Cardiac and plasma atrial natriuretic peptide after 9-day hindlimb suspension in rats.

To determine atrial natriuretic peptide (ANP) adaptation to simulated weightlessness, immunoreactive plasma (ir-NH2- and ir-COOH-terminals) and atrial (ir-COOH-terminal) ANP levels, atrial mRNA expression, immunoreactive cardiocyte ANP levels (ir-NH2- and ir-COOH-terminals), and ultrastructural observations of granules in atrial cardiocytes were assessed in male Wistar rats after a 9-day hindlimb suspension. Plasma ir-NH2- and ir-COOH-terminal ANP concentrations decreased by 17 (P < 0.05) and 37% (P < 0.05), respectively, in suspended rats. A concomitant ir-COOH-terminal ANP content reduction was also observed in left (31%; P < 0.01) and right atria (25%; P < 0.05). Atrial ANP mRNA expression was severely depleted in the right atrium and less so in the left atrium after 9 days of hindlimb suspension. Immunocytochemistry observations demonstrated lowered NH2- and COOH-terminal ANP immunoreactivities in left and right atria from suspended rats. A reduced number of storage granules (dense granules) in both atria was also noted on ultrastructural analysis. It was concluded that ANP biosynthesis, storage, and release were decreased after a 9-day hindlimb suspension.