Are mu-opioid receptors involved in the control of endothelin-1 release from the pituitary gland in normal and dehydrated rats?
The objective of the present study was to investigate whether the endogenous opioids are involved in the control of endothelin-1 release from the pituitary gland. To test this hypothesis we have measured the peripheral plasma concentration of ET-1 as well as the content of immunoreactive ET-1 (irET-1) in the pituitary in response to opioid receptors blockade in euhydrated and 24 h water-deprived Wistar-Kyoto rats. Placebo or naltrexone (50 micrograms/kg body wt.) were given i.v. in both groups. Trunk blood was collected to determine hematocrit, plasma sodium and ET-1 levels (RIA). Immunostaining of ET-1 in the whole pituitary glands was performed by colloidal gold labeling. The quantitative analysis of irET-1 was carried out under a light microscope using a computerized image analyzer (MultiScan).


RESULTS
(1) Twenty-four-hour dehydration resulted in marked increase of peripheral concentration of ET-1. Naltrexone injection induced a significant elevation of ET-1 plasma concentration in both, dehydrated and control animals. (2) The content of irET-1 in anterior and intermediate lobes of the pituitary in dehydrated rats was markedly higher than in control group. (3) Naltrexone injection caused a rapid and significant reduction irET-1 within the anterior, intermediate and posterior lobes in dehydrated and control animals.


CONCLUSIONS
(1) An elevation of irET-1 in the pituitary gland and peripheral circulation in dehydrated animals may play a role in maintaining of water-electrolyte balance. (2) The mu-opioid system appears to control the ET-1 release from the pituitary in normal and dehydrated animals.
Płonowski
A
A
Department of Neuroendocrinology, Medical Center of Postgraduate Education, Warsaw, Poland.
Szymańska-Debińska
T
T
Radzikowska
M
M
Baranowska
B
B
Woźniewicz
B
B
D000818:Animals; D003681:Dehydration / Q000378:metabolism*; D019332:Endothelin-1 / Q000097:blood / Q000378:metabolism*; D005260:Female; D006400:Hematocrit; D007150:Immunohistochemistry; D009271:Naltrexone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010902:Pituitary Gland / Q000378:metabolism*; D051381:Rats; D011921:Rats, Inbred WKY; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D012964:Sodium / Q000097:blood; D014882:Water-Electrolyte Balance
D019332:Endothelin-1; D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D009271:Naltrexone; D012964:Sodium