What effect will mannitol have on kidney function?

Diuretic. Manitol is a sugar like substance which is filtered by the kidney and not reabsorved like glucose would so it remains in the kidney tubules and it acts as a urine volume enhancer by pulling water and electrolytes into the urine space and ultimately out in the urine so as to be an osmotic diuretic. It is used mainly to reduce brain edema acutely.