Quantitative autoradiography of 125I-[Sar 1, Ile 8]-angiotensin II binding in the brain of spontaneously hypertensive rats

Abstract

The brain contains its own angiotensin II (AII) system. To better understand the role of central AII in cardiovascular regulation, we used 125I-[Sar 1, Ile 8]-AII ( 125I-SI-AII), radioactive AII antagonist, to autoradiographically localize putative AII receptor binding in many parts of the central nervous system of the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. With 125I-SI-AII binding on brain membrane preparations, Scatchard analysis indicated that K d values were from 0.10±0.04 nM to 0.13±0.05 nM, whereas B max values (femtomol/mg protein) were found to be from 6.95±1.60 to 15.52±4.99 among different brain regions studied. Various SI-AII receptor binding activities among brain regions revealed in this study were therefore most likely due to differences in AII receptor density with high affinity binding of 125I-SI-AII. Using 125I-SI-AII, specific binding for SI-AII was found in the nucleus tractus solitarius (NTS), paraventricular hypothalamic nucleus (PVN), subfornical organ (SFO), suprachiasmatic nucleus (SCN), area postrema, the dorsal motor nucleus of the vagus (DMX), and the nucleus of spinal tract of the trigeminal system (NSV). With quantitative receptor autoradiography in conjunction with radioactive standards, we have observed that the NTS possesses the highest SI-AII binding, followed by the PVN, SFO, NTS, DMX, and NSV. No significant differences were observed between the SHR and WKY rats in the SI-AII binding within the SFO, PVN and NTS. However, SHR at early hypertensive (7 weeks) and established hypertensive (16 weeks) stages contained significantly higher SI-AII bindings in the NSV, as compared to age-matched WKY rats. Furthermore, SHR rats also exhibited significantly higher SI-AII bindings in DMV at 16-week established hypertensive stage. It has been documented that the NSV and DMX are also abundant in opiate receptors. Therefore, AII in association with the opiate in the above two nuclei, may play important roles in the development and maintenance of hypertension in SHR rats.