INTRODUCTION

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URODILATIN is a 32-amino acid peptide isolated from human urine (39). Except for a four-amino acid NH₂-terminal extension, it is identical to atrial natriuretic peptide (ANP), indicating that urodilatin is a peptide probably produced from the same precursor as ANP (36). Urodilatin has not been detected in plasma (15), and it seems plausible that urodilatin is produced by renal tubular cells and secreted into the lumen of distal tubules and collecting ducts, where, by binding to ANP receptors, it may play a role in the regulation of sodium excretion (20). Early on it was reported that urodilatin, in contrast to ANP, was not inactivated by peptidase from dog kidney cortex membranes (22). These findings were supported by studies in rats demonstrating that inhibition of neutral endopeptidase was without significant effect on the metabolic clearance of urodilatin, whereas the clearance of ANP decreased markedly (1). Several studies have demonstrated that exogenous urodilatin exerts ANP-like effects on the kidney, i.e., a reduction of tubular sodium and water reabsorption, thereby increasing renal excretion (for review see Ref. 23). Microcatheterization studies in rats revealed that the two peptides are equally potent inhibitors of sodium transport in the inner medullary collecting duct (40). In experimental animals and humans, comparisons between the effects of equimolar doses of urodilatin and ANP indicate that the renal effects of urodilatin are more pronounced than those of ANP (7, 24, 37). Accordingly, we showed that also under conditions of similar increases in plasma immunoactivity of the two peptides, urodilatin produces natriuretic and diuretic effects stronger than those of ANP (7).

It has been reported that the rate of excretion of urodilatin in humans varies with sodium intake and that natriuretic and diuretic effects are produced at an infusion rate of 20 ng · kg¹ · min¹ without apparent depressant effects on the cardiovascular system (34). Bolus injections of urodilatin to healthy men produced more potent renal effects and smaller decreases in blood pressure than ANP (37). These findings have been taken to indicate that urodilatin could be useful in the treatment of electrolyte and fluid imbalance. Clinical investigations have suggested that urodilatin may possess a clinical potential, e.g., in the treatment of congestive heart failure (16) and acute renal failure after liver transplantation (13). A recent dose-response study in healthy men showed that infusion of urodilatin at 20 ng · kg¹ · min¹ for 60 min inhibited the renin-angiotensin-aldosterone system (RAAS) and produced a pronounced natriuretic response that was attenuated with higher doses (12). Detailed dose-response studies investigating the concomitant effects of urodilatin on vasoactive hormone concentrations and the cardiovascular system have not previously been presented, and it remains unsettled whether urodilatin in humans exerts dose-dependent, adverse effects on the cardiovascular system that, in higher doses, may counteract the natriuresis.

The aim of the present study was to investigate the dose-dependent effects of 2-h infusions of urodilatin in doses of 5, 10, 20, and 40 ng · kg¹ · min¹ on cardiovascular, endocrine, and renal function in healthy subjects. Furthermore, a newly developed RIA was used to assess the metabolism of the infused urodilatin.

	METHODS

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Subjects. Eight men (21-32 yr; 66.2-83.8 kg, mean 76.3 kg body wt; 172-191 cm, mean 184 cm height) were studied after they had given written informed consent. Each subject was investigated on five different occasions separated by intervals of 7 days. The subjects were supplied with a fixed diet containing 150 mmol sodium/day and were instructed to avoid strenuous physical activity and to abstain from smoking and from alcohol- and caffeine-containing drinks for 3 days before each study day. The protocol was approved by the Regional Scientific Ethical Committee of Copenhagen County (KA93264s) and the Danish National Board of Health (5312-408-1993).

