Although AM was first identified in pheochromocytoma tissue arising from adrenal medulla, immunoreactive AM and AM mRNA has been found to be ubiquitously distributed among various tissues (Fig. 6). Notably, high levels of AM mRNA have been detected in such cardiovascular tissues as atrium, aorta, kidney and lung, though the concentrations of immunoreactive AM in aorta, ventricle and kidney were less than 5% of that seen in adrenal gland (Sakata et al., 1993; Sakata et al., 1994, Sugo et al., 1994). It may be that AM biosynthesized in these tissues is rapidly and constitutively secreted into the blood and/or utilized as an autocrine or paracrine regulator (Bean et al., 1994). By contrast, AM synthesized in adrenal medulla is thought to be stored in the granules and secreted into a regulatory pathway. Thus, the biosynthetic and secretory systems of AM may be tissue-specific.
Immunohistochemical analysis demonstrated the presence of AM immunoreactivity in cardiac myocytes, vascular smooth muscle cells, endothelial cells, renal distal and collecting tubules, mucosal and glandular epithelia of the digestive, respiratory and reproductive systems as well as in the endocrine and neuroendocrine systems
(Martinez et al., 1995; Washimine et al., 1995). In the hypothalamus, AM staining was detected in the supraoptic nuclei and the magnocellular parts of the paraventricular nuclei (Ueta et al., 1995). In addition, immunoreactive AM is present in the blood, urine, cerebrospinal fluid and amniotic fluid (Kitamura et al., 1994; Macri et al., 1996; Nagata et al., 1998).
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