Atrial natriuretic factor and transgenic mice

Molecular and genetic aspects of guanylyl cyclase natriuretic peptide receptor-A in regulation of blood pressure and renal function

Natriuretic peptides (NPs) exert diverse effects on several biological and physiological systems, such as kidney function, neural and endocrine signaling, energy metabolism, and cardiovascular function, playing pivotal roles in the regulation of blood pressure (BP) and cardiac and vascular homeostasis. NPs are collectively known as anti-hypertensive hormones and their main functions are directed toward eliciting natriuretic/diuretic, vasorelaxant, anti-proliferative, anti-inflammatory, and anti-hypertrophic effects, thereby, regulating the fluid volume, BP, and renal and cardiovascular conditions. Interactions of NPs with their cognate receptors display a central role in all aspects of cellular, biochemical, and molecular mechanisms that govern physiology and pathophysiology of BP and cardiovascular events. Among the NPs atrial and brain natriuretic peptides (ANP and BNP) activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and initiate intracellular signaling. The genetic disruption of Npr1 (encoding GC-A/NPRA) in mice exhibits high BP and hypertensive heart disease that is seen in untreated hypertensive subjects, including high BP and heart failure. There has been a surge of interest in the NPs and their receptors and a wealth of information have emerged in the last four decades, including molecular structure, signaling mechanisms, altered phenotypic characterization of transgenic and gene-targeted animal models, and genetic analyses in humans. The major goal of the present review is to emphasize and summarize the critical findings and recent discoveries regarding the molecular and genetic regulation of NPs, physiological metabolic functions, and the signaling of receptor GC-A/NPRA with emphasis on the BP regulation and renal and cardiovascular disorders.

Association between human atrial natriuretic peptide Val7Met polymorphism and baseline blood pressure, plasma trough irbesartan concentrations, and the antihypertensive efficacy of irbesartan in rural Chinese patients with essential hypertension

BACKGROUND

Individual variations in the pharmacokinetics and pharmacodynamics of antihypertensive drugs are influenced by genetic and environmental factors. The ANP gene, which encodes the precursor of atrial natriuretic peptide (ANP), is among the candidate genes for genetic susceptibility to hypertension.

OBJECTIVE

This study examined the relationship between ANP Val7Met polymorphism (Single Nucleotide Polymorphism database ID: rs5063) and baseline blood pressure (BP), plasma trough irbesartan concentrations, and the antihypertensive efficacy of irbesartan in rural Chinese patients with essential hypertension.

METHODS

Patients with essential hypertension who had taken no antihypertensive medications within 4 weeks of study initiation received oral irbesartan 150 mg/d for 4 weeks. Genotyping was performed for all patients. BP was measured before dosing on the 1st and 28th days of treatment. Plasma irbesartan concentrations were measured using high-performance liquid chromatography with fluorescent detection. Antihypertensive efficacy was defined as attainment of a diastolic BP (DBP) <90 mm Hg (DBP analysis), a systolic BP (SBP) <140 mm Hg (SBP analysis), and a DBP <90 mm Hg and SBP <140 mm Hg (DBP and SBP analysis).

RESULTS

The study included 756 patients, 621 with the Val/Val genotype and 135 with the Val/Met+Met/Met genotypes. There were no significant differences in age, body mass index, sex, education level, occupation, alcohol consumption, or smoking status between the 2 groups. Patients with the Val/Met+Met/Met genotypes had a significantly lower mean baseline DBP compared with those with the Val/Val genotype (adjusted regression coefficient [SE]: -2.5 [1.0] mm Hg; P = 0.012) and significantly lower mean steady-state plasma trough irbesartan concentrations (adjusted regression coefficient: -12.6 [4.1]; P = 0.002). No significant association was found between antihypertensive efficacy and Val7Met polymorphism in the overall population, but in an analysis by baseline DBP status, patients with the Val/Met+Met/Met genotype a baseline DBP > or =100 mm Hg had significantly smaller reductions in DBP (adjusted regression coefficient: -5.7 [1.4] mm Hg; P < 0.001) and SBP compared with those with the Val/Val genotype and a baseline DBP > or =100 mm Hg (adjusted regression coefficient: -9.8 [2.9] mm Hg; P < 0.001).

CONCLUSION

The findings of this study suggest that in these rural Chinese patients with essential hypertension, ANP Val7Met polymorphism may be a genetic marker for baseline DBP, plasma irbesartan concentrations, and the antihypertensive efficacy of short-term irbesartan therapy.

Association between polymorphisms of the atrial natriuretic peptide gene and proteinuria: a population-based study

AIMS/HYPOTHESIS

In case-control studies, polymorphisms at the atrial natriuretic peptide gene (ANP) locus have been associated with presence of albuminuria in Type 1 and Type 2 diabetes. We evaluated the relationship between the ScaIand BstxI polymorphisms and albuminuria in the general population of the Mexico City Diabetes Study.

