Myocyte hypertrophy in neonatal rat heart cultures and its regulation by serum and by catecholamines

Effects of norepinephrine on neonatal rat cardiocyte growth and differentiation

Norepinephrine stimulates the growth in size of nondividing neonatal cardiocytes. During this time the neonatal cardiocyte is in a period of transition in which the cell can synthesize DNA and yet does not divide. Because the cell undergoes karyokinesis without cytokinesis the objective of this study was to determine whether the norepinephrine-induced growth in size of the neonatal cardiocyte was accompanied by an increase in a) the number of cardiocytes synthesizing DNA, b) the number of binucleate cardiocytes, and c) organized myofibrils. One- to four-d-old neonatal rat heart cells were isolated and placed in serum-free medium which was then supplemented with serum, norepinephrine, norepinephrine plus propranolol, or isoproterenol. After 4 d the number and size of the cells was determined using a Coulter counter. In other cultures cardiocytes were fixed on Days 0, 1, 2, and 4, and an increase in the number of binucleate cardiocytes was found in all treatment groups including controls. However, the rate of binucleation was faster in the norepinephrine group. It was also determined by proliferating cell nuclear antigen (PCNA) antibody staining that by Day 4, over 50% of the cardiocytes were in the cell cycle. The percentage of cells in which PCNA could be detected was higher in the norepinephrine and norepinephrine plus propranolol groups. Furthermore, there was a concomitant increase in the amount and organization of myofibrils in the catecholamine-treated cardiocytes.

Accelerated ribosome formation and growth in neonatal pig hearts

Rapid growth (5 mg dry heart/h) of the left ventricular free wall (LVFW) in the newborn pig heart accompanied by lack of growth of the right ventricular free wall (RVFW) represents a unique natural model of cardiac enlargement that is free of pathophysiological influences. By 3 days of life, LVFW was 71% larger than at 4 h of age. Rates of protein synthesis were measured during perfusion of isolated pig hearts with bicarbonate buffer containing glucose, lactate, insulin, and plasma concentrations of amino acids of an aortic pressure of 60 mmHg. In hearts from pigs that were 18 h of age, rates of protein synthesis were the same in RVFW and LVFW, but in 2-day-old pigs the rate was 52% greater in LVFW than RVFW. During the first 3 days of life, RNA content (mg/g) increased 3.4-fold faster in LVFW than RVFW. When RNA content was expressed per total heart portion, the increase was 7.9-fold greater. Because approximately 85% of total RNA is rRNA, these values indicated much more rapid formation of ribosomes in the LVFW than RVFW. When ribosome formation was measured in vitro in hearts from 48-h-old pigs, rates of formation were 39% greater in LVFW than RVFW, and at 18 h of age, ribosome formation was 40% faster in LVFW than RVFW. These findings indicated that formation of new ribosome preceded accelerated synthesis of total heart proteins. These findings indicated that rapid growth of LVFW compared with no growth of RVFW was associated with a 67% faster rate of ribosome formation and a 32% greater rate of protein synthesis.

Ultrastructural morphometric analysis of cultured neonatal and adult rat ventricular cardiac muscle cells

Neonatal and adult rat ventricular cardiac muscle cells cultured on laminin differed from similar myocytes grown on plastic in the amount and distribution of their mitochondria and transverse tubules. Point-count morphometry was used at the electron microscopic level to quantify these differences. Adult myocytes grown on laminin contained more mitochondria per unit volume than adult myocytes grown on plastic. No significant differences were observed in the volume percent of myofibrils in either adult or neonatal ventricular myocytes when grown on laminin and compared to those grown on plastic. The transverse tubule system in neonatal and adult myocytes was reduced significantly when both groups were cultured on laminin. Furthermore, neonatal and adult myocytes cultured on laminin were flatter than those cultured on plastic. This may indicate a relationship between the surface/volume ratio and transverse tubule development in cultured myocytes. These studies establish that point-count morphometry can be used to quantify changes in the organelle volume densities of cultured cardiac muscle cells.

Effects of chronic infusion of norepinephrine on cardiac structure, function, and biochemistry: physiologic versus pathologic hypertrophy

Ventricular hypertrophy should be divided into at least physiologic and patholgic states in order to clarify structural and functional clinical alterations. The elucidation of the structural, functional, and biochemical mechanisms of ventricular hypertrophy is vital to designing effective preventive and therapeutic measures for the hypertensive patient. Tissue markers may help differentiate pathologic from physiologic hypertrophy. Studies have established the concept that norepinephrine may be a myocardial cellular hypertrophying hormone. The studies ranged from the direct application of norepinephrine to isolated myocardial cells to the chronic subhypertensive infusion of norepinephrine into the conscious, free-roaming dog. Norepinephrine infusion can produce physiologic ventricular hypertrophy or a pathologic state of hypertrophic cardiomyopathy, the former by a three- to four-month infusion and the latter by an infusion of more than six months. The biochemical effect of subhypertensive infusion of norepinephrine was studied prior to the production of ventricular hypertrophy, thereby permitting the elucidation of the mechanism of the hypertrophic process. The biochemical stimulus for the production of myocardial cellular hypertrophy is postulated to be a diminution of cyclic AMP and a stimulation of alpha-1 receptors. Because the ventricular septum has the highest content of adenylate cyclase, which does not increase with cyclic AMP, these changes are postulated to be the biochemical basis for septal hypertrophy in the disease entity hypertrophic cardiomyopathy. A unique conscious-canine model for the production of a myocardial infarction capable of creating a controlled localized occlusion of the coronary artery is presented.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of catecholamines on fetal rat cardiocytes in vitro

Norepinephrine stimulates the growth in size of non-dividing, neonatal cardiac muscle cells, and it can stimulate the growth in numbers of dividing hepatocytes and endothelial cells in culture. The objective of this study was to test the hypothesis that in dividing fetal cardiocytes, norepinephrine would stimulate growth in cell number rather than in cell size. Fourteen-day fetal heart cells were placed in serum-free or serum-supplemented cultures in the presence or absence of norepinephrine (NE), NE plus propranolol, or isoproterenol for 4 days. Almost 90% of the cardiocytes in serum-supplemented medium were in the cell cycle as determined by proliferating cell nuclear antigen (PCNA) antibody staining during this period. In addition, between days 2 and 4 of culture, 35% and 40% of these cardiocytes were labeled with 3H-thymidine. After 4 days the cardiocytes increased in cell number in the serum-supplemented NE cultures as compared to serum-free cultures. In contrast, there was no significant change in cardiocyte volume between any of the groups examined. It was concluded that in dividing muscle cell populations the effect of norepinephrine was to enhance cell proliferation rather than to stimulate cell growth in size.

