Tubuloglomerular feedback and tubular reabsorption during acute potassium loading in rats

Acute hyperkalemia has been associated with changes in reabsorption, glomerular filtration rate (GFR), and autoregulation, which might represent altered tubuloglomerular feedback (TGF) responsiveness. Therefore, TGF responsiveness, segmental reabsorption of water, sodium and potassium, and single-nephron GFR were evaluated during acute potassium loading in male Sprague-Dawley rats. Rats receiving 300 mM KNO3, KHCO3, and KCl showed significantly increased plasma potassium levels and attenuation of stop-flow pressure responses 45-90 min after starting the potassium infusion compared with that observed in time controls and rats infused with 300 mM NaCl. Attenuation of TGF responsiveness could not be related to plasma and kidney angiotensin II levels. Segmental water and sodium handling and proximal to distal single-nephron GFR differences assessed in a time control group and a group receiving 300 mM KCl revealed no changes related to KCl infusion. However, late proximal and early distal potassium concentrations increased significantly from 4.7 +/- 0.2 to 6.3 +/- 0.3 mM (P < 0.01) and from 1.5 +/- 0.1 to 2.7 +/- 0.4 mM (P < 0.01), respectively. In summary, although attenuated TGF responsiveness was demonstrated at higher perfusion rates, this study does not support a significant role for either the TGF mechanism or changes in reabsorption upstream of the early distal tubule for the initiation of kaliuresis during acute potassium loading.