Developmental expression of p107 mRNA and evidence for alternative splicing of the p107 (RBL1) gene product

Expression of p107, a protein with homology to the retinoblastoma tumor suppressor (pRB), was monitored during murine development. Northern blot tissue surveys identified two transcripts of 4.9 and 2.4 kb that hybridized to a p107 cDNA clone. Expression of both transcripts was detected in fetal tissues, with particularly high levels in the liver and heart. In contrast, p107 transcripts were markedly decreased in most adult tissues examined. Molecular cloning analyses revealed that the 4.9- and 2.4-kb transcripts encoded proteins with deduced molecular masses of 119 and 68 kDa, respectively. Genetic mapping studies suggested that the two p107 transcripts arose by alternative splicing of a common precursor. The protein encoded by the 2.4-kb transcript lacks the spacer and B motif of the "pocket domain," a region of homology between p107 and pRB that is required for binding to cell cycle regulatory proteins. Structural modifications resulting from alternative splicing may thus confer functional diversity upon the 119- and 68-kDa proteins.

Strategies for myocardial repair

The therapeutic recourse for end-stage heart disease is currently limited to cardiac transplantation. The ability to augment cardiomyocyte number in an end-stage heart might facilitate myocardial function. Augmentation of cardiomyocyte number may be achievable by the targeted expression of cell cycle regulatory genes to the myocardium. Alternatively, intracardiac grafting of exogenous cardiomyocytes might also provide a viable approach to augment cardiomyocyte number. Potential strategies for heart muscle regeneration via gene therapy and cellular transplantation are discussed.

Blood pressure and fluid-electrolyte balance in ANF-transgenic mice on high- and low-salt diets

Transgenic mice overexpressing an atrial natriuretic factor (ANF) fusion gene (TTR-ANF) and their nontransgenic siblings were placed on a high- (8%) or low (< 0.008%)-salt diet for 14 days to determine whether the lifelong elevation of ANF in the transgenic animals compromised their ability to maintain fluid-electrolyte balance. Steady-state dietary intake and urinary output of sodium and chloride were not statistically different between TTR-ANF and control groups on either diet. By contrast, water intake and urine volume were markedly elevated in the TTR-ANF group on either diet. Arterial blood pressure, measured in anesthetized mice at the end of the dietary regimen, was significantly and similarly reduced in the TTR-ANF compared with control groups on either diet, although high salt intake was associated with increased pressure in both groups. Renal excretion of fluid and electrolytes was studied in the anesthetized mice before and after acute blood volume expansion. Although the absolute increase in fluid and electrolyte excretion was much greater on the high- than on the low-salt diet in both groups, TTR-ANF mice had an exaggerated response relative to controls on either diet. On the basis of these results, we conclude the following. 1) When they are stimulated, renal salt-conserving mechanisms are sufficiently powerful to overcome the expected natriuretic effects of chronic elevation of plasma ANF; however, the natriuretic potential of ANF can be expressed in the short term when the counterregulatory mechanisms are inactivated. 2) ANF exerts a chronic hypotensive effect that is independent of changes in renal salt excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of NGF within the heart of transgenic mice causes hyperinnervation, cardiac enlargement, and hyperplasia of ectopic cells

Nerve growth factor (NGF) supports the survival of developing sympathetic and a subpopulation of sensory neurons. In the adult it participates in maintenance of the neurotransmitter phenotype of responsive neurons. The amount of NGF synthesized by a given target tissue determines its final innervation density; those developing neurons that fail to receive sufficient NGF undergo apoptosis. In order to examine the ramifications of this principle in the context of a specific target organ, a transgenic mouse model was developed in which NGF expression was increased in developing and adult cardiac tissue by placing a NGF minigene under the transcriptional control of the cardiac-specific alpha-myosin heavy chain promoter. Transgenic mice developed cardiac enlargement secondary to both an increase in myocardial mass and the presence of an abundant ectopic cell population. Immunohistochemical analyses with the neural marker S-100 revealed staining of a subpopulation of ectopic cells, suggesting their derivation from the neural crest. Whereas immunostaining for the neuronal-specific protein neuron-specific enolase demonstrated labeling of another subpopulation of ectopic cells within the heart. Measurements of cardiac tissue catecholamine levels revealed a marked elevation in transgenic mice, consistent with sympathetic hyperinnervation. Analysis of mediastinal sympathetic ganglia revealed increases in both the size and the number of neurons. In this model, increased expression of NGF produced hyperinnervation of the heart, pathological cardiac growth, and the recruitment and/or expansion of an ectopic, neural crest-derived cell type.

