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Am J Physiol Regul Integr Comp Physiol 274: R1361-R1371, 1998;
0363-6119/98 $5.00
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Vol. 274, Issue 5, R1361-R1371, May 1998

K+-neutral amino acid symport of Bombyx mori larval midgut: a system operative in extreme conditions

B. Giordana1, M. G. Leonardi1, M. Casartelli1, P. Consonni1, and P. Parenti2

Departments of 1 Biology and 2 General Physiology and Biochemistry, University of Milan, 20133 Milan, Italy

The K+-dependent symporter for leucine and other neutral amino acids expressed along the midgut of the silkworm Bombyx mori operates with best efficiency in the presence of a steep pH gradient across the brush-border membrane, with external alkaline pH values up to 11, and an electrical potential difference (Delta psi ) of ~200 mV. Careful determinations of leucine kinetics as a function of external amino acid concentrations between 50 and 1,000 µM, performed with brush-border membrane vesicles (BBMV) obtained from the middle and posterior midgut regions, revealed that the kinetic parameter affected by the presence of a Delta pH was the maximal rate of transport. The addition of Delta psi caused a further marked increase of the translocation rate. At nonsaturating leucine concentrations in the solution bathing the external side of the brush-border membrane, leucine accumulation within BBMV and midgut cells was not only driven by the gradient of the driver cation K+ and Delta psi but occurred also in the absence of K+. The ability of the symporter to translocate the substrate in its binary form allows the intracellular accumulation of leucine in the absence of K+, provided that a pH gradient, with alkaline outside, is present. The mechanisms involved in this accumulation are discussed.

anterior, middle, and posterior regions; K+-amino acid symporters; extreme alkaline pH; transmembrane electrical potential


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