Spontaneous activity in transgenic mouse heart: comparison of primary atrial tumor with cultured AT-1 atrial myocytes

INTRODUCTION

We have generated transgenic animals that heritably develop atrial tumors composed of differentiated proliferating cardiomyocytes. Experiments were initiated to characterize the electrical properties of these cells.

METHODS AND RESULTS

We show that the primary atrial tumors are composed of discrete foci that exhibit spontaneous automatic activity. A direct correlation was observed between tumor size and firing rate of these foci. In addition to the primary atrial tumors, we examined the properties of cultured cardiomyocytes isolated from a transplantable transgenic tumor lineage (designated AT-1 cells). Cultured AT-1 cells are also spontaneously automatic. The action potential configuration from these preparations is similar to that observed in nontransgenic atrial cardiomyocytes, albeit somewhat more depolarized and of longer duration. As would be expected for cardiomyocytes of atrial origin, the transgenic cardiomyocyte preparations hyperpolarize during muscarinic stimulation due to increased K+ conductance mediated by a pertussis toxin sensitive G-protein. Assessment of pharmacologic blockage of the "if" pacemaker current suggests that the automaticity of both transgenic cardiomyocyte preparations may be of novel origin. In this context, the cultured AT-1 cells showed spontaneous behavior that was clearly of cellular origin; this activity was manifest as transient bursts of electrical activity followed by periods of electrical quiescence. This bursting pattern is unusual for normal adult cardiomyocytes, but has been observed in several other cell types. In the primary tumors, automatic behavior may arise from a similar cellular origin or alternatively from a microreentrant phenomena.

CONCLUSION

Primary tumors and AT-1 cells show essential atrial electrophysiology with important novel features.

Identification of SV40 large T-antigen-associated proteins in cardiomyocytes from transgenic mice

A cell line derived from transgenic mice expressing the SV40 large T-antigen oncogene in the heart was used to identify cardiomyocyte targets for T-antigen binding. A novel protein of molecular mass of 193 kDa was identified as an associated protein by virtue of its ability to be co-immunoprecipitated with multiple anti-T-antigen antibodies. Two previously described proteins, p120 and p53, were also observed to complex with T-antigen in transformed cardiomyocytes. In addition, several proteins that cross-reacted with either anti-T-antigen or anti-p53 antibodies were identified. Two of these proteins, of apparent molecular masses of 250 and 110 kDa, were only observed in cardiomyocytes. Expression of a third cross-reacting protein of a molecular mass of 180 kDa appeared to be dependent on the growth status of the cells. These proteins may be important constituents of the cardiomyocyte cell cycle, as well as potential cellular targets for myocardial regeneration.

Translation of heart preproenkephalin mRNA and secretion of enkephalin peptides from cultured cardiac myocytes

Rat ventricular cardiac muscle has previously been shown to contain exceptionally high levels of preproenkephalin mRNA (ppEnk mRNA). We have recently determined that the level of ppEnk mRNA is developmentally and hormonally regulated in rat ventricular cardiac muscle tissue and in cultured myocytes (J. P. Springhorn and W. C. Claycomb. Biochem. J. 258: 73-77, 1989). We demonstrate in the current study that heart ppEnk mRNA is structurally identical at the 5' end to brain ppEnk mRNA using a ribonuclease protection assay and that heart ppEnk mRNA can be translated in vitro using a rabbit reticulocyte lysate system. In vitro synthesized preproenkephalin peptides were immunoprecipitated with a polyclonal antibody directed to the carboxy-terminal seven amino acids of preproenkephalin. We have also established by radioimmunoassay that enkephalin-containing peptides are secreted from cultured neonatal and adult rat ventricular cardiac muscle cells. This secretion is linear with respect to time and can be stimulated by phorbol 12-myristate 13-acetate (PMA) and adenosine 3',5'-cyclic monophosphate (cAMP). It was determined by column chromatography that cAMP induced neonatal rat ventricular cardiac muscle cells to secrete Met5-enkephalin-Arg6-Phe7, whereas PMA plus 3-isobutyl-1-methylxanthine induced adult rat ventricular cardiac muscle cells to secrete Met5-enkephalin. These studies establish that ventricular heart muscle ppEnk mRNA can be translated and that enkephalin peptides are secreted from ventricular cardiac muscle cells.

