Published online before print August 2, 2001, doi: 10.1161/hh1601.095912
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© 2001 American Heart Association, Inc.
Integrative Physiology |
Targeted 
1A-Adrenergic Receptor Overexpression Induces Enhanced Cardiac Contractility but not Hypertrophy
From the Victor Chang Cardiac Research Institute (F.L., W.A.O., S.C., S.K., M.P.F., R.M.G.) and Cardiology Department (S.K., M.P.F., R.M.G.), St Vincent’s Hospital, Darlinghurst, New South Wales, Australia; Faculty of Medicine (M.P.F., R.M.G.), The University of New South Wales, Kensington, New South Wales, Australia; Bayer Corp (M.E.S.), Berkeley, Calif; and Baker Medical Research Institute (J.F.A., E.A.W.), Prahran, Victoria, Australia.
Correspondence to Robert M. Graham, Molecular Cardiology Unit, Victor Chang Cardiac Research Institute, 384 Victoria St, Darlinghurst 2010, New South Wales, Australia. E-mail b.graham{at}victorchang.unsw.edu.au
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Abstract |
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Activation of the
1A-adrenergic receptor (1A-AR)/Gq pathway has been implicated as a critical trigger for the development of cardiac hypertrophy. However, direct evidence from in vivo studies is still lacking. To address this issue, transgenic mice with cardiac-targeted overexpression of the 1A-AR (4- to 170-fold) were generated, using the rodent -myosin heavy chain promoter. Heterozygous animals displayed marked enhancement of cardiac contractility, evident from increases in dP/dtmax (80%, P<0.0001), dP/dtmax/LVPinst (76%, P<0.001), dP/dtmax:dP/dtmin (104%, P<0.0001), and fractional shortening (33%, P<0.05). Moreover, changes in the dP/dtmax–end-diastolic volume relationship provided load-independent evidence of a primary increase in contractility. Blood pressure and heart rate were largely unchanged, and there was a small increase in (-)norepinephrine-stimulated, but not basal, phospholipase C activity. Increased contractility was directly related to the level of receptor overexpression and could be completely reversed by acute 1A- but not ß-AR blockade. Despite the robust changes in contractility, transgenic animals displayed no morphological, histological, or echocardiographic evidence of left ventricular hypertrophy. In addition, apart from an increase in atrial natriuretic factor mRNA, expression of other hypertrophy-associated genes was unchanged. To our knowledge, these data provide the first in vivo evidence for an inotropic action of the 1A-AR.
Key Words: cardiac hemodynamics • phospholipase C activity • dP/dt • receptor signaling
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Introduction |
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Sympathetic control of cardiac function is mediated predominately by ß1-adrenergic receptors (ß1-ARs) acting via Gs and the adenylyl cyclase–protein kinase A pathway. Additionally, the heart contains
1-ARs that may also mediate increases in contractility, as well as changes in the electrophysiological properties and metabolic responses of the myocardium.1 All three 1-AR subtypes (1A, 1B, and 1D) have been identified in heart and isolated cardiomyocytes at the mRNA level, although only 1A- and 1B-ARs appear to be expressed as functional proteins in a variety of mammalian species ranging from the mouse to humans.1–3
Acutely, cardiac 1-AR activation results in polyphosphoinositide hydrolysis that is predominantly Gq-mediated and induces positive inotropic effects due to either an increase in cytosolic free Ca2+ levels or to increased responsiveness of the myofilaments to Ca2+.1,3 Cardiac 1-AR stimulation also leads to activation of the Na+-H+ exchanger and, as a result, activation of the Na+-Ca2+ exchanger, as well as Na+,K+-ATPase activation.1,3 Responses to 1-AR activation and expression of the various subtypes are developmentally regulated and show marked species and regional myocardial differences. Nonetheless, there is currently little evidence that cardiac 1-ARs play a major functional role under normal physiological conditions.1 In pathological settings, cardiac 1-ARs may function in a compensatory fashion to maintain cardiac inotropy when the ß-AR system is downregulated and uncoupled.3 On the other hand, cardiac 1-ARs, particularly of the 1A subtype, have also been