Polyamines and noradrenaline following myocardial hypertrophy

Right Ventricle Remodeling Metabolic Signature in Experimental Pulmonary Hypertension Models of Chronic Hypoxia and Monocrotaline Exposure

Introduction: Over time and despite optimal medical management of patients with pulmonary hypertension (PH), the right ventricle (RV) function deteriorates from an adaptive to maladaptive phenotype, leading to RV failure (RVF). Although RV function is well recognized as a prognostic factor of PH, no predictive factor of RVF episodes has been elucidated so far. We hypothesized that determining RV metabolic alterations could help to understand the mechanism link to the deterioration of RV function as well as help to identify new biomarkers of RV failure. Methods: In the current study, we aimed to characterize the metabolic reprogramming associated with the RV remodeling phenotype during experimental PH induced by chronic-hypoxia-(CH) exposure or monocrotaline-(MCT) exposure in rats. Three weeks after PH initiation, we hemodynamically characterized PH (echocardiography and RV catheterization), and then we used an untargeted metabolomics approach based on liquid chromatography coupled to high-resolution mass spectrometry to analyze RV and LV tissues in addition to plasma samples from MCT-PH and CH-PH rat models. Results: CH exposure induced adaptive RV phenotype as opposed to MCT exposure which induced maladaptive RV phenotype. We found that predominant alterations of arginine, pyrimidine, purine, and tryptophan metabolic pathways were detected on the heart (LV+RV) and plasma samples regardless of the PH model. Acetylspermidine, putrescine, guanidinoacetate RV biopsy levels, and cytosine, deoxycytidine, deoxyuridine, and plasmatic thymidine levels were correlated to RV function in the CH-PH model. It was less likely correlated in the MCT model. These pathways are well described to regulate cell proliferation, cell hypertrophy, and cardioprotection. These findings open novel research perspectives to find biomarkers for early detection of RV failure in PH.

Anti-hypotensive drug induced cardiotoxicity: an in vitro study

Cardiotoxic side effects of broad range of drugs have emerged as an important cause of developing cardiovascular complications, as patients recover from one disease but develop another. Both cardiovascular and non-cardiovascular drugs may lead to the toxicity in the heart. Many drugs were initially not screened for cardiotoxicity, which is now an essential concern for drug discovery. Levophed is used for treating hypotension in critical care patients. Being a neurotransmitter, its concentration increases significantly in stress conditions and administration of this drug to patients' results in developing acute as well as persistent cardiac complications. Therefore, understanding its concentration-mediated effects and identifying the toxic concentration will serve as a platform to develop interventions to prevent adverse drug effects. In the present study, concentration and time-dependent effects of Levophed in H9C2 cardiomyoblasts were studied in detail by various cytotoxicity assays. Norepinephrine as a Levophed substitute was used and apoptotic cellular death was characterized by Annexin V and TUNEL DNA fragmentation assays. Morphological alterations, growth inhibition, and cellular death were also studied in detail. We observed that Levophed induces concentration-mediated deleterious effects in cardiomyoblasts. In-depth analysis of these effects will help in designing strategies in near future to combat and reduce this drug-induced cardiac toxicity.

Correlation between endogenous polyamines in human cardiac tissues and clinical parameters in patients with heart failure

Polyamines contribute to several physiological and pathological processes, including cardiac hypertrophy in experimental animals. This involves an increase in ornithine decarboxylase (ODC) activity and intracellular polyamines associated with cyclic adenosine monophosphate (cAMP) increases. The aim of the study was to establish the role of these in the human heart in living patients. For this, polyamines (by high performance liquid chromatography) and the activity of ODC and N¹‐acetylpolyamine oxidases (APAO) were determined in the right atrial appendage of 17 patients undergoing extracorporeal circulation to correlate with clinical parameters. There existed enzymatic activity associated with the homeostasis of polyamines. Left atria size was positively associated with ODC (r = 0.661, P = 0.027) and negatively with APAO‐N¹‐acetylspermine (r = −0.769, P = 0.026), suggesting that increased levels of polyamines are associated with left atrial hemodynamic overload. Left ventricular ejection fraction (LVEF) and heart rate were positively associated with spermidine (r = 0.690, P = 0.003; r = 0.590, P = 0.021) and negatively with N¹‐acetylspermidine (r = −0.554, P = 0.032; r = −0.644, P = 0.018). LVEF was negatively correlated with cAMP levels (r = −0.835, P = 0.001) and with cAMP/ODC (r = −0.794, P = 0.011), cAMP/spermidine (r = −0.813, P = 0.001) and cAMP/spermine (r = −0.747, P = 0.003) ratios. Abnormal LVEF patients showed decreased ODC activity and spermidine, and increased N¹‐acetylspermidine, and cAMP. Spermine decreased in congestive heart failure patients. The trace amine isoamylamine negatively correlated with septal wall thickness (r = −0.634, P = 0.008) and was increased in cardiac heart failure. The results indicated that modifications in polyamine homeostasis might be associated with cardiac function and remodelling. Increased cAMP might have a deleterious effect on function. Further studies should confirm these findings and the involvement of polyamines in different stages of heart failure.

