Dual Serotonergic Regulation of Ventricular Contractile Force Through 5-HT2A and 5-HT4 Receptors Induced in the Acute Failing Heart
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 Gs-coupled 5-HT4 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. Ca2+ transients in CHF were measured in field stimulated fluo-4-loaded cardiomyocytes. A novel 5-HT2A receptor-mediated inotropic response was detected in acute failing ventricle, accompanied by increased 5-HT2A mRNA levels. Functionally, this receptor dominated over 5-HT4 receptors that were also induced. The 5-HT2A receptor-mediated inotropic response displayed a triphasic pattern, shaped by temporally different activation of Ca2+-calmodulin-dependent myosin light chain kinase, Rho-associated kinase and inhibitory protein kinase C, and was accompanied by increased myosin light chain-2 phosphorylation. Ca2+ transients were slightly decreased by 5-HT2A stimulation. The acute failing rat ventricle is, thus, dually regulated by serotonin through Gq-coupled 5-HT2A receptors and Gs-coupled 5-HT4 receptors.
- heart failure
- 5-HT2A receptor
- Ca2+-calmodulin-dependent myosin light chain kinase
- Rho-associated kinase
Direct 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-HT4 receptors and increased 5-HT4 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-HT2B 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 expression of various serotonin receptor subtypes is different at various stages of the disease.
We now report a composite inotropic serotonin response in failing left ventricle 3 days after MI, mediated predominantly through appearing 5-HT2A receptors but also through appearing 5-HT4 receptors. The 5-HT2A response involved increased myosin light chain (MLC)-2 phosphorylation.
Materials and Methods
An expanded Materials and Methods section is available in the online data supplement at http://circres.ahajournals.org.
Congestive Heart Failure Model
Animal care was carried out according to the NIH Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23, Revised 1996). As described,7 an extensive MI was induced in 320 g male Wistar rats by coronary artery ligation. Three days later, rats were included, provided lung weight >2.0 g and clinical signs of overt congestive heart failure (CHF) were present. Left-ventricular end diastolic (LVEDP) and systolic (LVSP) pressures were measured by catheterization.7 Some rats were included at 4, 7, and 42 days postinfarction for estimation of pD2 (−logEC50) values for serotonin (Figure 1). Sham-operated animals (Sham) underwent similar surgery without coronary artery ligation.
Isolated Papillary Muscles
Posterior left ventricular papillary muscles were prepared and the contraction-relaxation cycles (CRCs) were recorded and analyzed as previously described.8 Maximal development of force (dF/dt)max was used as an index of contractility, and inotropic responses to agonists were expressed by increases in (dF/dt)max. Lusitropic responses were expressed as reduction in RT (time to 80% relaxation−time to peak force [TPF]). The descriptive parameters at the end of the equilibration period were used as control values. Blockers of adrenergic (1 μmol/L prazosin; 1 μmol/L timolol) and muscarinic cholinergic (1 μmol/L atropine) receptors were present in the experiments and, where indicated, the 5-HT2A receptor blocker ketanserin (0.1 μmol/L) and the 5-HT4 receptor blocker GR113808 (1.0 μmol/L). Serotonin was added to the organ baths cumulatively (concentration-response curves) or as a single bolus (10 μmol/L). The inhibition constant Kb of ketanserin was calculated from the ratio of EC50 of serotonin in the absence and presence of ketanserin.
Quantitative Reverse-Transcription PCR
Left ventricular sample preparation, cDNA synthesis, and quantitative reverse-transcription PCR were performed as previously described.4
Phosphorylation of MLC-2 was determined by charged gel electrophoresis9 by using anti-ventricular MLC-2 monoclonal antibody, and the identity of the phosphorylated MLC-2 band was confirmed with a phospho-specific anti-MLC-2 (Ser19) antibody. MLC phosphorylation is reported as phosphorylated MLC-2 in percentage of total MLC-2.