implicated as primary mediators of cardiac hypertrophy and malignant arrhythmias.4–7 Thus, chronic exposure of cardiomyocytes to 1-AR agonists result in hypertrophy and is associated with a specific pattern of gene expression, most notably the induction of the ventricular-embryonic atrial natriuretic factor (ANF) gene, and the constitutively expressed myosin light chain-2 (MLC-2) gene, as well as the accumulation and assembly of contractile proteins into sarcomeric units.1,4 The signaling pathways mediating hypertrophic responses to 1-AR stimulation have not been fully elucidated. However, ANF gene expression and the morphological features of hypertrophy are generally thought to require Gq-mediated phospholipase C (PLC) activation.4 Not surprisingly, therefore, targeted overexpression of Gq in the myocardium results in marked induction of hypertrophic gene markers as well as contractile impairment, cardiac decompensation, and biventricular failure.8 Others have shown that the 1-AR hypertrophic response involves activation of members of the mitogen-activated protein kinase superfamily, as well as the PI-3-kinase/mTOR/p70S6k pathway.9,10
Although both the 1A and 1B subtypes have been reported to induce myocyte growth and hypertrophy, most studies favor the 1A-AR as the dominant subtype.1,4–6 This conclusion, however, is based largely on studies of rat heart or neonatal rat cardiomyocytes, a species that shows the most abundant expression of myocardial 1-AR (amounting to 50% of the total adrenergic receptor pool) and robust 1-AR–mediated inotropic responses.1,3 By contrast, in human myocardium, 1-ARs account for only 
15% to 20% of the total adrenergic receptor pool.1 Interestingly, cardiac-targeted overexpression of either the wild-type 1B-AR or a constitutively active mutant, in the mouse, a species that like humans shows very low levels of cardiac 1-AR expression,3 produces little, if any, hypertrophy, and either no change in contractility or impairment, rather than enhanced cardiac function,7,11–13 an effect due to promiscuous interaction with Gi and inhibition of the adenylyl cyclase pathway and to upregulation of G protein receptor–coupled kinase-2.12 Given the lack of in vivo data on the role of cardiac 1A-ARs, we elected in the present study to develop a mouse line with targeted overexpression of this 1 subtype. Based on studies of five independent lines thus developed, we report that cardiac overexpression of the 1A-AR results neither in cardiac hypertrophy nor in cardiac impairment. Rather, these animals demonstrate a marked increases in cardiac contractility that is directly related to the level of receptor overexpression and can be completely abolished by acute 1A-AR blockade.
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Materials and Methods |
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Generation of Transgenic Mice
Transgenic mice expressing the wild-type rat
1A-AR under the control of the rat -myosin heavy chain (MHC) promoter were generated using standard methodologies. Five transgenic lines were established and bred with FVB/N. Litter sizes and postnatal development were indistinguishable from those of wild-type animals. Heterozygous animals from at least the third generation, and at the ages indicated, were used for all studies, with their nontransgenic littermates (NTLs) serving as controls. All studies were performed with approval of the Lawrence Berkeley National Laboratory and St Vincent’s Hospital Institutional Review Board/Animal Ethics Committee. Supplementary information regarding the generation of the transgenic mice, details of the methodologies involved in their biochemical, morphometric and histological characterization, and details of other methodologies is available at www.circresaha.org.
Echocardiography
Studies were performed on mice anesthetized with 2.5% avertin (0.01 mL/g) (Aldrich Chemical Company, Inc), using a Hewlett-Packard Sonos 5500 ultrasonograph equipped with a 12-MHz phased-array transducer.
Hemodynamic Studies
Hemodynamics were determined by micromanometry using a 1.4F pressure transducer in mice anesthetized with xylazine (20 mg/kg) and ketamine (100 mg/kg). dP/dtmax–end-diastolic volume relationships14 were determined in additional mice (n=6 per group) by sonomicrometry during inferior vena caval occlusion.