Cardiomyocyte proliferation in mice expressing alpha-cardiac myosin heavy chain-SV40 T-antigen transgenes

To determine the proliferative potential of adult ventricular cardiomyocytes, we have generated transgenic mice that express the SV40 large T-antigen oncogene in the heart. A fusion gene comprised of the rat alpha-cardiac myosin heavy chain promoter and the SV40 early region was used to target oncogene expression to the myocardium. Expression of SV40 large T-antigen was observed in both atrial and ventricular cardiomyocytes in adult transgenic animals. T-antigen expression was associated with hyperplasia in the targeted cells. Immunohistological analysis indicated that the proliferating cells continued to express sarcomeric myosin. Electron microscopic examination demonstrated that cardiomyocytes in various stages of the cell cycle retained ultrastructural characteristics typical of mitotic cardiac muscle cells in vivo. Cardiomyocytes isolated from transgenic tumors were able to proliferate in culture and retained a differentiated phenotype, as evidenced by spontaneous contractile activity. Preliminary studies indicate that these cells can undergo a limited number of passages while retaining this differentiated phenotype. These studies demonstrate that both ventricular and atrial cardiomyocytes from transgenic mice proliferate in response to targeted T-antigen expression.

Gene expression and atrial natriuretic factor processing and secretion in cultured AT-1 cardiac myocytes

BACKGROUND

Studies were carried out to characterize several biochemical features of cultured AT-1 cells.

METHODS AND RESULTS

These cells were obtained from a transplantable atrial cardiomyocyte tumor lineage. Reverse transcriptase-polymerase chain reaction-based analyses demonstrated that the pattern of gene expression of cultured AT-1 cells was similar to that of adult atrial myocytes. AT-1 cells expressed atrial natriuretic factor (ANF), alpha-cardiac myosin heavy chain, alpha-cardiac actin, and connexin43. Radioimmunoassays verified that the cells synthesized, stored, and secreted ANF. Through size-exclusion, reversed-phase, and carboxymethyl-ion-exchange high-performance liquid chromatography, it was shown that cultured AT-1 cells stored ANF as pro-ANF (ANF-[1-126]), which was cosecretionally processed quantitatively to ANF-(1-98) and the bioactive 28-amino-acid ANF-(99-126). In addition, cultured AT-1 cells secreted ANF at almost a sixfold greater rate in response to endothelin-1, a potent secretagogue of ANF. KCl, metenkephalinamide, isoproterenol, phenylephrine, and 12-O-tetradecanoyl-phorbol-13-acetate also stimulated ANF release.

CONCLUSIONS

These studies, in combination with previous findings, demonstrated that cultured AT-1 cells, while maintaining the ability to proliferate, have retained functional, biochemical, and ultrastructural features that are characteristic of adult atrial myocytes.

Morphological characterization of cardiomyocytes isolated from a transplantable cardiac tumor derived from transgenic mouse atria (AT-1 cells)

We have developed a transplantable tumor lineage derived from transgenic mouse atrial cardiomyocytes that express the SV40 large T oncogene and have named these cardiomyocytes AT-1 cells. In this study, the transplantable tumors, freshly isolated tumor cardiomyocytes, and cultured tumor cardiomyocytes were examined using phase-contrast microscopy, autoradiography, and electron microscopy. The vast majority of the subcutaneous tumor cells, greater than 90% of the cellular mass of the tumor, exhibited sarcomeric banding. Ultrastructural characteristics typical of in vivo atrial cardiac muscle cells, including well-organized myofibrils, gap junctions, and atrial-specific cytoplasmic granules, were observed in in situ and in freshly isolated AT-1 cells. Those cells that did not contain some form of organized myofibrils were primarily vascular elements, such as endothelial cells. Labeling with [3H]thymidine indicated that greater than 90% of cultured AT-1 cells were synthesizing DNA; furthermore, many cells could be seen undergoing cell division. Electron microscopy revealed that the cultured AT-1 cardiomyocytes contained all of the above-described characteristics, including a well-developed transverse tubular system.

Proliferation in vivo and in culture of differentiated adult atrial cardiomyocytes from transgenic mice

Transgenic mice expressing atrial natriuretic factor-SV40 T-antigen fusion genes (ANF-TAG) developed unilateral right atrial tumors composed of differentiated dividing cardiomyocytes. The atrial tumors could be propagated as transplantable tumor lineages in syngeneic animals. Cardiomyocytes derived from ANF-TAG atrial tumors did not proliferate in tissue culture. However, cardiomyocytes derived from the transplantable tumor lines proliferated in culture, and these proliferating cardiomyocytes could be passaged in culture and recovered from frozen stocks. Cardiomyocytes from either tumor source were highly differentiated as determined by diverse functional and structural criteria. The cells continued to express numerous cardiac-specific proteins and retained ultrastructural features characteristic of cardiomyocytes including well-formed myofibrils, transverse tubules, and intercalated disks. In addition, the cultured cells displayed spontaneous electrical and contractile activities. These atrial tumor cardiomyocytes are a novel experimental resource for the identification of genes regulating the cardiomyocyte cell cycle.

