Effects of long-term enalapril and losartan therapy of hypertension on cardiovascular aldosterone.

BACKGROUND: Plasma aldosterone escape is found during long-term angiotensin-converting enzyme inhibitor therapy. Evidence for aldosterone production in cardiovascular tissues raised the question of whether or not aldosterone escape occurs in these tissues. METHOD: Spontaneously hypertensive rats were treated with enalapril (20 mg/kg/day) and losartan (50 mg/kg/day) for 20 weeks; untreated spontaneously hypertensive and Wistar rats were used as positive and normal controls, respectively. Ex vivo mesenteric artery and heart perfusion, high-performance liquid chromatography, and radioimmunoassay for aldosterone were performed. RESULTS: The results showed that enalapril failed to significantly inhibit aldosterone production in mesenteric artery, myocardium and plasma. Losartan significantly inhibited aldosterone production to that of Wistar rats in the mesenteric artery, myocardium and plasma. CONCLUSION: This study provides the first evidence that long-term angiotensin-converting enzyme inhibition therapy induces aldosterone escape in hypertensive cardiovascular tissues, and angiotensin II subtype 1 receptor antagonist does not induce aldosterone escape in mesenteric artery, myocardium and plasma of spontaneously hypertensive rats.     

自发性高血压大鼠心肌肥厚和心肌MAPK、AngⅡ的关系

放免法测定自发性高血压大鼠（ＳＨＲ）血浆及心肌血管紧张素Ⅱ（ＡｎｇⅡ）含量,凝胶内磷酸化法测定心肌丝裂素活化蛋白激酶活性（ＭＡＰＫ）,以心脏重／体重表示心肌肥厚程度。结果表明：与４个月的ＷＫＹ大鼠比较,４个月的ＳＨＲ血浆和心肌组织ＡｎｇⅡ及心肌ＭＡＰＫ活性分别增加了２１８．６％、１０１．２％和１０７．０％,心肌肥大程度严重,其中ＭＡＰＫ活性与心肌肥大程度呈明显正相关。提示４个月ＳＨＲ心肌肥厚可能是     

Myocardial changes after spironolactone in spontaneous hypertensive rats. A laser scanning confocal microscopy study

The present study aims to objectivate by laser scanning confocal microscopy, the cardiac structure of the spontaneously hypertensive rats (SHR) treated with different doses of spironolactone, either alone or in combination with an angiotensin converting enzyme inhibitor or with a calcium channel blocker. Thirty SHRs were divided into six groups and treated during 13 weeks as follows: control, spironolactone (5, 10 and 30 mg/kg/day), spironolactone + enalapril and spironolactone + verapamil. Spironolactone action on the SHR blood pressure (BP) was dose-dependent. The cardiac hypertrophy was affected by the treatment with spironolactone (high dose) or a combination of spironolactone and Enalapril. The myocardial structure was less affected by the spironolactone monotherapy (at all doses) showing hypertrophied cardiac myocytes, focal areas of the reactive fibrosis, inflammatory infiltrate. The treatment with spironolactone in combination with enalapril or verapamil prevented these alterations. In conclusion, the monotherapy with spironolactone had only a limited effect in the preservation of the myocardial structure and in the attenuation of the interstitial fibrosis in SHRs, even after high dose. This action on the myocardium is more efficient when the spironolactone (even in medium dose) was associated with enalapril or verapamil.     

The effects of spironolactone monotherapy on blood pressure and myocardial remodeling in spontaneously hypertensive rats: A stereological study

