Hypergravity decreases carbonic anhydrase‐reactivity in inner ear maculae of fish

Abstract

Previous investigations revealed that fish inner ear otolith growth depends on the amplitude and the direction of gravity. Both otolith total size, otolith bilateral size-asymmetry and the total and bilateral calcium-incorporation are also affected by gravity. Hypergravity, e.g., slows down otolith growth and diminishes bilateral otolith asymmetry as compared to 1 g control specimens raised in parallel. Since the enzyme carbonic anhydrase (CA) plays a prominent role in otolithic calcification, the reactivity of inner ear CA during otolith growth under hypergravity was investigated. CA-reactivity was demonstrated histochemically and densitometrically on sections of inner ear maculae of larval cichlid fish (Oreochromis mossambicus), that were kept for 6 hrs in a 3 g hypergravity centrifuge. The total unilateral macular CA-reactivity and the bilateral difference in CA between the left and the right maculae were significantly lower in 3 g animals than in 1g controls. The result is in complete agreement with previous studies indicating that a regulatory mechanism, which adjusts otolith size and asymmetry towards the gravity vector, acts via activation/deactivation of macular CA.

Mechanisms of otoconia and otolith development

“…In the mammalian inner ear, CA is widely expressed in the sensory and non-sensory epithelia (Lim et al, 1983; Pedrozo et al, 1997), especially in the embryonic endolymphatic sac. Inhibition of CA activity decreases HCO 3 − concentration and pH in the endolymphatic sac (Kido et al, 1991; Tsujikawa et al, 1993), and causes abnormal and reduced otoconia in the developing chick embryos (Anken et al, 2004; Kido et al, 1991). Alteration of macular CA is also associated with changes in zebrafish otolith growth (Anken et al, 2004).…”

“…Inhibition of CA activity decreases HCO 3 − concentration and pH in the endolymphatic sac (Kido et al, 1991; Tsujikawa et al, 1993), and causes abnormal and reduced otoconia in the developing chick embryos (Anken et al, 2004; Kido et al, 1991). Alteration of macular CA is also associated with changes in zebrafish otolith growth (Anken et al, 2004). The isoform CA2 and Pendrin may cooperate to maintain the normal function of the endolymphatic sac, where the two are co-expressed (Dou et al, 2004).…”

Influence of Magnitude and Duration of Altered Gravity and Readaptation to 1 g on the Structure and Function of the Utricle in Toadfish, Opsanus tau

“…However, Lim et al (1974) and Sondag et al (1995) in well-controlled studies could not demonstrate any change in the otoconia Ca +2 content, shape, size, or distribution in adult hamsters subjected to HG. More recently, Aceto et al (2015) found a decrease in otolith calcification after prolonged HG exposure in zebrafish, presumably the result of a regulation of carbonic anhydrase and other matrix protein productions ( Anken et al, 2004 ; Anken, 2006 ). As bone is adversely remodeled during space missions ( Demontis et al, 2017 ), calcium carbonate on the other hand might be deposited on existing otoconia to provide greater “weight” in an effort to restore the “gain” of gravity sensation; conversely, HG would lead to an ablation of otoconia mass to “rebalance” function.…”

Proteins Involved in Otoconia Formation and Maintenance

“…Injection of acetazolamide into the yolk sac of developing chick embryos alters and inhibits normal otoconial morphogenesis . Activation/deactivation of macular CA under different gravity is associated with changes in otolith sizes in fish (Anken et al 2004). Immunohistochemstry shows that CAII is coexpressed with pendrin in the same cells in the endolymphatic sac, suggesting that those two proteins may cooperate in maintaining the normal function of the endolymphatic sac (Dou et al 2004), which is an important tissue for endolymph production.…”