BETAINE TRANSPORT IN RABBIT RENAL BRUSH-BORDER MEMBRANE-VESICLES

Transport of the organic osmolyte betaine was characterized in brush-b order membrane vesicles (BBMV) isolated from rabbit renal cortex. Inwa rdly directed gradients of either Na+ or H+ supported concentrative up take in a manner consistent with the presence of parallel Na+-betaine and H+-betaine cotransport processes. Concentrative uptake occurred in the presence of membrane potential alone, indicating that betaine tra nsport is electrogenic. Accumulation of betaine was not dependent on c hloride in the medium. Whereas L-proline inhibited both the H+- and Na +-sensitive components of betaine transport, glycine blocked the H+-se nsitive pathway and had little effect on Na+-sensitive betaine transpo rt. Both pathways were adequately described by Michaelis-Menten kineti cs. Under Na+-gradient conditions (pH equilibrium), the maximal rate o f total betaine transport (J(max)) = 50.8 +/- 13.3 nmol . mg-1 . min-1 and the concentration of total betaine producing half-maximal uptake (K(t)) = 4.1 +/- 0.5 mM. Under H+-gradient conditions (Na+ free), J(ma x) = 102.5 +/- 10.5 nmol . mg-1 . min-1 and K(t) = 2.8 +/- 0.3 mM. Imp osition of both Na+ and H+ gradients increased J(max) (142 +/- 25.5 nm ol . mg-1 . min-1) to a level significantly greater than that noted in the presence of a Na+ gradient alone. We conclude that betaine transp ort in renal BBMV involves two distinct transport pathways that are di fferentiated on the basis of sensitivity to either Na+ or H+ and by th eir specificity to proline and glycine.