A Xanthine-Derivative K⁺-Channel Opener Protects against Serotonin-Induced Cardiomyocyte Hypertrophy via the Modulation of Protein Kinases

Abstract

This study investigated whether KMUP-1, a xanthine-derivative K⁺ channel opener, could prevent serotonin-induced hypertrophy in H9c2 cardiomyocytes via L-type Ca²⁺ channels (LTCCs). Rat heart-derived H9c2 cells were incubated with serotonin (10 μM) for 4 days. The cell size increased by 155.5%, and this was reversed by KMUP-1 (≥1 μM), and attenuated by the LTCC blocker verapamil (1 μM) and the 5-HT_2A antagonist ketanserin (0.1 μM), but unaffected by the 5-HT_2B antagonist SB206553. A perforated whole-cell patch-clamp technique was used to investigate Ca²⁺ currents through LTCCs in serotonin-induced H9c2 hypertrophy, in which cell capacitance and current density were increased. The LTCC current (I_Ca,L) increased ~2.9-fold in serotonin-elicited H9c2 hypertrophy, which was attenuated by verapamil and ketanserin, but not affected by SB206553 (0.1 μM). Serotonin-increased I_Ca,L was reduced by KMUP-1, PKA and PKC inhibitors (H-89, 1 μM and chelerythrine, 1 μM) while the current was enhanced by the PKC activator PMA, (1 μM) but not the PKA activator 8-Br-cAMP (100 μM), and was abolished by KMUP-1. In contrast, serotonin-increased I_Ca,L was blunted by the PKG activator 8-Br-cGMP (100 μM), but unaffected by the PKG inhibitor KT5823 (1 μM). Notably, KMUP-1 blocked serotonin-increased I_Ca,L but this was partially reversed by KT5823. In conclusion, serotonin-increased I_Ca,L could be due to activated 5-HT_2A receptor-mediated PKA and PKC cascades, and/or indirect interaction with PKG. KMUP-1 prevents serotonin-induced H9c2 cardiomyocyte hypertrophy, which can be attributed to its PKA and PKC inhibition, and/or PKG stimulation.

Introduction

Cardiac ventricular hypertrophy is a hallmark of progressive heart failure. Increased myocardial mass characteristic of the compensatory ventricular remodeling process is mainly attributed to hypertrophy of individual cardiomyocytes ¹. This process is initiated by a variety of stimuli, including paracrine and endocrine factors such as catecholamines, angiotensin II, endothelin, growth factors and inflammatory cytokines ². Additionally, serotonin (5-hydroxytryptamine, 5-HT) is believed to play a role in the regulation of cardiac growth in pathological conditions. Previous reports showed higher blood 5-HT levels in heart failure patients, which may be due to either clearance defect or enhanced secretion ^{3, 4}. Moreover, chronic treatment of 5-HT in rodents causes cardiac hypertrophy as well ^{5, 6}.

5-HT receptors are widely expressed in the cardiovascular system. In the heart, converging evidence shows that 5-HT_2A receptor expression and function are greatly altered during cardiac remodeling, supporting its role in the development of cardiac hypertrophy and failure. In rodent models of heart failure, a novel ventricular inotropic responsiveness to 5-HT appears, which is mediated in part through the 5-HT_2A receptor ⁷. In addition, the 5-HT_2A receptor is upregulated in human heart failure ⁷ and 5-HT_2A receptor blockade reverses hypertrophy during pressure overload in a mouse model of increased 5-HT levels ⁸. 5-HT_2A receptors trigger a hypertrophic response by 5-HT stimulation, through the involvement of transient receptor potential canonical 1 (TRPC1) channels ⁹ and calcineurin/NFAT activation ¹.