Isolation of cardiac ventricular myocytes from newborn rats by use of fractional elutriation centrifugation

Isolation of cardiac ventricular myocytes from newborn rats (0-4 days old) by use of elutriation centrifugation is described. By the use of fractional centrifugation, a homogeneous myocyte population with high purity was obtained without any further procedures, yielding about 10(6) myocytes/rat. When 48-hr monolayer cultures were established, the cells showed normal pulsatory contractions. A morphological evaluation of such cultures is given.

Serum-free, chemically defined medium to evaluate the direct effects of growth factors and inhibitors on proliferation and function of neonatal rat cardiac muscle cells in culture

Neonatal rat cardiac myocytes were isolated and cultured to evaluate the effects of growth factors and inhibitors on proliferation, survival, and functions in a serum-free medium. Insulin and transferrin in MCDB 107 nutrient medium elicited DNA and protein synthesis in cells on a fibronectin-coated culture surface in serum-free medium. Insulin was most effective on both DNA and protein synthesis in serum-free culture conditions. The serum-free, hormone-supplemented medium eliminated the contamination of noncardiac myocytes and supported the long-term survival (over 18 d) of cardiac myocytes. Dexamethasone was required to induce optimal contractility with or without insulin and transferrin. Serum contained both negative and positive effectors of DNA and protein synthesis of the cardiac myocytes. Concentrations of serum (above 5%) inhibited DNA and protein synthesis. Low density lipoprotein (LDL) accounted in part for the inhibitory activity. The serum-free culture system provides a useful model to elucidate the role of hormones, growth factors, and drugs in heart cell regeneration and function.

Supernormal ejection performance is isolated to the ipsilateral congenitally pressure-overloaded ventricle

Congenital left ventricular pressure overload is associated with "excessive" hypertrophy that leads to subnormal afterload (wall stress), permitting enhanced ventricular ejection performance. Whether congenital right ventricular pressure overload is associated with a similar phenomenon is uncertain. It is also unknown whether supranormal ejection performance affects only the overloaded ventricle or is a general process affecting both ventricles. Conflicting data exist about whether the hypertrophic process associated with pressure overload is induced primarily by local loading conditions or by neuroendocrine influences. If the former postulate is true, the hypertrophic response should be confined to the overloaded ventricle; if the latter is true, one might predict that both ventricles would be affected by a less specific response to circulating catecholamines. To help resolve these issues, both right and left ventricular performance was examined in seven patients with isolated congenital pulmonary stenosis (average pulmonary pressure gradient 78 +/- 13 mm Hg), six patients with isolated congenital aortic stenosis (average gradient 80 +/- 10 mm Hg) and six normal subjects. Right ventricular ejection fraction was increased in patients with pulmonary stenosis (61 +/- 2%) compared with the value in normal subjects (53 +/- 2%, p less than 0.01) and in patients with aortic stenosis (50 +/- 3%, p = 0.007). Left ventricular ejection fraction was increased in patients with congenital aortic stenosis (84 +/- 4%) compared with the value in normal subjects (70 +/- 4%, p less than 0.01) and in patients with congenital pulmonary stenosis (65 +/- 2%, p less than 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation of adult rat cardiac myocytes in cell culture

Cardiac myocytes isolated from adult rat hearts were grown on laminin coated culture dishes for more than a month. During this time, the cells underwent a morphological transformation which has also been referred to by others as cell remodeling (Guo J-X, Jacobson SL, Brown DL: Cell Mot Cytoskeleton 1986;6:291-304). This results in a change in myocyte morphology from its typical in vivo cylindrical shape to one which is more pleiomorphic. Despite the long-term change in morphology, myocytes expressed for differing lengths of time several aspects of the adult phenotype as evidenced by the following: 1) maintenance of cylindrical shape and/or evident cross-striations for the first 24-48 hours in culture, 2) reappearance of cross-striations during the second week in culture, 3) little or no spontaneous contractility for the first 4 days in culture, 4) expression of only the V1 isoform of myosin for at least 7 days, and 5) altered myosin isoform expression in response to changes in environmental conditions. These factors taken together suggest that in culture the adult cardiac myocyte remains a highly differentiated cell (as opposed to possible dedifferentiation) and maintains many of its previous in vivo characteristics. Such highly differentiated adult cells should be suitable as an in vitro system for studying the direct cellular effects of factors which regulate growth and differentiation of the in vivo heart.

Contraction modulates the capacity for protein synthesis during growth of neonatal heart cells in culture

Neonatal ventricular myocytes that were incubated in a well-defined serum-free medium containing 50 mM KCl did not contract and maintained stable cell size, as assessed by the protein/DNA ratio. The present study utilized KCl-arrested cells to examine the effect of constant rates of synchronous contraction in normal [K+]o (4 mM) as a physiological stimulus for myocyte growth. Cell growth increased following the onset of contraction when measured over 3 days. The rate of protein synthesis was accelerated in parallel by contraction, but the rate of protein degradation remained similar to rates in noncontracting cells. The capacity for protein synthesis was estimated by total RNA content and was increased in contracting as compared with KCl-arrested cells. This increase was accompanied by faster rates of RNA synthesis as determined from the incorporation of [3H]uridine into RNA and the specific activity of the cellular UTP pool. The rate of RNA degradation was accelerated during contraction but the difference between the rates of RNA synthesis and degradation resulted in net RNA accumulation of 49% after 3 days. These data demonstrated that 1) contractile activity stimulated myocyte growth through an increased capacity for protein synthesis and 2) the increased capacity for protein synthesis involved acceleration of the rate of RNA synthesis. Since enhancement of protein synthetic capacity is a common feature of myocyte hypertrophy in vivo and in vitro, this model can be used to examine the regulation of ribosome synthesis during hypertrophic growth.

