Semicircular canal fluid compartment morphology, ionic composition, and regulation in the oyster toadfish, Opsanus tau

The present studies address three aspects important to understanding the homeostasis and dynamics of inner fluids of the oyster toadfish, Opsanus tau. First, the ionic compositions of inner-ear fluids were characterized in vivo utilizing double barreled, ion-selective microelectrodes. Measurements were performed by introducing precalibrated microelectrodes into the perilymphatic fluid space, saccule, and utricle by small fistulas produced by electrocauterization. Results from the perilymph were consistent with other species, and its composition was similar to cerebrospinal fluid (Na + = 129 mM, K+ = 4.96 mM, Ca2+ = 1.83 mM). The ionic milieu of saccular and utricular endolymph resembled previously published values for elasmobranch fish but differed significantly from mammalian species. In particular, sodium was the dominant cation (Na+ in saccule 166 mM and 122 mM in utricle), and potassium, which is normally the dominant cation in mammalian species, was 51.4 mM and 47.7 mM in the saccule and utricle, respectively. More interestingly, the free calcium concentration was significantly higher in the toadfish (2.88 mM and 1.78 mM in the saccule and utricle, respectively). The difference in endolymph composition between different species may have interesting implications with regard to mechanoelectrical transduction by the sensory hair cells.; Second, a three-dimensional geometrical model of the membranous semicircular canals was developed. A novel orthogonal reconstruction technique was applied to histologically embedded membranous canals to reconstruct the three-dimensional geometry. Results were corrected for tissue shrinkage due to histological processing of the specimens by comparing dimensions before and after tissue fixation. The geometrical reconstructions indicate that the long slender membranous canals from the toadfish are similar to human canals in size. This work is the first complete model for membranous semicircular canal geometry and has proven fundamental to understanding the directional and temporal coding of the vestibular end organ as predetermined by the mechanics of fluid flow in the three-canal labyrinth.; Finally, a theoretical model of ion homeostasis was developed and applied to different regulatory cell spatial distributions and initial concentration perturbations within the toroidal geometry of the utricle and horizontal canal. This model is based on one-dimensional Fickian diffusion within a variable cross-sectional area duct coupled to a first-order, spatially distributed, ion regulation equation. The equations were discretized in space and time and solved numerically. These simulations indicate a considerable advantage in the rate of recovery to steady-state for evenly distributed versus spatially localized regulatory cell distribution. These results are due to the importance of diffusion as the rate-limiting step in endolymph ion homeostasis.