Statistical Analysis
Data are presented as mean±1 SEM. The number of mice used is indicated. All hemodynamic, echocardiographic, and ECG recordings and subsequent data analyses were performed without knowledge of the mouse genotype. Statistical analyses were performed using paired and unpaired Student’s t tests and ANOVA. Comparison of the dP/dtmax–end-diastolic volume relationships was made by multiple linear regression analysis.15 Receptor binding data were analyzed using the curve-fitting program PRISM. For comparisons of data fit to a one- or two-site model, the F test was used. In all analyses, a value of P<0.05 was considered significant.
An expanded Materials and Methods section can be found in the online data supplement available at http://www.circresaha.org.
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Results |
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Production of Transgenic Mice
Transgenic mice were generated using the rat
-MHC promoter to allow cardiac-restricted 1A-AR overexpression. The transgene was detected by polymerase chain reaction (PCR) and Southern blot analysis (data not shown). Five founder mice, designated A1A1 to A1A5, produced transgenic offspring at the expected mendelian frequency. Copy number in the different transgenic lines ranged from 5 to 70.
Transgene Expression
Expression (Northern blot analysis) was abundant in heart but absent in lung, liver, spleen, kidney, and brain (data not shown). Overexpression of 1A-AR protein was confirmed by Western blotting using a monoclonal antibody against a 1D4 C-terminal epitope (Figure 1A). A single 68-kDa species was detected in the cardiac membranes of all transgenic lines but not in those of their NTLs (Figure 1B), indicating that the receptor protein underwent appropriate posttranslational modification and was fully glycosylated. This was further demonstrated by immunofluorescence staining of ventricular cryosections using a polyclonal antibody against the endogenous C-tail of the 1A-AR. Labeling was clearly evident throughout the ventricles of transgenic mice but not in the ventricles of the NTLs, where the expression level of the endogenous 1A-AR is too low to be detected. In the transgenic animals, only surface cardiomyocyte labeling was evident (Figure 1C), confirming that the transgenic receptor protein was expressed appropriately on the cell membrane.
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By radioligand binding assays analysis, the NTLs expressed on average 17±0.5 fmol/mg protein (n=10) of endogenous 1-AR. These levels are similar to those reported by others3,16 in the mouse (5 to 18 fmol/mg), of which 25% are of the 1A and 75% of the 1B subtype,16 and in humans (12 fmol/mg), but are markedly lower than those found in rat cardiac membranes (90 fmol/mg).3 In transgenic cardiac membranes, 1A-AR expression was increased by 170- and 148-fold to 2976±30 and 1958±230 fmol/mg protein in the A1A1 and A1A4 lines, respectively. The remaining transgenic lines, A1A2, A1A3, and A1A5, demonstrated 66-, 112- and 4-fold receptor overexpression, respectively (Figure 1D). Competition radioligand binding studies were also used to show that the cardiac 1-ARs in the transgenic animals were predominantly of the 1A subtype (see the Table in the online data supplement available at www.circresaha.org).
Receptor-Mediated PLC Activation
As shown in Figure 2A, basal total [3H]inositol phosphates were not significantly different in left ventricular (LV) strips from the transgenic animals and their NTLs. In the transgenic tissues, (-)norepinephrine stimulation resulted in a small but significant increase in total [3H]inositol phosphates, to a level significantly above the basal value (P<0.05) and also significantly above that observed with (-)norepinephrine stimulation of the NTL tissue (P<0.05). The increases in total [3H]inositol phosphates with (-)norepinephrine stimulation were due to increases in the 1- (or 3-) and 4-isomers of inositol monophosphate, with smaller increases in inositol-1,4-bisphosphate. However, inositol-1,4,5-trisphosphate levels were unchanged (Figure 2A).
