Mechanisms of vestibular ion transport

摘要

It was initially considered that losses of body fluid and electrolytes, which occur upon exposure to microgravity, may alter vestibular Na,K-ATPase activity by mobilization of endocrine agents regulating fluid and electrolyte balance. The effect of alteration of serum aldosterone levels on Na,K-ATPase in ion transporting regions of the guinea pig inner ear was investigated by using a high Na low K diet to first significantly lower aldosterone. The results of these experiments are consistent with the hypothesis that the population of NA,K-ATPASE sites increases in response to aldosterone in ion transporting epithelia of the semicircular canal. In more recent studies, presumptive mineralocorticoid (type 1) receptors were investigated by measuring binding of 3H-aldosterone in the semicircular canal of male Harley guuinea pigs. These experiments demonstrated the presence of significant aldosterone binding, suggesting the existence of mineralocorticoid receptor sites. Thus, the vestibular tissues appear to be a target site for mineralocorticoid action. It was further demonstrated that NA,K-ATPase activity is elevated in dark cells of the utricle and ampullae (the dark cells have characteristic membrane infoldings on their basolateral surfaces, suggestive of fluid and ion transport). Vestibular organs were isolated from gerbil inner ear, and processed by the cytochemical method of Ernst for electron microscopic demonstration of Na,K-ATPase. No enzyme activity detectable with the cytochemical product was associated with dark cells of the utricle and ampullae. The saccular endolymphatic compartment is isolated from the utricle and ampullae, but communicates more freely with the cochlear duct. It seems likely that the cation content of saccular endolymph, in contrast to the situation for the utricle and ampullae, is maintained primarily by the marginal cells of the cochlear lateral wall. Finally, vestibular organs were isolated from gerbil inner ears by microdissection, labeled with 3Hquinuclidinyl benzilate at 1 nanomolar concetration, and processed for light-microscopic audoradiography. Audioradiographic observatitons indicated that the highest grain densities were associated with the Ampullae. Both in the ampullae and macular organs, audoradiographic label was concentrated within the sensory epithelium at a level corresponding to the synaptic poles of the hair cells. Additional label was found over nerve fibers passing through the connective tissue vbeneath the sensory epithelia. This suggests the presence of cholinergic receptor sites, presumably muscarinic, in vestibular sensory organs.