AQP2 and Menkes disease: Hence, we formulate the hypothesis that steroids and in particular vitamin D3 and/or oestradiol have a physiologically relevant inhibitory action upon AQP2 water channeling and that this regulation is directly linked to endolymphatic hydrops, MD symptoms, and other related inner ear pathophysiologies (Figure 4): in normal conditions, endolymphatic volume is stable (58) and because the cochlear canal endolymph is hyperosmotic compared to perilymph (9, 58, 79), membrane water permeability of the surrounding cells must be very low.