Renal Disposal

of Human

H. Vierhapper,

Atria1 Natriuretic

S. Gasic, P. Nowotny,

Peptide

in Man

and W. Waldhksl

In healthy men (n = 7) the renal fractional extraction of human atrial natriuretic factor (hANP) as determined by the renal venous catheter technique was approximately 50% both under basal conditions and during the administration of exogenous hANP. When arterial and venous plasma concentrations of hANP were maintained about tenfold above basal concentrations by a bolus- (100 pg) primed intravenous (IV) infusion (100 &g/h for 1 hour) of hANP, renal uptake of hANP increased from, basal, 11.2 2 6.7 pmol/min to 126.5 + 64.8 pmol/min (P < .05), while estimated renal plasma flow (ERPFI decreased by about 25% (P < .05). Total metabolic clearance rates (MCRs) of hANP, renal clearance rates, and production rates of hANP were 3.89 + 1.21 L/min, 0.42 * 0.18 L/min, and 76.1 * 52.7 pmol/min, respectively. In healthy men, one kidney accounts for about 10% of total hANP clearance. 0 1990 by W.B. Saunders Company.

U

SING THE HEPATIC venous catheter technique, we recently examined the splanchnic disposal of human atria1 natriuretic factor-(99- 126) (hANP)‘** in healthy men.3 To supplement these data an analogous series of experiments was performed to study the contribution of the kidneys to the metabolism of hANP in healthy men. MATERIALS AND METHODS Subjects Seven healthy non-obese male volunteers, aged 20 to 30 years, were carefully informed about the aim and the possible risks of the study and gave their written consent to participate. The protocol was reviewed and approved by the local ethical committee. No medication was permitted for at least 4 weeks prior to the study. In order to achieve homogeneity in sodium balance all subjects were told to consume their regular diet with 3 g of added salt (sodium chloride) per day (I g three times a day) for 3 days before the test. Protocol The studies were performed with the subjects in the recumbant position after a l2-hour overnight fast. Catheters were inserted percutaneously into a peripheral vein, an inguinal artery, and a renal vein under fluoroscopic control. The volunteers then received a bolus-primed, intravenous (IV) infusion of p-aminohippuric acid (PAH; Nephrotest, Biologische Arbeitsgemeinschaft GmbH, Lich/ Hessen, FRG; bolus of 550 mg followed by a continuous infusion of 27.5 mg/min). Following an equilibration period of 75 minutes, the volunteers received an additional bolus-primed infusion of hANP (hANaP; Chemie Bissendorf, Bissendorf, FRG) dissolved in Haemacccl (Behring, Marburg/Lahn, FRG); bolus of 100 fig, followed by a continuous infusion of 100 pg/h for I hour. Arterial and renal venous blood samples were collected simultaneously for the determination of plasma concentrations of hANP and of PAH at - 30, - 15,0, 15, 30,45. and 60 minutes. Analytical

Procedures

and Calculations

Renal plasma flow (ERPF) was estimated by the constant infusion of PAH. Fractional extraction of hANP was expressed as arteriorenal venous differences/arterial concentrations. Renal uptake of hANP was calculated as arteriorenal venous concentration differences times ERPF. The renal clearance rate was calculated as the renal uptake divided by the arterial concentrations. Calculation of the tot.31 metabolic clearance rate of hANP (MCR; infusion rate/ mean steady-state concentrations - basal concentrations) was based on arterial plasma concentrations of hANP estimated during the final 15 minutes of exogenous IV hANP (+45 and +60 minutes). Production rates of arterial plasma concentrations of hANP were calculated as the MCR x basal arterial plasma concentrations. Metabolism, Vol39,

No 4 (April), 1990: pp 341-342

Concentrations of PAH in plasma and urine were determined photometrically by a commercially available method (Nephrotest, Biologische Arbeitsgemeinschaft GmbH, Lich/Hessen, FRG). Plasma concentrations of hANP were determined by radioimmunoassay following pre-purification on Sep-Pak Cl8 cartridges as described previously.4 Results were corrected for individual recovery (68% t 7%, n = 144). Intra- and interassay coefficients of variation of this method are 5.4% and 7.5%, respectively. Data are presented as means * SD. ANOVA for sequential data and Duncan’s multiple range test were used for statistical evaluation.’ RESULTS Renal