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.

Culture and characterization of fetal human atrial and ventricular cardiac muscle cells

Atrial and ventricular cardiac muscle cells isolated from 14- to 18-wk old fetal human hearts were grown in culture and characterized. Once established in culture the flattened cells contracted spontaneously and possessed differentiated ultrastructural characteristics including organized sarcomeres, intercalated discs, and transverse tubules with couplings. Atrial granules were present in the cultured atrial cells. Some cultured ventricular myocytes also contained electron-dense granules associated with Golgi cisternae, which were similar in size and appearance to atrial granules. The cultured ventricular myocytes divided and expressed the genes for thymidine kinase, histone H4, myosin heavy chain, muscle-specific creatine kinase, atrial natriuretic factor, and insulin-like growth factor II. These results establish that differentiated fetal human heart muscle cells can be cultured in sufficient quantities for biochemical, molecular, and morphological analyses.

Differentiated membrane specializations and myofibrillar breakdown and recovery in cultured adult cardiac myocytes treated with TPA and diacylglycerol

Cultured adult rat ventricular cardiac muscle cells were treated with either 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or diacylglycerol (DAG) and observed by in situ transmission electron microscopy. Membrane specializations present in untreated cells (intercalated discs, transverse tubules, plasmalemmal couplings) were also present after TPA and DAG treatment. In the case of the transverse tubular system, there was morphological evidence for active growth. Our studies showed that myofilaments began to become disorganized after 12–24 h of TPA treatment and that after 2 days of exposure to TPA the breakdown of sarcomeres was essentially complete. Myocytes that were treated with TPA for 2 days and then allowed to recover in control medium for 5 days contained sarcomeres in various stages of reassembly. These data indicate that TPA-treated cardiac myocytes retain several membrane specializations, suggesting that there are separate controls for myofilament organization and the maintenance of these differentiated plasmalemmal regions. Furthermore, the ability of the myocytes to recover from TPA treatment may provide investigators with a useful model with which to study myofibrillogenesis.

Preproenkephalin mRNA expression in developing rat heart and in cultured ventricular cardiac muscle cells

Heart muscle tissue has previously been reported to have the highest content of preproenkephalin (ppEnk) mRNA of any tissue in the adult rat. We have determined that it is present in the ventricular cardiac muscle cells of the heart and is developmentally regulated. The expression of ppEnk mRNA was observed to be low throughout the first 2 weeks of postnatal development and decreases substantially during week 3. Expression was again low by week 4, but by adulthood (approx. 3 months), it reached a maximum. ppEnk mRNA was actively expressed in primary cardiac muscle cell cultures prepared from both neonatal and adult rats. Its steady-state content in cell cultures was observed to be increased by cyclic AMP and 3-isobutyl-1-methylxanthine. The phorbol ester phorbol 12-myristate 13-acetate elicited a transient effect (i.e. an increase was observed at 4 h and a return to control values by 24 h). We speculate that enkephalin may play a multi-functional role in the differentiation of neonatal cardiac muscle cells and in the terminally differentiated adult heart cell. We demonstrate that the primary culture systems employed in this study will be useful models with which to explore both transcriptional and translational regulation of ppEnk mRNA in the heart.

Atrial-natriuretic-factor mRNA is developmentally regulated in heart ventricles and actively expressed in cultured ventricular cardiac muscle cells of rat and human

Atrial-natriuretic-factor (ANF) mRNA is actively expressed in ventricular heart myocytes of neonatal rats and fetal and young (2.5-year-old) humans. In rat, ANF mRNA transcription is repressed by the 23rd day of postnatal development, and in human it is no longer detected by the 14th year of development. Its expression is observed in atrial myocytes of both of these species at all ages. ANF mRNA is expressed in primary cultures of ventricular cardiac muscle cells prepared from both neonatal-rat and fetal-human hearts. Surprisingly it is also very actively expressed in cultures of adult rat ventricular cardiac muscle cells. The effect of several hormones on the expression of ANF mRNA in rat and human myocyte cultures was evaluated. These studies demonstrate that ANF mRNA transcription is developmentally regulated in both rat and human heart ventricles and suggest that ventricular cardiac muscle-cell cultures may be useful in studying the regulation of the expression of this gene.

