Splanchnic disposal of human atrial natriuretic peptide in humans

Splanchnic

Disposal

of Human Atria1 Natriuretic

Peptide in Humans

H. Vierhapper, S. Gasic, P. Nowotny, and W. Waldhiusl In healthy men (n = 6). the splanchnic fractional extraction of human atrial natriuretic peptide (hANP), as determined by the hepatic venous catheter technique, was 75% under basal conditions resulting in a splanchnic uptake of hANP of 6.5 + 5.0 pmol/min. In spite of a drop (P c .05) in splanchnic fractional extraction to about 50%. splanchnic uptake of hANP rose to 66 to 99 pmol/min (P < .Ol) when pharmacologic plasma concentrations of hANP were induced during a bolus (100 fig)-primed intravenous (IV) infusion (I 00 wg/h; time, one hour) of hANP. This was accompanied by a fall in estimated hepatic blood flow (P -C .05). in pulmonary arterial pressure (P < .Ol), and, in each individual, in systemic BP. Total metabolic clearance rates, splanchnic clearance rates, and production rates of hANP were 4.5 + 2.2 L/min, 0.4 + 0.1 L/min, and 46.1 * 20.1 pmol/min, respectively. Thus, in healthy men, the splanchnic area accounts for approximately 10% of total hANP clearance. o 1988 by Grune & Stratton, Inc.

I

N HUMANS, the 2%amino acid polypeptide’ designated’ human atria1 natriuretic factor-(99-126) or hANP, may be of some importance in the regulation of fluid volume and BP.3-’ The compound’s pharmacologic effects and potential therapeutic use have recently attracted considerable attention 6-9but information on its metabolic fate in humans is scarce, and the importance of the splanchnic area in this context is as yet unknown. Using the hepatic venous catheter we have therefore directly examined the technique,” splanchnic disposal of hANP at physiological and pharmacological concentrations in healthy men. MATERIALS AND METHODS Subjects

Six healthy nonobese male volunteers, aged 21 to 31 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 (1 g, three times daily) for three days prior to the test. Protocol

The studies were performed with the subjects in the recumbent position after a 12-hour overnight fast. Catheters were inserted percutaneously into a peripheral vein and an inguinal artery, and into a right-sided hepatic vein under fluoroscopic control as described previously.” In four individuals, an additional balloontipped, flow-directed catheter was placed to a final position in a branch of the pulmonary artery, confirmed by fluoroscopy.” Following an equilibration period of 75 minutes, the volunteers received hANP (hANaP, Chemie Bissendorf, Bissendorf, FRG) dissolved in Haemaccel (Behring, Marburg/Lahn, FRG) in the form of a bolus (100 fig)-primed infusion (100 pgg/h, time, one hour). Blood samples were removed simultaneously for the determination of plasma concentrations of hANP from the arterial and hepatic venous catheters at -30, - 15, 0, 15, 30, 45, 60, 75, 90, 105, and 120 minutes. Arterial blood pressure (BP), pulmonary artery pressure (PAP), right atrial pressure (RAP), and heart rate (HR) were measured at the same time-intervals in four subjects. In three individuals, cardiac output (CO) was determined by the thermodilution method.” Analytic

Procedures

and Calculations

Hepatic plasma flow (EHPF) was estimated by the constant infusion of indocyanine green dye.” Fractional extraction of hANP Metabolism, Vol 37, No 10 (October), 1988: pp 973-975

was expressed as arterio-hepatic venous differences per arterial concentrations. Splanchnic uptake of hANP was calculated as arterio-hepatic venous concentration differences x EHPF. The splanchnic clearance rate was calculated as the splanchnic uptake divided by the arterial concentration. Calculation of the total metabolic clearance rate (MCR) of hANP (MCR: infusion rate per mean steady-state concentrations minus basal concentrations) was based on arterial plasma concentrations of hANP. Production rates of hANP were calculated as the MCR x basal arterial plasma concentrations. Plasma concentrations of hANP were determined by radioimmunoassay following prepurification on Sep-Pak Cl8 cartridges as described previously.g Results were corrected for recovery (68% + 7%, n = 144). Intra-assay and interassay coefficients of variation of this method are 5.4% and 7.5%, respectively. Data are presented as means r SD. ANOVA for sequential data and Duncan’s multiple range test were used for statistical evaluation.” RESULTS Hemodynamic

Effects

of hANP

Table 1 shows that in the four subjects in whom hemodynamic measurements were performed, hANP induced a fall in both systolic and diastolic pulmonary BP. No major changes were seen during the administration of hANP in HR and RAP, and the decrease in arterial BP from 124 + 4167 k 6 mm Hg to 108 + 32/60 + 19 mm Hg was not statistically significant. Nevertheless, one subject presented a dramatic fall in systolic BP to 60 mm Hg towards the end of the one-hour infusion period necessitating the immediate termination of the experiment. In two individuals, hANP induced a fall in cardiac output from 6.5 L/min to 5.8 L/min and from 7.4 L/min to 6.4 L/mitt, respectively, whereas cardiac output increased from 5.6 L/min to 7.3 L/min in a third subject.

