Ouabain-like immunoreactive substances exist in the hypothalamus and the adrenal medulla in rats

Abstract Since it has been demonstrated that one of the endogenous digitalis-like factors (EDLF) is ouabain, searches for the EDLF are focused on the endogenous ouabain in the mammalian body. We have developed a monoclonal, anti-ouabain antibody which similarly cross-immunoreacts with other digitalis glycosides such as ouabain, digoxin, and digitoxin; the epitope of this antibody was confirmed to be the partial structure important for cardiac compounds and neutralizing their pharmacological activity, Na + /K + -ATPase inhibition, in a competitive mode against K + . By using this monoclonal antibody, we immunohistochemically examined the distribution of the digitalis-like substances in the adrenal glands and the hypothalamus. The immunoreactivity was diffusely distributed in the cytoplasm of endocrine cells of the adrenal medulla, but not in the adrenal cortex. Immunoreactive neuronal somata and their proximal processes were found in the hypothalamic paraventricular nucleus, and the intensity of the immunostaining in the proximal processes was greater than in the somata. In the neurons of the supraoptic nucleus very weak immunoreactivity was observed. No immunoreactive material was detected in the other areas of the hypothalamus examined in this study. Present findings with a monoclonal, ouabain antibody suggest that the ouabain-like immunoreactive substances are exclusively distributed in the neural tissues such as the hypothalamus and the adrenal medulla. Although the physiological role of the substance is still uncertain, this particular distribution suggests that it may be a neurotransmitter implicated in the cardiovascular and endocrine functions.