Proportions of rat ANP-secreting cells that are cardiomyocytes and that synthesize the hormone.

Atrial natriuretic peptide (ANP) is released from the heart and participates in regulating blood pressure and volume. We recently developed a reverse hemolytic plaque assay to measure the release of ANP from individual rat atrial cardiocytes. This assay determines the total proportion of atrial cardiocytes committed to releasing ANP. We combined the plaque assay with immunocytochemistry (IC) and in situ hybridization (IS). Combining the plaque assay with IC for myosin revealed that 13.5 +/- 0.9% (%myosin+ and plaque forming; mean +/- SE, n = 4) of atrial cardiocytes are cardiomyocytes that release ANP. Combination of the plaque assay with IS for ANP mRNA showed that 16.6 +/- 0.6% (%in situ+ and plaque forming; mean +/- SE, n = 4) of the cardiocytes in the rat atria synthesize and release the hormone. Incubation of atrial cardiocytes with dexamethasone to stimulate ANP gene expression did not alter the total proportion of in situ-positive ANP-secreting cells. These data suggest that, within the total ANP-secreting population of the rat atria, only 33% of the secreting cells are cardiomyocytes. In addition, 68% of the ANP-secreting cells do not appear to synthesize the hormone. These results imply that muscle and nonmuscle cells are involved in secreting ANP and that cardiomyocytes synthesize the hormone.