Significance of urinary precession of sodium over simultaneously injected inulin

Publisher Website
Google Scholar
Abstract
After simultaneous intra-arterial injection of 22Na and of inulin into dogs, 22Na is well known to appear in urine ahead of inulin. This precession may be measured as a "clearance" derived from the rate of isotopic equilibration of plasma and urine under steady-state conditions. Precession clearance of 22Na amounted to 1-2% of renal plasma flow, a fraction similar to that reported for plasma flow through the inner medulla. Similar values were obtained for precession clearances of 45Ca and 82Br. Na precession clearance increased with elevation of blood pressure. Precession of these ions may be limited by medullary plasma flow and it reflects exchange between intra- and extratubular ions in the inner medulla. Additional support for this view is provided by the occurrence of rapid transtubular fluxes of 22Na, 45Ca, and 82Br under stop-flow conditions, all in the same general region proximal to the site of distal Na reabsorption. In contrast with the flow-limited precession of 22Na, precession of 35SO4 appears to be diffusion limited. Possible explanations for this observation are considered.