5-HT has even been shown to increase the magnitude of the L-type Ca²⁺ current (I_Ca,L) in human atrial myocytes via 5-HT₄ receptors ¹⁰, which may contribute to intracellular calcium overload and arrhythmic activity. The 5-HT₄ receptor is functionally present in the human atrium but not in the ventricle ¹¹. The present study used the H9c2 cell line, initiated by Kimes and Brandt (1976), established from spontaneous beating embryonic rat cardiac ventricle. Kimes and Brandt addressed that the beating activity ceased after the third “selective serial passage” ¹². This H9c2 cell line has properties similar to neonatal and adult cardiomyocytes ^{13, 14}. After undergoing differentiation, these cells can functionally express L-type Ca²⁺ channels (LTCCs) and ATP-sensitive K⁺ channels ^{13, 15}. Until now, the possible mechanism of serotonin-increased I_Ca,L has never been well described in H9c2 cells. We designed this study to determine whether protein kinases (i.e. PKA, PKC and PKG) are involved in the cascade of serotonin-induced rat heart-derived H9c2 cell hypertrophy model, using functional and electrophysiological studies.

A xanthine-derivative KMUP-1 has been demonstrated to increase cyclic nucleotides and stimulate K⁺ channels, resulting in relaxation in aortic, corporeal cavernosa and tracheal smooth muscles ^16-18, and to activate BK_Ca channels ¹⁹ and inhibit LTCCs ²⁰ in rat basilar artery myocytes. KMUP-1 was found to inhibit monocrotaline-induced chronic pulmonary arterial hypertension via cGMP-dependent inhibition of ROCK and modulation of K⁺ channels ²¹. It also attenuates isoprenaline-induced cardiac hypertrophy through the NO/cGMP/PKG pathway ²³. We have proved that it might have value in the management of cerebral vasospasm after subarachnoid hemorrhage ²⁴. However, it is not known whether KMUP-1 can modulate the influx of Ca²⁺ through LTCCs in H9c2 cardiomyocytes. The main objectives of this study were to characterize the role of protein kinases in serotonin-induced H9c2 cell hypertrophy and to elucidate the mechanisms by which KMUP-1 prevents 5-HT-induced ventricular myocyte hypertrophy via LTCCs mediated signaling pathways.

Materials and methods

Cell culture

The H9c2 cardiomyocyte-like cell line (ATCC CLR-1446; Rockville, MD) derived from rat embryonic myoblasts is commonly used as an in vitro model of cardiomyocyte biology ^{12, 14} since it shows similar hypertrophic and apoptotic responses as those seen in primary adult and neonatal cardiomyocytes ²⁵. Cells were plated onto collagen-coated culture dishes and cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) in a humidified atmosphere of 5% CO₂ and 95% air at 37^oC. Cells were used under the 20^th passage.

Serotonin-induced cell hypertrophy

H9c2 cells were performed under serum-free medium (DMEM with 0.1% FBS) and plated onto 6-cm dishes and stimulated with serotonin (1, 10 μM) for 4 days to measure their changes in cell surface area. Cell images were viewed with a digital camera fixed to a microscope (Nikon). The H9c2 cell surface area was analyzed for at least 30 cells per condition using the public ImageJ 1.47 software. To understand the mechanism of serotonin-induced H9c2 hypertrophy, the cells were exposed to the LTCC blocker verapamil (1 μM), the 5-HT_2A antagonist ketanserin (0.1 μM) and the 5-HT_2B antagonist SB206553 (0.1 μM) for 4 days. KMUP-1 was also applied in this experiment to observe its preventive effects on serotonin-induced H9c2 hypertrophy.

