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Angiotensin II and adrenocorticotropin release: mediation by endogenous corticotropin-releasing factor
Abstract:
Recent reports indicate that the main effect of systemically administered angiotensin II (AII) on ACTH release is probably due to some central nervous system mechanism. The present studies were designed to investigate whether the central action of AII on ACTH release is directly mediated through CRF. In order to test the participation of endogenous CRF in the AII-induced ACTH release in vivo, intact and pharmacologically blocked (pretreated with chlorpromazine-morphine-nembutal) female rats were injected iv with AII (8 nmol/100 g BW). Plasma levels of ACTH as well as CRF content in the median eminence (ME) and medial basal hypothalamus (MBH) were evaluated before and 1, 2.5, and 5 min after treatment. These responses were compared with the effect of 1 min ether exposure on hypothalamic CRF content and plasma ACTH levels in unanesthetized animals. AII injection and ether exposure increased plasma ACTH levels several-fold at both 2.5 and 5 min post treatment in intact rats. Conversely, AII failed to induce any significant increase in plasma ACTH levels in centrally blocked rats at any interval studied. On the other hand, AII injection and ether inhalation acted in similar fashion on CRF content in ME, inducing fast depletion at 1 min post treatment, recovering to control values at 2.5 min after injection, and finally, accumulating peptide at 5 min post treatment. In addition, CRF content in the MBH decreased significantly at 5 min, under both experimental conditions; AII had no effect on hypothalamic CRF content in centrally blocked rats. In vitro experiments using whole MBH (containing ME) fragments incubated with either neural peptides or high K+ solutions indicate that AII possesses a CRF releasing effect at concentrations of 10(-6) M or more. Conversely, other hypothalamic peptides, such as LHRH, TRH, and somatostatin did not induce significant release of CRF at any of the concentrations assayed (10(-7) to 10(-5) M). On the other hand, high K+ solutions released CRF in a concentration-related manner (15-60 mM). These studies suggest that the central effect of AII stimulation on ACTH release in vivo could be, at least in part, through the release of hypothalamic CRF into the portal circulation.