Potential approaches for myocardial regeneration

Cardiomyocytes in the adult mammal retain little or none of their developmental capacity for hyperplastic growth. As a consequence of this differentiated, nonproliferative phenotype, cardiomyocyte loss due to injury or disease is irreversible. Therapeutic intervention in end-stage diseased hearts is currently limited to cardiac transplantation. An increase in cardiomyocyte number in diseased hearts could improve function. Augmentation of the cardiomyocyte population may be achievable by the expression of regulatory proteins in the myocardium, or by intracardiac grafting of exogenous cardiomyocytes.

Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C

1. Beat-to-beat performance of myocardium is highly dependent on sarcomere length. The physiological basis for this effect is not well understood but presumably includes alterations in the extent of overlap between thick and thin filaments. Sarcomere length dependence of activation also appears to be involved since length-tension relationships in cardiac muscle are usually steeper than those in skeletal muscle. 2. An explanation recently proposed to account for the difference between length-tension relationships is that the cardiac isoform of troponin C (cTnC) has intrinsic properties that confer greater length-dependent changes in the Ca2+ sensitivity of tension than does skeletal troponin C (sTnC), presumably due to greater length-dependent changes in the Ca(2+)-binding affinity of cTnC. To test this hypothesis, transgenic mice were developed in which fast sTnC was expressed ectopically in the heart. This allowed a comparison of the length dependence of the Ca2+ sensitivity of tension between myocytes having thin filaments that contained either endogenous cTnC or primarily sTnC. 3. In myocytes from both transgenic and normal mice, the Ca2+ sensitivity of tension increased similarly when mean sarcomere length was increased from approximately 1.83 to 2.23 microns. In both cases, the mid-point (pCa50) of the tension-pCa (i.e. -log[Ca2+]) relationship shifted 0.12 +/- 0.01 pCa units (mean +/- S.E.M.) in the direction of lower Ca2+. 4. We conclude that the Ca2+ sensitivity of tension in myocytes changes as a function of sarcomere length but is independent of the isoform of troponin C present in the thin filaments.

Targeted expression of transforming growth factor-beta 1 in intracardiac grafts promotes vascular endothelial cell DNA synthesis

Intracardiac grafts comprised of genetically modified skeletal myoblasts were assessed for their ability to effect long-term delivery of recombinant transforming growth factor-beta (TGF-beta) to the heart. C2C12 myoblasts were stably transfected with a construct comprised of an inducible metallothionein promoter fused to a modified TGF-beta 1 cDNA. When cultured in medium supplemented with zinc sulfate, cells carrying this transgene constitutively secrete active TGF-beta 1. These genetically modified myoblasts were used to produce intracardiac grafts in syngeneic C3Heb/FeJ hosts. Viable grafts were observed as long as three months after implantation, and immunohistological analyses of mice maintained on water supplemented with zinc sulfate revealed the presence of grafted cells which stably expressed TGF-beta 1. Regions of apparent neovascularization, as evidenced by tritiated thymidine incorporation into vascular endothelial cells, were observed in the myocardium which bordered grafts expressing TGF-beta 1. The extent of vascular endothelial cell DNA synthesis could be modulated by altering dietary zinc. Similar effects on the vascular endothelial cells were not seen in mice with grafts comprised of nontransfected cells. This study indicates that genetically modified skeletal myoblast grafts can be used to effect the local, long-term delivery of recombinant molecules to the heart.

