Dynamics of intrarenal pressures and glomerular filtration rate after acetazolamide

The dynamics of intrarenal pressures, early distal tubular fluid conductivity (EDC), and renal flood flow (RBF) were studied in rats given acetazolamide (ACZ), an inhibitor of proximal reabsorption. Glomerular filtration rate (GFR) and end-proximal flow were estimated by clearances of 51Cr-EDTA and lithium. Proximal tubular pressure (Pprox) increased initially by 1.7 +/- 0.1 mmHg after ACZ, causing a decrease in the hydrostatic pressure difference across the glomerular membrane (delta P). EDC increased, and then RBF, glomerular capillary pressure (Pgc), Pprox, and star vessel pressures (Psv) dropped as a result of afferent vasoconstriction. Pprox decreased less than Pgc, resulting in a further decrease in delta P, which after 25-30 s reached a constant level 3-4 mmHg below control. After a transient increase the pressures declined to a new steady state, in which Pprox was equal to control, Pgc was decreased, and distal tubular pressure, end-proximal flow, and EDC were increased. GFR was depressed by 29%. The results indicate that the tubuloglomerular feedback mechanism controls Pgc and Pprox by afferent vasoconstriction, as well as efferent vasodilation. The data also indicate that proximal reabsorption rate is important in determining the changes in delta P by its effect on Pprox at least in the early transient phase.