Ca2+ Transients in Cardiomyocytes in Response to 5-HT2A Stimulation
Ca2+ transients were measured at 37°C in field stimulated rat cardiomyocytes, enzymatically isolated, loaded with fluo-4 acetoxymethyl ester (AM) and cells were studied with a Zeiss LSM 510 laser scanning microscope as previously described.10 The cells were superfused with a solution containing (in mmol/L); 4 KCl, 1 MgCl2, 145 NaCl2, 10 HEPES, 10 glucose, 1.8 CaCl2, 0.4 NaH2PO4 (pH 7.4).
Data are expressed as mean±SEM from n animals. P<0.05 was considered statistically significant (Student t test and nonparametric Mann-Whitney test). When appropriate, Bonferroni corrections were made.
All CHF rats had large antero-lateral infarcts and signs of CHF including tachypnea, pleural effusion, and pulmonary congestion (160±11% increase in lung weight versus Sham at 3 days). Hemodynamic data at 3 days suggested diastolic and systolic heart failure with increased LVEDP (334±7%) and decreased LVSP (22±4.5%) versus Sham. CHF papillary muscles revealed 28±1.8% decreased basal contractile force versus Sham. Animal characteristics are summarized in Table 1.
Serotonin Potency Is Lower in Acute Than Chronic CHF
During the study of chronic CHF hearts,4 some hearts harvested 3, 4, and 7 days after MI displayed inotropic effects of serotonin with lower potency compared with chronic (42-day) CHF, pD2 (−logEC50) values ranging from 6.56±0.15 (n=5) at 3 days to 7.50±0.08 (n=6) at 42 days post-MI (Figure 1). This 10-fold lower sensitivity to serotonin could reflect a different 5-HT receptor population in acute than chronic CHF, where 5-HT4 was the dominant receptor subtype.4 Accordingly, all further experiments were performed on acute (3-day) postinfarction failing hearts to systematically explore the possible contribution of different serotonin receptors to the inotropic response in acute CHF.
Both 5-HT2A and 5-HT4 Receptors Contribute to Inotropic Effects of Serotonin in Acute CHF
Serotonin (10 μmol/L) elicited a positive inotropic response in papillary muscles from acute CHF rats of 78.0±15.3% above control (n=7) compared with a negligible mechanical response in Sham hearts (2.2±1.0%, n=10, Figure 2A). The inotropic effect of serotonin in CHF was of similar magnitude as that of 10 μmol/L isoproterenol (76.8±10.2%, n=7), which was reduced compared with Sham (110.6±7.2%, n=10, P<0.05; Figure 2A). The lower sensitivity to serotonin in acute CHF (Figure 1) suggested involvement of a receptor with lower serotonin affinity, such as 5-HT2A, also known to be present in rat atria,2 possibly in combination with 5-HT4, which we had found in chronic failing ventricle.4 With the 5-HT4-selective antagonist GR113808 (1 μmol/L) and the 5-HT2A-selective antagonist ketanserin (0.1 μmol/L) present, the positive inotropic response to 10 μmol/L of serotonin was essentially abolished (0.4±0.4%, n=8; Figure 2B), indicating that these 2 receptors fully accounted for the serotonin effect. The individual contribution of 5-HT2A and 5-HT4 to the serotonin response was determined using the selective antagonists. In the presence of GR113808 (1 μmol/L), serotonin (10 μmol/L) elicited a positive inotropic response of 44.9±6.6% (n=11; Figure 2B), completely reversible by ketanserin (0.1 μmol/L) to −1.3±2.1% (P<0.05; Figure 2C) when added at steady-state serotonin response. In the presence of ketanserin (0.1 μmol/L), serotonin (10 μmol/L) elicited a positive inotropic response of 20.2±9.2% (n=8; Figure 2B), completely reversible by GR113808 (1 μmol/L) to −3.4±1.5% (P<0.05, Figure 2D) when given subsequently to serotonin, similar to the previously demonstrated 5-HT4 response in chronic CHF.4 Thus, acute failing hearts display a novel ventricular 5-HT2A-mediated inotropic effect, in addition to a 5-HT4-mediated inotropic effect.