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Hypertrophy Assessment
As shown in Table 1, although transgenic animals of the A1A1 line and their NTL controls showed the expected age-related increases (by 10% to 20%) in body weight, cardiac biventricular weight, and tibial length, neither these parameters nor the ratios of biventricular weight to body weight or biventricular weight to tibial length differed between the transgenic animals and their NTLs. In agreement with these gross morphology findings, cardiomyocyte cross-sectional area was also not significantly different in the transgenic animals as compared with their NTL controls (Table 1).
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Consistent with the known regulation of ANF transcription by a phenylephrine (ie, 1-AR) response element,17 compared with the NTLs, ANF mRNA expression was increased by 7.3- and 4.2-fold in the A1A1 and A1A4 lines, which have 170- and 148-fold 1A-AR overexpression, respectively, but not in the A1A5 line, which overexpresses the 1A-AR by only 4-fold (Figure 3). However, expression of other hypertrophy-associated genes, including MLC-2v, - and ß-MHC, SERCA2a, and -skeletal actin, as well as phospholamban (PLB), was unaltered (Figure 3).
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Echocardiographic and Hemodynamic Assessments
Echocardiographic evaluation of the A1A1 1A transgenics showed enhanced LV emptying observed in the 2D mode and virtual apposition of the LV posterior and anterior walls in the M-mode images (Figure 4B). Quantitatively, this was evidenced by a 33% increase (P<0.05) in fractional shortening, which was associated with a 72% decrease in LV internal dimension at end-systole and a lesser (8%) decrease in LV internal dimension at end-diastole (Table 2). Because dP/dtmax should decrease as a function of reduced LV end-diastolic volume,14 the fact that dP/dtmax increased in the transgenic animals (Table 3), despite a reduction in end-diastolic LV chamber dimension, indicates enhancement of contractility in these animals. This was further demonstrated by a significant leftward/upward shift (P<0.001) of the dP/dtmax–end-diastolic volume relationship in A1A1 transgenic animals (slope 252 792±41 207 mm Hg · s-1 · mL-1; intercept 0.012±0.004 mL; r2=0.93±0.02) as compared with their NTLs (slope 137 059±37 012 mm Hg · s-1 · mL-1; intercept 0.03±0.01 mL; r2=0.94±0.02). Cardiac output was not significantly different in the 1A transgenic animals. The finding of an increased ratio of wall thickness to chamber radius (h/r) and an increased ratio of LV mass to LV internal dimension at end-diastole, but no change in LV mass, is compatible with the reduced LV chamber size being due to markedly enhanced adrenergic drive in the 1A transgenics (Table 2).
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By micromanometry, neither systolic nor diastolic aortic pressure was altered in the 1A transgenics, whereas heart rate was 16% higher in the A1A1 line but not in the A1A5 line (Table 3). Consistent with enhanced LV fractional shortening on echocardiography, enhanced contractility was demonstrated by increased LV dP/dtmax and by increased dP/dtmax normalized for instantaneous pressure (by 80% and 76%, respectively, P<0.001) in the A1A1 1A transgenics (Figure 4A) but not in the A1A5 line. LV dP/dtmin was not significantly different from that in the NTLs in either line. As a result, the ratio of dP/dtmax to dP/dtmin was doubled from 0.93 in the NTLs to 1.9 in the 1A transgenics with 170-fold receptor overexpression (Table 3). Similar increases in LV dP/dtmax and the ratio of dP/dtmax to dP/dtmin were observed in the A1A2, A1A3, and A1A4 transgenics (data not shown). Although absolute LV dP/dtmin values were unaltered, the finding that the time constant of isovolumic relaxation (
) was prolonged by 40% in the A1A1 line indicates impaired relaxation (Table 3). LV end-diastolic pressure was significantly lower in the A1A1 transgenics, consistent with echocardiographic evidence of a lower end-diastolic volume in this line.