Patch-clamp electrophysiology

To measure the I_Ca through L-type Ca²⁺ channels ^{20, 26}, perforated whole-cell patch-clamp electrophysiology was used in rat heart-derived H9c2 cells. In brief, H9c2 cells treated with or without serotonin were detached with 0.25% trypsin-0.02% EDTA solution, the supernatant removed by centrifugation, and the pellets resuspended in 1 ml of bath solution containing (in mM): 135 tetraethylammonium (TEA)-Cl, 1.8 CaCl₂, 2 MgCl₂, 10 glucose, and 10 HEPES, (pH 7.4, Tris) using a fire polished glass pipette. The cells were put in a recording dish and then perfused with a bath solution. To minimize outward potassium currents, Cs⁺ rather than K⁺ was used in the pipette solution. A recording electrode was pulled from borosilicate glass (resistance: 3-5 MΩ), and the pipette was coated with sticky wax close to the tip to reduce capacitance and backfilled with pipette solution containing (in mM) 140 CsCl, 1 EGTA, 1 MgCl₂, 5 Na₂ATP, and 5 HEPES (pH 7.2, Tris). Nystatin (300 μg/ml) was included in the pipette solution for the perforated patch-clamp recordings. Serotonin-treated cells were clamped at -80 mV and depolarized to 0 mV for 300 ms to evoke whole-cell I_Ca. Voltage clamped cells were equilibrated for 15 min prior to experimentation. Membrane currents were recorded on the MultiClamp 700B amplifier (Molecular Devices Corporation, Sunnyvale, CA), filtered at 1 kHz using a low-pass Bessel filter, digitized at 5 kHz and stored on a computer for subsequent analysis with Clampfit 10.2. A 1 M NaCl-agar salt bridge between the bath and the Ag-AgCl reference electrode was used to minimize offset potentials. All electrical recordings were performed at room temperature. Following equilibration, serotonin-increased I_Ca was monitored in the presence and absence of KMUP-1 (0.1, 1, 10 μM). To ascertain whether PKA, PKC or PKG signaling was involved in the KMUP-1-inhibited I_Ca in serotonin (10 μM)-treated H9c2 cells, the activators and inhibitors of PKA (8-Br-cAMP, 100 μM and H89, 1 μM), PKC (phorbol 12-myristate 13-acetate (PMA), 100 μM and chelerythrine, 1 μM) and PKG (8-Br-cGMP and 100 μM, KT5823, 1 μM) were added in perfusate for this purpose.

Chemicals

Buffer reagents, chelerythrine, H-89, PMA, serotonin, TEA-Cl and verapamil were obtained from Sigma Aldrich Chemical Co. (St. Louis, MO). 8-Br-cAMP, 8-Br-cGMP, KT5823, ketanserin and SB206553 were obtained from Tocris Bioscience (Bristol, UK). All drugs and reagents were dissolved in distilled water unless otherwise noted. Chelerythrine, KT5823 and PMA were dissolved in DMSO at 10 mM. Serial dilutions were made in phosphate buffer solution with a final solvent concentration <0.01%.

Data analysis and statistics

Data were expressed as means ± SE; n indicates the number of cells. A repeated measure ANOVA was used to compare values at a given voltage. When appropriate, a Tukey-Kramer pairwise comparison was used for post hoc analysis. p values ≤0.05 were considered statistically significant.

Results

Effects on serotonin-induced H9c2 cells hypertrophy

Exposure of H9c2 cardiomyocytes to serotonin (1, 10 μM) caused time-dependent increases in cell surface area, an indicator of cell hypertrophy, and produced a marked hypertrophic response at day 4 (Fig. 1). Serotonin-induced H9c2 cell hypertrophy was significantly reversed by co-incubation with KMUP-1 (1-10 μM). Under the same situation, the LTCC blocker verapamil (1 μM) and the 5-HT_2A antagonist ketanserin (0.1 μM) also significantly attenuated the hypertrophic responses induced by serotonin, but not the 5-HT_2B antagonist SB206553 (0.1 μM) (Fig. 2).

Figure 1.

Effects of serotonin elicited H9c2 cardiomyocytes hypertrophy. H9c2 cells were cultured in serum-free medium and treated with 1 or 10 μM serotonin for 1-4 days. The cell surface areas (≥30 cells) were analyzed in each condition using ImageJ 1.47 software. Data are means ± SE, n=6. ^*P<0.05, ^**P<0.01 compared with control.

