THE SITE OF REABSORPTION IN THE KIDNEY TUBULE OF NECTURUS

Abstract

Necturus maculosus was chosen as most suitable for capsular and tubular catheterization because of the large size and superficial situation of the primary glomerular capsules and because of the accessibility and size of the tubules from these capsules. Sugar is present in protein-free fluid drawn from the glomerular capsules of spring Necturi. The blood sugar is about 60 mgm. per 100 cc. The bladder urine is sugar-free, as is fluid drawn from points 1/2 or even 1/3 the way down the proximal convoluted tubule. Fall and early winter animals usually have no blood-sugar, and the bladder urine, proximal tubular fluid and capsular fluid are all sugar-free. If the plasma sugar level is raised to 40-50 mgm. per 100 cc. by a subcutaneous injection of 2 cc. of 1% glucose, the capsular fluid contains sugar while the bladder urine and proximal tubular fluid are sugar-free. The site of sugar reabsorption is thus localized in the proximal convoluted tubule. There are 360-400 mgm. NaCl per 100 cc. of plasma in this form. Capsular fluid gives a heavy precipitate with AgNO3, while fluid from the distal end of the proximal tubule shows only a light cloud. Further evidence that the proximal tubule is the principal site of reabsorption of osmotically active substances is afforded by the observation that dog red cells stained with methylene blue, which lake in 0.2% NaCl, when injected into a glomerular capsule never lake in the capsular fluid but almost immediately disappear when they are washed down into the proximal tubule as filtration proceeds. Cells fixed in formalin are easily seen throughout the entire tubular system. The site of chloride reabsorption is thus also seen to be in the proximal convoluted tubule. There are only 17-26 mgm. NaCl per 100 cc. of urine.