In vivo demonstration of the absorptive function of the middle ear epithelium

The present study investigated in vivo fluid and ion transport across the middle ear epithelium. The tympanic membrane of rats was punctured under general anesthesia. A capillary tube was fitted to the external auditory canal and the bulla filled with various solutions. Middle ear (ME) fluid volume variations were then measured at constant pressure. When saline was used, a linear decrease of fluid volume was apparent. Replacement of sodium with a non-permeable cation (N-methyl-d-glucamin) reduced the absorption rate from 0.065 ± 0.008 to 0.019 ± 0.003 μl/min (P < 0.05, n = 6). Similarly, amiloride (10 ^-3 M), a sodium channel antagonist, reduced the absorption rate to 0.027 ± 0.006 μl/min (P < 0.05, n = 6). Net absorption was abolished when chloride was substituted with gluconate: -0.008 ± 0.004 μl/min (P < 0.02, n = 6), which might have been related (i) to the role of chloride as a diffusible anion through the paracellular pathway, or (ii) to the secretion of chloride through apical channels. However in this condition, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, a chloride channel blocker, did not affect the rate of fluid exchange -0.008 ± 0.007 μl/min (P = 0.75, n = 6). This model provides the first in vivo evidence for the absorptive function of the ME. Fluid introduced into the ME cavity disappears due to active transport through the mucosa. This process is sodium-dependent and can be hindered by high concentration of amiloride. The rate of absorption is high enough to allow total clearance of fluid from the cavity of the middle ear within 13 h. This process might play a role in the maintaining a fluid-free and gas-filled middle ear cavity.