A preparation of blood capillaries was isolated from the rete mirabile of the eel swim bladder. The capillaries, incubated for two hours at 37° C. in Krebs-Ringer bicarbonate buffer with 5 mM glucose, contained 29.3 ± 4.6 nmoles of sorbitol per gram and released in the medium 725 ± 60 nmoles of fructose per gram. When glucose in the medium was raised from 5 to 30 mM, capillary sorbitol and medium fructose rose by approximately 100 per cent. There were no accompanying changes in capillary water content as determined by the ratio of wet weight to dry weight and by the difference between water-3H space and inulin-l4C space. Respiration of cap Diary tissue was lower at 30 mM glucose than at 5 mM glucose. The addition of 30 mM mannitol to a medium containing 30 mM glucose did not restore the oxygen uptake by capillary tissue to values obtained with a medium containing 5 mM glucose alone. It is concluded that the polyol pathway is operative in vertebrate blood capillaries. At high medium glucose concentration, the activity of the pathway is enhanced, respiration is depressed, and water content is unchanged.

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