The endocrine function of the heart in adult mammals, including humans, resides on the ability of the striated cardiac muscle cells (cardiocytes) of the atrial wall to produce in a regulated manner the polypeptide hormones atrial natriuretic factor (ANF or ANP) and brain natriuretic peptide (BNP), which are referred to as natriuretic peptides (NP).  Their general function is to modulate cardiac preload and afterload by their effect on water and electrolyte balance and cardiovascular growth (For review see (6)).   There is some confusion in the literature as to the precise cell type where this secretory function resides, as well as to the peptide storage form and the location of the secretory function, which, of late, is of