Regional myocardial blood flow and metabolism at rest in mildly symptomatic patients with hypertrophic cardiomyopathy

Previous observations and clinical manifestations suggest the presence of ischemia in the disproportionately thickened septum of patients with hypertrophic cardiomyopathy. Metabolic consequences of ischemia can be demonstrated with positron emission tomography. Therefore, 10 patients with hypertrophic cardiomyopathy and an echocardiographic septum to posterior wall thickness ratio of 1.8 +/- 0.4 cm (range 1.3 to 2.5) were studied with the use of nitrogen (N)-13 ammonia, carbon (C)-11 palmitate and fluoro (F)-18 2-deoxyglucose as tracers of myocardial blood flow, fatty acid metabolism and exogenous glucose utilization. The results of positron emission tomography in 9 patients with hypertrophic cardiomyopathy were compared with those in 10 normal volunteers. In the hypertrophic cardiomyopathy group, observed myocardial activity of N-13 ammonia and C-11 palmitate in the septum was similar to that in the lateral wall. Septum to lateral wall tissue activity ratios averaged 1.04 +/- 0.15 for N-13 ammonia and 1.04 +/- 0.18 for C-11 palmitate, and were similar to those in the normal volunteers (0.98 +/- 0.07 and 0.98 +/- 0.03, respectively; p = NS). Myocardial clearance half-time and residual fraction of C-11 palmitate did not differ significantly between the septum and lateral wall. However, F-18 2-deoxyglucose uptake was significantly lower in the septum than in the lateral wall (15,768 +/- 4,314 versus 19,818 +/- 5,234 counts/pixel; p less than 0.003). The mean septum to lateral wall activity ratio of 0.83 +/- 0.21 was less than that observed in normal volunteers (0.92 +/- 0.07; p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of calcitonin gene-related peptide and substance P in cultured neonatal rat vagal sensory neurons

Nodose (inferior vagal sensory) ganglia were removed from neonatal rats, enzymatically dispersed using neutral protease, and maintained on previously dispersed rat atriacytes. After 7-10 days in culture, calcitonin gene-related peptide (CGRP) was present in 1-3 times the molar amount of substance P (SP). The content of SP was doubled by the addition of nerve growth factor (NGF) whereas CGRP was significantly less increased by 50% or less. The addition of forskolin increased SP and CGRP levels in cultures with or without NGF by 60-80 percent. Phorbol ester (PMA) did not alter SP content but significantly raised CGRP content by 40% in NGF supplemented cultures (P less than 0.001). Corticosterone, 0.01-0.1 microM, reduced SP content by 30% independently of NGF but had no effect on CGRP. These studies demonstrate that SP in vagal sensory neurons is more sensitive than CGRP to the effects of NGF or corticosterone. Both peptides are up-regulated by presumed increases in intracellular cyclic AMP, while CGRP (or CGRP neurons) may be independently regulated by protein kinase C.

Beta-adrenoceptor and adenylate cyclase regulation in cardiac myocyte growth

We studied the effect of growth on beta-adrenergic receptor properties of neonatal rat heart myocytes cultured in serum-free medium with transferrin and insulin. Growth was induced by addition of 1 microM (-)-norepinephrine for two days, 200 nM of the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) for two days, or 30 nM T3 for six days. The Kd values for beta-receptor binding (125I-ICYP) were unaffected by growth. The maximum number of beta-receptor binding sites calculated as sites/cell was increased 1.47-fold by T3 (p less than .005), but was decreased to 54% of control values by (-)-norepinephrine (p less than .005): TPA had no effect on either Kd or Bmax values. (-)-Isoproterenol-stimulated adenylate cyclase activity was augmented only in membranes from T3-treated cells and was reduced by 69% in membranes from (-)-norepinephrine treated cells. TPA had no effect on (-)-isoproterenol-stimulated adenylate cyclase activity. We conclude that the mechanisms controlling beta-adrenergic receptor number may be distinct from those controlling growth, since receptor number does not correlate with cell enlargement. Furthermore, in (-)-norepinephrine-stimulated growth, which we have shown previously is an alpha 1-adrenoceptor mediated response, beta-adrenergic signal transduction is modulated in a directionally opposite fashion.

Differentiation of cardiac myocytes after mitogen withdrawal exhibits three sequential states of the ventricular growth response

During cardiac myogenesis, ventricular muscle cells lose the capacity to proliferate soon after birth. It is unknown whether this developmental block to mitotic division and DNA replication might involve irreversible repression of the cellular oncogene c-myc. Ventricular myocytes from 2 d-old rats continued to differentiate in vitro during 15 d of mitogen withdrawal, as shown by the formation of cross-striations, increased proportion of the muscle isoenzyme of creatine kinase, stable expression of alpha-cardiac actin and myosin heavy chain mRNAs, and appropriate down-regulation of alpha-skeletal actin mRNA. After mitogen withdrawal for 2 d, serum evoked both DNA synthesis and mitotic division; after 7 d, DNA replication was uncoupled from cell division; after 15 d, DNA synthesis itself was markedly attentuated. These three distinct phenotypic states resemble the sequential properties of growth found in the neonatal rat heart in vivo. Despite failure to induce DNA replication or division after 15 d of mitogen withdrawal, serum elicited both c-myc and alpha-skeletal actin as found during hypertrophy of the intact heart. The results agree with previous evidence that one or more functional pathways that transduce the effects of serum factors may persist in older cardiac muscle cells, and indicate that irreversible down-regulation of c-myc cannot be the basis for the loss of growth responses.

