Recent developments in teleost renal transport

The teleost renal tubule continues to be a valuable tool with which to investigate epithelial membrane transport mechanisms and factors that control them. Several newer aspects of fish renal transport are reviewed, and hypothetical models incorporating these findings are proposed. Fluorescence microscopy has revealed a new Na-independent organic anion (OA) transport process and new information about the classical Na-dependent transporter. With the latter, OA enters the cytosol in exchange for a-ketoglutarate (αKG) at the basolateral membrane (BLM). Larger molecular weight OAs enter independently of (αKG and exit to the lumen on a brush-border membrane (BBM) ATP-dependent transporter. The Na-dependent OA transporter is inhibited by activation of protein kinase C (PKC). A primary messenger is dopamine whereas adrenergic stimulation increases OA secretion. Avid sulfate secretion by the proximal tubule involves two anion exchangers in series. Recent studies implicate carbonic anhydrase (CA) in facilitation of secretion through dehydroxylation of bicarbonate entering from the lumen in exchange for cellular sulfate. Hydroxyl ions resulting from this catalysis exchange for interstitial sulfate. Factors that activate PKC stimulate phosphate (Pi) secretion; those that activate protein kinase A stimulate Pi reabsorption. The flounder Na-coupled Pi transporter has been cloned (NaPi-II), kinetically characterized, and localized mainly to the BLM of proximal segment II (PII) and BBM of collecting duct. Morphology and ion distribution studies strongly implicate aggregation of Mg in cytosolic vesicles as a step in Mg secretion. These were found in PII and certain collecting duct cells. J. Exp. Zool. 283:653–661, 1999. © 1999 Wiley-Liss, Inc.