METHODS

Allele/genotype frequencies were analysed by PCR-RFLP analysis using ScaI (wild, A(2) vs mutated, A(1)) and BstxI (wild, C(708) vs mutated, T(708)) enzyme.

RESULTS

Among 1288 subjects, hypertension was present in 112 subjects, Type 2 diabetes in 191 and impaired glucose tolerance in 136; microalbuminuria was present in 464 subjects, and clinical proteinuria in 199. General frequencies were 0.93 and 0.96 for the wild alleles, and 0.07 and 0.04 for the mutated alleles, respectively for ScaI and BstxI. Frequency of A(1)was 0.08 in normoalbuminuric, 0.05 in microalbuminuric, and 0.05 in proteinuric patients (chi(2)=7.3, p=0.025). Frequency of T(708) was 0.06 in normoalbuminuric and 0.03 microalbuminuric and 0.03 in proteinuric subjects (chi(2)=8.1, p=0.017). By multivariate analysis, the associations between A(1)or T(708) allele and albuminuria were independent of age, sex, BMI, diabetes, and hypertension, (odds ratio (OR) 0.60, p=0.01, (OR) 0.51, p=0.004, respectively).

CONCLUSION/INTERPRETATION

In the general population of Mexico City, both polymorphisms of ANP are associated with albuminuria independently of hypertension, and could play a role in protecting subjects against development of albuminuria.

Increased blood pressure and loss of anp-induced natriuresis in mice lacking DARPP-32 gene

Atrial natriuretic peptide (ANP) is an important regulator of sodium metabolism and indirectly of blood pressure. Evidence has accumulated that ANP regulates sodium metabolism through a cascade of steps involving an increase in the level of cGMP, activation of cGMP-dependent protein kinase (PKG), and inhibition of renal tubular Na+, K+-ATPase activity. One of the major substrates for PKG is DARPP-32. In the present study we observed that ANP does not induce natriuresis in mice that lack DARPP-32. In contrast, there was a 4-fold increase in urinary sodium excretion following ANP administration to wild type mice. ANP as well as Zaprinast, a selective inhibitor of cGMP phosophodiesterase, inhibited renal Na+, K+-ATPase activity in wild type mice but had no such effect in mice lacking DARPP-32. Mean arterial blood pressure, measured in conscious animals, was significantly increased in DARPP-32 deficient mice as compared to wild type mice. The results confirm that DARPP-32 acts as a third messenger in the ANP signaling pathway in renal tissue and suggest an important role of DARPP-32 in the maintenance of normal blood pressure.

Polymorphisms in the hANP (human atrial natriuretic peptide) gene, albuminuria, and hypertension

Atrial natriuretic peptide (ANP) jointly affects kidney function and blood pressure homeostasis and is a candidate susceptibility gene for both essential hypertension and kidney disease. We evaluated the relation between the ScaI and BstXI polymorphisms of the human ANP (hANP) gene, hypertension, and albuminuria in a clinical cohort of 1033 subjects, including type 1 and type 2 diabetic patients, nondiabetic subjects with essential hypertension, and nondiabetic normotensive control subjects. Microalbuminuria was present in 15%, 29%, and 2%, respectively, of type 1 diabetic, type 2 diabetic, and nondiabetic patients. Macroalbuminuria was present in 9% of type 1 diabetics, 21% of type 2 diabetics, and 31% of nondiabetics. Prevalence of hypertension was 31%, 58%, and 61% in normoalbuminuric, microalbuminuric, and macroalbuminuric subjects, respectively (P<0.0001). Genotype distributions were in Hardy-Weinberg equilibrium in all 4 patient subgroups. The frequency of the ScaI mutated allele (A(1)) was significantly lower in hypertensive than in control subjects (11% versus 19%, P=0.018) and in patients with macroalbuminuria (5%) as compared with normoalbuminuric subjects (16%; P<0.0001). In a nominal logistic model adjusting for gender, age, obesity, diabetes, micro/macroalbuminuria, and hypertension, the A(1) allele was independently associated with macroalbuminuria (odds ratio, 0.57; confidence interval, 1.39 to 3.59; P=0.003) but not with hypertension. In the same model, the frequency of the BstXI mutated allele (T(708)) was increased in the presence of microalbuminuria (odds ratio, 2.25; confidence interval, 1.39 to 3.59; P<0.001). We conclude that the mutated genotypes of the ScaI polymorphism are negatively associated with overt nephropathy, whereas the mutated genotypes of BstXI polymorphism are positively associated with microalbuminuria. hANP gene variants may exert a protective effect against the development and progression of kidney damage in diabetes.