Protocol. Urine was collected for 24 h before each study day. The mean sodium excretion rates for the five series (placebo and 5, 10, 20, and 40 ng · kg¹ · min¹) were 165 ± 18, 122 ± 9, 166 ± 10, 156 ± 15, and 157 ± 11 (SE) mmol/24 h, respectively. After an overnight fast, the subjects arrived at the laboratory at 0800. They were confined to a half-seated position in a hospital bed (45° elevation of the head) except for briefly standing for voiding every 30 min. Venous cannulas were inserted into both cubital veins for infusions and withdrawal of blood, respectively. Water diuresis was induced by oral water intake (250 ml of tap water every 30 min); in addition, infusion of isotonic glucose an a rate of 150 ml/h was used for the administration of nuclides (see below). Steady state was assumed when urine flow rates approximately equaled water intake. Thereafter, urine was sampled for eight consecutive 30-min periods: two baseline periods, four infusion periods, and two recovery periods. Placebo or urodilatin in doses of 5, 10, 20, or 40 ng · kg¹ · min¹ was infused intravenously (0.1 ml · kg¹ · h¹). The infusions were administered in random order in a double-blind fashion. The coded preparations of placebo and urodilatin were supplied by HaemoPep Pharma (Hannover, Germany). Sodium and potassium concentrations were measured in all urine samples. In periods 2, 6, and 8, urine osmolality and urinary concentrations of albumin, cGMP, and urodilatin, as well as hematocrit, Hb, and plasma concentrations of protein, albumin, renin, ANG II, aldosterone, ANP, arginine vasopressin (AVP), and cGMP, were measured. Body weight was measured on arrival at the laboratory and at the end of periods 2, 6, and 8. The subjects were requested to report headache, dizziness, or nausea.

Cardiovascular measurements. Arterial blood pressure was measured every 10 min. Mean arterial pressure (MAP) was calculated as two-thirds of the diastolic pressure plus one-third of the systolic pressure. Stroke volume, heart rate (HR), and cardiac output were obtained by a transthoracic impedance technique with use of an impedance cardiograph (model NCCOM-3, BoMed Medical Manufacturing). Stroke volume was calculated automatically by use of the empirical formula originally described by Kubicek, as later modified by Bernstein (6). The cardiograph interfaced with a personal computer, and the parameters, calculated as the mean value from 12 cardiac cycles, were sampled continuously and saved for later calculations of hourly mean values. Recordings during voiding and for 10 min thereafter were excluded from the calculations. The reliability of impedance cardiography as applied in the present investigation has previously been confirmed (46). In physiological conditions, relative changes measured by impedance cardiography have been found to be concordant with results obtained by thermodilution, electromagnetic flowmeter, and application of Fick's principle (46).

Renal measurements. Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were measured by a constant-infusion technique with urine collections with use of ¹³¹I-labeled hippuran and ^99mTc-diethylenetriaminepentaacetate (DTPA), respectively. The lithium clearance method was used to assess effects of urodilatin on segmental tubular sodium reabsorption (43). Lithium carbonate (300 mg, 8.1 mmol) was ingested 12 h before the start of the experiments. Hourly clearance values for ¹³¹I-hippuran (ERPF), ^99mTc-DTPA (GFR), lithium (C_Li), and sodium (C_Na) were calculated from the urinary excretion rates and the mean values of plasma samples drawn 15 and 45 min into each clearance period. Segmental renal tubular reabsorption rates were calculated using the following formulas: absolute proximal reabsorption rate (APR) = GFR  C_Li; absolute distal reabsorption rate of sodium (ADR_Na) = C_Li  C_Na × P_Na (where P_Na is the plasma sodium concentration); fractional proximal reabsorption (FPR) = 1  C_Li/GFR; fractional distal reabsorption of sodium (FDR_Na) = 1  C_Na/C_Li.

Analytic methods. Activities of ¹³¹I-hippuran and ^99mTc-DTPA in plasma and urine were determined by a well counter. Plasma and urine concentrations of lithium were measured by atomic absorption spectrophotometry (model 403, Perkin-Elmer, Norwalk, CT). Urine concentrations of sodium and potassium were measured with a Bayer RA-XT instrument (Bayer, Tarrytown, NY). Plasma concentrations of sodium, potassium, albumin, and protein as well as Hb concentration and hematocrit were measured by a Bayer SMAC3 instrument. Concentrations of albumin in urine were measured by an enzyme immunoassay, as previously described (19). Osmolality in urine was measured by freezing-point depression (Osmomat 030D, Gonotec, Berlin, Germany).