Growth factors and TPA stimulate DNA synthesis and alter the morphology of cultured terminally differentiated adult rat cardiac muscle cells

Previous studies have established that the terminally differentiated ventricular cardiac muscle cell of the adult rat reinitiates semiconservative DNA replication when grown in culture (W. C. Claycomb and H. D. Bradshaw, Jr., 1983, Dev. Biol. 90, 331-337). Work reported here shows that several growth factors and chemicals will stimulate this DNA synthetic activity in a concentration-dependent manner. Autoradiographic experiments establish that this stimulated DNA synthesis is due to cells not previously synthesizing DNA being induced to enter the S phase of the cell cycle. By far the greatest stimulation (250%) is observed with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Fifty ng/ml is the optimal concentration, and the maximal effect is observed 5 days after adding TPA. TPA also substantially increases the protein content of the cultured myocytes. Diacylyglcerols (DAG) induce these same changes, indicating that the effect of TPA is mediated by protein kinase C. The morphology of the cultured cardiac muscle cells is profoundly altered by TPA and DAG. TPA- and DAG-treated myocytes spread more thinly on the surface of the culture flask, acquire multiple nuclei, and undergo nucleolar fragmentation. The myofibrillar ultrastructure of the treated cells becomes almost totally disorganized, and intermediate filaments and rough endoplasmic reticulum accumulate in the cytoplasm. These TPA results suggest a possible relationship between the degree of ultrastructural differentiation of the ventricular cardiac muscle cell and DNA synthetic activity. This easily altered cellular plasticity should be very useful for studies of the regulation of cardiac muscle cell proliferation and cell differentiation.

Proto-oncogene expression in proliferating and differentiating cardiac and skeletal muscle

We have examined the expression of 13 proto-oncogenes in proliferating and terminally differentiated cardiac and skeletal muscle. Total RNA was prepared from intact ventricular cardiac-muscle tissue and from purified ventricular cardiac-muscle cells of neonatal and adult rats and from cultured proliferating and terminally differentiated L6A1 rat skeletal-muscle cells. cDNA probes for histone H4, thymidine kinase, myosin heavy chain and M-creatine kinase were used to assess cellular proliferation and differentiation. Oncogenes c-myc, c-raf, c-erb-A, c-ras-H, c-ski, and c-sis were expressed in both proliferating and differentiated cardiac muscle tissue and cells, whereas c-myb expression was not observed in either. c-src was expressed only in neonatal cardiac muscle tissue and cells. c-fms, c-abl, and c-ras-K were expressed in tissue from both neonatal and adult animals but only in purified cells from neonatal animals. c-fes/fps was expressed only in neonatal cardiac muscles cells. c-fos expression was not observed in cardiac-muscle tissue from either neonatal or adult rats, but surprisingly was abundantly expressed in freshly isolated cardiac-muscle cells from animals of both ages. These results emphasize that biochemical analysis using intact cardiac-muscle tissue may not necessarily reflect muscle-specific cell processes. They also show that the expression of c-fos can be activated by the cell isolation procedure. c-myc, c-ski, c-ras-H, c-ras-K, c-abl, c-raf and c-erb-A were expressed in both proliferating and terminally differentiated skeletal-muscle cells, whereas c-myb, c-fos, c-src and c-fms transcripts were observed only in proliferating cells. c-fes/fps and c-sis were not expressed in dividing or fused skeletal-muscle cells. These results demonstrate unique tissue and cell-specific patterns of proto-oncogene expression and suggest that these genes may be involved with the regulation of cellular proliferation and terminal differentiation in striated muscle.

1,25-Dihydroxyvitamin D3 stimulates 45Ca2+ uptake by cultured adult rat ventricular cardiac muscle cells

