Dissociation of renal interstitial hydrostatic pressure and natriuresis of atrial natriuretic factor

The objective of these experiments was to test the hypothesis that renal interstitital hydrostatic pressure (RIHP) plays an important role in the natriuretic effect of atrial natriuretic factor (ANF) in anesthetized Wistar rats. Three groups of male Wistar rats were used in this study. Two groups were infused with different doses of ANF, and the third group was a time control. In all groups, one kidney was acutely decapsulated, and the contralateral kidney was used as control. Renal decapsulation was used to control RIHP. In one group, 3 μg·kg^-1·h^-1 of synthetic ANF were infused intravenously (iv) and produced a plasma level of ANF (P(ANF)) of 810 ± 186.5 pg/ml. This pharmacological dose of ANF produced a significant increase in RIHP of the control kidney from 9.5 ± 0.8 to 11.1 ± 1.3 mmHg (P < 0.05) but not in the decapsulated kidney [from 7.1 ± 0.6 to 8.1 ± 0.9 mmHg, not significant (NS)]. However, the changes in fractional excretion of sodium (FE(Na)) and urine flow rate (V̇) as a result of ANF infusion were similar in both kidneys. In the decapsulated kidney, FE(Na) and V̇ increased by 1.53 ± 0.41% and 26.21 ± 5.98 μl/min, respectively, from control to ANF infusion periods. In the control kidney, FE(Na) and V̇ increased by 1.60 ± 0.28% and 31.61 ± 5.87 μl/min, respectively, from control to ANF infusion periods. In the second group, 1 μg·kg^-1·h^-1 iv of synthetic ANF was infused and produced 165.2 ± 29.3 pg/ml of P(ANF). This physiological dose of ANF produced no significant increases in RIHP of decapsulated or control kidneys. In addition, the increase in FE(Na) and V̇ as a result of ANF infusion was similar in both kidneys. FE(Na) and V̇ increased by 0.82 ± 0.19% and 13.17 ± 2.14 μl/min for the decapsulated kidney and by 0.72 ± 0.24% and 16.72 ± 3.45 μl/min for the control kidney from control to ANF infusion periods. Thus ANF infusion produces a significant increase in RIHP only in pharmacological doses. When this increase in RIHP is prevented from occurring by acute renal decapsulation, and during the infusion of a physiological dose of ANF, the natriuresis and diuresis still occur in a similar degree. We conclude that elevations in RIHP are not essential for the ANF-induced natriuresis and diuresis.