Interactions between Na+ channels and Na+-HCO3- cotransporters in the freshwater fish gill MR cell:: a model for transepithelial Na+ uptake

Isolated mitochondria- rich ( MR) cells from the rainbow trout gill epithelium were subjected to intracellular pH ( pHi) imaging with the pH- sensitive dye BCECF- AM. MR cells were categorized into two distinct functional subtypes based on their ability to recover pHi from an NH4Cl- induced acidification in the absence of Na+. An apparent link between resting pHi and Na+-independent pHi recovery was made. We observed a unique pHi acidification event that was induced by extracellular Na+ addition. This further classified the mixed MR cell population into two functional subtypes: the majority of cells ( 77%) demonstrated the Na+-induced pHi acidification, whereas the minority (23%) demonstrated an alkalinization of pHi under the same circumstances. The focus of this study was placed on the Na+-induced acidification and pharmacological analysis via the use of amiloride and phenamil, which revealed that Na+ uptake was responsible for the intracellular acidification. Further experiments revealed that pHi acidification could be abolished when Na+ was allowed entry into the cell, but the activity of an electrogenic Na+-HCO3- cotransporter ( NBC) was inhibited by DIDS. The electrogenic NBC activity was supported by a DIDS-sensitive, Na+- induced membrane potential depolarization as observed via imaging of the voltage- sensitive dye bis- oxonol. We also demonstrated NBC immunoreactivity via Western blotting and immunohistochemistry in gill tissue. We propose a model for transepithelial Na+ uptake occurring via an apical Na+ channel linked to a basolateral, electrogenic NBC in one subpopulation of MR cells.