NA-MOSSAMBICUS ANALYZED BY CONFOCAL LASER-SCANNING MICROSCOPY( AND CA2+ HOMEOSTATIC MECHANISMS IN ISOLATED CHLORIDE CELLS OF THE TELEOST OREOCHROMIS)

Chloride cells (CCs; recognised by the vital mitochondrial stain DASPE I) and pavement cells (PCs) isolated from tilapia opercular epithelium were adhered to Cell-Tak-coated glass coverslips and loaded with fluo rescent probes for the measurement of intracellular concentrations of Na+ or Ca2+. Basal levels of cytosolic Na+ and Ca2+ ranged from 6.4 to 16.5 mmoll(-1) and from 76 to 110 nmoll(-1), respectively, and did no t differ between CCs and PCs. In CCs, inhibition of Na+/K+-ATPase by o uabain or Cu2+ increased intracellular [Na+]. Replacing extracellular Na+ with N-methyl-D-glucamine(+) led to a rise in cytosolic [Ca2+] tha t was dependent on the extracellular [Ca2+], indicating that a Na+/Ca2 + exchanger was operating in reverse mode (importing Ca2+). The forwar d mode of this exchanger could be demonstrated by inhibition with bepr idil. The CC has various pathways for passive Na+ influx: a tetrodotox in-sensitive pathway, an amiloride-sensitive pathway and other as yet unidentified pathways.