View larger version (13K): [in this window] [in a new window]   Fig. 1.   RIA for urodilatin. A: Standard curve. , Urodilatin; , human atrial natriuretic peptide (ANP); , rat ANP. B: HPLC separation of urodilatin immunoactivity measured by RIA. , Urine; , urine + urodilatin; , urodilatin assay standard (diluted 1:1,000). C: HPLC fractionation of 200 pmol of urodilatin immunoreactivity from 3 samples. , Synthetic urodilatin added to assay buffer; , synthetic urodilatin added to urine and immediately subjected to HPLC; , synthetic urodilatin added to urine 24 h before HPLC (urine sample was stored at room temperature).

Missing values. After occurrences of syncope during infusion of urodilatin at 40 ng · kg¹ · min¹, the National Board of Health withdrew permission for study of this dose. Therefore, only six of the eight subjects received the infusion of urodilatin at 40 ng · kg¹ · min¹. During infusion of the two highest doses, several subjects were unable to pass urine for one or more 30-min urine-sampling periods (all subjects during 40 ng · kg¹ · min¹ and 3 subjects during 20 ng · kg¹ · min¹). The mean clearance values (60 min) were therefore calculated from the subsequent urine samples. With infusion of 40 ng · kg¹ · min¹, two subjects were unable to pass urine in the 60-min recovery period. Therefore, mean clearance values of this period were calculated from four subjects. In case of lack of urine samples from period 6 or 8, osmolality, albumin, urodilatin, and cGMP were measured on samples from the previous period.

Statistical analysis. Values are means ± SE. Data were subjected to a two-way ANOVA for repeated measures within series and between series. In case of P < 0.05, the differences between baseline period and infusion periods and the differences between placebo and infusion series were evaluated by the Newman-Keuls test. P < 0.05 was considered to indicate significance. In case of apparent variance inhomogeneity, the data were subjected to logarithmic transformation before statistical evaluation.

	RESULTS

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Generally, infusion of urodilatin elicited dose-dependent changes in the measured parameters, but exceptions were observed, e.g., cardiac output decreased only by low doses of urodilatin and GFR apparently remained unchanged irrespective of dose.

Cardiovascular effects. MAP remained unchanged during infusions of placebo and 5 and 10 ng · kg¹ · min¹ but decreased with 20 and 40 ng · kg¹ · min¹ by 6 and 14%, respectively (Fig. 2); the decreases occurred concomitantly with increases in HR by 15 and 38% (Fig. 2). Stroke volume decreased with all doses of urodilatin and remained decreased throughout the 1-h recovery period: from 100 ± 9 to 90 ± 8 ml with 5 ng · kg¹ · min¹, from 105 ± 9 to 84 ± 7 ml with 10 ng · kg¹ · min¹, from 101 ± 13 to 81 ± 10 ml with 20 ng · kg¹ · min¹, and from 107 ± 14 to 87 ± 16 ml with 40 ng · kg¹ · min¹ (Fig. 2). Cardiac output decreased during infusion of 5 ng · kg¹ · min¹ (from 5.2 ± 0.3 to 4.8 ± 0.2 l/min) and 10 ng · kg¹ · min¹ (from 5.5 ± 0.3 to 4.6 ± 0.2 l/min) but remained unchanged during administration of the two higher doses (Fig. 2). However, in the recovery period after 20 ng · kg¹ · min¹, cardiac output decreased from 5.0 ± 0.3 to 4.6 ± 0.2 l/min.

View larger version (35K): [in this window] [in a new window]   Fig. 2.   Cardiovascular effects of urodilatin infusion. A: mean arterial pressure (MAP). B: heart rate (HR). C: stroke volume. D: cardiac output. Values are means ± SE; n = 8, except at 40 ng · kg1 · min1, where n = 6. Open bars, baseline; filled bars, 1st h of infusion; hatched bars, 2nd h of infusion; crosshatched bars, recovery. * Significantly different from baseline (P < 0.05). § Significantly different from placebo at same time (P < 0.05).

