PROPORTIONS OF RAT ANP-SECRETING CELLS THAT ARE CARDIOMYOCYTES AND THAT SYNTHESIZE THE HORMONE

Atrial natriuretic peptide (ANP) is released from the heart and partic ipates in regulating blood pressure and volume. We recently developed a reverse hemolytic plaque assay to mea sure the release of ANP from i ndividual rat atrial cardiocytes. This assay determines the total prop ortion of atrial cardiocytes committed to releasing ANP. We combined t he plaque assay with immunocytochemistry (IC) and in situ hybridizatio n (IS). Combining the plaque assay with IC for myosin revealed that 13 .5 +/- 0.9% (%myosin+ and plaque forming; mean +/- SE, n = 4) of atria l cardiocytes are cardiomyoctes that release ANP. Combination of the p laque assay with IS for ANP mRNA showed that 16.6 +/- 0.6% (%in situand plaque forming; mean +/- SE, n = 4) of the cardiocytes in the rat atria synthesize and release the hormone. Incubation of atrial cardioc ytes with dexamethasone to stimulate ANP gene expression did not alter the total proportion of in situ-positive ANP-secreting cells. These d ata suggest that, within the total ANP-secreting population of the rat atria, only 33% of the secreting cells are cardiomyocytes. In additio n, 68% of the ANP-secreting cells do not appear to synthesize the horm one. These results imply that muscle and nonmuscle cells are involved in secreting ANP and that cardiomyocytes synthesize the hormone.