A-type natriuretic peptide level in angiotensin II type 1a receptor knockout mice

A-type (atrial) natriuretic peptide (ANP) levels in the heart and plasma were examined by immunohistochemistry, electron microscopy, and radioimmunoassay (RIA) in angiotensin II type 1a receptor knockout (Agtr1a KO) mice. Additionally, the ANP mRNA level in the heart was measured using a real-time polymerase chain reaction (PCR) assay. The blood pressure in Agtr1a KO mice was significantly lower than that in wild-type (WT) mice. The number of ANP granules and ANP immunoreactivity in the auricular cardiocytes were significantly lower in Agtr1a KO mice than in WT mice. Ultrastructurally, the ventricular cardiocytes in Agtr1a KO mice occasionally had ANP-like granules, which were not present in WT mice. The plasma, auricular, and ventricular ANP and plasma cyclic guanosine monophosphate (cGMP) concentrations were significantly higher in Agtr1a KO mice than in WT mice. The ANP mRNA levels of the auricular and ventricular cardiocytes in the Agtr1a KO mice were almost twice as large as those in WT mice. The present data suggest that a notable increase in the ANP biosynthesis and release in the heart of Agtr1a KO mice may account for the reduction in blood pressure together with the lack of an AGTR1A receptor in this model.