The possible different serotonin sensitivity of these 2 components was addressed by serotonin concentration-response curves in the presence of the respective blockers (Figure 3A). In the presence of GR113808 (1 μmol/L), the serotonin concentration-response curve exhibited a pD2 value of 6.10±0.06 (n=7) and a maximal inotropic effect, at ≈10 μmol/L, of 24.3±5.4% above control. In contrast, the presence of ketanserin (0.1 μmol/L) revealed a pD2 value of 7.15±0.01 (n=5) and a maximal inotropic effect, at ≈1 μmol/L of serotonin, of 14.6±4.6% above control. The absence of antagonists resulted in an intermediate pD2 value of 6.56±0.15 (n=6) and a maximal inotropic effect, at ≈10 μmol/L of serotonin, of 22.4±4.6% above control. The 3 pD2 values were significantly different from each other (P<0.05). The higher maximal inotropic effect in the single-dose (10 μmol/L serotonin) compared with the concentration-response experiments may reflect desensitization during cumulative agonist addition in the latter.
5-HT2A-Mediated Inotropic Response to Serotonin in Acute Failing Hearts
The affinity of ketanserin for the putative 5-HT2A receptor was determined by constructing serotonin concentration-response curves in the presence of full 5-HT4 blockade by GR113808 (1 μmol/L) without and with 1.3 nmol/L or 5 nmol/L ketanserin (Figure 3B). Parallel curves with pD2 values of 6.10±0.06 (n=7), 5.70±0.01 (n=5), and 5.26±0.07 (n=7), respectively, with similar basal and maximum inotropic response (pooled average 20.2±2.7%, n=19) yielded an average inhibition constant (Kb) for ketanserin of 0.8 nmol/L (−logKb=9.1), in agreement with reported ketanserin affinity at 5-HT2A receptors.2
Qualitative Characteristics of Serotonin-Mediated Inotropic Effects in Acute CHF
In the presence of GR113808 (1 μmol/L), the time from serotonin addition to 50% and maximal 5-HT2A-mediated inotropic response was 180±13 seconds and 6 to 7 minutes, respectively (Figure 3C). The time-response curve after 5-HT2A stimulation followed a triphasic pattern with an initial fast component followed by a transient negative phase and subsequently a slowly developing sustained inotropic response (Figure 3C) similar to that described for α1-adrenoceptors.11 Serotonin did not significantly alter TPF and RT (Figure 2C, Table 2).
In the presence of ketanserin (0.1 μmol/L), the time from serotonin addition to 50% and maximal 5-HT4 receptor-mediated inotropic response was 29±1.7 seconds and ≈1 to 2 minutes, respectively (Figure 3C). Serotonin now reduced TPF and RT significantly, demonstrating a lusitropic effect in addition to the inotropic effect, similar to that of β-adrenoceptor stimulation (Figure 2D and Table 2).
Combined 5-HT2A– and 5-HT4-Mediated Effects
In the absence of GR113808 and ketanserin, the time from serotonin addition to 50% and maximal inotropic response was 42±10 seconds and 3 minutes, respectively (Figure 3C). Serotonin reduced TPF and RT slightly but not significantly in both the single bolus and concentration-response experiments (Table 2). This is different from the pure 5-HT4 effect in this study, characterized by a hastening of relaxation, comparable to that of β-adrenoceptor stimulation.8
Induction of 5-HT2A and 5-HT4 Receptor mRNA in 3-Day Postinfarction CHF
Real-time quantitative reverse-transcription PCR was used to measure changes in 5-HT2A and 5-HT4(b) mRNA levels normalized to GAPDH mRNA in CHF left ventricle compared with Sham (Figure 4). 5-HT2A mRNA increased 3.8-fold (P<0.05) and 5-HT4 mRNA increased 2.2-fold (P<0.05) in CHF compared with Sham, both in agreement with the functional data. Atrial natriuretic peptide (ANP) mRNA (CHF marker) was also significantly increased versus Sham.