Figure 2.

KMUP-1 prevented serotonin-induced H9c2 cardiomyocyte hypertrophy. H9c2 cells were cultured in serum-free medium and treated with 10 μM serotonin, 10 μM serotonin+0.1-10 μM KMUP-1, 10 μM serotonin+1 μM verapamil, 10 μM serotonin+0.1 μM ketanserin or10 μM serotonin+0.1 μM SB206553 for 4 days. The cell surface areas (≥30 cells) were analyzed in each condition using ImageJ 1.47 software. Data are means ± SE, n=6. ^** P<0.01 compared with control. ^## P<0.01 compared with serotonin group.

Cell capacitance and current density in serotonin-induced hypertrophic cells

The cell capacitance in serotonin-induced H9c2 cell hypertrophy (81.6±3.3 pF, n≥30) was larger than that of the control cells (serotonin-free; 65.0±3.8 pF, n≥30, p<0.05, Fig. 3A). Data also showed significant increases in LTCC current density in hypertrophic H9c2 cells (2.1±0.3 pA/pF, n≥30) compared to the control cells (0.7±0.1 pA/pF, n≥30, p<0.001, Fig. 3B).

Figure 3.

Cell capacitance and current density in serotonin induced H9c2 cardiomyocytes. (A) Average cell capacitance and (B) I_Ca,L current density obtained from serum-free conditions as a control and 10 μM serotonin-treated H9c2 cells under serum-free for 4 days as a hypertrophic model. Data are means ± SE, n=10. ^* P<0.05, ^*** P<0.001 compared with control.

Effects on I_Ca,L in serotonin-induced hypertrophic cells

Perforated patch-clamp was used to study the effect of I_Ca,L increases in serotonin (10 μM)-induced H9c2 cell hypertrophy. Depolarizing single pulse (0 mV for 300 ms) was applied from a hold potential of -80 mV to record the inward I_Ca,L current. Serotonin induced significant H9c2 cell hypertrophy and enhanced the I_Ca,L ~2.9-fold control at day 4. The increased I_Ca,L of hypertrophic H9c2 cells was inhibited by co-incubation with KMUP-1 (0.1-10 μM) in a dose-dependent manner. This current was also inhibited by co-treatment with verapamil (1 μM, ~60%) or ketanserin (0.1 μM, ~40%), but unaffected by SB206553 (0.1 μM) in hypertrophic H9c2 cells (Fig. 4).

Figure 4.

Effects of KMUP-1, verapamil, ketanserin and SB206553 on serotonin-increased I_Ca,L in hypertrophic H9c2 cells. (A) Representative recordings of I_Ca,L evoked by test pulse to 0 mV from a holding potential of -80 mV in H9c2 cells. Cells were treated with 10 μM serotonin, 10 μM serotonin+0.1-10 μM KMUP-1, 10 μM serotonin+1 μM verapamil, 10 μM serotonin+0.1 μM ketanserin or10 μM serotonin+0.1 μM SB206553 for 4 days. (B) Bar graph showing the relative I_Ca,L averaged from the recordings of trace (A). Data are means ± SE, n=6. ^*** P<0.001 compared with control. ^# P<0.05, ^## P<0.01, ^### P<0.001 compared with serotonin group.

Increased I_Ca,L involved PKC activation in serotonin-induced hypertrophic cells

To determine whether PKC plays a role in serotonin-increased I_Ca,L, the PKC activator PMA and inhibitor chelerythrine were used in this experiment. The possible mechanisms of KMUP-1 protection against serotonin-increased I_Ca,L were also examined. We monitored the I_Ca,L of serotonin (10 μM)-induced H9c2 hypertrophic cells in the presence of 1 μM chelerythrine, 1 μM KMUP-1, 1 μM PMA or 1 μM PMA plus 1 μM KMUP-1 for 4 days. As shown in Fig. 5, the PKC inhibitor chelerythrine abolished the increased I_Ca,L and the PKC activator PMA significantly enhanced this current in hypertrophic H9c2 cells. Enhanced I_Ca,L from serotonin plus PMA was also abolished by KMUP-1 co-incubation.

