Atrial natriuretic peptide (ANP)-granules in the guinea pig atrial and auricular cardiocytes: an immunocytochemical and ultrastructural morphometric comparative study.

The atrial natriuretic peptide (ANP) is a peptide hormone that is mainly produced in the cardiac atria, where it is stored within granules. It is known that the four regions of the atrial-auricular complex (two atria and two auricles) produce and store ANP in the granules. However, no report has been presented comparing the presence of ANP, and the number and diameter of atrial granules in the atria and auricles. ANP immunoreactivity was detected in cardiocytes from the four regions of the atrial-auricular complex. No differences were observed among the regions. The number of granules was greatest in the right atrium followed by the left atrium and left auricle and right auricle, in this order. The diameter of granules in the cardiocytes was significantly largest in the right atrium and reduced via the left auricle to the left atrium and right auricle. Both the number and diameter of the granules are larger in the right atrium in comparison with the other regions of the atrial-auricular complex, which leads to the supposition that this region is the one that most synthesizes and stores the ANP.