Proximal tubular function in transgenic mice overexpressing atrial natriuretic factor

A transgenic mouse model in which atrial natriuretic factor (ANF) expression is targeted to the liver was used to study intrarenal adjustments to the chronically elevated hormone level. Such animals, designated TTR-ANF, are characterized by reduced arterial blood pressure but similar sodium excretion compared with nontransgenic siblings. Proximal tubular micropuncture gave the following results: single-nephron filtration rate = 12.7 +/- 1.1 vs. 15.6 +/- 1.9 nL/min (TTR-ANF versus nontransgenic, ns); end-proximal tubular fluid/plasma concentration ratio of inulin = 1.93 +/- 0.09 vs. 1.97 +/- 0.15 (ns); fractional reabsorption of sodium = 45.5 +/- 2.8 vs. 46.0 +/- 3.8% (ns); fractional reabsorption of chloride = 33.6 +/- 3.3 vs. 32.4 +/- 4.1% (ns). These data indicate that life-long elevation of plasma ANF concentration was not associated with significant alteration in single-nephron filtration rate and proximal tubular function. We conclude that compensatory antinatriuretic mechanisms, localized downstream from the proximal tubule, can prevent ANF natriuresis.

Tumor suppressor gene expression during normal and pathologic myocardial growth

Previous studies have identified several host proteins (p53, p107, and p193), which form prominent complexes with SV40 T antigen in transformed cardiomyocytes. Expression of p53 and p107 was monitored during normal and pathologic growth in nontransformed murine myocardium. Both genes were expressed at relatively high levels in embryonic cardiomyocytes. Transcript levels decreased markedly during the process of cardiomyocyte terminal differentiation and were very low or undetectable in adult animals. In contrast, retinoblastoma transcripts were observed at low levels throughout myocardial development. None of the tumor suppressor genes examined were transcriptionally activated during acute myocardial overload or isoproterenol-induced myocardial hypertrophy. The potential role of tumor suppressor gene product expression in myocardial development and pathology is discussed.

Assessment of donor bone marrow cell-derived chimerism in transplantation tolerance using transgenic mice

Allospecific skin graft prolongation can be induced in mice using antithymocyte globulin and allospecific donor bone marrow cells (DBMC). This enhancement may be due to the persistence of chimeric cells of donor origin in the host. In this study, we systematically investigated DBMC-derived chimerism in various lymphoid and nonlymphoid tissues over time. To do this, transgenic mice were used as a source of DBMC to clearly distinguish chimerism due only to the injected DBMC. Chimerism in various tissues was assessed at several times points after DBMC infusion by polymerase chain reaction amplification of tissue DNA using transgene specific primers. A cDNA probe specific for the transgene was used to demonstrate DBMC-derived chimerism in polymerase chain reaction products by the method of Southern. Although chimerism was initially detectable in most tissues tested 1 day after DBMC infusion, the presence of chimeric cells generally diminished over time. At 4 weeks or longer, chimerism was consistently confined to recipient skin. Furthermore, the chimeric cells in recipient skin persisted even after the allograft was rejected. In contrast to chimerism in recipient skin, chimerism became undetectable in donor skin as early as 2 weeks after DBMC infusion. The loss of chimerism in donor skin showed a temporal correlation with a reduction of chimerism in host bone marrow and lymphoid tissues that preceded rejection in all experiments by a range of 7-14 days. The use of DBMC from transgenic mice allowed a unique opportunity to monitor the kinetics of DBMC-derived chimerism. The presence of chimerism in the skin of mice in temporal association with chronic allograft rejection suggests that chimerism per se is not a reliable index of allogeneic unresponsiveness.

Formation of nascent intercalated disks between grafted fetal cardiomyocytes and host myocardium

Fetal cardiomyocytes isolated from transgenic mice carrying a fusion gene of the alpha-cardiac myosin heavy chain promoter with a beta-galactosidase reporter were examined for their ability to form stable intracardiac grafts. Embryonic day 15 transgenic cardiomyocytes delivered directly into the myocardium of syngeneic hosts formed stable grafts, as identified by nuclear beta-galactosidase activity. Grafted cardiomyocytes were observed as long as 2 months after implantation, the latest date assayed. Intracardiac graft formation did not induce overtly negative effects on the host myocardium and was not associated with chronic immune rejection. Electron microscopy revealed the presence of nascent intercalated disks connecting the engrafted fetal cardiomyocytes and the host myocardium. These results suggest that intracardiac grafting might provide a useful approach for myocardial repair, provided that the grafted cells can contribute to myocardial function.

