Natriuretic peptide receptors in the human endolymphatic sac

Objective: To examine human endolymphatic sac (ELS) tissue for atrial natriuretic peptide (ANP) receptor subtypes A, B, and C. Design: Pilot study. Methods: Immunohistochemical analysis of human ELS tissue specimens. The ANP receptors were characterized using the peroxidase/antiperoxidase method and polyclonal antibodies directed against each receptor subtype. The identity of the stain regarding receptor subclass was masked from the observer. Human kidney tissue known to contain all 3 receptor subtypes was used as a control. Presence of the receptor subclasses was confirmed using reverse transcriptase-polymerase chain reaction (RT-PCR) techniques. Subjects: Samples of ELS tissue were obtained at autopsy from 3 fresh cadaver specimens (6 ears) and as surgical specimens froth 3 patients (1 for immunohistochemical analysis and 2 for RT-PCR) undergoing acoustic neuroma resection using the translabyrinthine approach. Results: The ANP type B receptors demonstrated moderate to strong reactivity in all 7 specimens, and mild to moderate staining to the ANP type C receptor was also noted. No appreciable reactivity to the ANP type A receptor was detected using immunohistochemical techniques. All 3 receptor subclasses were detected using RT-PCR. Conclusions: The ANP receptors are found within the human ELS, with a predominance of ANP type B based on the intensity of staining. The ANPs may be involved in fluid homeostasis in the inner ear. Based on these findings, C-type natriuretic peptide may be a more effective peptide within the human ELS for fluid regulation because its binding affinity is virtually exclusive for the ANP type B receptor.