Role of proto-oncogenes in myocardial hypertrophy

A question of major clinical significance in cardiology is the nature of the signals that initiate and maintain the various types of myocardial hypertrophy, either in response to hemodynamic loading or in the absence of altered load. This review suggests that the proto-oncogene model, a concept derived from the study of cancer, can be very useful in identifying these signals. The proto-oncogene model conceives of cell growth regulation in terms of a limited number of classes of critical regulatory proteins: growth factors, growth factor receptors, intracellular transducing proteins and ribonucleic acid (RNA) transcription factors. Growth of all cells has dissociable components: hypertrophy (growth in size), deoxyribonucleic acid synthesis, mitosis and cytokinesis. Hypertrophy may be the end result of activation of RNA transcription. The various types of hypertrophy could reflect transcription of specific myocyte genes in response to different growth factors. At least 1 member of each functional class of proto-oncogenes has been detected in the myocardium or myocytes, or both. The alpha 1-adrenergic receptor has been shown to be a growth factor receptor and to regulate RNA transcription. Continued work on proto-oncogenes in myocytes may open the way to manipulate the growth of these cells.

Substance P content in cultured neonatal rat vagal sensory neurons: the effect of nerve growth factor

To begin to study the factors regulating the synthesis and release of substance P (SP) in the sensory vagus nerve, cultures of neonatal rat nodose ganglia were developed. In microexplant cultures, obtained from small fragments of nodose ganglia, SP was present in low amounts: after 3 weeks, 141 +/- 36 pg per well, 10 ganglia equivalents per well. To enhance neuron survival, nodose ganglia were enzymatically dissociated using neutral protease. Estimated survival at 5 days was 20-30%, with 800-1200 surviving neurons per plated ganglion, and decreased slowly thereafter. Specific SP immunostaining was present in 10-20% of neurons, mostly of small diameter (18-22 micron). SP content was low for 5 days then rose progressively after 14 days to 80-150 pg per plated ganglion. The addition of nerve growth factor (NGF, 100 ng/ml) to the culture medium did not alter neuron survival. However, SP content was doubled in the presence of NGF, or fell rapidly to one-half control levels following its withdrawal: e.g. following 12 days in culture with NGF 1185 +/- 176 pg/well vs NGF withdrawn day 8-12, 592 +/- 118 pg/well, mean +/- S.D., P less than 0.01. Somatostatin, present in one-sixth the amount of SP, was unaltered by NGF. In subsequent studies, plating of neurons onto previously dissociated rat atriacytes increased survival by 50% but did not alter SP content per surviving neurons. These studies demonstrate that SP is present in dissociated cultures of rat vagal sensory neurons; the quantities and estimated net synthesis rate correspond to previous observations in vivo. The studies also demonstrate that SP content but not neuron survival are regulated by NGF in nodose ganglion neurons. This model may prove valuable for the study of SP and other sensory neuropeptides in this important class of visceral afferent neurons.

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.

Pulmonary vascular extraction of circulating norepinephrine in infants with bronchopulmonary dysplasia

Although the pulmonary circulation in infants with advanced bronchopulmonary dysplasia (BPD) is characterized by abnormal structure and vasoreactivity, metabolic lung functions have not been studied in these infants. To test the hypothesis that patients with severe BPD may have abnormal metabolic lung function, we assessed the pulmonary vascular extraction of circulating norepinephrine in six children with BPD during cardiac catheterization. Plasma norepinephrine levels were measured from simultaneously drawn mixed venous (main pulmonary artery) and left atrium or femoral artery samples. In comparison with four infants with mild heart disease without pulmonary hypertension, we found that infants with BPD extract proportionately less norepinephrine than non-BPD infants [-7 +/- 50% (BPD) versus +27 +/- 6% (non-BPD); P less than 0.001, t test]. Three infants with BPD had higher arterial than mixed venous concentrations of plasma norepinephrine, suggesting net production across the lung. Plasma catecholamine levels and percent extraction correlated poorly with cardiac index and systemic and pulmonary vascular resistance indices. However, this study group was characterized by a high incidence of pulmonary (6/6) and systemic (4/6) hypertension, left ventricular hypertrophy (4/6), and subsequent death (3/6). We conclude that infants with severe BPD and pulmonary hypertension have decreased pulmonary vascular clearance or net production of circulating norepinephrine, but links between altered pulmonary catecholamine metabolism and pulmonary hypertension, or other cardiovascular abnormalities associated with BPD, remain speculative.

Effect of age on the development of cardiac hypertrophy produced by aortic constriction in the rat

To test the hypothesis that the capacity to develop left ventricular (LV) hypertrophy might diminish with advancing age, we examined the hypertrophic response to ascending aortic constriction in 3 groups of adult Fischer 344 rats (9 months, 18 months, and 22 months of age). Aortic constriction was created so that aortic cross-sectional areas would be the same for the 3 groups of rats. Four weeks after imposition of aortic constriction, there was no significant difference in peak LV pressure, peak-to-peak and mean systolic pressure gradients between left ventricle and aorta, cardiac output, LV minute work, or cross-sectional area of the aortic constrictions in the 3 groups. In 9-month-old aortic-constricted rats, LV dry wt (LVDW)/body wt, LVDW/tibial length, and myocyte width increased by 23% (p less than 0.01), 14% (p less than 0.01), and 27% (p less than 0.01), respectively, compared with sham-operated rats. In contrast, in 18-month-old and 22-month-old aortic-constricted rats, LVDW/body wt and LVDW/tibial length were unchanged compared with sham-operated controls, and increases in myocyte width were only modest 4 weeks following constriction. RNA concentration in the myocardium 5 days after constriction increased by 21% (p less than 0.001) in 9-month-old rats but showed no significant rise in 18-month-old rats. These results suggest that advancing age is associated with a diminished capacity to develop myocardial hypertrophy in response to acute pressure overload and that a reduced ability to synthesize protein may be one of the major contributing factors to a diminished capacity for hypertrophy in advanced age.