The aim of this study was to evaluate the cardiac structure of spontaneously hypertensive rats (SHRs) treated with different doses of spironolactone. Twenty SHRs were separated into four groups and treated for 13 weeks, as follows: one control group and three spironolactone treatment groups receiving doses of 5, 10 or 30 mg/kg/day. The spironolactone treatment either attenuated or prevented the tendency for increased blood pressure. However, the myocardial structure was not significantly affected by the spironolactone monotherapy treatment (all doses); it showed hypertrophied cardiac myocytes, focal areas of reactive fibrosis, inflammatory infiltrate and a decrease in the density of intramyocardial microvessels. None of the cardiac myocyte stereological parameters in the left ventricular myocardium showed significant differences among the SHR groups. The cardiac myocyte volume density was around 80%, the cardiac myocyte surface density varied from 3.6 to 4.1 x 10(4) mm2/mm3 and the cardiac myocyte mean cross-sectional area varied from 351 to 415 micro m2. The connective tissue volume density of the SHRs treated with the highest dose of spironolactone was 75% lower than in the control SHRs, and this was the only significant difference found for this parameter among SHR groups. The intramyocardial vessels showed some differences when the control SHRs and the other SHRs were compared. The lowest intramyocardial vessel volume density was found in the control group (more than 20% lower than that in the treated SHRs), but no significant difference was detected among the treated SHRs (all doses). The intramyocardial vessel length density (Lv[v]) and surface density (Sv[v]) showed a similar tendency, being significantly greater in the treated SHRs than in the control rats. The Lv[v] was 45% greater in the high-dose spironolactone group than in the control group, and it was 28% greater in the high-dose spironolactone SHRs than in the other treated SHRs. The Sv[v] was 50% greater in the high-dose spironolactone SHRs than in both control and low-dose spironolactone SHRs. Long-term spironolactone monotherapy showed a partial effect in the preservation of intramyocardial vessels and also in the attenuation of interstitial fibrosis.     

Role of atrial natriuretic peptide in mineralocorticoid escape phenomenon in patients with primary aldosteronism

The withdrawal effect of spironolactone treatment on natriuresis was studied in relation to atrial natriuretic peptide (ANP) in five patients with primary aldosteronism due to adenoma. The patients had been treated with spironolactone for 2-3 months before they were admitted. After admission, blood pressure, body weight, and urinary excretion of sodium were measured daily. Venous samples were obtained twice a week for measurements of plasma levels of ANP, plasma renin activity (PRA), and plasma concentrations of aldosterone (PAC), cortisol, and deoxycorticosterone. The study was performed for 7 days during the treatment with spironolactone and for 18 days after stopping the administration. Plasma volume was determined two times, during the control period and on the 13th day after stopping spironolactone. Urinary sodium excretion decreased initially and returned to the control levels successively. Body weight and plasma volume increased, and blood pressure rose steadily. PRA and the plasma concentrations of cortisol and deoxycorticosterone decreased significantly (P less than 0.05); however, high levels of PAC did not alter significantly. Plasma ANP levels increased significantly (P less than 0.05) from 26 +/- 4 pg/ml during the control period to 195 +/- 47 pg/ml on the 13th day after stopping spironolactone. The data of the urinary sodium excretion showed the escape from sodium-retaining effect of aldosterone, and this escape could be explained by the increase in plasma ANP. Furthermore, ANP might contribute to the decrease in cortisol and deoxycorticosterone in plasma because of the direct inhibitory action of ANP on steroidogenesis.     

The therapeutic effect of rosuvastatin on cardiac remodelling from hypertrophy to fibrosis during the end-stage hypertension in rats

End-stage hypertensive heart disease is an increasing cause of cardiac mortality. Therefore, the current study focused on the cardiac remodelling from hypertrophy to fibrosis in old-aged spontaneously hypertensive rats (SHRs), and explored the therapeutic effects of Rosuvastatin and its possible mechanism(s) of action. Spontaneously hypertensive rats at age 52 weeks were randomly divided into three groups, the first two to receive Rosuvastatin at a dose of 20 mg/kg/day and 40 mg/kg/day, respectively, and the third to receive placebo, which was to be compared with Wistar-Kyoto as controls. After 2-month treatment, SBP, heart to body weight ratio (HW/BW%) and echocardiographic features were evaluated, followed by haematoxylin and eosin and Masson trichrome staining in conjunction with qPCR of foetal gene expressions. Transferase-mediated dUTP nick-end labelling assay and immunofluorescent labelling for active caspase-3 were used to detect the apoptotic cardiomyocytes. Signaling pathways involved were examined by using western blot. Old-aged SHR developed end-stage hypertensive heart disease characterized by significant enhancement of HW/BW%, LVAWd and LVPWd, and decreased LVEF and LVFS, accompanied by cardiomyocytes enlargement and fibrosis along with activation of foetal gene programme. Cardiac apoptosis increased significantly during the transition process. Rosuvastatin reduced hypertrophy significantly via AT(1) Receptor-PKCβ2/α-ERK-c-fos pathway; protected myocardium against apoptosis via Akt-FOXO1, Bcl-2 family and survivin pathways and consequently suppressed the caspase-3 activity. The present study revealed that old-aged SHRs developed cardiac remodelling from hypertrophy to fibrosis via cardiac apoptosis during the end stage of hypertensive heart disease. These pathological changes might be the consequence of activation of AT(1) Receptor-PKCβ2/α-ERK-c-fos and AKT-FOXO1/Bcl-2/survivin/Caspase3 signaling. Rosuvastatin effectively attenuated the structural changes by reversing the signaling transductions involved.     

