Evidence from several rodent models has suggested that a reduction of either atrial natriuretic peptide or its receptor in the heart affects cardiac remodeling by promoting the onset of cardiac hypertrophy. The atrial natriuretic peptide receptor mediates signaling at least in part via the generation of intracellular cyclic GMP. To directly test whether accumulation of intracellular cyclic GMP conveys protection against cardiac hypertrophy, we engineered transgenic mice that overexpress a catalytic fragment of constitutively active guanylate cyclase domain of the atrial natriuretic peptide receptor in a cardiomyocyte-specific manner. Expression of the transgene increased the intracellular concentration of cyclic GMP specifically within cardiomyocytes and had no detectable effect on cardiac performance under basal conditions. However, expression of the transgene attenuated the effects of the pharmacologic hypertrophic agent isoproterenol on cardiac wall thickness and prevented the onset of the fetal gene expression program normally associated with cardiac hypertrophy. Likewise, expression of the transgene inhibited the hypertrophic effects of abdominal aortic constriction, since it abolished its effects on ventricular wall thickness and greatly attenuated its effects on cardiomyocyte size. Altogether, our results suggest that cyclic GMP is a cardioprotective agent against hypertrophy that acts via a direct local effect on cardiomyocytes. Left ventricular hypertrophy (LVH)1 results from the activation of multiple signaling pathways by either mechanical or neurohumoral stimuli (1, 2). A great number of studies have used animal models of transgenesis or gene inactivation to test the possible roles of these pathways in the induction of LVH in vivo. However, the contributions of possible negative regulators of LVH have so far received much less attention. Recently, several lines of evidence have suggested that atrial natriuretic peptide (ANP) and endothelial nitric-oxide synthase may act as such negative regulators, since 1) blood-pressure-independent LVH is present in mice with either general (3) or cardiomyocyte-restricted (4) inactivation of natriuretic peptide receptor A (NPRA); 2) cardiomyocyte-specific expression of NPRA partially rescues the cardiac hypertrophic phenotype seen in NPRA-null mice (5); 3) we have shown that cardiac mass and ventricular expression of ANP were both associated (in an inverse fashion) with a naturally occurring allele of natriuretic peptide precursor A (i.e. the gene that codes for ANP) (6 -8); and 4) overexpression of an endothelial nitric-oxide synthase transgene attenuates isoproterenol-induced LVH (9). The common denominator between ANP and endothelial nitric-oxide synthase is that many (if not most) of their biologic effects are mediated by cGMP (10, 11). Interestingly, it has also been shown that cGMP protects cultured neonatal cardiomyocytes against the effects of hypertrophic agents in vitro (12-14). However, it remains to be proven that cGMP has similar actions in vivo.To add...

Abstract-Cardiac left ventricular hypertrophy (LVH) is commonly associated with hypertension, but its variance is determined for more than 50% by blood pressure-independent genetic factors. Because it constitutes one of the most important risk factors for cardiovascular mortality, we have performed a genome-wide scan of the F2 progeny of crosses between inbred WKY and WKHA rats to detect quantitative trait loci (QTL) linked to cardiac mass. In addition to left ventricular mass (LVM), we also measured left ventricle (LV) concentration of atrial natriuretic factor (ANF), because we have previously established that there was a genetic link between these 2 traits in the same animal cross. We found 2 contiguous QTL on chromosome 5 that were linked to either LVM (logarithm of odds [LOD]ϭ3.5) or log n (LV ANF) (LODϭ12). The 1-LOD support intervals of both QTL shared a region overlapping the locus of natriuretic peptide precursor A (Nppa) (ie, the ANF-coding gene). We found by sequencing 2 single nucleotide polymorphisms (SNPs) within the first 650 bp of the Nppa minimal promoters of the genes from both strains. One of these SNPs increased the transcriptional activity of the Nppa minimal promoter in transfected neonatal cardiomyocytes in keeping with the higher LV concentration of ANF observed in WKY versus WKHA rats. Key Words: left ventricular hypertrophy Ⅲ genetics Ⅲ atrial natriuretic factor Ⅲ natriuretic peptide precursor A gene Ⅲ quantitative trait locus C ardiovascular diseases are the principal cause of mortality and morbidity in industrialized countries. In recent years, left ventricular hypertrophy (LVH) has emerged as a powerful independent risk factor for cardiovascular mortality and morbidity. 1,2 In humans, cardiac mass may be determined in part by environmental factors (including diet habits and level of physical activity) 3 as well as underlying disease state (including diabetes, coronary insufficiency, renal failure, and increased cardiac workload). 2,4,5 However, the sum of all these factors can account for only a small fraction of the variance of cardiac mass, 6 a fact suggesting the existence of heritable genes that modify cardiac mass independently of the other factors. Accordingly, studies performed with 254 pairs of twins of Caucasian origin have estimated that genetic factors accounted for 63% of the total variance in boys and 71% in girls. 7 Likewise, a study using 23 inbred strains of rats and crosses between them has estimated that the portion of the variance of cardiac mass that was genetically determined ranged from 45% to 65%. 8 In humans, the distribution of cardiac mass is mostly continuous within populations of unselected normotensive or hypertensive patients, 9 which defines this variable as a complex quantitative trait. Such traits typically result from the action of multiple genes that each have modest effects on their own and thus are difficult to detect by linkage analysis in human populations. 10 One experimental alternative is to perform crosses of inbred (ie, genetically pure) an...