Figure 5.

Effects of KMUP-1 on serotonin-increased I_Ca,L via the PKC pathway in hypertrophic H9c2 cells. (A) Representative recordings of I_Ca,L evoked by test pulse to 0 mV from a holding potential of -80 mV in H9c2 cells. Cells were treated with 10 μM serotonin, 1 μM chelerythrine, 10 μM serotonin+1 μM chelerythrine, 10 μM serotonin+1 μM KMUP-1, 10 μM serotonin+1 μM PMA or 10 μM serotonin+1 μM PMA+1 μM KMUP-1 for 4 days. (B) Bar graph showing the relative I_Ca,L averaged from the recordings of trace (A). Data are means ± SE, n=8. ^** P<0.01, ^*** P <0.001 compared with control. ^###P<0.001 compared with the serotonin group. ^✝P <0.001 compared with the serotonin+PMA group.

Increased I_Ca,L involved PKA activation in serotonin-induced hypertrophic cells

To ascertain the role of PKA in serotonin-increased I_Ca,L and KMUP-1's effects on this current, the PKA activator 8-Br-cAMP and inhibitor H-89 were used. We monitored the I_Ca,L of serotonin (10 μM)-induced H9c2 hypertrophic cells in the presence of 1 μM H-89, 1 μM KMUP-1, 100 μM 8-Br-cAMP or 100 μM 8-Br-cAMP plus 1 μM KMUP-1 for 4 days. The PKA inhibitor H-89 alone inhibited the control current ~25%. The PKA activator 8-Br-cAMP combined with serotonin did not further enhance the I_Ca,L current, and this effect was abolished by KMUP-1. Serotonin-increased I_Ca,L was attenuated by H-89 or KMUP-1 (Fig. 6).

Figure 6.

Effects of KMUP-1 on Serotonin-increased I_Ca,L via the PKA pathway in hypertrophic H9c2 cells. (A) Representative recordings of I_Ca,L evoked by test pulse to 0 mV from a holding potential of -80 mV in H9c2 cells. Cells were treated with 10 μM serotonin, 1 μM H-89, 10 μM serotonin+ 1 μM H-89, 10 μM serotonin+1 μM KMUP-1, 10 μM serotonin+100 μM 8-Br-cAMP or 10 μM serotonin+100 μM 8-Br-cAMP+1 μM KMUP-1 for 4 days. B) Bar graph showing the relative I_Ca,L averaged from the recordings of trace (A). Data are means ± SE, n=8. ^* P<0.05, ^*** P <0.001 compared with control. ^## P <0.01, ^### P <0.001 compared with the serotonin group. ^✝P <0.001 compared with the serotonin+8-Br-cAMP group.

Increased I_Ca,L involved PKG inhibition in serotonin-induced hypertrophic cells

To determine whether PKG is involved in serotonin-increased I_Ca,L and KMUP-1's effects on this current, the PKG activator 8-Br-cGMP and inhibitor KT5823 were used. We monitored the I_Ca,L of serotonin (10 μM)-induced H9c2 hypertrophic cells in the presence of 100 μM 8-Br-cGMP, 1 μM KT5823, 1 μM KMUP-1 or 1 μM KT5823 plus 1 μM KMUP-1 for 4 days. The PKG activator 8-Br-cGMP attenuated the I_Ca,L in hypertrophic H9c2 and control cells, but the PKG inhibitor KT5823 showed no significant effects in both cells. KMUP-1 inhibited serotonin-increased I_Ca,L which was partly reversed by KT5823 (Fig. 7).

Figure 7.

