Renal medullary electrolytes: effects of furosemide assessed by studies in vivo of electrical admittance.

In anaesthetized rabbits electrical admittance (a reciprocal of impedance) of the kidney in situ was recorded using electrodes located in the cortex, outer medulla, inner medulla and papilla. Renal haemodynamics, clearances and Na+ concentration in tissue slices were also determined. Admittance changes in response to i.v. furosemide, 1.5 or 3 mg/kg body weight, and to 15% mannitol infusion, reflected changing interstitial electrolyte concentration and, indirectly, changes in tubular reabsorption of NaCl. The large dose of furosemide and mannitol infusion decreased admittance in all renal zones whereas the small dose affected only the inner medulla and papilla. The rapid onset of the fall in admittance of the inner medulla, even in absence of changes within the outer medulla, suggests that the drug's action is not confined to the thick ascending limb but includes the thin ascending segment.