Light and electron microscopic localization of atrial natriuretic peptide in the heart of spontaneously hypertensive rat

Atrial natriuretic peptide (ANP) is a newly discovered peptide hormone present mainly in the atria. We investigated the occurrence and distribution of ANP immunoreactivity in the myocardiocytes of the ventricles of spontaneously hypertensive rats by use of immunocytochemistry at both light and electron microscopic level. ANP immunoreactivity was found in the specific granules in the cytoplasm of the cardiocytes in the subendocardium and the myocardium of the ventricles, as well as in the atria. The specific granules found in the ventricles of hypertensive rats were similar in size, shape, and ANP immunoreactive content to those in the atria. The abundance of ANP immunoreactivity in the left ventricle is greater than that in the right, and appears to increase with increasing severity of hypertension. Conversely, the overall content of ANP in the atria of hypertensive rats was decreased when compared with that in age-matched normotensive rats. The present findings indicate that ventricles may become a major source for ANP synthesis and release during hypertension, and may play important roles in cardiac endocrine pathology and cardiac hypertrophy.