Aging phenotype(s) in kidneys of diabetic mice are p66ShcA dependent.

The p66ShcA protein controls cellular responses to oxidative stress, senescence, and apoptosis. Here, we test the hypothesis that aging phenotype(s) commonly associated with the broad category of chronic kidney disease are accelerated in diabetic kidneys and linked to the p66ShcA locus. At the organ level, tissue stem cells antagonize senescent phenotypes by replacing old dysfunctional cells. Using established methods, we isolated a highly purified population of stem cell antigen-1-positive mesenchymal stem cells (Sca-1+ MSCs) from kidneys of wild-type (WT) and p66 knockout (p66 KO) mice. Cells were plated in culture medium containing normal glucose (NG) or high glucose (HG). Reactive oxygen species (ROS) metabolism was substantially increased in WT MSCs in HG medium in association with increased cell death by apoptosis and acquisition of the senescent phenotype. DNA microarray analysis detected striking differences in the expression profiles of WT and p66 KO-MSCs in HG medium. Unexpectedly, the analysis for p66 KO-MSCs revealed upregulation of Wnt genes implicated in self-renewal and differentiation. To test the in vivo consequences of constitutive p66 expression in diabetic kidneys, we crossed the Akita diabetic mouse with the p66KO mouse. Homozygous mutation at the p66 locus delays or prevents aging phenotype(s) in the kidney that may be precursors to diabetic nephropathy.

Molecular biology of protein kinase C signaling in cardiac myocytes.

The PKC family of serine/threonine kinases have been implicated in a diverse array of cellular responses. Adult cardiac myocytes express multiple PKC isozymes, which participate in the response of muscle cells to extracellular stimuli, modulate contractile properties, and promote cell growth and survival. Recently, the classification of this ubiquitous family of signaling molecules has been expanded from three to four subfamilies. This review will focus on the application of pharmacologic and molecular approaches to explore the biology of cardiac PKC isozymes. The availability of transgenic mice and peptide PKC modulators have been instrumental in identifying target substrates for activated cardiac PKC isozymes, as well as the identification of specific isozymes linked to distinct growth characteristics and cell phenotype. The rapid growth of knowledge in the area of PKC signaling and PKC substrate interactions, may result in the development of therapeutic modalities with the potential to arrest or reverse the progression of cardiovascular diseases.

Angiotensin II promotes glucose-induced activation of cardiac protein kinase C isozymes and phosphorylation of troponin I.

Activation of the protein kinase C (PKC) family is a potential signaling mechanism by which high ambient glucose concentration modulates the phenotype and physiological function of cells. Recently, the cardiac renin angiotensin system (RAS) has been reported to promote PKC translocation in the diabetic heart via the angiotensin (ANG) II type 1 receptor (AT-1R). To evaluate the molecular events coupled with high glucose-induced PKC translocation and to examine the role of endogenously released ANG II in myocyte PKC signaling, primary cultures of adult rat ventricular myocytes were exposed to normal (5 mmol/l) or high (25 mmol/l) glucose for 12-24 h. Western blot analysis indicated that adult rat ventricular myocytes coexpress six PKC isozymes (alpha, beta(1,) beta(2,) delta, epsilon, and zeta). Translocation of five PKC isozymes (beta(1), beta(2), delta, epsilon, and zeta) was detected in response to 25 mmol/l glucose. Inhibition of phospholipase C with tricyclodecan-9-yl-xanthogenate blocked glucose-induced translocation of PKC-beta(2), -delta, and -zeta. Inhibition of tyrosine kinase with genistein blocked glucose-induced translocation of PKC-beta(1) and -delta, whereas chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane N,N,N,'N'-tetraacetic acid blocked translocation of PKC-beta(1) and -beta(2). Enzyme-linked immunosorbent assay performed on culture media from myocytes maintained in 25 mmol/l glucose detected a twofold increase in ANG II. Addition of an AT-1R antagonist (losartan; 100 nmol/l) to myocyte cultures blocked translocation of PKC-beta(1), -beta(2), -delta, and -epsilon. Phosphorylation of troponin (Tn) I was increased in myocytes exposed to 25 mmol/l glucose. Losartan selectively inhibited Tn I serine phosphorylation but did not affect phosphorylation at threonine residues. We concluded that 1) 25 mmol/l glucose triggers the release of ANG II by myocytes, resulting in activation of the ANG II autocrine pathway; 2) differential translocation of myocyte PKC isozymes occurs in response to 25 mmol/l glucose and ANG II; and 3) AT-1R-dependent PKC isozymes (beta(1), beta(2), delta, and epsilon) target Tn I serine residues.