Hemodynamics in transgenic mice with overexpression of atrial natriuretic factor

The circulatory effects associated with lifelong plasma atrial natriuretic factor (ANF) elevation were examined by generating transgenic mice, which constitutively express a fusion gene consisting of the transthyretin promoter and the ANF structural gene. These mice have chronically elevated ANF levels as compared with their nontransgenic siblings. Transgenic animals exhibited immunoreactive ANF levels that were nearly fivefold higher than those measured in nontransgenic littermates. Systemic and regional hemodynamics and blood volumes were explored by using modifications of the reference microsphere and dilution techniques. Mean arterial pressure was reduced by 24 mm Hg, associated with a 27% reduction in total heart weight. This chronic reduction in blood pressure was due to a 21% reduction in total peripheral resistance, whereas cardiac output, stroke volume, and heart rate were not significantly altered, despite a 15% elevation in plasma volume. Transgenic mice displayed reductions of 35%, 33%, 32%, and 19% in muscle, skin, brain, and renal vascular resistance, respectively, whereas coronary and splanchnic resistances were not significantly altered. The findings complement earlier data from chronically infused normotensive mammals and suggest that these mice are an excellent model for investigating the effects of lifelong ANF elevation.

Assessment of cardiomyocyte DNA synthesis during hypertrophy in adult mice

The ability of cardiomyocytes to synthesize DNA in response to experimentally induced cardiac hypertrophy was assessed in adult mice. Isoproterenol delivered by osmotic minipump implantation in adult C3Heb/FeJ mice resulted in a 46% increase in heart weight and a 19.3% increase in cardiomyocyte area. No DNA synthesis, as assessed by autoradiographic analysis of isolated cardiomyocytes, was observed in control or hypertrophic hearts. A survey of 15 independent inbred strains of mice revealed that ventricular cardiomyocyte nuclear number ranged from 3 to 13% mononucleate, suggesting that cardiomyocyte terminal differentiation is influenced directly or indirectly by genetic background. To determine whether the capacity for reactive DNA synthesis was also subject to genetic regulation, cardiac hypertrophy was induced in the strains of mice comprising the extremes of the nuclear number survey. These data indicate that adult mouse atrial and ventricular cardiomyocytes do not synthesize DNA in response to isoproterenol-induced cardiac hypertrophy.

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.

Skeletal troponin C reduces contractile sensitivity to acidosis in cardiac myocytes from transgenic mice

Depressed contractile function plays a primary role in the pathophysiology of acute myocardial ischemia. Intracellular acidification is an important factor underlying the inhibition of force production in the ischemic myocardium. The effect of acidosis to depress contractility is markedly greater in cardiac as compared to skeletal muscle; however, the molecular basis of this difference in sensitivity to acidosis is not clearly understood. In this report, we describe transgenic mice that express the fast skeletal isoform of troponin C (sTnC) in cardiac muscle. In permeabilized single cardiac myocytes the shift in the midpoint of the tension-pCa relationship (i.e., pCa50, where pCa is -log[Ca2+]) due to lowering pH from 7.00 to 6.20 was 1.27 +/- 0.03 (n = 7) pCa units in control cardiac TnC (cTnC) expressing myocytes and 0.96 +/- 0.04 (n = 11) pCa unit in transgenic cardiac myocytes (P < 0.001). The effect of pH to alter maximum Ca(2+)-activated tension was unchanged by TnC isoforms in these cardiac myocytes. In a reciprocal experiment, contractile sensitivity to acidosis was increased in fast skeletal muscle fibers following extraction of endogenous sTnC and reconstitution with purified cTnC in vitro. Our findings demonstrate that TnC plays an important role in determining the profound sensitivity of cardiac muscle to acidosis and identify cTnC as a target for therapeutic interventions designed to modify ischemia-induced myocardial contractile dysfunction.

