ANP in regulation of arterial pressure and fluid-electrolyte balance: lessons from genetic mouse models

Genomic sequence and cardiac expression of atrial natriuretic peptide in cats

OBJECTIVE

To determine the nucleotide and amino acid sequence of atrial natriuretic peptide (ANP) in cats and its typical regions of cardiac expression.

ANIMALS

5 healthy adult mixed-breed cats.

PROCEDURE

Total RNA was extracted from samples obtained from the left and right atrium, left and right ventricle, and interventricular septum of each cat. The RNA was used to produce cDNA for sequencing and northern blot analysis. Genomic DNA was extracted from feline blood samples. Polymerase chain reaction primers designed from consensus sequences of other species were used to clone and sequence the feline ANP gene.

RESULTS

The feline ANP gene consists of 1,072 nucleotides. It consists of 3 exons (123, 327, and 12 nucleotides) separated by 2 introns (101 and 509 nucleotides). It has several typical features of eukaryotic genes and a putative steroid-response element located within the second intron. Preprohormone ANP consists of 153 amino acids. The amino acid sequence of the active form of feline ANP (ANP-30) is identical to that of equine, bovine, and ovine ANP-30 and differs from that of human, canine, and porcine ANP-28 only by 2 carboxy-terminal arginine residues. The ANP mRNA was detected only in the left and right atria.

CONCLUSIONS AND CLINICAL RELEVANCE

The genetic and protein structure and principal regions of cardiac expression of feline ANP are similar to those of other species. Results of this study should be helpful in future studies on the natriuretic response in cats to diseases that affect cardiovascular function.

Targeted disruption of the gene for natriuretic peptide receptor-A worsens hypoxia-induced cardiac hypertrophy

Targeted disruption of the gene for natriuretic peptide receptor-A (NPR-A) worsens pulmonary hypertension and right ventricular hypertrophy during hypoxia, but its effect on left ventricular mass and systemic pressures is not known. We examined the effect of 3 wk of hypobaric hypoxia (0.5 atm) on right and left ventricular pressure and mass in mice with 2 (wild type), 1, or 0 copies of Npr1, the gene that encodes for NPR-A in mice. Under normoxic conditions, right ventricular peak pressure (RVPP) was greater in 0 than in 2 copy mice, but there were no genotype-related differences in carotid artery PP (CAPP). The left ventricular free wall weight-to-body weight (LV/body wt) ratio was greater in 0 than in 2 copy mice and there was a trend toward a greater right ventricular weight-to-body weight (RV/body wt) ratio. Three weeks of hypoxia increased RVPP and RV/body wt in all genotypes. The increase in RVPP was similar in all genotypes (11-14 mmHg), but the hypoxia-induced increase in RV/body wt was more than twice as great in 0 copy mice than in 2 copy mice (1.11 +/- 0.06 to 2.65 +/- 0.46 vs. 0.96 +/- 0.04 to 1.4 +/- 0.09, P < 0.05). Chronic hypoxia had no effect on CAPP in any genotype and did not effect LV/body wt in 1 or 2 copy mice, but increased LV/body wt 41% in 0 copy mice. We conclude that absent expression of NPR-A worsens right ventricular hypertrophy and causes left ventricular hypertrophy during exposure to chronic hypoxia without increasing pulmonary or systemic arterial pressure responses.

New developments in mechanisms of obesity-induced hypertension: role of adipose tissue

Hypertension develops in almost 60% of obese individuals. Apart from the recent observation of obesity-associated structural changes in kidney structure that may lead to enhanced tubular sodium reabsorbtion, reports of paracrine and hormonal factors derived from adipose tissue have prompted speculations about the role of adipose tissue in the pathophysiology of obesity-induced hypertension. We summarize recent data on leptin's sympathoexcitatory actions, the possible influence of adipose tissue on atrial natriuretic peptide levels, and the formation of vasoactive substances, such as angiotensin II and nonesterified fatty acids, by adipocytes. The mechanisms discussed herein may contribute to the typical findings in obesity-induced hypertension, including volume expansion, sodium retention, enhanced sympathetic nervous system activity, increased activity of the systemic renin-angiotensin system, low atrial natriuretic peptide levels, and disturbed glucose and insulin metabolism. Together, these data strengthen the hypothesis that adipose tissue is potentially a major regulator of cardiovascular-renal function.