Overexpression of insulin-like growth factor-1 attenuates the myocyte renin-angiotensin system in transgenic mice.

Constitutive overexpression of insulin-like growth factor-1 (IGF-1) in myocytes protects them from apoptosis and interferes with myocyte hypertrophy in the normal and pathological heart. Conversely, angiotensin II (Ang II) triggers cell death and promotes myocyte hypertrophy. Moreover, activation of p53 upregulates the cellular renin-angiotensin system (RAS). Therefore, IGF-1 overexpression in FVB.Igf+/- mice may downregulate the local RAS through the attenuation of p53 and p53-inducible genes. On this basis, p53 DNA binding activity to angiotensinogen (Aogen), bax, and the AT1 receptor was determined in left ventricular myocytes from FVB.Igf-/- and FVB.Igf+/- mice. The quantity of Bax, Bcl-2, Aogen, and AT1 receptor in these cells was evaluated. The presence of Mdm2-p53 complexes was also established. Finally, Ang II levels in myocytes were measured. Upregulation of IGF-1 in myocytes was associated with a protein-to-protein interaction between Mdm2 and p53, which attenuated p53 transcriptional activity for bax, Aogen, and AT1 receptor. Similarly, the amount of Bax, Aogen, and AT1 receptor proteins in these cells decreased. In contrast, the expression of Bcl-2 remained constant. The downregulation of Aogen in myocytes from FVB.Igf+/- mice was characterized by a reduction in Ang II. In conclusion, IGF-1 negatively influences the myocyte RAS through the upregulation of Mdm2 and its binding to p53. This may represent the molecular mechanism responsible for the effects of IGF-1 on cell viability and myocyte hypertrophy in the nonpathological and pathological heart in vivo.

Emerging concepts in antihypertensive therapy: the benefits of angiotensin II blockade.

Essential hypertension affects more than 40 million Americans, or one in four adults. The prevalence of hypertension is greater among the African-American population, with a distressingly high rate of end-organ complications. Although diabetes mellitus has surpassed hyper tension as the dominant etiology of end-stage renal disease in the United States, kidney failure secondary to hypertensive nephrosclerosis remains a significant problem, particularly among African Americans. During the past decade, a shift in the paradigm for the renin-angiotensin system (RAS) has evolved from a circulating vasoactive cascade toward angiotensin II (ANG II) formation at the cellular level. The molecular components of the RAS have been identified in cells, documenting the existence of an autocrine tissue RAS, as well as the presence of enzymes, which catalyze the formation of ANG II by angiotensin-converting-enzyme-independent pathways, providing new targets for therapeutic intervention. The latter challenge has important clinical implications, in view of recent evidence implicating ANG II in pathologic cell growth and cell death and fundamental events in the remodeling of the vascular wall and myocardium in the setting of hypertension. This review focuses on ANG II as a major determinant of end-organ damage in essential hypertension; the benefits of ANG II blockade at the end-organ level, which appear to be independent of the blood pressure-lowering effect; and the emerging role for ANG II receptor antagonists as first-line agents in the treatment of essential hypertension.

Angiotensin II activates programmed myocyte cell death in vitro.

To determine whether angiotensin II (Ang II) can induce apoptosis of neonatal ventricular myocytes, these cells were exposed to 10(-9) M Ang II for 24 h in vitro and the effects of this intervention on programmed myocyte cell death were examined by the terminal deoxynucleotidyl transferase assay and DNA gel electrophoresis. Ang II resulted morphologically in a 2.5-fold increase in the percentage of myocytes with double strand cleavage of the DNA and biochemically in the formation of DNA fragments equal in size to mono- and oligonucleosomes. Moreover, Ang II stimulation was characterized by a 37% increase in resting level of intracellular calcium and the activation of calcium-dependent endogenous endonuclease. In contrast, pH-dependent endogenous endonuclease was not enhanced by the addition of Ang II. Ang II-induced DNA damage was inhibited by the AT1 receptor antagonist, losartan. Similarly, the calcium chelator, BAPTA-AM, prevented Ang II-mediated cell death. Conversely, the calcium ionophore, A23187, triggered programmed cell death. Finally, the selective AT2 receptor subtype blocker, PD123319, failed to reduce myocyte apoptosis. In conclusion, ligand binding of AT1 receptors may initiate programmed myocyte cell death via an elevation in cytosolic calcium and the stimulation of calcium-dependent endogenous endonuclease.