Molecular Mechanisms of the 5-HT2A-Mediated Inotropic Response
Effects of MLC Kinase Inhibition
MLC kinase (MLCK) is involved in the α1-adrenoceptor-mediated positive inotropic response in rat and human heart.12 The 5-HT2A-mediated mechanical response pattern in CHF resembles the α1-adrenoceptor response in rat.11 We investigated whether the 5-HT2A response was dependent on the same signal transduction pathway. The MLCK inhibitor ML-9 (50 μmol/L) essentially abolished the sustained positive inotropic 5-HT2A-mediated serotonin response (3.7±1.5%, n=6 with versus 44.9±6.6%, n=11 without, P<0.05; Figure 5A and 5B). The initial fast positive component was also inhibited by ML-9 (1.2±0.3% with versus 8.7±0.9% without, P<0.05; Figure 5A), whereas the transient negative component was enhanced (-8.4±2.5% with versus 2.5±1.5% without, P<0.05, Figure 5A). The β-adrenoceptor-mediated response to isoproterenol was not inhibited by ML-9 (82.6±3.6%, n=6 with versus 76.8±10.2% without, n=7; not significant). ML-9 reduced basal contractility by 26.9±2.1% (n=6, P<0.05), consistent with previous results.12
Effects of Calmodulin Inhibition
The Ca2+-binding protein calmodulin activates MLCK and regulates smooth muscle contraction, but its involvement in myocardial Gq-coupled receptor-mediated inotropic effects has not been explored. The calmodulin antagonist W-7 (50 μmol/L) significantly inhibited the sustained positive inotropic 5-HT2A effect (13.8±6.7%, n=6 with W-7 versus 44.9±6.6%, n=11 without, P<0.05; Figure 5A). The initial positive component was decreased (3.7±0.9% versus 8.7±0.9%, P<0.05; Figure 5A) and the transient negative component unaltered (Figure 5A). The β-adrenoceptor-mediated response to isoproterenol was not significantly different from control (58.7±9.7, n=6 with W-7 versus 76.8±10.2%, n=7 without). W-7 decreased basal contractile force by 27.7±6.7%, n=6.
Effects of Rho-Associated Kinase Inhibition
Rho-associated kinase (ROCK) regulates MLC phosphorylation and the α1-adrenoceptor-mediated response through both inactivation of myosin phosphatase and direct phosphorylation of MLC-2.12,13 The selective ROCK inhibitor Y-27632 (50 μmol/L) significantly attenuated the sustained positive 5-HT2A-mediated inotropic effect when added 45 minutes before 10 μmol/L of serotonin (17.7±4.9%, n=6 with versus 44.9±6.6%, n=11 without, P<0.05; Figure 5A and 5B). The initial positive and the transient negative components were not significantly changed by Y-27632 (Figure 5A). The β-adrenoceptor-mediated response was not significantly affected by Y-27632 (86.9±3.7%, n=6, with versus 76.8±10.2%, n=7 without). Y-27632 decreased basal contractile force by 32.0±2.2%, n=6. When Y-27632 was added after 5-HT2A stimulation, the response was reversed from 42.0±9.8% to −12.1±5.5% (n=5, P<0.05). This corresponds to 78% reversal of the inotropic 5-HT2A response when corrected for the cardiodepressive effect of Y-27632.
Effects of Protein Kinase C Inhibition
The negative inotropic component of the α1-adrenoceptor response in rat and mouse heart is mediated through the diacylglycerol-protein kinase C (PKC) signal transduction pathway.14,15 A 5-HT2A-mediated negative inotropic component has not been previously demonstrated. Bisindolylmaleimide I (BIM) (10 μmol/L), an inhibitor of all PKC isoforms, completely abolished the transient negative 5-HT2A response (Figure 5A), yielding an almost monophasic time course (Figure 5A). In addition, the initial and the sustained positive inotropic responses were increased to 17.6±1.6% (n=5, versus 8.7±0.9% without BIM, n=11, P<0.05) and 50.6±4.7% (n=5 versus 44.9±6.6%, n=11 without BIM, not significant; Figure 5A), respectively. BIM increased basal contractility by 11.5±3.1%. Gö6976 (10 μmol/L), a selective inhibitor of Ca2+-dependent PKC (α,β) isoforms,16 did not significantly change the triphasic 5-HT2A response (Figure 5D), whereas the relatively selective PKCδ inhibitor rottlerin17 (1, 3, and 5 μmol/L) demonstrated a concentration-dependent inhibition of the transient negative 5-HT2A component (Figure 5D), possibly implicating PKCδ in this negative component.
