Immunohistochemical Localization of AT(1a), AT(1b), and AT(2) Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT(1a), AT(1b), and AT(2). mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT(1a) and AT(2) receptors in adrenal zona glomerulosa and medulla. AT(1b) immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT(1a) receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT(1b) and AT(2), but not AT(1a), immunofluorescence was observed in the anterior pituitary. Stellate cells were AT(1b) positive while ovoid cells were AT(2) positive. In the brain, neurons were AT(1a), AT(1b), and AT(2) positive, but glia was only AT(1b) positive. Highest levels of AT(1a), AT(1b), and AT(2) receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.