Angiotensin II induces apoptosis of adult ventricular myocytes in vitro.

To determine whether angiotensin II (Ang II) activates the suicide program of myocytes, primary cultures of adult rat ventricular myocytes were exposed to 10(-9) M of Ang II, for 24 h. Ang II resulted in a five-fold increase in programmed myocyte cell death (PMCD) documented by the terminal deoxynucleotidyl transferase assay and confirmed by DNA agarose gel electrophoresis. Ang II stimulation was associated with translocation of the epsilon and delta isoforms of protein kinase C (PKC) which was coupled with an increase in cytosolic Ca2+ in the cells. The PKC inhibitor chelerythrine abolished Ang II-mediated increases in cytosolic Ca2+ and PMCD. Similarly, pretreatment of cells with the intracellular Ca2+ chelator BAPTA/AM inhibited the formation of DNA strand breaks. Conversely, the Ca2+ ionophore A23187 markedly increased PMCD. Finally, the AT1 receptor antagonist, losartan, completely blocked Ang II-induced PMCD, whereas the AT2 receptor antagonist, PD123319, did not attenuate this phenomenon. In conclusion, ligand binding of AT1 receptors on myocytes triggers PMCD by a mechanism involving PKC-mediated increases in cytosolic calcium, which result in internucleosomal DNA fragmentation.

Coronary artery stenosis in rats affects beta-adrenergic receptor signaling in myocytes.

OBJECTIVE: The purpose of this study was to determine whether the early chronic ischemic cardiomyopathy produced by non-occlusive coronary artery constriction was characterized by alterations in the regulation of beta-adrenoreceptor (beta-AR) signaling. METHODS: Coronary artery narrowing was surgically induced in rats and the animals sacrificed at 7 and 14 days. The changes in the biochemical properties of the multiple components of the beta-AR pathway were examined in enzymatically dissociated myocytes. RESULTS: Coronary stenosis, involving an average 55% reduction in luminal diameter, was associated with left ventricular failure and right ventricular dysfunction at both time intervals. A decrease in the quantity of beta-AR was detected at 7 days and preceded the loss of high-affinity binding sites. This regulatory modification was characterized by a reduction in beta 1 and beta 2 receptors and a shift in the isoproterenol dose response curve indicating a functional correlation between the decrease in beta-AR and attenuated inotropic support of the myocardium. The percentage of beta-AR binding agonist with high affinity decreased significantly at 14 days along with a further reduction in the density of beta 1 and beta 2 receptors. Reconstitution studies with cyc S49 lymphoma cells did not detect an impairment of Gs alpha functional activity, but the quantity of Gi alpha was increased at both intervals. Finally, activation of the catalytic unit of adenylyl cyclase by forskolin and GTP was not altered by coronary stenosis, however, basal cyclic AMP in myocytes was depressed at 14 days. CONCLUSIONS: Coronary stenosis induces distinct and progressive modifications in the beta-AR signaling cascade which may contribute to the impaired ventricular performance in this model of myocardial ischemia.

Myocardial infarction alters myofilament calcium sensitivity and mechanical behavior of myocytes.

To determine whether myocardial infarction leads to alterations in myofilament isometric tension as a function of Ca2+ concentration, unloaded shortening velocity, and sarcomere compliance, these properties were examined in skinned myocytes 7 days after coronary artery occlusion. Changes in myofilament proteins were also evaluated Myocardial infarction was characterized by a 10-15% reduction in myofilament isometric tension at submaximum Ca2+ levels in the physiological range. However, developed tension at maximum activation was unaltered. Conversely, unloaded shortening velocity was decreased by 31% in the remaining viable cells, whereas resting tension was increased by 30-40%. The regulatory protein troponin I content was reduced, but phosphorylation of troponin I and troponin T was increased. Myosin isoenzymes and troponin T contents were not altered. In conclusion, molecular responses occurred acutely after myocardial infarction, and these adaptations may depress the mechanical behavior of the unaffected cells, contributing to acute impairment in global cardiac pump function beyond that resulting from myocyte loss.

