Angiotensin Peptides in Brain and Pituitary of Rat and Sheep

Abstract

Several lines of evidence indicate that angiotensin peptides may be formed in the brain, where angiotensin II (Ang II) and angiotensin-(1-7) (Ang-(1-7)) may function as neurotransmitters. However, there is considerable controversy concerning the identity and levels of angiotensin peptides in the brain. We have used a novel high performance liquid chromatography-based radioimmunoassay to measure Ang-(1-7), Ang II, Ang-(1-9) and Ang I in various brain regions and in the pituitary of the rat and sheep. We also studied the effect of different methods of tissue extraction, and the effect of the converting enzyme inhibitor ramipril, on angiotensin peptide levels in the rat hypothalamus. The levels of Ang-(1-7), Ang II, Ang-(1-9) and Ang I were low (<25 fmol/g) in all brain regions examined, except for the sheep median eminence and cerebellar cortex where Ang II levels were 385±116 and 193±37 fmol/g (mean ± SEM, n = 6), respectively. Pituitary Ang II levels were 103±13 fmol/g in the rat and 63±18 fmol/g in the sheep. The levels of Ang-(1-7), Ang-(1-9) and Ang I were much lower than those of Ang II in brain and pituitary. Ang-(1-7) levels in the rat hypothalamus were low (<6 fmol/g) but methods of extraction which involved freezing and thawing of the tissue resulted in substantially higher levels of this peptide. Ang II levels in the rat hypothalamus (18±3 fmol/g) were reduced to undetectable levels (<6 fmol/g) by ramipril administration. The low levels of angiotensin peptides in the hypothalamus and brainstem indicate that if these peptides function as neurotransmitters in these regions, then they are of particularly low abundance. Moreover, our results indicate that the high levels of Ang-(1-7) reported previously for rat hypothalamus may be artefactual, due to the method of tissue extraction.