Taurine, a sulfonic acid analog of beta-alanine and an important osmoregulator in mammals, undergoes a renal adaptive response to altered dietary intake of sulfur amino acids. Dietary taurine (LTD) restriction induces a higher initial rate Vmax of NaCl-induced taurine symport in rat renal brush border membrane vesicles (BBMV) and enhanced reabsorption of filtered taurine as compared to rats fed a normal taurine diet (NTD). A high taurine diet (HTD) enhances urinary taurine excretion and induces a decline in the Vmax of the same symporter. Taurine is translocated by means of a 2Na:l Cl:l taurine quaternary complex 'carrier' and an inside negative potential, but the precise details of transport are unclear, as are the mechanisms signaling the adaptive response. Evidence in BBMV and continuous cultured renal cell lines (LLC-PK1 and MDCK) suggest that the adaptive response may be triggered by changes in renal intracellular taurine content and that increased or reduced import of carrier relates to cell membrane trafficking or import by the microtrabecular network, because transporter import into membranes is blocked by colchicine. New protein synthesis is also important since cycloheximide blocks the adaptive response. The mechanisms responsible for the adaptive response and maintenance of taurine homeostasis will be further examined both in continuous renal cell culture and in BBMV by exploring the following hypotheses: ( 1 ) The signal for the renal adaptive response relates to intracellular rather than extracellular taurine content. ( 2 ) The renal adaptive response demonstrates an asymmetric polarity with enhanced (or reduced) uptake across the apical membrane and enhanced (or reduced) efflux across the basolateral surface. ( 3 ) A rapid adaptive response represents changes in cellular trafficking or """"""""shuttling"""""""" of the taurine symporter, but the prolonged (8-14 day) response requires new protein synthesis. ( 4 ) The renal adaptive response as well as taurine symport activity requires first chloride and then sodium or taurine interaction with the carrier prior to translocation across the apical membrane. ( 5 ) The renal adaptive response may serve to enhance both taurine and chloride uptake by renal epithelium. ( 6 ) The prolonged adaptive response involves transcription of increased amounts of messenger RNA.
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