Identification and activation of autocrine renin-angiotensin system in adult ventricular myocytes.

To date, the demonstration that the molecular components of the renin-angiotensin system (RAS) are present in adult ventricular myocytes is lacking. In addition, whether the RAS is upregulated under conditions of overload and myocyte hypertrophy in vivo remains to be determined. By employing an in vivo model of ischemic cardiomyopathy in rats, we document that adult myocytes express genes for renin, angiotensinogen, angiotensin-converting enzyme (ACE), and angiotensin II (ANG II) receptors. Moreover, renin, ACE, and ANG II receptor mRNAs increased in stressed myocytes undergoing cellular hypertrophy. At the protein level, the percentage of myocytes containing renin, ANG I, and ANG II was significantly increased in the overloaded heart. The number of binding sites for ANG II per myocyte also markedly increased under this setting. These results provide direct evidence of the existence of a myocyte RAS, which may be activated in pathological states of the heart to support myocyte growth and contractile function.

Fibroblast proliferation during myocardial development in rats is regulated by IGF-1 receptors.

To determine whether the growth of cardiac fibroblasts during development is modulated by the insulin-like growth factor (IGF)-1 receptor (IGF-1R), the expression of IGF-1, IGF-2, and IGF-1R was determined in fibroblasts from fetal and postnatal hearts. The expression of proliferating cell nuclear antigen (PCNA) and DNA polymerase-alpha was also evaluated in combination with the estimation of DNA replication. In comparison with fetal hearts, at postnatal day 21, fibroblast expression of IGF-1R mRNA, IGF-2, PCNA, and DNA polymerase-alpha was reduced by 77, 70, 80, and 86%, respectively. Moreover, IGF-1R protein decreased by 48% at 21 days. Bromodeoxyuridine labeling decreased by 88 and 89% in the left and right ventricle, respectively, at this time. Two different antisense oligodeoxynucleotides to IGF-1R reduced DNA replication by 60 and 44% in fibroblasts in culture. In addition, this intervention markedly attenuated the growth response of fibroblasts to IGF-1 or serum. In conclusion, the IGF-1R system appears to play a major role in the regulation of fibroblast growth in the heart in vivo.

Alterations in angiotensin II receptor mediated signal transduction shortly after coronary artery constriction in the rat.

OBJECTIVE: The aim of the study was to determine the effect of coronary artery constriction on the density of angiotensin II receptors and on the effector responses coupled with these receptors on myocytes one week after surgical induction of coronary artery stenosis in rats. METHODS: After induction of coronary artery stenosis and following the estimation of global cardiac performance, myocytes were enzymatically dissociated and radioligand binding studies were performed. In addition, the isotonic contractile performance, cytosolic calcium transients, and angiotensin II stimulated inositol phosphate generation in myocytes were measured in the presence and absence of the angiotensin II receptor subtype antagonist losartan. RESULTS: After documenting left ventricular failure and right ventricular dysfunction, the expression and density of angiotensin II receptors in left ventricular myocytes were evaluated and found to be increased 3.1-fold and 4.1-fold, respectively. Corresponding increases in right ventricular myocytes were 3.6-fold and 4.5-fold. In contrast, the quantity of the regulatory protein Gq alpha was not altered in either ventricle. Angiotensin II did not increase the generation of total inositol phosphates in left and right ventricular myocytes at maximum stimulation. However, the threshold for the formation of inositol phosphates was lowered in left ventricular myocytes of coronary narrowed rats. Measurements of single cell mechanics indicated that angiotensin II stimulation markedly improved the depression in myocyte function biventricularly. This inotropic effect was coupled with the restoration of cytosolic calcium. CONCLUSIONS: The upregulation of angiotensin II receptors on myocytes in this model of global ischaemia may be a compensatory mechanism ameliorating myocyte contractility in an attempt to sustain ventricular pump function.