Effects of KMUP-1 on Serotonin-increased I_Ca,L via the PKG pathway in hypertrophic H9c2 cells. A) Representative recordings of I_Ca,L evoked by test pulse to 0 mV from a holding potential of -80 mV in H9c2 cells. Cells were treated with 10 μM serotonin, 100 μM 8-Br-cGMP, 1 μM KT5823, 10 μM serotonin+100 μM 8-Br-cGMP, 10 μM serotonin+1 μM KT5823, 10 μM serotonin+1 μM KMUP-1 or 10 μM serotonin +1 μM KMUP-1+1 μM KT5823 for 4 days. B) Bar graph showing the relative I_Ca,L averaged from the recordings of trace (A). Data are means ± SE, n=8. ^* P<0.05, ^*** P <0.001 compared with control. ^# P <0.05, ^### P <0.001 compared with the serotonin group. ^✝P <0.005 compared with the serotonin+KMUP-1 group.

Discussion

This study is the first to use patch-clamp electrophysiology to investigate the role of PKA, PKC and PKG in serotonin-induced H9c2 cardiomyocyte hypertrophy and to examine the protective mechanisms of KMUP-1 against cardiomyocyte hypertrophy. Serotonin induced H9c2 cell hypertrophy, which increased cell size and I_Ca,L, was reversed by the K⁺ channel opener KMUP-1, attenuated by the LTCC blocker verapamil and the 5-HT_2A antagonist ketanserin, but unaffected by the 5-HT_2B antagonist SB206553. We also found that serotonin-increased I_Ca,L was reduced by the PKA or PKC inhibitor and the PKG activator, and was enhanced by the PKC activator but not affected by the PKG inhibitor. Interestingly, while KMUP-1 blunted serotonin-increased I_Ca,L, this was partly reversed by the PKG inhibitor. In light of our results, we suggest that serotonin increases I_Ca,L mainly by activating the 5-HT_2A receptor-mediated PKA and PKC cascades, and partly by interacting with PKG. KMUP-1 prevents serotonin-induced H9c2 cardiomyocyte hypertrophy primarily by PKA and PKC inhibition and/or PKG stimulation.

Previous reports have demonstrated that H9c2 cell membranes expressed cardiac and skeletal types of LTCCs. The difference between the Ca²⁺ current of cardiac and skeletal muscle is related to different mechanisms of depolarization-contraction coupling ^{14, 27}. Hescheler et al. (1991) also pointed out that H9c2 cells show morphological characteristics similar to immature embryonic cardiomyocytes but preserve several elements of the electrical and hormonal signal pathway found in adult cardiac cells. This cell line may be useful as a model for cardiomyocytes in terms of transmembrane signal transduction. To understand H9c2 cardiomyocyte hypertrophy, cardiac I_Ca,L was our target in this study. Therefore, we suggest that serotonin-induced hypertrophy in H9c2 cells provides advantages to understand the progression of pathogenic myocardial hypertrophy in humans.

A previous report showed that 5-HT_2A receptors mediate the receptor-dependent hypertrophic activity of serotonin in cardiac cells. The role of these receptors in cardiac hypertrophy has been supported by various results in vivo. In a mouse model of increased plasma serotonin, 5-HT_2A but not 5-HT_2B receptors were involved in pressure overload hypertrophy ⁸. Other studies also showed that 5-HT_2A receptors mediated positive inotropic response in ventricular myocytes and were overexpressed in different models of cardiac hypertrophy, along with human heart failure ^{7, 28}. Additionally, 5-HT_2A receptor blockade reverses hypertrophy during pressure overload and reduces left ventricular hypertrophy ^{8, 29}. 5-HT_2B receptors have also been implicated in ventricular hypertrophy. It has been shown that 5-HT_2B receptors on cardiac fibroblasts mediate the release of IL-6 and indirectly induce cardiomyocyte hypertrophy ³⁰. In this study, serotonin-increased cell size and I_Ca,L were not affected by the pharmacological (SB206553) blockade of 5-HT_2B function, which suggested that 5-HT_2B receptors could not have an important role in H9c2-mediated hypertrophy.

