Influence of salt intake on atrial natriuretic peptide gene expression in rats

Mammalian atrial cardiocytes produce and secrete a peptide hormone called atrial natriuretic peptide (ANP), which causes diuresis, natriuresis, a decrease of blood pressure and inhibition of smooth muscle contraction, renin and aldosterone release. Despite a variety of investigations, the influence of salt intake on the transcriptional regulation and the circulating levels of ANP is still not clear. This study sought to examine the influence of different levels of long-term dietary salt intake on the expression of the ANP gene in different regions of the rat heart. To this end male Sprague-Dawley rats were fed either a high-salt (4% saline chow) or a low-salt (0.02% saline chow) diet for 5, 10 or 20 days. Another group had access to normal chow and received 1% NaC1 as a drinking solution for 10 days. During the experiment sodium and chloride uptake via food or drinking solutions was monitored and sodium and chloride excreted in the urine was determined. ANP mRNA levels in the atria and the ventricles were assayed. We found that no form of the different salt diets had influence on cardiac ANP mRNA levels at any of the times examined though excretion rates of sodium and chloride differed by a factor of 200. Also plasma immunoreactive ANP remained unaltered. At the same time plasma renin activities were markedly increased in rats fed a low-salt diet and substantially suppressed in rats fed a high-salt diet, suggesting the efficacy of the salt diet. Our findings suggest that ANP gene expression and secretion of the active hormone are probably not involved in salt balance during chronic salt loading given by dietary food or by 1% NaCl in the drinking water.