MLC-2 Phosphorylation Levels in Papillary Muscles After 5-HT2A Stimulation
In the basal state, phosphorylated MLC-2 (all values given as percentage of total MLC-2) was significantly lower in CHF (14.3±2.7%, n=3; Figure 5C) compared with Sham (23.8±1.1%, n=3, P=0.03). Serotonin (10 μmol/L; 1 μmol/L GR113808 present) led to a 1.79-fold increase in phosphorylated MLC-2 (25.6±2.0%, n=4, P=0.02) that was abolished when muscles were treated with inhibitors of MLCK (ML-9; 16.0±0.3%, n=3) or ROCK (Y-27632; 15.8±1.1%, n=4) but not with the PKC inhibitor BIM (25.9±4.7%, n=4).
5-HT2A-Mediated Effects on Ca2+ Transients in Isolated Cardiomyocytes
The 5-HT2A effects on Ca2+ transients in CHF were measured with a laser scanning confocal microscope in field-stimulated cardiomyocytes loaded with fluo-4. Figure 6 shows representative line scan diagrams illustrating Ca2+ transients recorded from CHF myocytes before and during selective 5-HT2A stimulation with serotonin (10 μmol/L; 1 μmol/L GR113808 present). The 5-HT2A stimulation data were corrected for a 6.3% decrease in maximal systolic fluorescence normalized to diastolic fluorescence (F/F0) over time in nonstimulated myocytes (not shown). 5-HT2A stimulation induced an 8.9% reduction in the Ca2+ transients (P<0.05). As a control, isoproterenol (10 μmol/L) induced a >50% increase in the Ca2+ transient.
We demonstrate, for the first time, induction of functional 5-HT2A serotonin receptors in acute failing cardiac ventricle, accompanied by increased 5-HT2A mRNA expression. Stimulation of Gq-coupled 5-HT2A receptors elicits a triphasic inotropic response, resulting from temporally distinct activation of Ca2+-calmodulin-dependent MLCK, ROCK, and inhibitory PKC and increases MLC-2 phosphorylation. 5-HT2A stimulation did not increase Ca2+ transients. We also found induction in acute CHF of functional 5-HT4 receptors and 5-HT4 mRNA, previously documented in chronic CHF.3,4 Quantitatively, the 5-HT2A response dominates over the 5-HT4 response in acute failing ventricle.
Two independent pharmacological criteria identify the functional non-5-HT4 receptor in acute CHF as 5-HT2A. First, a hallmark of 5-HT2A receptors is low affinity for serotonin, translating into low potency in functional assays. The GR113808-resistant serotonin response showed the same potency as that of 5-HT2A in rat atria,2 ≈10-fold lower than the potency of the ketanserin-resistant response (5-HT4; Figure 3A). Second, the high affinity for the 5-HT2A-selective antagonist ketanserin (pKb=9.1), as determined by the shifts of the serotonin concentration-response curves, is only compatible with the 5-HT2A. For example, with a pKi of 7.3 and 5.5 for rat 5-HT2C18 and 5-HT2B receptors,19 respectively, the ketanserin concentrations (1.3 nmol/L and 5.0 nmol/L) used here would not move the serotonin concentration-response curves for 5-HT2B or 5-HT2C-mediated responses. Similarly, the presence of 0.1 μmol/L ketanserin would cause <1% and 10% decreases in a 5-HT2B and 5-HT2C response, respectively. Thus, among 5-HT receptors, only 5-HT2A and 5-HT4 receptors regulate contractility in the acute failing ventricle.