Coronary artery constriction in rats affects the activation of alpha 1 adrenergic receptors in cardiac myocytes.

OBJECTIVE: To determine whether alpha 1 adrenergic receptor mediated myocyte contractility and growth are depressed acutely after non-occlusive coronary artery narrowing, the left coronary artery was constricted in rats and mechanical behaviour, cytosolic calcium, and regulation of alpha 1 adrenergic receptors were examined in myocytes seven days later. METHODS: Coronary artery stenosis was surgically induced in rats and following the estimation of global cardiac performance myocytes were enzymatically dissociated and radioligand binding studies were performed. In addition, the isotonic contractile performance, cytosolic calcium transients and noradrenaline stimulated inositol phosphate generation in myocytes were measured in the presence of WB 4101 or after chlorethylclonidine treatment. RESULTS: Estimations of cell mechanics in vitro established that peak shortening was decreased by 36% and 18% in left and right ventricular myocytes of coronary stenosed rats. Time to peak shortening was prolonged by 29% in left and 20% in right myocytes, whereas velocity of shortening was decreased by 27% in left myocytes. These alterations were associated with increases in cell length and width, indicative of myocyte hypertrophy. In addition, coronary stenosis was accompanied by reductions in the expression of alpha 1a and alpha 1b receptor subtypes in myocytes. alpha 1 Adrenergic receptor density and noradrenaline stimulated phosphoinositol turnover were decreased by 30% and 34% in left myocytes. alpha 1a Adrenergic receptor subtype mediated cytosolic calcium concentration and myocyte mechanical performance were also impaired in left myocytes only. The alpha 1a adrenergic receptor subtype antagonist WB 4101 abolished noradrenaline stimulated inositol phosphate generation in myocytes, whereas chlorethylclonidine at large doses only partially inhibited this response. CONCLUSIONS: In conclusion, coronary narrowing leads to defects in the regulation of alpha 1 adrenergic receptors on myocytes which are coupled with attenuation in the transmission of signals, possibly affecting myocyte cell function and ongoing reactive cellular hypertrophy.

Efficacy of angiotensin-converting enzyme inhibition and AT1 receptor blockade on cardiac pump performance after myocardial infarction in rats.

To determine whether cardiac unloading by inhibition of angiotensin I (AI) to AII conversion by captopril or blockade of the AII receptor (AT1) by losartan was more effective in prevention of the detrimental hemodynamic consequences of myocardial infarction (MI), inhibition of metabolic production of AII by captopril was compared with blockade of AT1 with losartan in Sprague-Dawley rats with large MI. Infarcts were created by surgical occlusion of the left main coronary artery and oral drug therapy initiated immediately and continued until hemodynamic evaluation seven days later. Heart weight was unchanged in untreated infarcted animals, whereas captopril reduced heart weight in control animals and losartan increased heart weight in infarcted animals. Left ventricular (LV) peak systolic blood pressure (SBP) was lower in treated and untreated infarcted animals. Although captopril reduced end-diastolic pressure (EDP) to a greater degree than losartan, all infarcted group showed an increase in this parameter with respect to similarly treated controls. LV peak rates of pressure increase and decay in infarcted hearts were decreased significantly more by captopril than by losartan administration. Captopril also impaired right side cardiac function more than losartan when peak rate of pressure increase was evaluated. Thus, inhibition of the effects of AII during cardiac failure improved but did not normalize cardiac pump performance.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiocyte as a target for parathyroid hormone in end-stage renal disease.

Approximately 50% of the annual mortality in patients with end-stage renal disease (ESRD) is attributed to cardiovascular-related events. Multiple factors, including volume overload, hypertension, electrolyte abnormalities, and the presence of comorbid diseases, such as diabetes mellitus, may have an adverse effect on left ventricular function in ESRD. The purpose of this brief review is to advance the hypothesis that parathyroid hormone (PTH) is a cardiotoxin and a potential mediator of cardiac dysfunction in uremia. Recent studies have provided evidence that cardiocytes possess a distinct class of binding sites for PTH, and the PTH receptor has recently been cloned. Furthermore, the PTH receptor may be coupled to more than one effector pathway. Finally, the possibility that a PTH-related protein autocrine system may be present in cardiocytes and the implications of this signaling pathway on cardiocyte function are discussed.