Effects of spironolactone and eprosartan on cardiac remodeling and angiotensin-converting enzyme isoforms in rats with experimental heart failure

Angiotensin-converting enzyme (ACE)-2 is a newly described enzyme with antagonistic effects to those of the classical ACE (ACE-1). Both ANG II and aldosterone play an important role in the pathophysiology of congestive heart failure (CHF) and in the adverse cardiac remodeling during its development. In this study, we examined the effects of experimental CHF induced by an aortocaval fistula (ACF) and of its treatment with ANG II and aldosterone inhibitors on the relative levels of ACE-1 and ACE-2. We also compared the effects of spironolactone, an aldosterone antagonist, and eprosartan, an ANG II receptor antagonist, on heart hypertrophy and fibrosis in rats with ACF. Spironolactone (15 mg x kg(-1) x day(-1) ip, via minipump) or eprosartan (5 mg x kg(-1) x day(-1) ip, via minipump) was administered into rats with ACF for 14 and 28 days. Specific antibodies were used to determine the protein levels of myocardial ACE-1 and ACE-2. ACF increased the cardiac levels of ACE-1 and decreased those of ACE-2. Heart-to-body weight ratio significantly increased from 0.30 +/- 0.004% in sham-operated controls to 0.50 +/- 0.018% and 0.56 +/- 0.044% (P < 0.001) in rats with ACF, 2 and 4 wk after surgery, respectively, in association with increased plasma levels of aldosterone. The area occupied by collagen increased from 2.33 +/- 0.27% to 6.85 +/- 0.65% and 8.03 +/- 0.93% (P < 0.01), 2 and 4 wk after ACF, respectively. Both spironolactone and eprosartan decreased cardiac mass and collagen content and reversed the shift in ACE isoforms. ACF alters the ratio between ACE isoforms in a manner that increases local ANG II and aldosterone levels. Early treatment with both ANG II and aldosterone antagonists is effective in reducing this effect. Thus ACE isoform shift may represent an important component of the development of cardiac remodeling in response to hemodynamic overload, and its correction may contribute to the beneficial therapeutic effects of renin-angiotensin-aldosterone system inhibitors.     

Synthesis and biological evaluation of novel potent angiotensin II receptor antagonists with anti-hypertension effect

A series of novel angiotensin II type 1 receptor antagonists were prepared. Radioligand binding assay suggested that compounds 1b and 1c could be recognized by the AT(1) receptor with an IC(50) value of 1.6 ± 0.09 nM and 2.64 ± 0.7 nM, respectively. In vivo anti-hypertension experiments showed that compounds (1a, 1b, 1c, 1e) elicited a significant decrease in SBP and DBP of spontaneous hypertensive rats (SHRs). The antihypertensive effects maintained for 10 h, which indicated that these compounds had a favorable blood pressure-lowering effect. Acute toxicity testing suggested that the LD(50) value of compound 1b was 2316.8 mg/kg which was lower than valsartan (LD(50)=307.50 mg/kg) but higher than losartan (LD(50)=2248 mg/kg). So they could be considered as novel anti-hypertension candidates and deserved for further investigation.     

Influence of spironolactone on urinary prostaglandin E2 and kinin excretion in rats