Stimulation of Gq-coupled 5-HT2A receptors in acute failing ventricle elicits a rather complex temporally regulated inotropic response, different from that of Gs-coupled 5-HT4 receptors. Composed of an initial positive, a transient negative and a sustained positive phase, the response resembles that of the α1-adrenoceptor stimulation in rat.11 In contrast to cAMP-dependent effects on cardiac contractility (eg, 5-HT4, β-adrenoceptor), the 5-HT2A response develops slowly (Figure 3) and is characterized by a symmetrical change in the CRC with unchanged or slightly prolonged TPF and RT (Figure 2C and Table 2), qualitative characteristics typical of cardiac Gq-coupled receptors (eg, α1-adrenoceptors and endothelin receptors). Despite a robust positive inotropic response in acute failing hearts, we observed no increase in Ca2+ transients after 5-HT2A stimulation, in contrast to a >50% increase after activation of β-adrenoceptors. Accordingly, the 5-HT2A-mediated positive inotropic response is probably attributable to an increased myofilament Ca2+ sensitivity in acute failing ventricle.
In smooth muscle, where phosphorylation of MLC-2 is necessary for force production, G-protein-coupled receptor-mediated MLCK activation and MLC phosphatase inhibition determine the MLC-2 phosphorylation state and, thus, contraction.20 In cardiac muscle, MLC-2 phosphorylation has been shown to be important for regulation of contraction by increasing myofibrillar Ca2+ sensitivity.21 Furthermore, regulation of cardiac contractility through Gq-coupled receptors, eg, α1-adrenoceptor and endothelin receptors, is mediated through a Ca2+-sensitizing mechanism assumed to involve increased MLC-2 phosphorylation.12,22,23 On this background and given the similarity of 5-HT2A and α1-adrenoceptor inotropic responses, we wanted to determine possible involvement of MLC-2 phosphorylation.
The specific MLCK inhibitor ML-924 selectively inhibits cAMP-independent inotropic effects (eg, of phenylephrine, endothelin) in heart tissue,12 indicating that these effects are mediated via increased MLC-2 phosphorylation. Similarly, the novel ventricular 5-HT2A-mediated inotropic response was almost abolished by ML-9. Furthermore, the 5-HT2A-mediated increased phosphorylation of MLC-2 at Ser19 was also inhibited by ML-9. We found no inhibition of the β-adrenoceptor-mediated effect by ML-9, nor was the 5-HT4-mediated inotropic effect inhibited by ML-9 (not shown). Accordingly, we suggest that the sustained positive inotropic response to serotonin through 5-HT2A receptors in acute CHF depends on MLCK-mediated phosphorylation of MLC-2. This was further supported by the observed attenuation of the positive 5-HT2A response by inhibition of calmodulin, which is involved, for example, in activation of MLCK (Figure 5A). The present study is the first to demonstrate a 5-HT2A-mediated increase in MLC-2 phosphorylation at Ser19 in failing ventricular tissue. The decrease in basal MLC-2 phosphorylation in CHF compared with Sham is in accordance with a previous study.25
In smooth muscle, the Rho/ROCK pathway contributes to agonist-mediated contraction mainly through inhibition of myosin phosphatase, thus increasing phosphorylated MLC-2.26 ROCK is activated by Gq-coupled receptor signaling via the small G-protein RhoA.27 The pyridine derivative, Y-27632, is a selective ROCK inhibitor,26 and there is now increasing evidence that the Rho/ROCK pathway is involved in agonist (eg, phenylephrine)-induced inotropic effects.12,28 In the present study, Y-27632 inhibited the 5-HT2A-mediated positive inotropic effect both when added before and at steady-state response to serotonin and abolished the increased MLC-2 phosphorylation, suggesting involvement of the Rho/ROCK pathway in both increased MLC-2 phosphorylation and positive inotropic response through the novel ventricular 5-HT2A receptor. In contrast, Y-27632 did not inhibit the cAMP-dependent β-adrenoceptor-mediated inotropic response, consistent with previous reports.12,29 Both ML-9 and Y-27632 decreased basal contractility in failing ventricular tissue, demonstrating an importance of MLCK and ROCK in the maintenance of basal cardiac contractility.