Atrial natriuretic factor and transgenic mice

Atrial natriuretic factor (ANF) is a peptide hormone that induces potent but transient hypotensive and natriuretic responses on short-term administration. The role of the hormone in long-term cardiovascular regulation has remained elusive in part because of the temporal limitations of long-term infusion models and the extremely short half-life of the molecule in vivo. To circumvent these temporal limitations, a transgenic mouse model was developed that exhibits lifelong elevated plasma ANF levels. These mice are chronically hypotensive, with arterial pressures averaging 20 to 30 mm Hg less than those observed in nontransgenic siblings. In contrast, no obvious natriuretic or diuretic phenotype was observed in transgenic animals housed in metabolic cages. Thus, the mice adequately compensate for the renal effects but not the hemodynamic effects of the hormone. The ANF transgenic mice provide a tractable model system with which to study the consequences of long-term alterations of ANF expression in vivo.

Differentiation and long-term survival of C2C12 myoblast grafts in heart

We have assessed the ability of skeletal myoblasts to form long-term, differentiated grafts in ventricular myocardium. C2C12 myoblasts were grafted directly into the heart of syngeneic mice. Viable grafts were observed as long as 3 mo after implantation. Immunohistological analyses revealed the presence of differentiated myotubes that stably expressed the skeletal myosin heavy chain isoform. Thymidine uptake studies indicated that virtually all of the grafted skeletal myocytes were withdrawn from the cell cycle by 14 d after grafting. Graft myocytes exhibited ultrastructural characteristics typical of differentiated myotubes. Graft formation and the associated myocardial remodeling did not induce overt cardiac arrhythmia. This study indicates that the myocardium can serve as a stable platform for skeletal myoblast grafts. The long-term survival, differentiated phenotype, and absence of sustained proliferative activity observed in myoblast grafts raise the possibility that similar grafting approaches may be used to replace diseased myocardium. Furthermore, the genetic tractability of myoblasts could provide a useful means for the local delivery of recombinant molecules to the heart.

Cardiopulmonary responses to chronic hypoxia in transgenic mice that overexpress ANP

Elevated plasma atrial natriuretic peptide (ANP) levels have been shown to blunt pulmonary hemodynamic responses to chronic hypoxia, but whether elevated circulating ANP levels negatively feedback on cardiac expression of the ANP gene is unknown. Using a recently developed strain of transgenic mouse (TTR-ANF) that expresses a transthyretin promoter-ANP fusion gene in the liver, we studied the effect of chronically elevated plasma ANP levels on cardiac hypertrophic and pulmonary hemodynamic responses and expression of the endogenous cardiac ANP gene during chronic hypoxia. Plasma ANP levels were 10-fold higher in TTR-ANF mice than in their non-transgenic littermates. After 3 wk of hypobaric hypoxia (0.5 atm), right ventricular hypertrophy and pulmonary hypertension had developed in both groups of mice, but TTR-ANF mice had lower right ventricle-to-left ventricle plus septum weight ratios (0.39 +/- 0.01 vs. 0.45 +/- 0.02), right ventricular systolic pressures (25 +/- 2 vs. 29 +/- 2 mmHg), and lung dry weight-to-body weight ratios (0.48 +/- 0.03 vs. 0.57 +/- 0.01 mg/g) and less muscularization of peripheral pulmonary vessels (8.3 +/- 1.4 vs. 17.4 +/- 2.5%) than nontransgenic controls. Right atrial and ventricular steady-state ANP mRNA levels were the same in both groups of mice under normoxic and hypoxic conditions despite much higher plasma ANP levels and less pulmonary hypertension in TTR-ANF mice. We conclude that chronically elevated plasma ANP levels attenuate the development of hypoxic pulmonary hypertension in mice but do not suppress cardiac expression of the endogenous ANP gene under normoxic conditions nor blunt the upregulation of right ventricular ANP expression during chronic hypoxia.