Effects of chronic progressive myocardial hypertrophy on indexes of cardiac autonomic innervation

The development of cardiac hypertrophy is associated with marked changes in cardiac autonomic innervation. Significant and sustained reductions of myocardial catecholamine stores and activities of tyrosine hydroxylase and dopamine beta-hydroxylase have been reported in models of acutely induced ventricular hypertrophy. Conversely, activity of choline acetyltransferase, a marker of parasympathetic nervous function, shows transient increases during the development of acute right ventricular hypertrophy. The potential physiological importance of these changes prompted us to examine a clinically more relevant model of slowly progressive ventricular hypertrophy. Application of a loose band around the pulmonary artery of weanling guinea pigs resulted in a growth-related progressive right ventricular pressure overload. Right ventricular weight-to-body-weight ratio was increased significantly and progressively at 9 and 18 weeks in banded animals (0.92 +/- 0.05 and 1.31 +/- 0.11 mg/g, respectively, p less than 0.01) compared with sham-operated controls (0.55 +/- 0.02 and 0.59 +/- 0.01 mg/g, respectively) but showed no further gain at 27 weeks (1.41 +/- 0.10 mg/g). Activities of tyrosine hydroxylase and dopamine beta-hydroxylase remained unchanged in all experiment groups, while right ventricular contents of norepinephrine in banded animals at 18 and 27 weeks exhibited sustained and progressive increases (2.45 +/- 0.11 and 3.40 +/- 0.19 micrograms/right ventricle, respectively) over controls (1.80 +/- 0.13 and 2.40 +/- 0.22 micrograms/right ventricle, respectively, p less than 0.01). The activity of choline acetyltransferase was markedly elevated in banded animals at 18 weeks (32.6 +/- 2.7 nmol/hr/right ventricle) but returned to baseline by 27 weeks (22.8 +/- 1.4 nmol/hr/right ventricle).(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac myocyte hypertrophy is associated with c-myc protooncogene expression

The mechanism of hormonally induced cell hypertrophy is unknown. Stimulation of cardiac myocytes by alpha 1-adrenergic agents, phorbol esters, and serum induces an increase in the cell size of nondividing cardiac myocytes in primary culture. Expression of the c-myc gene, known to be increased in growth factor-induced cell division, was studied in this model of cell hypertrophy. The alpha-adrenergic agonist norepinephrine (0.002-20 microM) increased levels of c-myc-encoded mRNA to 10-fold over control levels. This increase was detectable at 30 min, peaked at 2 hr, and returned to baseline by 6 hr after stimulation. The norepinephrine response was abolished by the alpha 1-antagonist terazosin (2 microM) but was not affected by the beta-adrenergic antagonist propranolol (2 microM) and was only slightly (25%) attenuated by the alpha 2-adrenergic antagonist yohimbine (2 microM). Serum and the phorbol ester tumor promoter phorbol 12-myristate 13-acetate also enhanced c-myc expression in cardiac myocyte cultures. These findings show that the induction of cardiac myocyte hypertrophy is associated with enhanced expression of the c-myc gene and suggest that hormonally induced cell hypertrophy and cell division share common mechanistic pathways.

Sympathetic innervation alters growth and intrinsic heart rate of fetal rat atria maturing in oculo

The influence of sympathetic innervation on the growth and intrinsic rate of beating established by fetal rat heart was studied by culturing fetal atrial tissue in sympathetically innervated and denervated anterior eye chambers of adult Sprague-Dawley rats. One anterior eye chamber in each host rat was sympathetically denervated by removing the ipsilateral superior cervical ganglion. In oculo, atrial grafts were vascularized by blood vessels sprouting from the iris and innervated by sympathetic and parasympathetic fibers from the ground plexus of the iris. Innervation was assessed by light-activated efferent nerve stimulation to the grafts that changed their rates of beating. The norepinephrine contents of 16 atria cultured for 2.5 months in sympathetically innervated and denervated eye chambers were 5.7 +/- 1.1 ng/implant vs. 0.2 +/- 0.07 ng/implant (mean +/- SEM), indicating permanent sympathetic denervation of the anterior eye chamber and the implanted atria. By 8 weeks in oculo, atria maturing in sympathetically innervated anterior eye chambers were 86% larger than those in denervated eye chambers (2.22 +/- 0.29 vs. 1.19 +/- 0.13 mm2); the weight of innervated transplants was over 3 times that of noninnervated grafts (2.35 +/- 0.75 vs. 0.76 +/- 0.21 mg). After implanted atria had ceased growing rapidly (2.5 months in oculo), bipolar electrodes were implanted adjacent to the cornea to record impulses from atrial grafts while host rats were unanesthetized. The dark-adapted baseline heart rates of sympathetically innervated and noninnervated atria were virtually identical (289 vs. 290 bpm). Graft intrinsic heart rate was estimated by combined beta-adrenergic and muscarinic receptor blockade with atenolol (1.0 mg/kg) and methylatropine (10 micrograms/kg). Sympathetically innervated transplants had lower intrinsic heart rates than noninnervated atria (134 +/- 25 vs. 213 +/- 12 bpm). These data suggest that sympathetic innervation of the developing heart influences both growth and intrinsic rate of beating.

Control of myocardial tissue components and cardiocyte organelles in pressure-overload hypertrophy of the cat right ventricle

Previous studies have demonstrated that there is a disproportionate increase in connective tissue in right ventricular myocardium subjected to pressure-overload hypertrophy associated with depressed cardiac contractility. While the myocardium is primarily responsive to load, the aim of the present study was to determine whether catecholamines also modulate the response of myocardial tissue components and cardiocyte organelles in pressure-overload-induced cardiac hypertrophy. Four experimental groups of cats were examined: a sham-operated control group, a group which had their pulmonary arteries banded in order to induce a pressure overload, a group which had been subjected to the same pressure overload, but in addition had beta-adrenoceptor blockade produced prior to and during the pressure overloading, and a group which had been subjected to the same pressure overload, but in addition had alpha-adrenoceptor blockade produced prior to and maintained during the pressure overloading. As in our previous study, there was a significant and equivalent degree of right ventricular hypertrophy in all experimental groups with pressure overload when assessed either as the ratio of right ventricular weight to body weight or as cardiocyte cross-sectional area. At the light microscopic level, the disproportionate increase in the volume density of myocardial connective tissue seen in banded animals was completely prevented by either alpha- or beta-adrenoceptor blockade. At the electron microscopic level, there was a reduction in the mitochondrial and myofibrillar volume fractions following beta-adrenoceptor blockade. The results of this study provide evidence for a modulatory role of catecholamines in the control of myocardial connective-tissue proliferation in pressure-overload-induced cardiac hypertrophy. There is also evidence to support the role of the adrenergic nervous system in regulating cardiocyte subcellular organelles, independent of the regulation of cardiocyte size.