From The Division of Clinical Endocrinology and Diabetes Mellitus, I. Medizinische Universitiitsklinik, Wien. Austria. Supported by the ‘%onds zur Forderung der wissenschaftlichen Forschung &terreichs,” Grant No. P6539M. Address reprint requests to H. Vierhapper. MD, Division of Clinical Endocrinology and Diabetes mellitus, I. Medizinische Universitiits-Klinik. Lazarettgasse 14. A-1090 Wien. Austria. o 1988 by Grune & Stratton, Inc. 0026-0495/88/3710-0012$03.00/0

973

VIERHAPPER ET AL

974

Table 1. Effect of a Bolus-Primed Infusion of hANP on HR and on Arterial, Right Atrial, and Pulmonary Blood Pressure HR lbeats/min)

Time (mitt)

in Healthy Men (n = 4)

ArterialBP LmmHg) Svstolic

PulmonaryArtery Pressure(mm Hg)

Diastolic

RAP

Systolic

Basal

62 r 7

124 f 4

67 * 6

4&2

24 + 3

10 f 2

+15

642

15

122 + 6

67 f 6

3+1

20 f 2’

8?

+30

78 + 18

121 f 4

70 * 8

3&2

19 * 3t

8 * 2t

+45

56 f 6

108+32

60+

19

3+2

18 + 3t

6 + 2t

+60

61 ?9

108 f 32

60+

19

2r3

17 * 3t

6 + 2T

+75

72 + 25

116 f 14

68 * 7

4+3

19 f 2t

8+

+90

68 + 25

113 + 12

65 f 3

3+1

19 f 2t

9&O

+105

70*

18

116 + 7

66 * 2

4+2

20 * 2t

8 & 2*

+ 120

76+

17

117 f 15

65 + 5

4*1

20 * 2’

9+1

lt

1’

Values represent the mean ? SD (n = 4). lP c .05 as compared with basal values. tP < .Ol as compared with basal values.

Metabolic Clearance Rate and Production Rate of hANP

Based on the arterial plasma concentrations of hANP estimated during the final 15 minutes of exogenous intravenous (IV) hANP (+45 and +60 minutes) the MCR of hANP was 4.5 2 2.2 L/min. Production rates of hANP were 46.1 * 20.2 pmol/min. Splanchnic Disposal of hANP

In the basal state, the marked differences in arterial and hepatic venous concentrations of hANP indicated a splanchnit fractional extraction of approximately 75% and a splanchnic uptake of hANP of approximately 8.5 pmol/min (Tables 2 and 3). Both arterial and hepatic venous plasma concentrations of hANP were markedly elevated during the infusion of the peptide and promptly returned to basal concentrations thereafter. In spite of a marked quantitative rise in splanchnic uptake of hANP during the infusion of hANP, splanchnic fractional extraction and splanchnic clearance rate decreased and did not return to basal values within 60 minutes after the end of the infusion, whereas EHPF, though also decreased during administration of exogenous hANP, returned to basal values after the end of the infusion of hANP. This fall in EHPF was seen in each Table 2. Arterial and Hepato-Venous Concentrations and Splanchnic Fractional Extraction of hANP Before, During, and

subject including the individual in whom we observed a rise in CO. DISCUSSION

Since the description of the structure of hANP,’ an increasing number of investigations have dealt with both plasma hANP concentrations in healthy humans as well as in various pathological conditions such as heart failure, cirrhosis of the liver, and hypertension, as well as with the pharmacological effects of this peptide following its IV administration.3’9 Concerning the peptide’s elimination, data recently obtained in vitro have suggested that a considerable share of hANP receptors may be biologically silent and may be related to the peripheral clearance of hANP.14 A rapid metabolism of hANP (t’/2 = 2.5 minutes) in humans is apparent from the determination of its plasma concentrations following its IV administration.” In the present study, a total MCR of hANP of 4.5 * 2.2 L/min was calculated from arterial plasma samples. In regard to the arterial-venous differences in the concentrations of hANP, this value is not at variance with the lower MCR (2.4 L/min) recently calculated from venous plasma samples.‘s Although it has been suggested that there might be a rapid metabolism Table 3. EHPF, Splanchnic Uptake, and Splanchnic Clearance Rate of hANP Before, During, and After IV Administration of the Peptide in Healthy Men