Spironolactone was administered to spontaneously hypertensive rats (SHRs) in order to examine the urinary excretions of prostaglandin E2 (PGE2) and kinin. Thirteen SHRs were divided into 2 groups: 0.1 ml of sesame oil was administered to one group (the spironolactone-lactone-untreated group, n = 6) and 20 mg of spironolactone in 0.1 ml of sesame oil was administered to the other group (the spironolactone-treated group, n = 7) by the subcutaneous route for 10 days in succession. Determinations were then made of the body weight, blood pressure, urine volume, and excretion levels of Na, K, kinin and PGE2 in the 24-hour urine. After the animals had been killed by decapitation, blood samples were drawn for determination of the plasma renin activity (PRA). The results obtained indicated decreased blood pressure and increased urinary Na excretion in the spironolactone-treated group. On the other hand, the PGE2 excretion level in the 24-hour urine decreased markedly immediately after administration of spironolactone (p less than 0.05) and was maintained at lower levels up to the end of the experiment. However, the 24-hour urinary kinin levels showed similar changes in both the spironolactone-treated group and the untreated group with no significant difference between them. These findings suggest that spironolactone has a suppressive effect on urinary PGE2 excretion, the activity of which is not mediated by kinin production in the kidneys but is the result of a direct action of spironolactone itself.     

Mineralocorticoid receptor blockade improves diastolic function independent of blood pressure reduction in a transgenic model of RAAS overexpression

There is emerging evidence that aldosterone can promote diastolic dysfunction and cardiac fibrosis independent of blood pressure effects, perhaps through increased oxidative stress and inflammation. Accordingly, this investigation was designed to ascertain if mineralocorticoid receptor blockade improves diastolic dysfunction independently of changes in blood pressure through actions on myocardial oxidative stress and fibrosis. We used young transgenic (mRen2)27 [TG(mRen2)27] rats with increases in both tissue ANG II and circulating aldosterone, which manifests age-related increases in hypertension and cardiac dysfunction. Male TG(mRen2)27 and age-matched Sprague-Dawley rats were treated with either a low dose (～1 mg·kg(-1)·day(-1)) or a vasodilatory, conventional dose (～30 mg·kg(-1)·day(-1)) of spironolactone or placebo for 3 wk. TG(mRen2)27 rats displayed increases in systolic blood pressure and plasma aldosterone levels as well as impairments in left ventricular diastolic relaxation without changes in systolic function on cine MRI. TG(mRen2)27 hearts also displayed hypertrophy (left ventricular weight, cardiomyoctye hypertrophy, and septal wall thickness) as well as fibrosis (interstitial and perivascular). There were increases in oxidative stress in TG(mRen2)27 hearts, as evidenced by increases in NADPH oxidase activity and subunits as well as ROS formation. Low-dose spironolactone had no effect on systolic blood pressure but improved diastolic dysfunction comparable to a conventional dose. Both doses of spironolactone caused comparable reductions in ROS/3-nitrotryosine immunostaining and perivascular and interstitial fibrosis. These data support the notion mineralocorticoid receptor blockade improves diastolic dysfunction through improvements in oxidative stress and fibrosis independent of changes in systolic blood pressure.     

Comparison of Surgery and Prolonged Spironolactone Therapy in Patients with Hypertension,Aldosterone Excess,and Low Plasma Renin

The effect of prolonged preoperative treatment with spironolactone has been studied in a series of 67 patients with hypertension, aldosterone excess, and low plasma renin. In the series as a whole a highly significant reduction in both systolic and diastolic pressures was achieved, with no evidence of escape from control during therapy lasting several years in some cases. The drug was equally effective in controlling blood pressure in patients with and without adrenocortical adenomata. Occasional unresponsive patients were encountered in both groups; pretreatment blood urea levels in these were significantly higher than in the responsive patients. The hypotensive effect of spironolactone usually predicted the subsequent response to adrenal surgery.Spironolactone in all cases corrected plasma electrolyte abnormalities; significant increases in total exchangeable (or total body) potassium and significant reductions in total exchangeable sodium, total body water, extracellular fluid, and plasma volumes were seen. Plasma urea rose during treatment and there was a slight fall in mean body weight. Significant increases in peripheral venous plasma renin and angiotensin II concentrations occurred during treatment.In two patients no increase in aldosterone secretion rate was found during treatment, although plasma aldosterone rose in three of four subjects studied.Severe side effects were rare; in only two of the 67 patients did the drug have to be stopped.In addition to its routine preoperative use, spironolactone can now be advised as long-term therapy in selected patients.     