Mechanism of Enhanced Contractility
We next investigated the possibility that the hypercontractile phenotype of the 1A transgenics was due to promiscuous interaction with the cardiac ß-AR/Gs/adenylyl cyclase pathway. These studies revealed that ß-AR density (56.5±0.5 fmol/mg, 1A transgenics; 58.7±2.4 fmol/mg, NTLs) and both basal and maximal (-)isoproterenol-stimulated cAMP generation (Figure 2B) were unaltered in the 1A transgenics. Also, activation of LV strips with the 1-agonist phenylephrine (in the presence of the ß-antagonist (±) propranolol) failed to increase cAMP generation in either the 1A transgenics or their NTLs (Figure 2B). These in vitro findings were also confirmed in in vivo studies, in which the ß-AR agonist (-)isoproterenol was administered to 2-month-old animals of the A1A1 line, both before and after complete ß-AR blockade with (±)propranolol. As expected, administration of isoproterenol to NTL controls produced a marked increase in dP/dtmax from 8037±805 to 14 424±842 mm Hg/s (P<0.0001) and a slight increase in dP/dtmin from -7721±568 to -9086±755 mm Hg/s (P<0.05), effects that were completely inhibited by (±)propranolol (Figure 5A). By contrast, ß-AR stimulation was unable to increase the already enhanced contractility of the 1A-AR transgenics, and both dP/dtmax and dP/dtmin remained unaltered in these animals with induction of ß-blockade (Figure 5A).
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and TG animals (
). *P<0.001, NTL versus TG.
The specificity of the 1A-transgenic cardiac phenotype and its dependence on 1A-AR signaling was evaluated by producing selective 1A-AR blockade with KMD-3213. As shown in Figure 5B, infusion of KMD-3213 to 1A transgenics caused a gradual decline in dP/dtmax over 3 minutes, to values not significantly different from those in NTLs. By contrast, neither cardiac contractility (Figure 5B) nor blood pressure or heart rate (data not shown) was significantly altered with administration of KMD-3213 to the NTL controls.
Finally, we also evaluated if expression of various receptor-coupled effectors or contractile proteins was altered in the 1A transgenics. As shown in Figure 3C, neither total PLB expression nor that of two phosphorylated PLB species was altered in the 1A transgenics. Similarly, expression of SERCA2a, troponin I, MLC, the Na+-Ca2+ exchanger, and NHE1 was also unchanged.
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Discussion |
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The novelty of this work lies in the following. First, to our knowledge, this is the first study to specifically show an inotropic action of the
1A-AR in vivo. Second, the dissociation of inotropy from hypertrophy indicates that these two receptor-coupled responses are mediated by distinct signaling pathways. Third, it demonstrates that despite the 1A- and 1B-ARs both being Gq-coupled, their downstream-activated pathways differ. This is evident from the finding that only cardiac-targeted overexpression of the 1A-AR results in enhanced contractility. Fourth, it demonstrates that receptor-linked PLC activation, per se, does not mediate enhanced cardiac contractility. Thus, receptor-linked inotropy and PLC activation are clearly uncoupled in the 1A- versus 1B-AR overexpressors. Fifth, it questions the role of the cardiac 1A-AR as a mediator of hypertrophy, a conclusion based entirely on in vivo studies of the rat and in vitro studies of rat cardiac tissue and cardiomyocytes.1,3–6 Sixth, as a corollary, and given that human myocardium, like that of the mouse, but unlike that of the rat, expresses only very low levels of 1-ARs, it also questions the relevance of the rat as a model of human cardiac hypertrophy.