Increased sensitivity to alpha-adrenoceptor stimulation but intact purinergic and muscarinergic effects in prehypertensive cardiac hypertrophy of spontaneously hypertensive rats

The effects of phenylephrine, isoprenaline and adenosine, (-)-N6-phenylisopropyladenosine (PIA) or carbachol alone and in the presence of isoprenaline on force of contraction were studied in isolated electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and age-matched Wistar control rats. In SHR an increased heart to body weight ratio was observed when blood pressure was not yet elevated. During this stage of the syndrome (i.e. between the 27th and 35th day of life) phenylephrine was about 3.4 times more potent to increase force of contraction in SHR (mean EC50: 2.8 mumol l-1) than in control rats (mean EC50: 9.4 mumol l-1). The positive inotropic effect of isoprenaline was similar in SHR and control rats. Also no difference could be detected in the isoprenaline-antagonistic effect of adenosine, the adenosine receptor agonist PIA or carbachol. We conclude that an increased sensitivity to cardiac alpha-adrenoceptor stimulation might be related to prehypertensive cardiac hypertrophy in SHR.

Cardiac involvement in Friedreich's ataxia: a clinical study of 75 patients

To establish the prevalence and to characterize the types of cardiac involvement in Friedreich's ataxia, 75 consecutive patients (39 male and 36 female), aged 10 to 66 years (mean 24) were prospectively studied. Electrocardiograms were performed in all patients, vectorcardiograms in 34 and echocardiograms in 58. Electrocardiographic and vectorcardiographic abnormalities occurred in 69 (92%) of the 75 patients. Electrocardiograms revealed ST-T wave abnormalities in 79%, right axis deviation in 40%, short PR interval in 24%, abnormal R wave in lead V1 in 20%, abnormal inferolateral Q waves in 14% and left ventricular hypertrophy (voltage and repolarization criteria) in 16%. Echocardiograms revealed concentric left ventricular hypertrophy in 11%, asymmetric septal hypertrophy in 9% and globally decreased left ventricular function in 7%. Progression from a normal echocardiogram to concentric left ventricular hypertrophy, asymmetric septal hypertrophy or globally decreased left ventricular function was identified in one patient in each category, although the study was not designed for longitudinal follow-up. Two patients died, and necropsy revealed in both a minimally dilated but flabby left ventricle. On the basis of electrocardiographic and vectorcardiographic and echocardiographic data, 95% of patients had one or more disorders. The most common abnormality was segmental myocardial "dystrophy" (electrocardiographic QRS initial force abnormalities), but global left ventricular hypokinesia occurred more often than previously recognized.(ABSTRACT TRUNCATED AT 250 WORDS)

Load regulation of the properties of adult feline cardiocytes. The role of substrate adhesion

We have recently described rapid and reversible changes in cardiac structure, function, and composition in response to surgical load alteration in vivo. In the present study, we used a simple, well-defined in vitro experimental model system, consisting of terminally differentiated quiescent adult cat ventricular cardiocytes maintained in serum-free culture medium, to assess more definitively the role of loading conditions in regulating these same biological properties of heart muscle. Cardiocytes considered to be externally loaded were adherent throughout their length to a protein substrate, such that the tendency for the ends of the cells to retract was prevented. Cardiocytes considered to be unloaded were not adherent to a substrate and, thus, were free to assume a spherical shape. Cardiocyte structure and surface area were assessed, in initially identified cells, both by serial light microscopy and by terminal electron microscopy. Cardiocyte function was assessed in terms of the ability to exclude trypan blue, to remain quiescent with relaxed sarcomeres containing I-bands, and to shorten in response to electrical stimulation. Cardiocyte composition was first assessed by quantitative gel electrophoresis of proteins and then by microfluorimetric measurement of ribonucleic acid, protein, and deoxyribonucleic acid. In addition, cardiocyte incorporation of [3H]thymidine into deoxyribonucleic acid and [3H]uridine into ribonucleic acid were measured. Loading via substrate adhesion was found to be very effective in terms of each of these measurements in retaining the differentiated features of adult cardiocytes for up to 2 weeks in culture; unattached and thus unloaded cardiocytes quickly dedifferentiated. Conditions thought to stimulate cardiac growth, including catecholamine stimulation, were found to be ineffective. These experiments demonstrate that external load has a primary role in the maintenance of the basic differentiated properties of adult mammalian cardiocytes.

Influence of plating density on individual cell growth, cell division and differentiation of neonatal rat heart primary cultures

The influence of plating cell density of an originally enriched myocardial cell population has been studied in neonatal rat heart cells in culture. Low density (LDM) is defined as a density (24 h after plating) of 209 +/- 44 cells/mm2 (mean +/- SEM) and is compared with high density (HDM), 419 +/- 67 cells/mm2. Cell growth is evaluated by the total cell number, the percentage of myocardial cells (M) in culture (PAS method) and the protein content per cell. Some differentiation parameters such as beating rates, glycogen concentration, enzymatic activities (cytochrome C oxidase and glycogen phosphorylase) are studied with time in culture (48, 96 and 192 hr). High density was designed to yield a complete confluency of the cells within 24 hr after plating and to minimize cell division of the non-muscle cells (F). At high density, cell division of F cells is effectively limited, thus leading to a more stable model regarding the cell density per plate and the percentage of M cells: 85.7 +/- 4% and 33.4 +/- 6% in LDM cultures compared with 86.5 +/- 4.7% and 51.7 +/- 9.8% in HDM cultures at 24 and 192 hr (mean +/- SEM). Heart cells increase similarly in size with age in culture in both groups. In HDM cultures the spontaneous contractions begin sooner (24 hr) than in LDM cultures and are more rapidly synchronized. The beating rate is higher in HDM cultures between 48 and 96 hr; however, after this time it falls in HDM and does not fall in LDM. Thus the overgrowth of muscle cells by non-muscle cells is not responsible for loss of beating with time in culture but more likely high density could be a limiting factor for isotonic contraction. There is more glycogen per myocyte in LDM than in HDM cultures. The cell density influences the enzymatic activities of cytochrome C oxidase and glycogen phosphorylase. The cytochrome oxidase activity is higher in HDM cultures than in LDM cultures at 96 hr whereas glycogen phosphorylase activity is higher in LDM cultures at time 96 and 192 hr. In LDM cultures, the ratio cytochrome C oxidase/glycogen phosphorylase decreases with time in culture from 1.685 +/- 0.680 at 48 hr to 0.780 +/- 0.290 at 192 hr but not in HDM cultures (2.13 +/- 0.36 and 1.64 +/- 0.34 respectively). Thus plating density influences properties of heart cell cultures with regard to the overgrowth of the F-cell population and the differentiated state of M cells.

