Elevated renal perfusion pressure does not contribute to natriuresis induced by isotonic saline infusion in freely moving dogs

Abstract

The study was designed to determine to what extent moderate elevation of renal perfusion pressure (RPP) via the mechanism of 'pressure natriuresis' contributes to the natriuresis induced by acute i.v. saline loading. Nine Beagle dogs maintained on ample sodium intake (5.5 mmol (kg body mass)(-1) day(-1)) were chronically equipped with an aortic occluder to servocontrol RPP, a bladder catheter to measure renal function, and catheters for measurement of RPP and mean arterial blood pressure (MABP). A swivel system allowed free movement in the kennel during experiments. Isotonic saline loading (500 ml in 100 min) was studied as follows: with and without servocontrol of RPP, and these two protocols repeated in the presence of angiotensin-converting enzyme inhibition (ACEI, Enalapril, 2 mg (kg body mass)(-1)). Saline loading increased MABP by about 12 mmHg and sodium excretion from about 28 micromol min(-1) up to about 350 micromol min(-1). Without ACEI, servocontrol of RPP at 10% below control 24 h MABP slightly delayed the onset of the saline-induced natriuresis, but did not reduce peak sodium excretion or cumulative sodium excretion. The slight delay most probably resulted from pressure-controlled renin release because, with ACEI, servocontrol of RPP did not delay or reduce the saline-induced natriuresis. In conclusion, pressure natriuresis does not contribute to the natriuresis following acute saline loading.

Figure 1. 20-min mean values of systemic mean arterial blood pressure (MAPB) and renal perfusion pressure (RPP)

Values are mean ±s.e.m. of 9 dogs studied in 4 protocols during 3 control periods (C1-C3), 5 periods with Na⁺ loading by i.v. infusion (I1–I5) and 2 postinfusion periods (P1, P2). Grey bar indicates period of infusion. RPP was free to change with MABP in protocols fRPP and fRPP + E (E = Enalapril), and was servocontrolled at pre-set reduced levels in protocols scRPP and scRPP + E. Individual pre-set levels of servocontrol of RPP were used to account for the interindividual variability of control MABP. For these protocols, RPP is plotted separately (error bars mirror individual pre-set levels). Significant differences obtained by GLM ANOVA for repeated measurements followed by Duncan's multiple comparison test: * significant versus fRPP, ‡ significant versus scRPP; within each protocol, MABP was higher during I2 through P2 as compared with control periods.

Figure 2. Urinary sodium excretion (A) throughout the experiments, and cumulative urinary sodium excretion at the end of the respective infusion and postinfusion periods (B)

Grey bars indicate period of infusion. Values are mean ±s.e.m. of 9 dogs studied in 4 protocols. For protocols and periods see legend to Fig. 1. Significant differences: * significant versus fRPP, ‡ significant versus scRPP; within each protocol, sodium excretion was higher during I1 through P2 as compared with control periods.

Figure 3. GFR (exogenous creatinine clearance, A), fractional sodium excretion (B) and fractional lithium excretion (C)

Grey bars indicate period of infusion. Values are mean ±s.e.m. of 9 dogs studied in four protocols. For protocols and periods see legend to Fig. 1. Significant differences: * significant versus fRPP, ‡ significant versus scRPP; within each protocol, GFR was higher during I3 through P2, and Fe_Na and Fe_Li were higher during I2 through P2 as compared with control periods.

Figure 4. Plasma renin activity (PRA, log axis, A), and plasma aldosterone concentration (PAC, B)

Grey bars indicate period of infusion. Values are mean ±s.e.m. of 9 dogs studied in four protocols. For protocols and periods see legend to Fig. 1. Significant differences: * significant versus fRPP, † significant versus fRPP + E, ‡ significant versus scRPP; within each protocol, PRA was lower in I5 and P2 as compared with control periods; in scRPP, PAC was lower in I5 and P2 as compared with control periods.