Residues 2-25 of phospholamban are insufficient to inhibit Ca2+ transport ATPase of cardiac sarcoplasmic reticulum

We have used sarcoplasmic reticulum (SR) vesicles isolated from AT-1 cardiomyocytes to examine the mechanism of phospholamban (PLB) inhibition of the Ca2+ pump of cardiac SR. By immunoblotting with monoclonal antibodies, we observed that these SR vesicles contained a normal amount of the cardiac isoform of the Ca2+ pump (SERCA 2) but only a trace level of PLB. A monoclonal antibody that recognized amino acid residues 9-17 of PLB had no significant effect on Ca2+ transport by AT-1 SR vesicles, but it increased Ca2+ transport into mouse ventricular SR vesicles greater than 10-fold by reversing PLB inhibition of SERCA 2 at low ionized Ca2+ concentration. To further explore the domains of PLB responsible for SERCA 2 inhibition, we examined the effect of a PLB synthetic peptide consisting of amino acid residues 2-25 on Ca2+ uptake by AT-1 SR vesicles. Even at concentrations as high as 0.44 mM, no significant effect of the peptide was observed. Based on these results, we conclude that the cytoplasmic domain of PLB, containing the phosphorylation sites, by itself is insufficient to inhibit the Ca2+ pump and that the transmembrane region, which stabilizes the pentamer, is also essential for Ca2+ transport regulation.

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.

Long-term survival of AT-1 cardiomyocyte grafts in syngeneic myocardium

The long-term viability of cardiomyocyte grafts in the adult myocardium was tested. AT-1 cardiomyocytes, a differentiated tumor line derived from transgenic mice expressing an atrial natriuretic factor-simian virus 40 T antigen fusion gene, were grafted directly into the myocardium of syngeneic animals. Viable grafts were detected as long as 4 mo postimplantation. Thymidine uptake studies suggested that the grafted cardiomyocytes retained mitotic activity. The presence of AT-1 cardiomyocyte grafts and the associated myocardial remodeling were not accompanied by overt cardiac arrhythmia. Electron microscopic analyses showed that the majority of the grafts were juxtaposed directly to the host myocardium and were not encapsulated. This study indicates that the myocardium can serve as a stable platform for cells that have been manipulated in vitro and suggests that cardiomyocyte grafts may provide a useful means for the local delivery of recombinant molecules to the heart. The long-term survival of the AT-1 cardiomyocytes in the heart also raises the possibility that similar grafting approaches may be used to replace diseased myocardium.

Effect of potassium infusion on renal function in ANF-transgenic mice

Transgenic mice expressing an ANF fusion gene in the liver were used to study renal function before and during an intravenous KCl load. These animals are characterized by a 10- to 20-fold elevation in plasma ANF concentration, and by a reduction in arterial blood pressure by 20-30 mm Hg, compared to nontransgenic littermates. Before the KCl infusion, renal excretions of fluid, sodium, potassium, and chloride were not different from corresponding values in the nontransgenic sibling mice. Glomerular filtration rates were slightly lower in the transgenic animals. During the KCl infusion, diuresis, saluresis, and kaliuresis were found in both groups. However, salt and water excretion, but not potassium excretion, were significantly greater in the transgenic group. In a separate series, plasma aldosterone concentrations were significantly higher in the transgenic, compared to the nontransgenic mice. These data are interpreted as indicating that antinatriuretic mechanisms, including aldosterone-dependent sodium reabsorption in the cortical collecting tubule, can counteract the effect of ANF to inhibit sodium reabsorption in the medullary duct system, thus allowing maintenance of salt balance. Furthermore, a reduced tubular flow rate at the aldosterone-sensitive site would ensure normal potassium excretion despite the elevated mineralocorticoid level. During KCl infusion, the known increase in tubular delivery of salt and water to the duct would allow full expression of the downstream ANF effect, accounting for the relatively greater diuresis and saluresis in the transgenic mice. We conclude that both renal and adrenal actions of ANF can be rendered ineffective by countervailing mechanisms, suggesting an explanation for the apparent lack of biological activity of endogenously elevated plasma NAF in some disease states.

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.