Previous studies of cardiac-targeted 1-AR-overexpression, involving the wild-type 1B subtype or a constitutively active mutant, demonstrated either no change or impairment of cardiac contractility.11–13,18 In contrast, as shown here, 1A-AR overexpression markedly enhances contractility. This was evidenced by (1) increased echocardiographic fractional shortening with a decrease in LV dimension at both end-diastole and end-systole; (2) increased dP/dtmax, which could not be further augmented by ß-AR stimulation, and increased dP/dtmax normalized for instantaneous LV pressure despite decreased baseline end-diastolic volume; and (3) a significant leftward/upward shift of the relationship between dP/dtmax and end-diastolic volume, a load-independent index of contractility.14
Although dP/dtmax was markedly increased in the 1A-transgenic animals, dP/dtmin, a measure of cardiac relaxation, was unchanged. As a result, the ratio of dP/dtmax to dP/dtmin doubled in the transgenic animals, a finding not observed in 1B-AR, PLB knockout, or ß2-AR models of enhanced cardiac contractility.13,19,20 In contrast, in animals with genetically induced hypertrophic cardiomyopathy, an increase in this ratio has been noted,21 suggesting a relative disparity between contraction and relaxation phenomena. In keeping with this interpretation, the time constant of isovolumic relaxation () was prolonged in the 1A transgenics. Further studies in older animals are currently being undertaken to explore the mechanism of this altered relaxation and to ascertain if, long term, it results in cardiac dysfunction. It is of interest, however, that preliminary hemodynamic studies of 12-month-old 1A transgenics of the A1A1 and A1A4 lines demonstrate persistence of both the hypercontractile phenotype and lack of hypertrophy.
Several lines of evidence indicate that the 1A-transgenic phenotype is due to receptor overexpression and not to indirect effects, resulting, for example, from promiscuous G protein or effector coupling or to crosstalk with other signaling pathways. Thus, the enhanced contractility is directly related to overexpression of the 1A-AR and can be eliminated by acute 1A-specific receptor blockade. Moreover, the enhanced contractility is not associated with a change in ß-AR density or either basal or (-) isoproterenol-stimulated adenylyl cyclase activity and is unaltered by maximal ß-AR blockade. Because ß-AR stimulation increases both the rate of cardiac contraction and relaxation, the finding that dP/dtmin is not increased in the 1A transgenics also argues against crosstalk with ß-ARs in this model. Finally, in contrast to ß-AR activation, stimulation with the 1-agonist phenylephrine does not increase cAMP generation (Figure 3B).
The 1A-AR–coupled signaling pathway mediating increased contractility in the transgenic animals is currently unclear, given that expression of a variety of receptor-coupled effectors and contractile proteins is unaltered in the 1A transgenics (Figure 3). Consideration of this question must take into account the unchanged or impaired contractility observed in mice with cardiac-targeted overexpression of the wild-type 1B-AR or a constitutively active mutant, despite both an increase in basal PLC activity7,11,12 and a more marked increase in (-)norepinephrine-stimulated activity (230% increase in total inositol phosphates),7 than that observed in the 1A transgenics (+101%). Moreover, neither the 1A (Figure 2) nor the 1B overexpressors7 showed changes in inositol-1,4,5-trisphoshate (IP3). Thus, it is unlikely that inositol phosphates are primarily responsible for the contractile phenotype observed in the 1A transgenics. In any case, concentrations of cardiac IP3 receptors are low and are not localized primarily to Ca2+ stores, responses to IP3 are slow and weak, and the Ca2+ so generated does not contribute to Ca2+-induced Ca2+ release.7 Thus, IP3 liberated by 1-AR activation of PLC is unlikely to be an important regulator of intracellular Ca2+ concentration under physiological conditions.