L-Arginine and its metabolites in kidney and cardiovascular disease

L-Arginine is a semi essential amino acid synthesised from glutamine, glutamate and proline via the intestinal-renal axis in humans and most mammals. L-Arginine degradation occurs via multiple pathways initiated by arginase, nitric-oxide synthase, Arg: glycine amidinotransferase, and Arg decarboxylase. These pathways produce nitric oxide, polyamines, proline, glutamate, creatine and agmatine with each having enormous biological importance. Several disease are associated to an L-arginine impaired levels and/or to its metabolites: in particular various L-arginine metabolites may participate in pathogenesis of kidney and cardiovascular disease. L-Arginine and its metabolites may constitute both a marker of pathology progression both the rationale for manipulating L-arginine metabolism as a strategy to ameliorate these disease. A large number of studies have been performed in experimental models of kidney disease with sometimes conflicting results, which underlie the complexity of Arg metabolism and our incomplete knowledge of all the mechanisms involved. Moreover several lines of evidence demonstrate the role of L-arg metabolites in cardiovascular disease and that L-arg administration role in reversing endothelial dysfunction, which is the leading cause of cardiovascular diseases, such as hypertension and atherosclerosis. This review will discuss the implication of the mains L-arginine metabolites and L-arginine-derived guanidine compounds in kidney and cardiovascular disease considering the more recent literature in the field.

Altered myocardial acetylcholine and norepinephrine concentrations in right ventricular hypertrophy and failure

The progression of cardiac hypertrophy and failure is associated with marked changes in cardiac autonomic innervation, and there are sympathetic-parasympathetic interactions in the regulation of cardiac function. Although the indexes of sympathetic innervation have been found to be depressed with the development of heart failure, those of parasympathetic innervation have not yet been fully investigated. In order to better understand changes in markers of autonomic innervation associated with cardiac hypertrophy and failure, we measured the myocardial acetylcholine (ACh) store as a parasympathetic marker and the norepinephrine (NE) store as a sympathetic marker in pressure-overloaded right ventricular hypertrophy in rats. Two weeks after the injection of monocrotaline, significant right ventricular hypertrophy occurred. Three weeks after, severe right ventricular hypertrophy with no sign of heart failure occurred, and 4 weeks after, overt heart failure developed. In the right heart of monocrotaline rats, NE concentrations tended to increase at 1 week, returned to baseline at 2 weeks, decreased to one-half of the control values at 3 weeks, and then fell to 14% of the controls at 4 weeks. ACh concentrations in the right heart tended to increase at 1 week and exhibited a significant increase (136% and 129% of the controls in the right atrium and ventricle, respectively) at 2 weeks. As with NE, ACh concentrations in the right atrium and ventricle decreased to 76% and 54% of the controls at 3 weeks, and continued to decrease to 22% and 24% of the controls at 4 weeks after monocrotaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Red blood cell polyamine level changes following heart transplantation in man

Follow-up of orthotopic heart transplanted patients has revealed the existence of abnormally high red blood cell (RBC) spermidine (Spd) levels during the first two months after surgical procedure (A-period). From the third month after heart transplantation (B-period), RBC Spd concentrations went back to normal values in early cardiac rejection (ECR) patients. During A- and B-periods, significantly higher Spd levels and Spd/Spm ratios were observed in late cardiac rejecting (LCR) patients than in ECR ones. The lack of a direct relationship between the histological grade of rejection and RBC Spd levels leads us to consider these polyamine blood levels as a new biological instrument in the diagnosis of heart rejection.