Disposal

of hANP

(Tables

1 and 21

During the infusion of hANP both arterial and renal venous plasma concentrations of hANP were increased about tenfold (P < .Ol). However, the difference between arterial and renal venous concentrations was maintained by a tenfold increase in the renal uptake from, basal, 11.2 + 6.7 pmol/ min to 126.5 k 64.8 nmol/min (P < .05). Renal fractional extraction and renal clearance rate of hANP did not change during the administration of exogenous hANP, whereas ERPF decreased by about 30% from, basal, 1,226 + 7 17 mL/min to 832 f 217 mL/min (P < .OS). MCR

and Production

Rate of hANP

The MCR of hANP was 3.89 2 1.21 L/min. Production rates of hANP were 76.1 + 52.7 (range, 21.0 to 176.7 pmol/min). DISCUSSION Studies performed in the isolated, perfused kidney of the rabbit and in the intact dog6 provided evidence for the considerable role of the kidney in the removal of ANP from the circulation, at least in these animal models. Substantial degradation of hANP has also been demonstrated in the rat kidney, and the major proteolytic cleavage sites within the peptide’s structure have been identified.’ In humans, extraction of hANP from arterial plasma during one passage

From the Division of Clinical Endocrinology and Diabetes Mellitus, I. Medizinische Universitiitsklinik. Wien, Austria. Address reprint requests to: H. Vierhapper. MD, Division of Clinical Endocrinology and Diabetes mellitus. I. Med. Univ.-Klinik. Lazaretrgasse 14, A-1090 Wien, Austria. o 1990 by W.B. Saunders Company. 0026-0495/90/3904-0002$3.00/O 341

VIERHAPPER ET AL

342

Table 1. Arterial

and Renal Venous Concentrations

Fractional Extraction Administration

Basal (x)

Table 2.

ERPF, Renal Uptake, and Renal Clearance

Before, During, and After IV Administration

Arterial

VWlOUS

18.7 2 a.4

9.0 + 5.2

Rate of hANP

of the Peptide in

Healthy Men

of the Peptide in Healthy Men

PlasmaConcentration of hANP Ipmol/L) Time Imin)

and Renal

of hANP Before, During, and After IV

Renal Fractional Extraction1%)

TIITW (mm) Basal (x)

50 * 21

+15

ERPF tmL/mm) 1,226?

717

Renal Uptake of hANP (pmol/minj

Renal ClearanceRate (ml/mint

11.2 f 6.7

532 * 178

923 * 303

89.1

f 68.5

375 * 215

+15

230.0

? 99.8*

125.4

r 85.6’

43 + 26

+30

928 * 471

126.5

t 64.8’

444

+30

212.9

+ 137.3*

104.5

r 59.7’

46 f 18

+45

032 + 240’

66.5

+ 40.1

363 ? 155

+60

832 f 217’

104.6

+ 95.1

465 + 243

+45

174.9

* 64.1,

100.1

+ 46.7*

43t

+60

194.0

+ 03.4’

84.4

+ 26.3’

53 _+ 18

15

+ 298

NOTE. N = 7. Values are mean + SD.

NOTE. N = 7. Values are mean ? SD.

lP < .05 as compared with basal values.

lP -c .Ol as compared with basal values.

through the circulation of the kidney has been shown to be approximately 50%.’ These data are confirmed and supplemented by the present study. The results indicate a renal fractional extraction of hANP of approximately 50% both under basal conditions and during the infusion of exogenous hANP. The contribution of each kidney to the overall MCR of ANP is about 10%. and thus roughly comparable to the share of the splanchnic area.’ The kidney is regarded as an important target organ for the effects of hANP.9 but apparently the peptide’s elimination within a vascular bed does not necessarily go together with its local physiological importance. The enhanced blood flow to the splanchnic area and the kidneys may contribute to to the enhanced clearance of hANP in the supine as compared with the upright position.” The relevance of our data, which were obtained in healthy men, obviously cannot be extrapolated to patients with renal disease where analogous experiments have yet to be performed. In regard to cardiac insufficiency where renal function tends to be diminished, reduced renal clearance of

hANP could contribute to the increase in the peptide’s plasma concentrations, although the available evidence suggests that increased cardiac secretion is the primary determinant in this regard.” Calculation of the MCR of hANP was feasible since similar plasma concentrations (both arterial and venous) during the last 15 minutes of the peptide’s infusion indicated steady-state conditions. This is in keeping with our previous report.’ However, due to local legal reasons, the infusion of a large amount of unlabeled hANP rather than of a tracerdose of labeled material had to be employed for the calculation of the MCR of hANP. Therefore, we cannot exclude the possibility that the calculated MCR may have been influenced by the experimental procedure and may differ from those found during physiological conditions. Keeping these limitations in mind, the MCR of hANP calculated from the present data is comparable to that found in our previous series of experiments and the decrease in estimated renal plasma flow is in keeping with the reported decrease in splanchnic blood flo~.‘,‘~

REFERENCES

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