MECHANISM OF FLUID SECRETION COMMON TO AGLOMERULAR AND GLOMERULAR KIDNEYS

Isolated renal proximal tubules of sea water fish net secrete fluid in vitro. The principal electrolytes in secreted fluid are Na, Cl, Mg an d S. Transepithelial voltages may be lumen-negative or -positive by a few millivolts, and transepithelial resistances are low partly due to high paracellular Na and Cl permeabilities. Transepithelial electroche mical potentials indicate secretion of Mg into the tubule lumen by act ive transport. As Mg concentration in secreted fluid rises, Na concent ration falls. Surprisingly, these observations of fluid secretion are made in glomerular and aglomerular proximal tubules, suggesting a fund amental mechanism common to both. Central to this commonality appears to be their behavior as open Donnan systems. Mg actively secreted into the tubule lumen from which it cannot diffuse back into the peritubul ar medium causes the transepithelial secretion of diffusible Na and Cl . Water follows by osmosis. Since there is flow out of the distal end of the tubule Donnan equilibrium is not attained. Instead, a dynamic D onnan system is maintained, driven by active transport of Mg. A mathem atical model of tubular electrolyte and fluid secretion confirms the o peration of this open, dynamic Donnan system in aglomerular and glomer ular proximal tubules.