Norepinephrine-induced enlargement of nucleus in cultured myocardial cells

The change in the nuclear size of neonatal rat myocardial cells was evaluated under the culture conditions of exposure to norepinephrine (NE). Daily administration for 1 week of 0.2, 2, and 20 ng/ml NE induced a significant increase in nuclear size as a result of the dose-dependent quality of the nuclei. NE also stimulated a beating response in the cultured myocardial cells because of this dose dependency. A good correlation was found between the two markers and the NE dose dependency. Single or mononucleated myocardial cells often appeared in the NE-treated groups. Ornithine decarboxylase activity was not acutely stimulated even by 20 ng/ml NE. These observations suggest that administration of NE induces nuclear enlargement and enhances nuclear function through the stimulation of beating. Further, there may not be a direct relation between nuclear enlargement and the polyamine synthesis pathways.

Abnormal exercise hemodynamics in patients with normal systolic function late after aortic valve replacement

We studied the hemodynamic response to supine bicycle exercise in 20 patients late (10 +/- 2 years) after aortic valve replacement (for aortic stenosis in 12 patients, aortic insufficiency in six patients, and for combined stenosis and insufficiency in two patients). The pulmonary artery wedge pressure was obtained with a pulmonary artery balloon catheter, and left ventriculography was performed by digital-subtraction angiography after injection of radiographic contrast into the pulmonary artery. These patients were compared with 11 control subjects with no or minimal cardiac disease studied routinely for evaluation of chest pain in whom left ventricular end-diastolic pressure and a direct contrast ventriculogram were obtained. Compared with the control population, the study population had similar left heart filling pressures (7 +/- 3 vs 9 +/- 3 mm Hg, NS), but higher left ventricular ejection fractions (75 +/- 7% vs 67 +/- 7%, p less than .02) and higher left ventricular muscle mass indexes (106 +/- 28 vs 85 +/- 9 g/m2, p less than .01). Elevated myocardial muscle mass led to lower systolic wall stress in the study population than in the control subjects (254 +/- 65 vs 320 +/- 49 10(3).dynes/cm2, p less than .01) and might explain the higher ejection fraction observed. Fourteen patients had a normal response to exercise (with left heart filling pressures of 16 +/- 4 vs 18 +/- 2 mm Hg for control subjects, NS; and left ventricular ejection fraction of 77 +/- 8% vs 73 +/- 5% for control subjects, NS). However, while the remaining six patients had a normal exercise left ventricular ejection fraction (72 +/- 9%, NS), they had an abnormal rise in left heart filling pressure (33 +/- 8 mm Hg, p less than .01). Preoperatively these patients also had higher left ventricular mid- and end-diastolic pressures at similar diastolic volumes, suggesting a decrease in chamber compliance. Thus, late after aortic valve replacement there is a subgroup of patients who, despite normal hemodynamics and normal left ventricular systolic function as assessed by the left ventricular ejection fraction at rest, have an abnormal response to exercise characterized primarily by a substantial rise in left heart filling pressures. Preoperatively this group also has a decrease in diastolic chamber compliance despite nearly normal left ventricular ejection fractions. This abnormality appears to result from a primary derangement of diastolic function that is not evident at rest.

Treatment with polyamine synthesis inhibitors reduces the positive inotropic effect of ouabain, noradrenaline and calcium

Polyamines (putrescine, spermidine and spermine) are considered to act as intracellular second messengers by increasing Ca++ influx and mobilizing intracellular calcium. On the other hand, intracellular Ca++ increase is the common final step of the mechanism of action of many inotropic agents. To discover whether the functional integrity of the cardiac ornithine decarboxylase (ODC)/polyamine system is necessary to cope with a stimulated inotropism, we studied the effect of ouabain, noradrenaline, and calcium on ventricle strips obtained from rats treated with polyamine synthesis inhibitors. The combined administration of methylglioxal bis (guanylhydrazone) (MGBG) (single i.p. injection of 50 mgkg-1) and of alpha-di fluoromethylornithine (DFMO) (100 mgkg-1 every 12 h for 7 consecutive days) caused a 62.5% inhibition of ventricular ODC activity, and a significant decrease of the ventricular content of putrescine and spermidine (-59.5%, and -40.1%, respectively). While the basal isometric tension developed by ventricle strips obtained from rats treated with MGBG+DFMO was similar to that developed by ventricle strips from controls, the response to ouabain (1 microM), noradrenaline (10 microM), or Ca++ (3.6 mM) was significantly reduced. It cannot be excluded that effects of MGBG unrelated to the inhibition of polyamine synthesis may have also concurred in part to influence the effect of ouabain, Ca++ and noradrenaline adversely. However, the present results seem to indicate that the heart response to inotropic agents requires an efficient ODC/polyamine system, polyamines probably being involved in calcium ion movements or affecting the Ca++ sensitivity of contractile proteins.

