The endolymphatic sinus is a possible detector of endolymph volume status

Abstract

In the paper, in question (Salt and Rask-Andersen, 2004), we presented direct measurements of hydrostatic pressure and potassium concentration from the luminal space of the endolymphatic sac during pressure manipulations of the perilymph. Negative pressures (i.e. suction) applied to the perilymphatic space induced ionic, hydrostatic pressure and resting potential changes in the sac while positive pressures typically did not. Thus, our evidence for a one-way valve between the vestibule and the endolymphatic sac, which is now questioned by Wit and Hofman, was based on experimental measurements. As anatomic data in the same study showed the membrane of the endolymphatic sinus was in close proximity to the opening of the endolymphatic duct into the vestibule, we proposed that the membrane of the endolymphatic sinus was the anatomic structure acting as the valve. Wit and Hofman provide no new data to question the experimental findings of our paper yet they pose the question "Does the endolymphatic sinus function as a one way valve?" Since they have no experimental data conflicting with our original findings and suggest no alternative anatomic structure to act as the valve that was demonstrated experimentally, we can find no rational basis to doubt our original conclusions that the sinus membrane can act as a valve between the endolymphatic spaces of the vestibule and the endolymphatic duct.Wit and Hofman go on to question the concept that the endolymphatic sinus could be involved in endolymph volume regulation. On the basis of their calculations, they conclude that "perilymph pressure has no influence [on endolymph flow]" and that "the model for the regulation of endolymph flow through the sinus, as proposed by Salt and Rask-Andersen (2004); Salt and Rask-Andersen (2005) has to be rejected". We strongly disagree with both of these statements. The flow explanation described in their Figs. 2 and 3 that their subsequent arguments appear to disprove, appears to be of Wit and Hofman's own making and is completely different from the interpretation we presented in our papers (Salt and RaskAndersen, 2004;Salt, 2005). We have never suggested that perilymph pressure changes could induce a flow between the sacculus and the endolymphatic sinus in the manner presented in their Fig. 3. This is simply a "straw man argument" in which the concept they disprove is of their own making and has no bearing on the interpretation we presented in our paper.In order to clarify the situation, we present in Fig. 1 a more complete explanation of the pressures and flows involved in our original interpretation. Fig. 1a diagrams the membranous endolymphatic structures (thin lines) within the non-compliant bony otic capsule (heavy lines). Since neither endolymph or perilymph are flowing at a high rate, it is reasonable to assume that perilymph pressure (P p ) and endolymph pressure (P e ) are uniform throughout each fluid space of the system. To our knowledge, no pressure differences between different parts of the endolymphatic system (P 1 -P 2 in W...

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