After IV Administration of the Peptide in Healthy Man PlasmaConcentration of hANP tpmol/L) Arterial

VetlOW

11.7 + 6.0

2.7 + 1.5

Time tmin) Basal

Splanchnic Fractional Extraction1%) 76+

10

Time (mini

EHPF (mL/minl

Splanchnic Uptake of hANP (pmol/minl

Basal

947 + 165

+15

973 + 196

98.7

8.5 t 5.0 f 58.08

Splanchnic ClearanceRate (mL/min) 731 f 175 433 + 226t 370 * 1771

+15

249.9

f 154.8*

142.0

+ 134.7*

45 f 23T

+30

736 + 173t

79.4 f 66.4*

+30

202.3

+ 174.1’

82.4

& 82.7t

49 f 23T

+45

733 f 106t

56.2

+ 42.7t

341 + 151’

+45

160.0

* 83.7*

80.2

f 59.lt

47 * 23t

+60

747 + 12t

76.0

+ 64.9’

349 + 155’

+60

177.0

f 137.2’

75.9

+ 56.2t

53 + 16

+75

899 + 195

2.3 + 1.5

114 * 21’

+75

18.6 * 8.6

15.5 * 6.3

15 + 5’

792 i

3.5 + 2.7

352 + 180.

+90

9.7 + 3.9

5.6 & 2.5

42 + lot

+90 +105

972 * 370

1.8 + 1.6

342 + 229*

+105

6.3 * 2.2

3.9 + 2.6

40 f 32*

+120

913 + 443

2.0 + 1.6

433 f 402t

+120

5.7 * 2.3

3.9 f 2.5

36 + 25*

183

Values represent the mean + SD (n = 6).

Values represent the mean + SD (n = 6)

lP < .Ol as compared with basal values.

P < .05 as compared with basal values.

TP < .05 as compared with basal values.

975

SPLANCHNIC DISPOSAL OF HANP

and/or uptake of hANP across a wide variety of vascular beds,15 this problem has not as yet been studied in humans. Experiments using perfused lungs of guinea pigsI or rabbits” have demonstrated removal of ANP in the pulmonary circulation, but this effect was not reproduced in intact animals.‘6*‘8 Renal uptake of hANP is considerable in the rabbitI and possibly in humans (unpublished data). This does not come as a surprise since the kidney is regarded as an important target organ for the pharmacological and possibly the physiological effects of hANP.3-9 Little attention has, however, been given until now to the splanchnic area in regard to the metabolism of hANP, although the presence of binding sites on hepatocytes could suggest a metabolic role of ANP in that organI The aim of the present investigation was to define the role of the splanchnic region in the metabolism of endogenous and exogenous ANP. Based on an overall MCR of hANP of 4.5 L/min, the data of our study indicate that during exogenous hANP administration the splanchnic area accounts in healthy humans for about 0.4 L/min, ie, 10% of the overall hANP disposal. Both the physiological significance of this finding and the potential alterations in hANP metabolism in hepatic diseases are unclear at present. In regard to the effect of exogenous hANP on splanchnic blood flow, our data, which were obtained by hepatic

catheterization and the constant infusion of indocyanine green dye, are in keeping with a recent report by Biollaz et al,” who observed a fall in hepatic blood flow by 21% during an infusion of 1 Kg hANP/kg. Since we determined EHPF but not CO in each of our volunteers, we cannot definitively comment on the interdependence of these two hemodynamic variables. While this question needs to be addressed in a larger group of subjects, it is noteworthy that we observed a fall in EHPF even in one individual who simultaneously presented an increased CO. Thus, local effects of hANP on vascular resistance which, at least in the rat, vary among different vascular beds and depend on the mode of administration of hANP,*’ also may have contributed to the observed fall in EPHF. However, since our experimental protocol did not include the measurement of blood flow in other vascular beds, this explanation is, at present, hypothetical. In keeping with previous reports, hANP induced a fall in peripheral’,’ and in pulmonary BP.**Special attention should be given to the pronounced hypotensive reaction observed in one individual during the infusion of hANP, which confirms an experience previously reported’ and cautions against the use of too-large doses of hANP. In conclusion, the results of our study indicate an important role for the splanchnic area in the disposal of endogenous hANP in healthy humans.

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