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)

Measurement of myosin adenosinetriphosphatase and myosin content in cultured heart cells

An assay specific for myosin ATPase in whole-cell extracts of cultured heart cells has been developed. Myosin ATPase is measured by the production of Pi from ATP in the presence of high ionic strength (0.5 M KCl) at pH 9.1. Enzyme activity is maximal with 10 mM CaCl2 and completely inhibited with 5 mM MgCl2. Spontaneously beating myocytes grown in the presence of 10% newborn calf serum and 0.1 mM 5-bromo-2'-deoxyuridine show a significant rise in myosin ATPase between Days 1 and 4 in culture. The measurement of myosin ATPase allows for the quantitation of cellular myosin content, and can be used to assess changes in myosin content that occur during growth, development, and cellular repair.

Development of heart cells in culture: studies using an affinity purified antibody to a myosin light chain

Cultured neonatal rat heart cells can be used to study the factors that regulate cardiac contractility and myocyte development in vitro. An antibody to the 26,000 dalton light chain of myosin (MLC1), has been produced and purified on a Sepharose 4B affinity column prepared with rat heart myosin. When primary cultures of myocytes are studied by indirect immunofluorescence using this antibody a predictable pattern of myofibrillar structure is observed to develop over 72 h. This myosin cytoskeleton is highly organized and the myosin fibrils exhibit cross striations. The antibody does not stain non-muscle heart cells and there is no evidence for myocyte division in culture. The qualitative immunofluorescent pattern of myosin organization is the same in both spontaneously beating and in non-contracting cells.

Stimulation of hypertrophy of cultured neonatal rat heart cells through an alpha 1-adrenergic receptor and induction of beating through an alpha 1- and beta 1-adrenergic receptor interaction. Evidence for independent regulation of growth and beating

Catecholamines may be one of the molecular signals linking increased circulatory demand to myocardial hypertrophy, and I have found previously that norepinephrine stimulates hypertrophy of cultured neonatal rat heart muscle cells through an alpha 1-adrenergic receptor. Since catecholamine stimulation of contractility is believed to be under beta-adrenergic control, I asked whether these cultured heart cells had dual pathways regulating growth and contractility through alpha- and beta-adrenergic receptors, respectively. I examined the effect of adrenergic agents on hypertrophy and beating of myocytes in serum-free cultures. Hypertrophy was defined as an increase in myocyte surface area and in cell protein content, measured by a radioisotopic method, and chronotropic activity was examined visually. Norepinephrine and epinephrine were equipotent stimulants of hypertrophy and beating, increasing cell protein and area 1.5- to 2-fold, and the proportion of beating cells from 5% or less to 95%. Response maxima occurred 24-48 hours after exposure, and EC50 were 20-200 nM. Studies with other agonists (phenylephrine, methoxamine, clonidine, isoproterenol, dopamine) and antagonists (prazosin, terazosin, yohimbine, propranolol, betaxolol, ICI 118,551) indicated that hypertrophy was mediated through an alpha 1-adrenergic receptor, whereas the induction of beating required both alpha 1- and beta 1-receptor activation. Hypertrophied cells with minimal beating were produced by alpha-stimulation, alone. In contrast, alpha-plus beta-stimulation in the presence of cycloheximide to inhibit protein synthesis resulted in maximum beating but no hypertrophy. These findings imply that growth and beating can be regulated independently through separate cellular pathways.

Hemodynamic versus adrenergic control of cat right ventricular hypertrophy

The purpose of this study was to determine whether cardiac hypertrophy in response to hemodynamic overloading is a primary result of the increased load or is instead a secondary result of such other factors as concurrent sympathetic activation. To make this distinction, four experiments were done; the major experimental result, cardiac hypertrophy, was assessed in terms of ventricular mass and cardiocyte cross-sectional area. In the first experiment, the cat right ventricle was loaded differentially by pressure overloading the ventricle, while unloading a constituent papillary muscle; this model was used to ask whether any endogenous or exogenous substance caused uniform hypertrophy, or whether locally appropriate load responses caused ventricular hypertrophy with papillary muscle atrophy. The latter result obtained, both when each aspect of differential loading was simultaneous and when a previously hypertrophied papillary muscle was unloaded in a pressure overloaded right ventricle. In the second experiment, epicardial denervation and then pressure overloading was used to assess the role of local neurogenic catecholamines in the genesis of hypertrophy. The degree of hypertrophy caused by these procedures was the same as that caused by pressure overloading alone. In the third and fourth experiments, beta-adrenoceptor or alpha-adrenoceptor blockade was produced before and maintained during pressure overloading. The hypertrophic response did not differ in either case from that caused by pressure overloading without adrenoceptor blockade. These experiments demonstrate the following: first, cardiac hypertrophy is a local response to increased load, so that any factor serving as a mediator of this response must be either locally generated or selectively active only in those cardiocytes in which stress and/or strain are increased; second, catecholamines are not that mediator, in that adrenergic activation is neither necessary for nor importantly modifies the cardiac hypertrophic response to an increased hemodynamic load.

Adrenergic receptor characteristics of cardiac myocytes cultured in serum-free medium: comparison with serum-supplemented medium

Neonatal rat ventricular myocytes cultured in serum-free medium coexpress both alpha 1 and beta 1 receptors as determined by radioligand binding studies. In cells exposed to serum for 48 hr surface area increased 3.69 fold, but the maximum number of binding sites ([125I]-iodocynanopindolol) only increased 1.5 fold from 12956 +/- 7579 to 19676 +/- 5181 sites/cell (n = 5, p less than .05) yielding a value of 2.48 sites/um2 for cells grown in serum-supplemented medium compared with 6.96 sites/um2 for cells grown in serum-free medium. Thus serum-induced hypertrophy is associated with a decrease in beta 1 receptor density relative to cell size; however, adenylate cyclase response is unaffected. This cell culture system constitutes an excellent model for studying interventions that may influence the regulation of cardiac myocyte hypertrophy by nonhemodynamic factors, particularly through the adrenergic receptor system.

