Renal Tubular Permeability to Radioactive Sodium, Bromide and Iodide Isotopes

Abstract

Na²⁴, Br⁸² or I¹³¹ together with glomerular indicators were injected i. v. during stop-flow experiments 12 minutes after clamping of ureteral catheter in 10 dogs. Analyses of serial urine portions collected 3 minutes after the injection was started, revealed a specific activity of Na²⁴ and Br⁸² nearly as high as that of plasma in urine portions delayed in the proximal tubules. The specific activity of I¹³¹ was much lower. These observations suggested that proximal tubules are highly permeable to sodium and bromide ions and less permeable to iodide ions.

The significance of passive flux of sodium under conditions when concentration gradients, electrical potential gradients and colloid osmotic pressure are changed, is discussed. It is concluded that it is only when concentration gradients increase as during mannitol diuresis that passive flux of NaCl is likely to contribute to the natriuresis.