Metabolic aspects of the development of experimental cardiac hypertrophy

In three models of cardiac hypertrophy the significance of catecholamines and the adenylate cyclase-cyclic AMP-system was examined. Two approaches were utilized: 1. The time course of cyclic AMP alterations was correlated with the changes in adenine nucleotide and protein biosynthesis. 2. The effect of beta-receptor blockade on the obligatory increase in adenine nucleotide and protein synthesis was evaluated. In isoproterenol-elicited cardiac hypertrophy, the elevation of the cyclic AMP content was one of the earliest metabolic alterations preceding the enhancement of the biosynthesis of adenine nucleotides and proteins. beta-Receptor blockade with propranolol abolished the increase in adenine nucleotide synthesis. In pressure-induced cardiac hypertrophy due to constriction of the abdominal aorta, catecholamines and the adenylate cyclase-cyclic AMP-system were found not to play a significant role. In triiodothyronine-elicited hypertrophy, the cyclic AMP level was increased very early, but beta-receptor blockade did not prevent hypertrophy nor the enhancement of cardiac adenine nucleotide biosynthesis, although the positive chronotropic and inotropic effects of triiodothyronine were abolished. This result can best be interpreted to indicate a direct effect of triiodothyronine on myocardial carbohydrate metabolism including the pentose phosphate pathway.

Increased right ventricular wall thickness in left ventricular pressure overload: echocardiographic determination of hypertrophic response of the "nonstressed" ventricle

Left ventricular hypertrophy in left ventricular pressure overload occurs in response to excessive work load imposed on the left ventricle by increased impedance to ejection. Right ventricular hypertrophy may occur in patients with these findings, but has been considered to be secondary to pulmonary hypertension. To determine the frequency of right ventricular hypertrophy and its relation to increased left ventricular wall thickness in patients with left ventricular pressure overload, right ventricular wall thickness was measured using M-mode echocardiography with two-dimensional echocardiographic guidance in 65 patients with left ventricular pressure overload; 49 patients had essential hypertension and 16 had aortic valve stenosis. These measurements were compared with data from 13 patients with "thin-walled" dilated cardiomyopathy and 20 normal subjects. Average right ventricular wall thickness in hypertensive patients (7 +/- 2 mm) and patients with aortic stenosis (6 +/- 2 mm) was significantly greater than that in normal subjects (4 +/- 1 mm) and patients with dilated cardiomyopathy (4 +/- 1 mm) who had normal left ventricular wall thickness, even though left ventricular mass was increased in all patient groups. Increased right ventricular wall thickness was present in 40 (80%) of 49 patients with hypertension and 10 (63%) of 16 patients with aortic stenosis. The magnitude of increase in right ventricular wall thickness was linearly correlated (r = 0.76, p less than 0.005) with left ventricular wall thickness, but was not associated with pulmonary hypertension. It is concluded that increased right ventricular wall thickness is common in patients with left ventricular pressure overload, is directly related to increases in left ventricular wall thickness, and is independent of right ventricular hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in left ventricular mass during a double-blind study with chlorthalidone and slow-release nifedipine

The presence of a possible correlation between changes in left ventricular mass of hypertensive patients and the degree of blood pressure reduction with different antihypertensive drugs has been investigated in 40 outpatients by M-mode echocardiography. Ten of these, with blood pressure in normal limits with different antihypertensive treatment had their therapy changed in chlorthalidone 25 mg/day without any run-in (Group A); other 30 patients, with a previously uncontrolled blood pressure, after a 14 day run-in, were randomly allocated to chlorthalidone 25 mg/day (Group B), slow release nifedipine 20 mg/day (Group C) or placebo (Group D). At the end of the eight week treatment period a further decrease in systolic blood pressure was observed in Group A without changes in ventricular mass; an highly significant decrease in both systolic and diastolic blood pressure was observed in B and C but only patients on chlorthalidone changed their ventricular mass; no change in both blood pressure and ventricular mass was observed on placebo. As changes in ventricular mass are not correlated with blood pressure reduction, we conclude that other, not well defined factors, apart from the decrease in duration and degree of left ventricular systolic wall tension, may be responsible for reversal of left ventricular hypertrophy.