In the present study we extend previous in vivo findings that KMUP-1 was observed to prevent isoprenaline-induced cardiac hypertrophy via activation of NO/cGMP/PKG and inhibition of ERK1/2/calcineurin A signaling cascades ²³. Increased cGMP production prevents cardiac hypertrophy has not been entirely elucidated but it probably involves PKG. PKG inhibits LTCCs thus reducing Ca²⁺ influx and blocking calcineurin-mediated activation of nuclear factor of activated T cell (NFAT), a transcription factor that is obligatory for hypertrophy ^{23, 31}. This study not only assessed the role of PKG but also PKA and PKC in 5-HT-induced H9c2 cardiomyocyte hypertrophy. Data obtained from patch-clamp recordings, we suggest that PKC plays a major role whereas PKA or PKG plays a relatively minor role in 5-HT-induced H9c2 hypertrophy. This work attempted to explain the connection of 5-HT receptors and intracellular protein kinases in H9c2 hypertrophy induced by serotonin. Another possible hypertrophic mechanism for 5-HT transporter (SERT) needs to be investigated further ^{1, 32}.

To our knowledge, this report is the first to describe the relationship between serotonin-increased I_Ca,L and the transduction pathway of protein kinases in hypertrophic H9c2 cells using patch-clamp recordings. The novel finding of this report is that serotonin increased the I_Ca,L currents, a marker of cell hypertrophy, which was attenuated by PKA and PKC inhibitors (H-89 and chelerythrine) and enhanced by the PKC activator PMA. Serotonin-mediated I_Ca,L effects were also reduced by the PKG activator 8-Br-cGMP, but unaffected by the PKG inhibitor KT5823. Thus, we suggest that serotonin-induced H9c2 hypertrophy can be directly attributed to 5-HT_2A receptor-mediated PKA and PKC pathways, indirectly interacting with PKG. The K⁺-channel opener KMUP-1 reversed or blunted serotonin-mediated cell hypertrophy and I_Ca,L currents, and therefore its mechanism of action could be due to the modulation of these intracellular cascades of protein kinases (Fig. 8). Previously, we have demonstrated that KMUP-1 inhibits Ca²⁺ entry via voltage-dependent LTCCs in rat basilar arteries, which is attributed to its modulation of the PKC pathway ²⁰. In this study, the single concentration of protein kinase inhibitors/activators and receptor antagonists used are considered for their selectivity and specificity ³³.

Figure 8.

Proposed mechanisms of KMUP-1 modulation of serotonin-induced hypertrophic H9c2 cardiomyocytes involving the protein kinase cascade. Serotonin-induced H9c2 cell hypertrophy was reversed by the K⁺ channel opener KMUP-1, attenuated by the LTCC blocker verapamil and the 5-HT_2A antagonist ketanserin, but unaffected by the 5-HT_2B antagonist SB206553. Collectively, serotonin-increased I_Ca,L could be mainly due to activated 5-HT_2A receptor-mediated PKA and PKC cascades, and partly attributed to interaction with PKG. Accordingly, KMUP-1 blocks serotonin-induced H9c2 hypertrophy, which can be attributed to its PKA and PKC inhibition or PKG stimulation.

In summary, this study presents evidence that PKA and PKC play a key role in regulating serotonin-mediated cell hypertrophy and I_Ca,L changes via activation of 5-HT_2A receptors in H9c2 cardiomyocytes. This investigation is also the first to use patch-clamp electrophysiology to study KMUP-1 prevention of serotonin-induced cardiomyocyte hypertrophy by blocking PKA- and PKC-mediated Ca²⁺ influx through LTCCs, and by stimulating PKG-inhibited Ca²⁺ influx. Finally, we suggest that KMUP-1 could be of value in controlling cardiac hypertrophy due to serotonin overstimulation under pathophysiologic conditions.