Cardiac fibrogenesis in magnesium deficiency: a role for circulating angiotensin II and aldosterone

Mechanisms underlying cardiac fibrogenesis in magnesium deficiency are unclear. It was reported earlier from this laboratory that serum from magnesium-deficient rats has a more pronounced stimulatory effect on cell proliferation, net collagen production, and superoxide generation in adult rat cardiac fibroblasts than serum from rats on the control diet. The profibrotic serum factors were, however, not identified. This study tested the hypothesis that circulating angiotensin II may modulate cardiac fibroblast activity in hypomagnesemic rats. Male Sprague-Dawley rats were pair-fed a magnesium-deficient (0.0008% Mg) or -sufficient (0.05%) diet for 6 days, and the effects of serum from these rats on [3H]thymidine and [3H]proline incorporation into cardiac fibroblasts from young adult rats were evaluated in the presence of losartan, an angiotensin II type 1 (AT1) receptor antagonist, and spironolactone, an aldosterone antagonist. Losartan and spironolactone markedly attenuated the stimulatory effects in vitro of serum from the magnesium-deficient and control groups, but the inhibitory effects were considerably higher in cells exposed to serum from magnesium-deficient animals. Circulating and cardiac tissue levels of angiotensin II were significantly elevated in magnesium-deficient animals (67.6% and 93.1%, respectively, vs. control). Plasma renin activity was 61.9% higher in magnesium-deficient rats, but serum angiotensin-converting enzyme activity was comparable in the two groups. Furthermore, preliminary experiments in vivo using enalapril supported a role for angiotensin II in magnesium deficiency. There was no significant difference between the groups in serum aldosterone levels. The findings suggest that circulating angiotensin II and aldosterone may stimulate fibroblast activity and contribute to a fibrogenic response in the heart in magnesium deficiency.     

Alpha-tocopherol supplementation favorable effects on blood pressure, blood viscosity and cardiac remodeling of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) were separated into two groups (n = 6 per group) and, since 5 months old, received alpha-tocopherol (alpha-tocopherol acetate120 IU) or vehicle by daily gavage for 2 weeks. Blood viscosity, blood pressure (BP) and myocardial remodeling were analyzed. The SHRs treated with alpha-tocopherol showed a significant reduction of BP and a major reduction of blood viscosity in comparison with the control SHRs. The cardiac hypertrophy indices showed some differences when the two SHR groups were compared, the LV mass index was not different between the groups; however, the cardiomyocyte size was more than 20% smaller in SHRs treated with alpha-tocopherol than in control SHRs (P < .05). The intramyocardial vessels distribution was more than 45% greater in alpha-tocopherol-treated SHRs than in control rats, significantly improving the vessels-to-myocytes ratio in treated SHRs than in control SHRs (P < .05). In conclusion, present findings strongly suggest a beneficial effect of alpha-tocopherol supplementation to genetically hypertensive rats. This was observed by a reduction of both blood viscosity and BP, and a consequent cardiomyocyte hypertrophy in treated SHRs; an improvement of vessels-to-myocytes ratio in these rats was also observed.     

Intact female stroke-prone hypertensive rats lack responsiveness to mineralocorticoid receptor antagonists

Data from the Framingham Heart Study suggest that women may be more sensitive to the deleterious cardiovascular remodeling effects of aldosterone. Previous studies from our laboratory have shown that chronic treatment with spironolactone, a mineralocorticoid receptor (MR) antagonist, decreases ischemic cerebral infarct size and prevents remodeling of the middle cerebral artery (MCA) in male spontaneously hypertensive stroke-prone rats (SHRSP). Therefore, we hypothesized that MR antagonism would reduce ischemic infarct size and prevent MCA remodeling in female SHRSP. Six-week-old female SHRSP were treated for 6 wk with spironolactone (25 or 50 mg.kg(-1).day(-1)) or eplerenone (100 mg.kg(-1).day(-1)) and compared with untreated controls. At 12 wk, cerebral ischemia was induced for 18 h using the intraluminal suture occlusion technique, or the MCA was isolated for analysis of passive structure using a pressurized arteriograph. MR antagonism had no effect on infarct size or passive MCA structure in female SHRSP. To study the potential effects of estrogen, the above experiments were repeated in bilaterally ovariectomized (OVX) female SHRSP treated with spironolactone (25 mg.kg(-1).day(-1)). Infarct size and vessel structure in OVX SHRSP were not different from control SHRSP. Spironolactone had no effect on infarct size in OVX SHRSP. However, MCA lumen and outer diameters were increased in spironolactone-treated OVX SHRSP, suggesting an effect of estrogen. Cerebral artery MR expression, assessed by Western blotting, was increased in female, compared with male, SHRSP. These studies highlight an apparent sexual dimorphism of MR expression and activity in the cerebral vasculature from hypertensive rats.     