PKC, activated, for example, by Gq-coupled receptors through phospholipase Cβ activation, plays a pivotal role in smooth muscle contraction by regulating the Ca2+ sensitivity through MLC-2 phosphorylation.30 In the present study, PKC inhibition by BIM did not inhibit the positive inotropic 5-HT2A response or alter the phosphorylation state of MLC-2 after serotonin stimulation. In contrast, BIM inhibited the negative inotropic component of the 5-HT2A response, which is in agreement with reported involvement of PKC in α1-adrenoceptor-mediated negative inotropic effects in mouse and rat heart.15,31 BIM, an isoform-nonselective PKC inhibitor32 produced a moderate increase in basal contractile force, probably reflecting the presence of a constitutive PKC activity suppressing contractility in failing ventricular tissue, consistent with previous reports.33 Whereas Gö6976 (10 μmol/L) which selectively inhibits the Ca2+-dependent PKC isoenzymes (PKCα: IC50=2.3 nmol/L; PKCβI: IC50=6.2 nmol/L) without affecting the kinase activity of Ca2+-independent isoforms (PKCδ, ε, ζ, μ)16 did not influence the triphasic 5-HT2A response, rottlerin (1, 3, and 5 μmol/L) dose dependently inhibited the transient negative inotropic component without affecting the positive components. The PKC inhibitor rottlerin can, to some extent, differentiate between different isoforms with at least 10-fold selectivity for PKCδ (IC50≈3 μmol/L) compared with other isoforms (PKCα, β, γ, μ: IC50≈30 μmol/L; PKCε, ζ, η: IC50 ≈80 to 100 μmol/L).17 Taken together, this suggests a possible involvement of PKCδ in the transient negative 5-HT2A response, in agreement with the observed PKCδ-dependent α1-adrenergic negative inotropic response in mice.15
We did not observe an involvement of the mitogen-activated protein kinase (MAPK) pathways in the 5-HT2A-mediated positive inotropic response. The presence of the p38 MAPK inhibitor (SB203580, 10μmo/L) or the ERK cascade inhibitor (PD98059, 10 μmol/L) did not affect the 5-HT2A-mediated inotropic response (data not shown). These results are in agreement with previous reports demonstrating no involvement of MAPK pathways on α1-adrenoceptor-mediated inotropic effects.12
In both acute and chronic failing ventricle, serotonin alone elicited an inotropic response of comparable size to the attenuated β-adrenoceptor-mediated effect. This emphasizes the dramatic alterations of cardiac serotonin responsiveness and the ability to modulate cardiac contractility after MI and CHF. Induction of functional 5-HT4 and 5-HT2A receptors in the acute failing postinfarction ventricle may be considered a compensatory mechanism to rescue contractile function after massive loss of viable myocardium. Whether sufficient levels of endogenous serotonin are present to activate serotonin receptors in the heart is currently unknown. In cardiac tissue serotonin has been identified in vascular beds, mast cells, and sympathetic nerve endings,34,35 and increased plasma levels of serotonin in CHF and hypertensive disease and after MI have been reported.36–38
In conclusion, the acute failing rat ventricle is dually regulated by serotonin through cAMP-independent mechanisms via Gq-coupled 5-HT2A receptors and cAMP-dependent mechanisms via Gs-coupled 5-HT4 receptors analogous to what is observed for the adrenoceptors (α1 and β). The mechanism of the 5-HT2A-mediated inotropic response involves phosphorylation of MLC-2 at Ser19 similar to stimulation of α1-adrenoceptors. A further challenge is to clarify the importance of the myocardial serotonergic system and the conditions and signals evoking these changes in the failing heart.
Supported by The Norwegian Council on Cardiovascular Diseases, The Research Council of Norway, Anders Jahre’s Foundation for the Promotion of Science, The Novo Nordisk Foundation, and The Family Blix foundation. Line Solberg and Nils Tovsrud provided technical assistance.
↵*Both authors contributed equally to this work.
Original received February 23, 2005; revision received June 13, 2005; accepted June 29, 2005.
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