Abstract-Most cardiovascular traits of interest can be defined as "complex traits," with the first step in the identification of genetic factors affecting such traits being the detection of quantitative trait loci (QTLs). Animal models have proven particularly useful in this regard. However, only very few of the QTLs identified to date have led to the identification of candidate genes. We describe an example of our own work where the combination of anatomical and a biochemical intermediate phenotypes have led to the identification of the natriuretic peptide precursor A (Nppa) gene as a candidate gene for left ventricular hypertrophy (LVH). Combined with the power of comparative genetics, these strategies will continue to improve the chances of finding candidate genes for cardiovascular traits such as susceptibility to heart diseases, hypertension, and hypertension-induced end-organ damage. ''C omplex traits" can be defined as traits in which (unlike Mendelian traits) there is not a simple one-to-one relationship between genotype and phenotype. 1 Although the identification of gene mutations linked to Mendelian traits is still a challenging task, it is a relatively achievable goal and depends mostly on the availability of informative pedigrees. Thus, as of September 2001, more than 900 syndromes have been entered in the "Online Mendelian Inheritance in Man" (OMIM) database as being linked to gene mutations (http://www.ncbi.nlm.nih. gov/Omim/). However, most cardiovascular traits of interest, including susceptibility to heart diseases, hypertension, and hypertension-induced end-organ damage, can be defined as complex traits. Genetic studies of multifactorial disorders in human populations remain challenging because of the multiplicity of genes underlying complex phenotypes, the modesty of the effect of each gene, and the heterogeneity that occurs within human populations. 2 Investigators have therefore relied on alternative strategies. In particular, efforts have been made to identify quantitative trait loci (QTLs) in crosses of inbred animals (mostly mice or rats), because alleles in the progeny of such crosses originate from only 2 possible sources (corresponding to the 2 parental strains), and a large numbers of chromosomal markers have become available for these models.Recently, a reanalysis of linkage data obtained in 7 sets of intercrosses between 5 different inbred rat strains identified 67 QTLs for 39 blood pressure-related phenotypes. 2 These QTLs clustered in 15 independent genomic regions (on chromosomes 1 to 15), covering a distance of 567 centimorgan (cM), which corresponds to Ϸ30% of the rat genome. These regions might not include all existing QTLs, because other ones have also been identified on chromosome 16 to 20, as well as on sex chromosomes. 3 Positional cloning requires a QTL to be mapped to a region as small as 1 cM 4 (equivalent to roughly 1 million bases in rodents), so there is still a long way to go between identification of a QTL and identification of a gene. In fact, a critical examinat...

In vivo, the normal heart obtains at least 60% of its energy from lipids and the remainder from glucose. Several lines of evidence indicate that an increase in the utilization of glucose [at the expense of fatty acids (FA)] may play a role in the genesis of hypertrophy. Primary cultures of neonatal cardiomyocytes have been used extensively to study the phenotype of these cells as well as their responses to hormonal hypertrophic agents. Unfortunately, such cultures are most typically cultured in glucose-rich FA-free media, and thus might be hypertrophied to start with. We therefore tested the effects of FAalbumin complexes on three different surrogate end points of hypertrophy of cardiomyocytes. Oleate-albumin complexes decreased the baseline values of all three variables, and increased the relative response of these variables to administration of norepinephrine. Oleate:palmitate-albumin complexes also affected all three variables and their responses to norepinephrine, but the effects differed somewhat from that of oleatealbumin complexes.Our results suggest that addition of long-chain FA, by providing conditions that more closely resemble physiological situations, may optimize the expression of hypertrophic responses in such cells. However, the differences between the effects of oleate and oleate:palmitate also suggest that the precise composition of FA may affect the phenotype of cardiomyocytes and how these cells respond to hypertrophic agents. -Zahabi, A., and C. F. Deschepper. Long-chain fatty acids modify hypertrophic responses of cultured primary neonatal cardiomyocytes.