Relationship between the in-situ activity of ornithine decarboxylase and contractile function of the rabbit papillary muscle

The relationship between isometric contraction and myocardial ornithine decarboxylase (ODC; EC.4.1.1.17) activity was studied in right ventricular papillary muscles isolated from rabbits. ODC specific activity and polyamine content were significantly increased in papillary muscles contracting isometrically 90 times per minute at the apex of the length-tension relationship for 4 to 5 hours when compared with paired non-contracting muscles or isometrically contracting muscles stimulated at 30 times per min. The increase in ODC activity appeared to be due to new protein synthesis since cycloheximide blocked the increase in ODC activity without affecting isometric function. Thus, the present results suggest that increased contractile demands of the heart may stimulate the synthesis of myocardial ODC, increase ODC activity and polyamine content.

Biochemical regulators in cardiac hypertrophy

In recent years research has shown that muscle is capable of reacting to mechanical stimuli by altering biochemical processes. Myocardium is probably the source of a biochemical factor, or factors which activate myocardial protein synthesis. In experimentally induced cardiac hypertrophy adaptive alterations have been shown to occur not only in the adrenal medulla but also in the adrenal cortex. Finally, detection of cross reactivity between digitalis glycosides and a number of steroid hormones has succeeded. We assume that such cross reactivity indicates the existence of an endogenic factor of steroid character, which is produced in the adrenal gland and functions as an endogenic cardiotonic agent. During experimental cardiac hypertrophy its synthesis is possibly increased. We propose the term "endocardin" or "endocardiotonin" for this agent.

Polyamine metabolism in muscles of mice and rats

Polyamine biosynthesis in different types of muscle was studied in mice and rats. A sex difference of polyamine biosynthesis in the gastrocnemius of the mouse was demonstrated. Ornithine decarboxylase activity was found to be several-fold higher in the gastrocnemius of the male mouse than in that of the female. Orchiectomy resulted in a decline of enzyme activity in the gastrocnemius. This effect was reversed by the administration of testosterone. The elevation of ornithine decarboxylase activity in the gastrocnemius by testosterone was reflected in an increased content of the polyamines in the muscle. Muscles of other types, i.e. soleus, heart and urinary bladder were shown to be virtually unresponsive to testosterone treatment. Neither were the muscles of the rat, including gastrocnemius, found to be affected by the androgen.

Polyamine levels and diamine oxidase activity in hypertrophic heart of spontaneously hypertensive rats and of rats treated with isoproterenol

Polyamine levels and diamine oxidase (EC 1.4.3.6) activity were studied in hypertrophic heart of spontaneously hypertensive rats as well as in the heart of Wistar rats during the development and regression of cardiac hypertrophy induced by isoproterenol administration. In spontaneously hypertensive rats, putrescine content and diamine oxidase activity were higher than those found in normotensive Kyoto-Wistar control rats. During the development of cardiac hypertrophy induced by isoproterenol, there was an increase in polyamine content and diamine oxidase activity. The administration of cycloheximide or actinomycin D prevented the increase in diamine oxidase activity during the first 24 h after isoproterenol administration, demonstrating that the rise in diamine oxidase activity was due to synthesis of new enzyme. Following the cessation of isoproterenol treatment, cardiac hypertrophy regressed and polyamine levels and diamine oxidase activity diminished toward control values. The administration of aminoguanidine to isoproterenol-treated rats caused in the heart an inhibition of diamine oxidase activity that led to an increase in putrescine level beyond the values found in animals given isoproterenol alone. The results suggest that the enhancement of diamine oxidase activity plays a role in the regulation of putrescine level in hypertrophic heart.

Cardiac hypertrophy: its characteristics as a growth process

The causal relation between cardiac function and growth is analyzed in this review article. Three different levels of development are discussed: cytodifferentiation, embryogenesis and postnatal development. The earliest stage of cardiac morphogenesis, that is, the appearance of cell-specific proteins and of spontaneous contractions, appears to be independent of hemodynamic forces. Also, the first major morphologic transformation of the primitive heart, looping, is the intrinsic property of the heart itself. However, at any later stage of life, hemodynamic function in both health and disease is closely coupled to cardiac growth.

