Background: Head-down suspension (HDS) of rats has been used as a model for the simulation of a microgravity environment. Natriuretic peptide family including atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptides (CNP), and their receptors are responsible for body fluid homeostasis and blood pressure control. However, little is known about the long-term responses and the simultaneous observations of both natriuretic peptide family and their receptors following HDS. This study was analyzed the regulation of cardiac, hypothalamic and renal natriuretic peptides and their receptors syntheses to 4 weeks of HDS in rats. Methods: Unanesthetized, unrestrained, male Sprague-Dawley rats were subjected to either a horizontal position (control rats) or a -45° head-down tilt using the tail-traction technique (HDS rats). This study observed the cardiac, hypothalamic and renal syntheses of natriuretic peptides as a expression of ANP, BNP and CNP mRNA. The mRNA expressions of A-type natriuretic peptide receptor (NPR-A), B-type NPR (NPR-B) and clearance receptor (NPR-C) were also determined. The expressions of natriuretic peptide and their receptor mRNA were measured by reverse transcription-polymerase chain reaction with [32P]-dCTP following 4 weeks of HDS in both control and HDS rats. Results: After 4 weeks of HDS, the expressions of ANP mRNA were significantly decreased in the right atrium and the kidney, and showed the non-significant decreasing trend in the left atrium and the hypothalamus of HDS rats. BNP mRNA expressions were decreased in the heart especially in the left atrium, while its mRNA in the left ventricle was showed the non-significant increasing trend following 4 weeks of HDS. The expressions of CNP mRNA were increased in the hypothalamus and showed the non-significant increasing trend in the kidney of the HDS rats. After 4 weeks of HDS, NPR-A mRNA expressions were decreased in the kidney and hypothalamus, but NPR-C mRNA expression was showed the non-significant increasing trend in the kidney of HDS rats compared with controls. Conclusion: These results suggest that the regulation of natriuretic peptide mRNA following 4 weeks of HDS exerts to maintain the blood volume through an attenuation of syntheses in natriuretic peptide family of the heart and the kidney.
Head-down suspension, ANP, BNP, CNP, NPR-A, NPR-B, NPR-C, mRNA