Reabsorption of betaine in Henle's loops of rat kidney in vivo

This study was designed 1) to localize and 2) to characterize betaine reabs orption from the tubular lumen in rat kidney in vivo, and 3) to test whethe r reabsorption is modulated by the diuretic state. [C-14]betaine (+ [H-3]in ulin) was microperfused through the proximal convoluted tubule (PCT) and mi croinfused into late proximal (LP) and early distal (ED) tubules, long loop s of Henle (LLH), and vasa recta of the rat in vivo et situ, and the fracti onal recovery of the C-14 label was determined endproximally (PCT) and in t he final urine, respectively. [C-14]betaine was not reabsorbed during ED mi croinfusion, whereas fractional reabsorption during LP microinfusion was 82 % at 0.06 mM betaine and decreased gradually to 4.8% at 60 mM. L-Proline ha d lower Michaelis-Menten constant (K-m) and sarcosine a higher K-m than bet aine. Chronic, but not acute, diuresis inhibited betaine reabsorption in He nle's loops. Fractional [C-14]betaine reabsorption in PCT was much smaller than that during LP microinfusion. [C-14]betaine (7.28 mM) microinfused 1) into LLH was reabsorbed to 30% and 2) into vasa recta appeared in the ipsil ateral urine to a much higher extent than contralaterally. In both cases, n o saturation was detected at 70 mM. We conclude that betaine is reabsorbcd by mediated transport from descending limbs of short Henle's loops by a pro line-preferring carrier in a diuresis-modulated manner. In the deep medulla , bidirectional blood/urine betaine transport exists.