[The regulation of protein synthesis in heart muscle. Biochemical data, stimulative and inhibitory factors and their clinical significance (author's transl)]

The regulation of protein synthesis in heart muscle has been investigated by many authors under both normal and pathological conditions. This review summarizes the evidence for the dependence of normal heart protein synthesis from normal serum levels of insulin, amino acids, fatty acids and glucose. A decreased serum concentration of these substances causes an inhibition of heart muscle protein synthesis by 30--60%. Various drugs and other chemical lead to similar impairments of heat muscle protein synthesis. The resulting imbalance between synthesis and degradation of myocardial proteins with their half-times of 5--12 days gradually leads to a decrease in their myocellular concentration with a consequent impairment of myocardial function. Finally, the biochemial sequences are described which represent the important pathogenetic mechanisms in the development of heart muscle hypertrophy and in the adriamycin-induced cardiomyopathy.

Changes in cAMP concentrations during chronic cardiac hypertrophy

Mild pulmonic stenosis in the dog, where right ventricular peak systolic pressure was increased approximately 150% at the time of sacrifice, induced 100% or more increase in right ventricular free wall weight by 3 weeks postoperative. Accompanying cardiac hypertrophy at these postoperative times, there was a decrease in both tissue PO2 levels and cAMP concentrations in the hemodynamically stressed ventricle, the right ventricle. Myosin ATPase activity was elevated as well as the velocity of contractile element shortening. The hemodynamically nonstressed left ventricle did not hypertrophy at these early postoperative times.

Regulation of ribonucleic acid synthesis by polyamines. Reversal by spermine of inhibition by methylglyoxal bis(guanylhydrazone) of ribonucleic acid synthesis and histone acetylation in rabbit heart

The relationship between polyamines and RNA synthesis was studied by considering the action of spermine on histone acetylation in perfused heart. In addition, the effect of methylglyoxal bis(guanylhydrazone), inhibitor of putrescine-activated S-adenosylmethionine decarboxylase activity, on RNA and polyamine specific radioactivity and on acetylation of histone fractions was also investigated in perfused heart. Different concentrations of spermine and/or methylglyoxas bis(guanylhydrazone) were injected into the heart, 15 min after beginning the perfusion. The results demonstrate that spermine stimulates the specific radioactivity of RNA of subcellular fractions. Acetylation of the arginine-rich histone fractions, involved in the regulation of RNA transcription, is enhanced by spermine. The perfusion with methylglyoxal bis(guanylhydrazone) causes a decrease in the specific radioactivity of polyamines and RNA, and in acetylation of histone fractions. However, spermine is able to reverse the methylglyoxal bis(guanylhydrazone) inhibition when injected simultaneously. From these results we may assume a possible role for spermine in the regulation of RNA transcription.

Ornithine decarboxylase in cardiac hypertrophy in the rat

Ornithine decarboxylase, a possible rate-limiting enzyme in the synthesis of polyamines, was assayed in hearts of normal rats, sham-operated rats, and rats subjected to aortic constriction. In the hearts of rats with constricted aortas, significantly increased enzyme activity compared with that in the hearts of sham-operated rats was observed 2, 4, 6, and 8 hours and 3, 5, and 10 days after operation, and two peaks in activity occurred--one at 4 hours and the other at 5-10 days. Increased ornithine decarboxylase activity was one of the earliest changes associated with cardiac hypertrophy. The changes in enzyme activity correlated well with the subsequent hypertrophy in the hearts of rats with aortic constriction and with the regression in the hearts of sham-operated rats, suggesting a role for polyamines in the regulation of cardiac growth. The early increase in ornithine decarboxylase activity in hearts of rats with aortic constriction was inhibited by actinomycin D, 8-azaguanine, and cycloheximide, indicating that RNA and protein synthesis are involved in the process. Actinomycin D given 30 minutes after operation or even at the time of aortic constriction failed to inhibit the increase in the enzyme activity, suggesting that the transcription required for the increase occurs early after operation. Cycloheximide given 1 hour before the rats were killed markedly decreased the enzyme activity, and the estimated half-life of cardiac ornithine decarboxylase was comparable to that of the reported short-lived liver ornithine decarboxylase. The study suggests that the synthesis of ornithine decarboxylase is influenced at the stage of transcription by mechanical stress on the myocardium and that polyamines might have a regulatory role in cardiac hypertrophy and regression.