This study tested the hypothesis that 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and its previously described cardiac receptors play roles in regulating intracellular calcium homeostasis in cardiac muscle cells. This question was addressed by assessing whether 1,25-(OH)2D3 influences 45Ca2+ uptake by homogeneous cultures of adult rat ventricular cardiac muscle cells. Twenty-four h prior to the measurement of 45Ca2+ uptake, the cells were transferred to serum-free medium ([Ca2+], 1.0 mM) containing 1.0 nM 1,25(OH)2D3 or vehicle. The cells were then incubated with 45Ca2+ for periods up to 60 min at room temperature, followed by removal of excess external 45Ca2+ by washing repeatedly with La3+. Pretreating the cells with 1,25-(OH)2D3 caused 3-fold stimulation (p less than 0.005) of 45Ca2+ uptake. Stimulation of 45Ca2+ uptake required a prolonged (8-12 h) exposure to 1,25-(OH)2D3, suggesting a receptor-mediated phenomenon. Concentrations of 0.01-10 nM 1,25-(OH)2D3 yielded a dose-response curve which peaked at 1.0 nM and decreased at higher concentrations. Steroid specificity was established by the failure of 1.0 nM levels of 25-hydroxyvitamin D3, estradiol-17 beta, and progesterone to change 45Ca2+ uptake. Sucrose gradient analysis confirmed the presence of a specific 3-4 S 3H-1,25-(OH)2D3 binding component both in freshly isolated and in cultured ventricular cardiac muscle cells. The stimulatory effect of 1,25-(OH)2D3 on 45Ca2+ uptake was abolished by the concomitant incubation of the cells with cycloheximide or actinomycin D, demonstrating a requirement for protein and nucleic acid synthesis. In conclusion, these data demonstrate that 1,25-(OH)2D3 stimulates 45Ca2+ uptake in adult ventricular cardiac muscle cells by a mechanism resembling a receptor-mediated phenomenon.

Electrophysiological properties of cultured adult rat ventricular cardiac muscle cells

Action and resting potential characteristics of isolated adult rat myocardial cells maintained in culture for 10-28 days are described. Resting potentials averaged -76.3 +/- 2 mV in 5 mM extracellular [K+] ([K+]o). Resting potentials changed by 54.3 mV/decade change in [K+]o for concentrations greater than 5 mM. The average maximum rate of rise of action potential (Vmax) was 117.7 +/- 10 V/s with overshoots of 34.6 +/- 2.5 mV. Action potential durations (APD) to 0 and -40 mV and full repolarization were 21.8 +/- 3.9, 36.3 +/- 6.0, and 206 +/- 16.9 ms respectively. Action potential configurations were qualitatively similar to those previously reported by others for rat myocardial tissue or freshly dissociated cells. Tetrodotoxin (4 micrograms/ml) decreased Vmax to less than 24 V/s and decreased overshoot and APD. Isoproterenol (10(-8) M) decreased APD with slight elevation of the overshoot. Verapamil (10(-5) to 10(-4) M) depressed overshoot and plateau while slowing the final phase of repolarization. Verapamil (10(-4) M) depressed upstroke velocity and blocked excitability. While APDs recorded from these cultured cells are apparently longer than those reported by others for rat myocardial APDs, the values of all other electrophysiological parameters recorded are within the limits previously reported for normal rat myocardial tissue. These data indicate that adult rat myocardial cells maintained in tissue culture for 10-28 days possess electrophysiological properties and responses to pharmacological agents similar to adult rat myocardial tissue or undamaged freshly isolated cells.

Culture of atrial and ventricular cardiac muscle cells from the adult squirrel monkey Saimiri sciureus

Atrial and ventricular cardiac muscle cells isolated from the adult squirrel monkey Saimiri sciureus were cultured and characterized by light and transmission electron microscopy (TEM). Freshly isolated cells were striated and cylindrical in shape and, when placed in culture, rounded up and lost their highly organized morphology. With prolonged time in culture, they spread out on the surface of the culture flask and reacquired many of the internal ultrastructural characteristics of their in vivo atrial and ventricular cardiac muscle cell counterparts. Autoradiographic experiments indicated that both atrial and ventricular myocytes synthesized DNA when grown in culture. In some binucleated atrial cells only one nucleus became labelled. These studies show that it is now possible to culture cardiac muscle cells isolated from an adult primate.

Ultrastructure of cultured atrial cardiac muscle cells from adult rats

Atrial cardiac muscle cells enzymatically isolated from adult rats were maintained in culture for 0-17 days and examined by transmission electron microscopy (TEM). Cells were stained with conventional TEM stains as well as with osmium ferrocyanide and tannic acid. Our results show that cultured adult atrial cells are capable of in vitro ultrastructural reorganization and possess differentiated ultrastructural characteristics including specific atrial granules, sarcomerically arranged myofilaments, appropriately organized sarcoplasmic reticulum (both junctional and nonjunctional), and intercalated disc components. In addition, the cultured atrial cells also possess rare, but ultrastructurally typical, elements of the transverse tubular system. These can be identified on the basis of size, location, association with internal junctional sarcoplasmic reticulum, and accumulation of extracellular tracer. Atrial muscle cells are capable of reestablishing a myotypic ultrastructure, although they have a considerably less complex and organized in vitro ultrastructure than similarly cultured adult ventricular myocytes. This lessened in vitro ultrastructural specialization is in accord with the in vivo comparative ultrastructure of atrial vs. ventricular myocytes.

