Abstract
Atrial natriuretic peptide (ANP) is stored in atrial granules primarily as a larger molecular weight precursor (pro-ANP), which is believed to be rapidly converted to an active peptide of 28 amino acids during or shortly after secretion. A tissue kallikrein-like serine protease has been suggested as a potential processing enzyme. In the present immunocytochemical study, using specific monoclonal antibodies, we found that esterase A, a kallikrein-like serine protease, was demonstrable in rat atrial myocytes and in ventricular myocytes, and was capable of cleaving pro-ANP to yield a low molecular weight product. Using colloidal gold immunocytochemistry at the electron microscopic level, we have found esterase A in atrial myocytes, both in granules and in another subcellular site that corresponds to sarcoplasmic reticulum. Double-label electron microscopic immunocytochemical results indicated that esterase A can co-localize with ANP in granules of atrial myocytes.