Mineralocorticoids act centrally to regulate blood-borne tumor necrosis factor-alpha in normal rats

Excessive mineralocorticoid receptor (MR) stimulation induces neurohumoral excitation and cardiac and vascular fibrosis. In heart failure (HF) rats, with excessive neurohumoral drive, central infusion of the MR antagonist spironolactone (SL) decreases blood-borne TNF-alpha. This study aimed to determine whether DOCA, a precursor of aldosterone, acts centrally to stimulate TNF-alpha production in normal rats. DOCA (5 mg sc daily for 8 days) induced a progressive increase in TNF-alpha beginning on day 3 and increased tissue TNF-alpha in hypothalamus, pituitary, and heart but not in other brain and peripheral tissues harvested on day 9. A continuous intracerebroventricular infusion of SL (100 ng/h) blocked the plasma TNF-alpha response. Oral SL (1 mg/kg) blocked the plasma and tissue TNF-alpha responses. Thus DOCA increases TNF-alpha in brain, heart, and blood in normal rats. Activation of brain MR appears to account for the increase in plasma TNF-alpha. These findings have important implications for the understanding of pathophysiological states (e.g., HF, hypertension) characterized by high circulating levels of aldosterone.     

Altered secretion of corticosteroids and prolactin in adrenal regeneration hypertensive rats.

To assess the possible role of mineralocorticoids in the onset and maintenance of hypertension in adrenal regeneration hypertensive (ARH) rats, the change in plasma mineralocorticoids, with adrenal regeneration after enucleation in ARH rats was investigated and compared with those in unilaterally nephroadrenalectomized, 1% saline-fed (UNA) rats, sham-operated, 1% saline-fed (1% NaCl) rats and water-fed (water) rats. Plasma aldosterone was determined by RIA and the other mineralocorticoids were measured by HPLC. How plasma PRL, a marker of central dopaminergic activity, affected aldosterone secretion was determined by RIA. In ARH, plasma corticosterone (B), 18-OH-DOC and aldosterone levels 2 weeks after operation were as low as 20-30% of corresponding values, but the plasma DOC level was almost 100% of the corresponding value in the other groups. Four weeks after operation plasma B increased to a level comparable with that in the other groups and the plasma aldosterone level remained low. However, plasma DOC and 18-OH-DOC levels 4 weeks after operation were as high as 120-200% of corresponding values in the other groups. Six weeks after operation, the plasma aldosterone level returned to a value comparable with that in UNA and 1% NaCl and plasma DOC and 18-OH-DOC levels returned to corresponding values in the other groups. The plasma PRL level 4 weeks after operation was significantly lower in ARH than in the other groups. These results suggest that transient DOC and 18-OH-DOC increases observed in ARH may be important in the onset of hypertension, while other factors may be involved in its maintenance and that the transient central dopaminergic hyperactivity observed in ARH may be responsible for a delayed return from aldosterone deficiency.     

Alterations of load-induced p38 MAP kinase activation in failing rat hearts.

Hemodynamic load-induced cardiac p38 mitogen-activated protein kinase (MAPK) activation was studied in normotensive control Dahl rats (n = 10) and hypertensive Dahl rats with heart failure (n = 16). The isolated heart from each animal was stretched on a Langendorff apparatus at an equivalent diastolic wall stress, and the p38-MAPK activity of the left ventricular (LV) myocardium was analyzed by immunoprecipitation-kinase assay. Compared to the control hearts, the stretch-induced p38-MAPK activities were significantly decreased, and inversely correlated with the LV diameter (r = -0.73, P < 0.01). Chronic treatment with an angiotensin II AT1-receptor antagonist, valsartan (10 mg/kg/day), ameliorated cardiac function and remodeling process in the failing hearts, which was associated with an improvement of the p38-MAPK activities. Thus, the mechano-signal transduction of p38-MAPK pathway is downregulated in the failing hearts, along with progressive ventricular remodeling. The data also suggest that the beneficial effects of the AT1-receptor antagonists are potentially mediated by the restoration of cardiac growth-related signal transduction.