EXPRESSION OF MESSENGER-RNAS ENCODING ALPHA-SUBUNIT AND BETA-SUBUNIT ISOFORMS OF NA,K-ATPASE IN THE VESTIBULAR LABYRINTH AND ENDOLYMPHATIC SAC OF THE RAT

The distribution of mRNAs coding for three different isoforms of the a lpha and two of the beta subunit of Na,K-ATPase was studied in the rat vestibular system using in situ mRNA hybridization. The dark cells of the utricular macula and of the ampullae of the semicircular canals e xpressed high levels of mRNA encoding the alpha 1 and beta 2 isoforms of the Na,K-ATPase, a composition that in the cochlea has been uniquel y found in the stria vascularis. However, in the dark cells it was cou pled with a weak expression of beta 1. The sensory epithelia of the ve stibular system showed alpha 1 and beta 1 expression at much higher le vels than in the cochlear sensory epithelium. Weak expression limited to the alpha 1, beta 1, and beta 2 isoforms was observed in the endoly mphatic sac, contrasting previous cytochemical results which suggested extensive Na,K-ATPase activity to the sac. The results support the wi dely held hypothesis that the vestibular dark cells play a role simila r to that of the stria vascularis in endolymph production. They indica te that the ion transport requirements of the vestibular sensory epith elia may be different than those in the cochlea. They also suggest tha t the endolymphatic sac may not be a major site of inner ear ion excha nge. (C) 1994 Academic Press, Inc.