Ion transport mechanisms responsible for K + secretion and the transepithelial voltage across marginal cells of stria vascularis in vitro

Abstract

It has long been accepted that marginal cells of stria vascularis are involved in the generation of the endocochlear potential and the secretion of K +. The present study was designed to provide evidence for this hypothesis and for a cell model proposed to explain K + secretion and the generation of the endocochlear potential. Stria vascularis from the cochlea of the gerbil was isolated and mounted into a micro-Ussing chamber such that the apical and basolateral membrane of marginal cells could be perfused independently. In this preparation, the transepithelial voltage ( V t ) and resistance ( R t ) were measured across marginal cells and the resulting equivalent short circuit current ( I sc ) was calculated ( I sc = V t R t ). Further, K + secretion ( J K +, probe ) was measured with a K +-selective vibrating probe in the vicinity of the apical membrane. In the absence of extrinsic chemical driving forces, when both sides of the marginal cell epithelium were bathed with a perilymph-like solution, V t was 8 mV (apical side positive), R t was 10 ohm-cm 2 and I sc was 850 μA/cm 2 ( N = 27). J K +,probe was outwardly directed from the apical membrane and reversibly inhibited by basolateral bumetanide, a blocker of the Na + /Cl − /K + cotransporter. On the basolateral but not apical side, ouabain and bumetanide each caused a decline of V t and an increase of R t suggesting the presence of the Na,K-ATPase and the Na + /Cl − /K + cotransporter in the basolateral membrane. The responses to [Cl −] steps demonstrated a significant Cl − conductance in the basolateral membrane and a small Cl − conductance in the paracellular pathway or the apical membrane. The responses to [Na +] steps demonstrated no significant Na + conductance in the basolateral membrane and a small Na + or nonselective cation conductance in the apical membrane or paracellular pathway. The responses to [K +] steps demonstrated a large K + conductance in the apical membrane. Apical application of 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) and basolateral elevation of K + caused an increase in V t and a decrease in R t consistent with stimulation of the apical K + conductance. Similar observations have been made in vestibular dark cells, which suggest that strial marginal cells and vestibular dark cells are homologous and transport ions by the same pathways. Taken together, these observations are incompatible with a model for the generation of the endocochlear potential which ascribes the entire potential to the strial marginal cells [Offner et al. (1987) Hear. Res. 29, 117–124]. However, the data are compatible with the notion that marginal cells contribute not more than a few mV to the endocochlear potential as suggested by Salt et al. [Laryngoscope 97, 984–991, 1987].