acute failing ventricle, accompanied by increased 5-HT 2A mRNA levels. Functionally, this receptor dominated over 5-HT 4 receptors that were also induced. The 5-HT 2A receptor-mediated inotropic response displayed a triphasic pattern, shaped by temporally different activation of Ca 2ϩ -calmodulin-dependent myosin light chain kinase, Rho-associated kinase and inhibitory protein kinase C, and was accompanied by increased myosin light chain-2 phosphorylation. Ca 2ϩ transients were slightly decreased by 5-HT 2A stimulation. The acute failing rat ventricle is, thus, dually regulated by serotonin through G q -coupled 5-HT 2A Key Words: serotonin Ⅲ heart failure Ⅲ 5-HT 2A receptor Ⅲ Ca 2ϩ -calmodulin-dependent myosin light chain kinase Ⅲ Rho-associated kinase D irect cardioexcitation by the neurotransmitter and vasoactive mediator serotonin (5-hydroxytryptamine [5-HT]) was believed until recently to be restricted to atria because of the lack of positive inotropic effects in nondiseased ventricular tissues from different species. 1,2 In contrast to previous reports, we recently discovered functional 5-HT 4 receptors and increased 5-HT 4 mRNA levels in chronic failing human and rat ventricle, demonstrating altered cardiac serotonin responsiveness in chronic heart failure. 3,4 Recent studies also demonstrated involvement of 5-HT 2B receptors in cardiac hypertrophy and failure, emphasizing the pathophysiological relevance of serotonin in cardiac disease. 5 An extensive acute myocardial infarction (MI) causes a substantial loss of viable myocardium within a few hours and leads to progressive cardiac dysfunction. Consequently, a variety of compensatory mechanisms are initiated, such as increased neurohumoral drive, hypertrophy, and possibly reactivation of fetal genes, to rescue myocardial function. 6 Heart failure is a progressive disease and gene expression, as well as phenotype, will be different at various stages. It is not known whether serotonin receptor expression increases gradually or is even higher in the acute phase. It is possible that the pattern of expre...

The reduced power of the failing heart can be ascribed to a combination of reduced force and slower contraction. We hypothesized that these two properties are due to different cellular mechanisms. We measured contraction parameters both in vivo and in isolated left ventricular (LV) cardiomyocytes from a rat model of post infarction congestive heart failure (CHF). ECG was measured simultaneously with echocardiography and LV pressure, respectively. Shortening and shortening velocity (SV) in isolated cardiomyocytes were measured during different stimulation protocols. LV end diastolic pressure (LVEDP) was 24.6 +/- 0.7 mmHg in CHF. LV systolic pressure was decreased by 20%, maximum rate of pressure development in the LV (+dP/dtmax) by 36% and time in systole increased by 20% in CHF compared to sham. Electrical remodelling occurred in CHF cells, which were depolarized and had prolonged action potentials (AP) compared to sham cells. Fractional shortening (FS) was increased in CHF compared to sham independent of stimulation protocol. Larger FS was accompanied by increased sarcoplasmic reticulum (SR) Ca2+ load and depended on the electrical remodelling. Time to peak contraction (TTP) was increased in CHF compared to sham cells, but in contrast to FS, TTP was only slightly affected when the cells were stimulated with sham APs and sham diastolic membrane potential (DMP). Contraction duration (corresponding to systolic duration) was 25% longer in CHF than in sham independent on stimulation protocol. We conclude that electrical remodelling affecting DMP and AP duration (APD) significantly affects the size of contraction, whereas the mechanism for slowing of contraction in CHF is different.

Congestive heart failure (CHF) induces changes in the neurohumoral system and gene expression in viable myocardium. Several of these genes encode G protein-coupled receptors (GPCRs) involved in mechanisms which compensate for impaired myocardial function. We used real-time quantitative RT-PCR (Q-RT-PCR) to investigate the expression of mRNA encoding 15 different GPCRs possibly involved in CHF, and the effect of normalisation to GAPDH mRNA (GAPDH) or 18S rRNA (18S). CHF was induced in rats by coronary artery ligation, with sham-operated controls (Sham). After 6 weeks, mRNA expression in viable left ventricular myocardium was determined using both 18S and GAPDH as the normalisation standard. An apparent 30% reduction in GAPDH mRNA levels vs. 18S in CHF compared to Sham, although not significant in itself, influenced the interpretation of regulation of other genes.Thus, levels of mRNA encoding receptors for angiotensin II (AT(1)), endothelin (ET(A), ET(B)) and the muscarinic acetylcholine (mACh) receptor M(1) increased significantly in CHF only when normalised to GAPDH. Levels of mRNA encoding the mACh receptors M(3) and M(4) and the serotonin receptors 5-HT(2A) and 5-HT(4) increased, whereas alpha(1D)-adrenoceptor mRNA decreased in CHF irrespective of the normalisation standard. No significant change was detected for M2 and M5 mACh receptors or alpha(1A)-, alpha(1B)-, beta(1)- or beta(2)-adrenoceptors. Q-RT-PCR is a sensitive and powerful method to monitor changes in GPCR mRNA expression in CHF. However, the normalisation standard used is important for the interpretation of mRNA regulation.

Systolic heart failure may be due to too few cardiomyocytes, or to reduced contractile function of the heart cells. In the latter situation the myocardial function is impaired and this condition is called myocardial failure. The pathophysiological mechanism behind this cellular defect is not known, but Ca2+ handling is altered. Although the most important trigger of sarcoplasmatic reticulum (SR) Ca2+ release, the L-type Ca2+ current, seems to be unaltered, SR Ca2+ load is reduced in human heart failure. This could explain the reduced contractility observed in failing hearts. Three possible mechanisms have been suggested to explain the reduction in SR Ca2+ load. They are leak through the SR Ca2+ release channel (RyR), impaired SR Ca2+ ATPase (SERCA) function and increased Na+/Ca2+-exchanger (NCX) function. Leak through RyR is not consistently found. Increased NCX function is probably secondary to a change in Ca2+ handling, and thus not a primary mechanism, but blockade of the NCX might have therapeutic potential. Reduced SERCA function is probably a primary mechanism for the observed systolic dysfunction, and further insight is to be gained through studies in genetically modified models.

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