Isolation and culture of the terminally differentiated adult mammalian ventricular cardiac muscle cell

We have carried out systematic studies to optimize and standardize methodology to isolate and culture the adult rat ventricular cardiac muscle cell. Four hearts were perfused simultaneously with a calcium-free medium containing collagenase. The ventricular tissue was then minced and further digested to liberate individual cells. Approximately 16 million rod-shaped muscle cells were obtained. The plating efficiency has been greatly improved by culturing the cells in a conditioned medium prepared from a rabbit corneal cell line. This medium also contained added fetal bovine serum, essential and nonessential amino acids, vitamins, insulin, transferrin, and 25 trace minerals. The culture flasks were precoated with rat-tail collagen. Fibroblast contamination was virtually eliminated by including cytosine arabinoside in the medium during the first 7 d of culture. After this time the cells could be cultured in the absence of serum in a chemically defined medium composed of MEM, vitamins, nonessential amino acids, and trace minerals. They continued to contract spontaneously and do well in this medium for at least 3 d thereafter. This improved methodology resulted in a reproducible culture system with improved plating efficiency. It provided a new and unique system to study the structure and function of the adult mammalian ventricular cardiac muscle cell.

Culture of the terminally differentiated ventricular cardiac muscle cell. Characterization of exogenous substrate oxidation and the adenylate cyclase system

Oxidation of several exogenous substrates by cultured adult rat ventricular cardiac muscle cells has been assessed. Unlike freshly isolated cardiac muscle cells which oxidize glucose preferentially, the cultured cells more closely resemble metabolically the in situ heart and the isolated perfused heart, in that their preference for exogenous substrates is in the order of fatty acid greater than glucose. This switch in metabolic preference from glucose to fatty acid is complete within 12 h after placing freshly isolated cells in culture. Glucose oxidation is stimulated by insulin and isoproterenol and inhibited by beta-hydroxybutyrate and octanoate. The adenylate cyclase system has also been examined in these cultured cells. Isoproterenol, norepinephrine and epinephrine stimulate the accumulation of cyclic adenosine 3':5'-monophosphate (cyclic AMP) in a concentration-dependent manner. The order of potency is isoproterenol greater than norepinephrine approximately equal to epinephrine. This stimulation is potentiated by 1-isobutyl-3-methylxanthine and inhibited by 1-propranolol.

Acquisition of multiple nuclei and the activity of DNA polymerase alpha and reinitiation of DNA replication in terminally differentiated adult cardiac muscle cells in culture

Terminally differentiated ventricular cardiac muscle cells isolated from the adult rat and maintained in cell culture have been observed to acquire multiple nuclei. In one cultured myocyte as many as 10 nuclei have been counted. Apparently, these multiple nuclei are formed by DNA replication followed by karyokinesis; the cells must then fail to complete mitosis and divide. To investigate whether DNA synthesis was occurring, the cells were cultured in the presence of [3H]thymidine and then processed for autoradiography. Mononucleated, binucleated, and multinucleated cells incorporate [3H]thymidine into DNA as evidenced by the high concentration of silver grains over their nuclei. Peak periods of incorporation were observed to occur at 10- to 12-day intervals; at 11, 23, and 33 days after initially placing the cells in culture. When the cells were maintained in the presence of [3H]thymidine continuously from Day 7 to Day 17 of culture, 23% of the cells became labeled. If the cells were cultured continuously for 30 days in the presence of [3H]thymidine, from Day 10 to Day 40, 56% of the cells were labeled. Isopycnic gradient analysis indicates that this thymidine incorporation was into DNA that was being replicated semiconservatively; these experiments did not eliminate the possibility, however, that this incorporation was due to amplification of specific genes, such as those coding for the contractile proteins. The activity of DNA polymerase alpha also returns to these cells. These studies demonstrate that the terminally differentiated mammalian ventricular cardiac muscle cell, previously thought to have permanently lost the capacity to replicate DNA during early development, is able to reinitiate semiconservative DNA replication when grown in culture.