Increased Na+-taurine symporter in rat renal brush border membranes: preformed or newly synthesized?

Abstract

The renal adaptive response to a varied intake of sulfur amino acids is demonstrated by an increase in the initial rate of Na+-taurine symport (cotransport) by rat renal brush border membrane vesicles (BBMVs) after 8-14 days of a low methionine diet. A high (3%) taurine diet reduces Na+-taurine symport. Fasting for 3 days, which depletes renal tubule cell taurine content, also enhances Na+-taurine symport both initially (15 s) and throughout the overshoot. In this study we examine the possibility that a rapid-onset adaptive response is expressed in BBMV, with the increased Na+-taurine symport reflecting the incorporation of preformed symporter into membranes rather than new synthesis. Rats fed the low methionine diet for 14 days were placed on the high taurine diet for 12-18 h; Na+-taurine symport activity fell by 40%. Fasting for 4 h restored low methionine diet levels of Na+-taurine symport activity (92 pmol.mg protein-1.15 s-1), defining a rapidly induced rise in uptake. Colchicine (0.6 mg) was injected prior to fasting in a group of rats because it blocks the incorporation (import) of preformed symporter into the membrane. Animals injected with colchicine had a pattern of BBMV uptake similar to that found in animals switched to the high taurine diet for 18 h. This agent blocked the rapidly induced rise in uptake. Feeding with the high taurine diet for 4 h caused a fall in uptake of 16.5%; colchicine blocked this reduction in uptake. These results indicate that the nephron can respond rapidly to changes in the intake of amino acids, conserving taurine in periods of nutrient lack and excreting excess taurine within 4 h in periods of surfeit. This rapid response is expressed at the brush border surface. The use of cholchicine indicates that the increase or reduction in Na+-taurine symport activity is due to incorporation (import) of transporter into the BBMV rather than to de novo synthesis.