Asymmetric septal hypertrophy and sympathetic overactivity in normotensive hemodialyzed patients

Sympathetic activity has been evaluated in 23 chronic uremic normotensive patients on long-term hemodialysis. M-mode and bidimensional echocardiographic finding of asymmetric septal hypertrophy (ASH) was shown in seven (30.4%). Sympathetic function was assessed by determining arterial plasma norepinephrine (plasma NE) and epinephrine (plasma E) in supine and upright positions, both before and after dialysis. After dialysis standing caused a significant increase in plasma NE levels in the patients with ASH in comparison to the patients without ASH and the control group. A significant decrease in mean blood pressure (mBP) and a sharp heart rate (HR) increase were detected in the patients without ASH, whereas mBP and HR were unchanged in the patients with ASH. Predialysis serum creatinine and fasting triglycerides were found to be significantly higher in the group with ASH. These results suggest that sympathetic overactivity may play a role in the development of interventricular septum hypertrophy. This increased neurosympathetic responsiveness is probably related to the counteraction of the postural dialysis-induced hypotension.

Atrophy reversal and cardiocyte redifferentiation in reloaded cat myocardium

We have recently described rapid cardiac atrophy in response to decreased load. The present study was designed to determine whether this atrophy is solely a degenerative response of damaged myocardium or is, instead, an adaptive response of viable myocardium. A discrete portion of cat myocardium was unloaded by severing the chordae tendinae of a single right ventricular papillary muscle. One week later, the muscle was reloaded by attachment of its apex to the ventricular free wall. This allowed the load to be removed and restored without altering the blood supply, innervation, or frequency of contraction of the tissue. In myocardium unloaded for 1 week, the cardiocyte cross-sectional area and the volume densities of mitochondria and myofibrils decreased significantly. Large areas of cytoplasm were devoid of organelles, and the few remaining myofilaments were oriented in a variety of directions rather than longitudinally within the cell. Upon reloading for 1 week, the cardiocyte cross-sectional area, volume density of mitochondria, and ultrastructural organization all returned to normal. The volume density of the myofibrils increased toward control, and they reoriented with respect to the long axis of the cardiocyte. The contractile function of the papillary muscles, which was depressed as early as 1 day after unloading and almost absent at times later than 3 days after unloading, returned to normal after 2 weeks of reloading. This study demonstrates that adult mammalian myocardium responds to unloading with a marked loss of cellular differentiation, organization, and function which is fully reversible with reloading. This plasticity in response to load may well be the basic mechanism responsible for the development and maintenance of normal cardiac structure and function.

Cardiac glycoside receptors in cultured heart cells--I. Characterization of one single class of high affinity receptors in heart muscle cells from chick embryos

Binding of (3H)-ouabain and ouabain-induced inhibition of the sodium pump and of the (Na+ + K+)-ATPase have been characterized in cultured cardiac muscle and non muscle cells, as well as in cardiac cell membranes--all obtained from chick embryos. In both cell types, ouabain binds to a single type of binding sites in a temperature-dependent manner. The association rate but not the dissociation rate, is lowered by K+; specific binding is lost after heat-denaturation of the cells. Binding parameters (association and dissociation rate constants, activation energies for association and dissociation) are similar in muscle and non muscle cells. The dissociation constant of specific ouabain binding is 1.5 X 10(-7)M in cardiac muscle cells, and 1.9 X 10(-7)M in cardiac non muscle cells, the binding capacity being 2.6 and 2.1 pmoles/mg protein respectively. Specific binding of ouabain to the cells is coupled to inhibition of the sodium pump, as can be seen from ouabain-induced inhibition of active (86Rb+ + K+)-uptake, decrease in cellular K+, and increase in cellular Na+ (EC50 = 10(-7)-10(-6)M). The data obtained with cardiac cells are in good agreement with results found for ouabain binding (dissociation constant 4.3 X 10(-7)M) and (Na+ + K+)-ATPase inhibition (EC50 = 1.4 X 10(-6)M) in cardiac cell membranes prepared from the same tissue. Due to the experimental evidence it is concluded that the binding site for ouabain is identical with the cardiac glycoside receptor of these cells. In cardiac non muscle cells, binding of ouabain to its receptor is strictly coupled to inhibition of active K+-transport in a stoichiometric manner. In cardiac muscle cells, however, active K+-transport is inhibited by less than 10% when up to 40% of cardiac glycoside receptors have bound ouabain. It is assumed that this non-stoichiometric coupling of receptor occupancy and sodium pump inhibition in cardiac muscle cells may prevent substantial changes of Na+- and K+-contents in the heart in the presence of therapeutic levels of cardiac glycosides.

Norepinephrine-stimulated hypertrophy of cultured rat myocardial cells is an alpha 1 adrenergic response

We have shown recently that norepinephrine stimulates muscle cell hypertrophy in primary cultures from the neonatal rat ventricle and that this stimulation is not blocked by the beta adrenergic antagonist propranolol. The present study was done to define the adrenergic specificity of the myocyte hypertrophic response to norepinephrine. 90% pure, single-cell cultures of nongrowing myocytes were maintained in serum-free medium 199 with transferin and insulin. Myocyte size was quantitated 48 h after addition of adrenergic agents, by measuring cell volume, cell surface area, and cell protein. L-norepinephrine increased myocyte size to a maximum 150% of control; half-maximum effect was obtained at a concentration of 0.2 microM. This increase in cell size was inhibited by the nonselective alpha adrenergic antagonist phentolamine and by the alpha 1 adrenergic antagonists prazosin and terazosin; it was not inhibited by propranolol or by the alpha 2 adrenergic antagonist yohimbine. The beta adrenergic agonist isoproterenol did not increase cell size. Thus, norepinephrine-stimulated hypertrophy of cultured rat myocardial cells is an alpha 1 adrenergic response.