Effect of Angiotensin II on Proximal Tubular Reabsorption in Normal Humans

The effects of angiotensin II (AII) on proximal tubular reabsorption have been evaluated in 6 healthy volunteers under normal salt and water balance. One-hour clearance periods were performed before, during and after the infusion of pressor doses of AII; in 3 of the 6 subjects, the study was repeated with lower doses of AII. Glomerular filtration rate (GFR) and renal plasma flow (RPF) were determined by the clearances of inulin and PAH, and the fractional excretion of lithium (FELi) was considered as an index of proximal sodium reabsorption. The effects of AII on the fractional excretion of beta 2 microglobulin (FE beta 2M) were also studied. Both doses of AII decreased GFR and RPF and increased the filtration fraction (FF); the modifications of these parameters, as well as the reduction of FELi and the fractional excretion of sodium (FENa) and the increase of plasma aldosterone and of plasma atrial natriuretic peptide (ANP), were more evident with pressor doses of AII, which increased the blood pressure from 129/83 to 142/95 mm Hg (p less than 0.01). AII did not modify FE beta 2M in either study. During AII, FELi decreased less than FENa and both were closely and inversely related to the variations of FF, whilst no relationship was present between FE beta 2M and FF. These results suggest that, in normal humans, the AII-induced rise of FF may be an important factor, even if not the only one, in enhancing the proximal reabsorption of lithium and thus of sodium, whilst it does not affect the absorption of beta 2M.