Other possible candidates for the increased contractility include the transient outward K+ current (Ito) and the Na+-H+ exchanger, which are inhibited and activated, respectively, by 1-AR–stimulated protein kinase C (PKC) activation.1 Suppression of Ito prolongs action potential duration and increases cystolic calcium transients, whereas activation of the Na+-H+ exchanger results in cystolic alkalization and sensitization of the myofilaments to intracellular Ca2+. However, the activator of PKC, sn-1,2-diacylglycerol (DAG), is generated in equimolar amounts to inositol phosphates as a result of PLC-mediated cleavage of inositol phospholipids. Thus, DAG release and consequent PKC activation would be expected to be lower for the 1A than the 1B transgenics. Subtype-specific differences in receptor signaling may, therefore, depend on activation of different PKC isoforms by the two 1-ARs.22 It is also of note in this regard, that in addition to interaction with the Gq/PLCß pathway, the 1B but not the 1A-AR couples via the high-molecular-weight G protein Gh to PLC
.23
In addition to activation of the Na+-H+ exchanger, phosphorylation of regulatory myosin light chains (RLCs) also results in an increased rate of force generation due to sensitization of the myofilaments to Ca2+, as well as enhanced myofibrillar organization,24–26 and transgenic mice in which the wild-type RLCs are replaced by a mutant protein that is resistant to phosphorylation develop biventricular dilation and a decrease in dobutamine-stimulated dP/dtmax.24 Phosphorylation of RLCs is catalyzed by myosin light chain kinase, Rho kinase, and PKC. Because these enzymes are all activated by 1-AR stimulation,24,25 enhanced RLC phosphorylation may contribute to the hypercontractile phenotype observed in the 1A transgenics.
Despite compelling evidence to implicate the 1A-AR/Gq pathway in cardiac hypertrophy and hypertrophic gene induction,1,3–6 the 1A transgenics failed to develop hypertrophy. It is also of interest in this regard, that studies both in cultured rat cardiomyocytes and in the intact rat5,6 indicate that expression of the 1A-AR, but not 1B-AR or 1D-AR, is itself increased by 1-AR and other hypertrophic stimuli, including aortic banding.5 Failure of the 1A transgenics to develop hypertrophy is, thus, quite surprising and contrasts with the marked hypertrophy and remodeling observed in mice with cardiac overexpression of Gq8 or other Gq-coupled receptors, such as the angiotensin II type I receptor.27 To resolve this paradox, species differences in cardiac 1A-AR signaling, in the transcriptional regulation of the 1A-AR or both, must be invoked. Interestingly, it has been reported recently that expression of the mouse 1A-AR does not increase with hypertrophy in vivo,18,28 indicating very different regulation of 1A gene expression in mouse, as compared with rat, cardiomyocytes. Nevertheless, in the present study, given that expression of the 1A transgene was sustained because it was under the control of a heterologous promoter, and that enhanced 1A-AR signaling persisted, altered transcriptional regulation of the mouse 1A-AR gene cannot account for the failure to induce hypertrophy.
In keeping with the lack of hypertrophy observed here in vivo in adult 1A-transgenic animals, Deng et al29 have also found that neonatal mouse cardiomyocytes are resistant to 1-AR–stimulated hypertrophy, despite displaying intact ß-AR–mediated and T3-activated hypertrophic responses. Thus, our studies, as well as those of others, indicated that cardiac 1A-AR–coupled hypertrophic pathways present in the rat may not be present in the mouse. Nonetheless, the finding of enhanced cardiac contractility and increased ANF mRNA expression indicate that 1A-AR–coupled signaling is intact in the mouse and, importantly, that the pathway mediating receptor-coupled inotropic responses is preserved.
In summary, the cardiac 1A transgenic model developed in the present study questions the role of the 1A-AR as a mediator of cardiac hypertrophy while demonstrating mediation of an unexpectedly robust increase in contractility. This potentially provides unique opportunities for understanding biochemical pathways relevant to these two responses, pathways that may also be important for the regulation of cardiac function in humans.
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Acknowledgments |
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This work was supported in part by grants (9938388, 970982, and 980199) from the National Health and Medical Research Council of Australia (R.M.G.), a Postgraduate Medical Research Scholarship (PM97S0254) from the National Heart Foundation, Australia (F.L.), and a Royal Australasian College of Surgeons/Royal College of Surgeons of England Research Exchange Fellowship and National Heart Foundation, Australia Postgraduate Medical Research Scholarship (W.A.O.).
Received December 15, 2000; accepted July 12, 2001.
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References |
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