Upregulation of IGF1, IGF1-receptor, and late growth related genes in ventricular myocytes acutely after infarction in rats.

To determine the effects of acute myocardial infarction on the expression of insulin-like growth factor1 (IGF1) and insulin-like growth factor1 receptors (IGF-1R) on the surviving myocytes of the left and right ventricles, large infarcts were produced in rats and the animals sacrificed 2 days later. Hemodynamic measurements of left and right ventricular pressures, +dP/dt and -dP/dt, and central venous pressure documented that coronary occlusion was associated with a severe impairment of cardiac function. By employing reverse transcriptase polymerase chain reaction (RTPCR), a low level of expression of IGF-1R mRNA was detected in myocytes from sham-operated rats. Acute myocardial infarction was found to enhance by nearly twofold the message for IGF-1R in viable myocytes biventricularly. Moreover, IGF1 mRNA increased 4.3-fold and 9.4-fold in left and right myocytes, respectively. In order to establish whether the upregulation of IGF1 and IGF-1R with infarction was coupled with induction of late growth related genes, which are known to be implicated in DNA replication and mitotic division, proliferating cell nuclear antigen (PCNA) and histone-H3 expression was assessed by Northern blot and RTPCR. The level of expression of PCNA mRNA was found to be increased 3.9-fold and 2.4-fold in left and right myocytes, respectively from infarcted hearts. Corresponding increments in histone-H3 mRNA were 25.5-fold and 5.3-fold, respectively. However, PCNA protein as detected by immunoperoxidase staining was restricted to a limited number of myocyte nuclei adjacent to the necrotic myocardium of the left ventricle. In conclusion, acute myocardial infarction is associated with enhanced expression of IGF1 and IGF-1R on stressed myocytes, and this phenomenon may activate genes essential for DNA synthesis, possibly affecting myocyte growth. These processes may be fundamental for the reconstitution of tissue mass and amelioration of function after infarction.

Myocyte loss and left ventricular failure characterise the long term effects of coronary artery narrowing or renal hypertension in rats.

OBJECTIVE: The aim was to determine the effects of chronic coronary artery narrowing and two kidney, one clip renal hypertension alone and in combination on ventricular function and myocardial morphology. METHODS: Left coronary stenosis and renal artery clipping were surgically induced in rats and pump dynamics, systolic and diastolic wall stress, cardiac anatomy, and the changes in number and size of left ventricular myocytes were examined 11-13 weeks later. RESULTS: Ventricular failure evolved with each intervention: left ventricular end diastolic pressure was raised, whereas +dP/dt, -dP/dt, stroke volume, cardiac output, and cardiac index were reduced. Calculated ventricular systolic wall stress increased nearly 70% in the three experimental conditions. By contrast, diastolic wall stress was augmented 6.1-fold with coronary stenosis, 4.0-fold with hypertension, and 4.4-fold with combined treatment. These differences were due to variable preservations of wall thickness between the groups. Left ventricular weight expanded 26%, 35%, and 32% with stenosis, hypertension, and a combination of the two, whereas diastolic cavitary volume increased 57%, 35%, and 49%. Corresponding increases in systolic chamber volumes were 156%, 122%, and 154%. Finally, myocyte loss in the ventricle was 25%, 25%, and 37% in coronary narrowing, renal hypertension and a combination of the two with concomitant enlargements of the unaffected myocytes of 47%, 63%, and 65%. CONCLUSIONS: Decompensated eccentric ventricular hypertrophy developed as a result of coronary artery narrowing, renal hypertension, or the two in combination. Coronary artery narrowing, however, may have a greater maladaptive effect on ventricular function than systemic hypertension, and coronary stenosis and hypertension combined because of the more extensive chamber and wall remodelling which sustained greater increases in diastolic wall stress.

Regulation of angiotensin II receptors on ventricular myocytes after myocardial infarction in rats.

