The regulatory mechanism of endothelin on the secretion of the pituitary horones

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1. (1) The distribution of endothelin-3 (ET3)-like immunoreactivity in the bullfrog pituitary was investigated immunohistochemically. ET3-immunoreactive (ET3-ir) cells were found to correspond to cells showing LH- and FSH -immunoreactivities. In the male pituitary, only a few ET3-ir cells were observed. (2) Immuno-electron microscopic analysis using colloidal gold revealed the co-existence of ET3-like substance (s) and gonadotropins within the same secretory granules. These results suggest the possible participation of ET3 in regulating pituitary function as an autocrine and/or paractrine hormone.2. Antisera were raised against amino-acid residues 425-439 of the rat ET B receptor (ET_B receptor). Massive ET_B receptor immunoreactive fibers were observed in the median eminence and organum vasculosum lamina terminalis of rats. Double labeling studies with antiluteinzing hormone-releasing hormone (LHRH) antiserum revealed that ET_B receptor-like immunoreactivity associated with LHRH-immunoreactive fibers in these areas. However, ET_B receptor-like immunoreactivity was not detected in LHRH-immunoreactive somata. This result suggests that ET affects the LHRH neuronal systems via ET_B receptors on the axon at the terminal field.3. Effects of ET-1 on the release of vasopressin (VP) from the isolated neural lobe of the rat were investigated. In addition, effects of angiotensin II (ANG II) were also investigated, because it colocalizes with ET-1 and VP in the same secretory granules in the axon terminals. ET-1 induced an increase in VP release, followed by a decline to basal levels upon its withdrawal. The VP release was also stimulated by ANG II.These results strongly suggest that ET-1 and ANG II act directly on the neural lobe to stimulate the release of VP.However, the sites of action of these peptides are not clear.