Chronic elevation of calmodulin in the ventricles of transgenic mice increases the autonomous activity of calmodulin-dependent protein kinase II, which regulates atrial natriuretic factor gene expression

Although isoforms of Ca2+/calmodulin-dependent protein kinase II (CaMKII) have been implicated in the regulation of gene expression in cultured cells, this issue has yet to be addressed in vivo. We report that the overexpression of calmodulin in ventricular myocytes of transgenic mice results in an increase in the Ca2+/calmodulin-independent activity of endogenous CaMKII. The calmodulin transgene is regulated by a 500-bp fragment of the atrial natriuretic factor (ANF) gene promoter which, based on cell transfection studies, is itself known to be regulated by CaMKII. The increased autonomous activity of CaMKII maintains the activity of the transgene and establishes a positive feed-forward loop, which also extends the temporal expression of the endogenous ANF promoter in ventricular myocytes. Both the increased activity of CaMKII and transcriptional activation of ANF are highly selective responses to the chronic overexpression of calmodulin. These results indicate that CaMKII can regulate gene expression in vivo and suggest that this enzyme may represent the Ca2+-dependent target responsible for reactivation of the ANF gene during ventricular hypertrophy.

Ectopic expression of active processed form of atrial natriuretic peptide in skeletal myoblasts

Atrial natriuretic peptide (ANP) is a cardiac hormone that elicits a profound diuresis, natriuresis, and hypotension. As a preliminary study toward ANP gene therapy of cardiovascular disorders, we have cloned a cDNA for mouse preproANP and carried out expression studies in muscle cells. The expression cassette, which was flanked by ITRs from AAV-2, consisted of HCMV IE enhancer/promoter, preproANP gene, and polyadenylation signal from bovine growth hormone. We transfected this expression vector into primary skeletal myoblasts and examined the following points: (1) secretion of immunoreactive ANP, (2) biological activity, and (3) nature of secreted ANP(s). The conditioned media from cells transfected with ANP vector had significantly higher levels of irANP in comparison to mock control. The secreted irANP had biological activity as confirmed by the elevated level of intracellular cGMP in human umbilical vein endothelial cells. Reverse-phase HPLC analysis showed that the processed form of ANP was the predominant form. These results demonstrate that preproANP gene could be ectopically expressed and correctly processed in skeletal myoblasts, which has implications for development of muscle-based ANP gene therapy.

The guanylyl cyclase family of natriuretic peptide receptors

Guanylyl cyclases are cytoplasmic and membrane-associated enzymes that catalyze the conversion of GTP to cyclic GMP, an intracellular signaling molecule. Molecular cloning has identified a multigene family encoding both soluble and particulate forms of the enzymes. Diffusible agents such as nitric oxide and carbon monoxide activate the soluble guanylyl cyclases. The particulate members of the family share a characteristic domain arrangement, with a single transmembrane span separating a variable extracellular ligand-binding domain from a conserved intracellular regulatory and cyclase catalytic domain. Seven members of the particulate guanylyl cyclase family have been identified, and they include the receptors for natriuretic peptides and Escherichia coli heat-stable enterotoxin. Recently, animal models have been developed to study the role of natriuretic peptides and their guanylyl cyclase-coupled receptors in renal and cardiovascular physiology.

Atrial natriuretic peptide participates in the regulation of vestibular blood flow

Atrial natriuretic peptide (ANP) is a cardiac hormone which exerts natriuretic, diuretic and vasorelaxant effects. Among the many organs and vascular beds populated with ANP receptors (Genest and Cantin, 1988) are the vestibular and auditory organs of the inner ear (Lamprecht and Meyer zum Gottesberge, 1988). The purpose of the current study was to assess the potential influence of ANP on vestibular blood flow in the guinea pig. The inner ear was exposed with a posterior-lateral approach medially through the mastoid cortex. The laser Doppler probe was placed adjacent to the ampulla of the posterior semicircular canal. Baseline measurements of mean blood pressure (BP), heart rate (HR) and vestibular blood flow were established. ANP dissolved in physiologic saline was infused intravenously at concentrations of 15, 150 or 300 ng/kg/min at 10 microliters/min for 30 min. Measurements were recorded during the infusion and for a recovery period of 65 min. The control group was treated equivalently and infused with 0.15 M NaCl. Baseline BP and HR for all animals were 40.1 +/- 6.67 and 190 +/- 15.7, respectively. BP, HR and vestibular blood flow remained stable during the baseline, control and recovery conditions of saline infused subjects. Infusion of ANP (15 ng/kg/min) induced a mild elevation of BP followed by a small decrease in pressure during the post-infusion period. Vestibular blood flow showed a decrease to approximately 20% below baseline during infusion and stabilized at this level during the recovery period. Infusion of higher concentrations of ANP (150 and 300 ng/kg/min) induced a similar pattern of BP change in a dose-dependent manner. Vestibular blood flow, however, evidenced significant elevations during the post-infusion periods for both concentrations. These increases (22% and 26%, for 150 and 300 ng/kg/min, respectively) were significantly different from vestibular blood flow changes in the saline and low dose groups. The HR remained stable for baseline, infusion and recovery periods for each of the ANP infused subjects. This investigation demonstrates the systemic and local effects of ANP suggest a possible role for ANP in local regulation of vestibular blood flow.