To determine the effects of acute myocardial infarction on the regulation of angiotensin II (Ang II) receptors and contractile performance of left and right ventricular myocytes, coronary artery ligation was surgically induced in rats, and Ang II receptor density and affinity and the mechanical properties of surviving muscle cells were examined 1 week later. Physiological determinations of cardiac pump function revealed the presence of ventricular failure, which was associated at the cellular level with a depression in the velocity of myocyte shortening and relengthening, a prolongation of time to peak shortening, and a reduction in the extent of cell shortening. These abnormalities in single-cell function were more prominent in left than in right ventricular myocytes. Cellular hypertrophy was documented by increases in cell length and width, which were also greater in the spared myocytes of the infarcted left ventricle. Reactive hypertrophy was accompanied by a 1.84- and 1.85-fold increase in the density of Ang II receptors on left and right myocytes, respectively. On the other hand, the affinity of Ang II receptors for the radiolabeled antagonist was not altered. However, Ang II-stimulated phosphoinositol turnover was enhanced by 3.7- and 2.5-fold in left and right myocytes, respectively, after infarction. Ventricular myocytes were found to possess the AT1 receptor subtype exclusively. In conclusion, myocardial infarction leads to impairment in the contractile behavior of the remaining cells and to the activation of Ang II receptors and effector pathway associated with these receptors, which may be involved in the reactive growth adaptation of the viable myocytes.

Alterations in ANG II responsiveness in left and right myocardium after infarction-induced heart failure in rats.

To determine the effects of coronary arterial occlusion on the contractile response of the heart to angiotensin II (ANG II) administration, large infarcts were surgically induced in Sprague-Dawley rats at 2 mo of age. Forty-eight hours later, hearts from experimental animals presented a hemodynamic profile indicative of left ventricular failure and right ventricular dysfunction and revealed a loss of mass of 49.3 +/- 10.8% of the left ventricle inclusive of the interventricular septum. Plasma renin activity was found to be decreased by 48% in animals with occlusion of the left main coronary artery. Left and right posterior papillary muscles removed from these same hearts were evaluated mechanically in the presence and absence of ANG II. Contractile performance was impaired in left ventricular myocardium from infarcted rats as evidenced by the inability to attain developed tension similar to that seen in control rats. In addition, peak rates of tension rise and decay were significantly depressed. A reduction in contraction duration was also found in experimental animals, limiting the active state of the myocardium. ANG II resulted in a depression in the force-generating ability of left and right papillary muscles of control and experimental animals. Importantly, the negative inotropic effect of ANG II affected the left and right myocardium from infarcted rats by nearly twofold and threefold more than the corresponding muscles from controls. Morphometric evaluation revealed the absence of damage in both papillary muscles from control hearts and in the right muscles from experimental animals. However, necrotic tissue comprised 28.3 +/- 9.8% of left papillary muscles obtained from infarcted ventricles. It is concluded that ANG II administration resulted in reduced mechanical performance of rat myocardium. Coronary arterial ligation potentiated this phenomenon, and such a negative effect may have implication in infarction induced heart failure in vivo.

ANG II receptors, c-myc, and c-jun in myocytes after myocardial infarction and ventricular failure.

To determine the relationship between reactive cardiac hypertrophy and the expression of angiotensin II (ANG II) receptors in surviving myocytes after infarction, large infarcts were produced in rats that were killed 2-3 days later. Measurements of global ventricular dynamics indicated that left ventricular failure and right ventricular dysfunction occurred in experimental animals. These alterations in ventricular pump function were associated with increases in ventricular weight-to-body weight ratio, indicative of developing cardiac hypertrophy. Such a response was coupled with a 6.6-fold increase in ANG II receptor mRNA in myocytes from the left ventricle. A 2.3-fold increase in the expression of ANG II receptor in myocytes from the right ventricle was also found. Radioligand binding assay documented a 44% increase in the density of ANG II receptors on left ventricular myocytes of infarcted hearts. To establish whether the induction of genes commonly associated with myocyte hypertrophy was present, the message for c-myc and c-jun was biventricularly assessed. Myocardial infarction was accompanied by overexpressions of c-myc and c-jun that were more prominent in left than in right ventricular myocytes. In conclusion, the enhanced expression of ANG II receptor and its receptor protein and c-myc and c-jun in myocytes may participate in the reactive growth processes of these cells after infarction.