Metabolism of urodilatin. Because the ANP immunoreactivity in plasma was measured by an antibody against ANP that cross-reacts 100% with urodilatin, the ANP immunoreactivity results represent the sum of the plasma concentrations of ANP and urodilatin. Compared with placebo values of 83 ± 9 pg/ml, ANP immunoreactivity increased dose dependently with the four infusion rates to 107 ± 9, 157 ± 9, 283 ± 27, and 594 ± 76 pg/ml, respectively, and returned to preinfusion level within the recovery period (Table 1). Under the assumption that the infusions of urodilatin did not change the plasma concentrations of ANP, the plasma concentrations of urodilatin were calculated as the difference between baseline and infusion values of ANP immunoreactivity corrected for the extraction recovery and for the different mass of urodilatin. Thus the estimated urodilatin concentrations were 21 ± 6, 80 ± 8, 208 ± 32, and 539 ± 89 pg/ml with infusion rates of 5, 10, 20, and 40 ng · kg¹ · min¹, respectively. With the assumption that metabolic clearance is identical to the rate of infusion (steady state), the corresponding metabolic clearance rates were calculated by dividing the estimated plasma concentration by the appropriate infusion rate and were 405 ± 111, 135 ± 16, 117 ± 22, and 85 ± 17 ml · kg¹ · min¹, respectively.

Endocrine effects. Plasma concentrations and the urinary excretion rates of cGMP followed the same pattern as ANP immunoreactivity, with the noticeable exception that these variables remained elevated in the recovery periods (Table 1).

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Renal hemodynamics. ERPF decreased by infusion of all doses of urodilatin by 13, 23, 22, and 37% for 5, 10, 20, and 40 ng · kg¹ · min¹, respectively, and remained decreased in the recovery period (Fig. 3). None of the urodilatin doses changed GFR measurably, except for a small decrease in the recovery period after 40 ng · kg¹ · min¹ (n = 3; Fig. 3). Consequently, filtration fraction increased dose dependently from a placebo value of 21 ± 1% to a maximum of 28 ± 1% (Fig. 3).

View larger version (41K): [in this window] [in a new window]   Fig. 3.   Renal hemodynamic effects of urodilatin infusion. A: effective renal plasma flow (ERPF). B: glomerular filtration rate (GFR). C: filtration fraction. D: lithium clearance. See Fig. 2 legend for explanation of bars and symbols.

Segmental tubular reabsorption rates. C_Li increased significantly with all doses of urodilatin from a placebo value of 32 ± 2 to 37 ± 1, 38 ± 2, 40 ± 3, and 40 ± 3 ml/min at 5, 10, 20, and 40 ng · kg¹ · min¹, respectively (Fig. 3). APR remained unchanged except for a small but significant decrease in the recovery period after the highest dose (n = 3; Fig. 4). However, all four doses of urodilatin decreased FPR significantly from a placebo value of 72 ± 1 to 69 ± 2, 67 ± 2, 68 ± 2, and 66 ± 2%, respectively (Fig. 4). ADR_Na was increased by all doses of urodilatin from 4.2 ± 0.2 to 4.9 ± 0.2, 4.9 ± 0.2, 5.0 ± 0.4, and 5.2 ± 0.3 mmol/min, respectively (Fig. 4). Despite this increase in absolute reabsorption, FDR_Na decreased from a placebo value of 94.9 ± 0.5 to 94.3 ± 0.7, 92.6 ± 0.9, 90.6 ± 1.0, and 92.2 ± 0.7% (Fig. 4). The value of the lowest dose was significantly different from the baseline value (within series) but not from the placebo value (between series).

View larger version (40K): [in this window] [in a new window]   Fig. 4.   Segmental tubular reabsorption rates. A: absolute proximal reabsorption (APR). B: fractional proximal reabsorption (FPR). C: absolute distal reabsorption of sodium (ADRNa). D: fractional distal reabsorption of sodium (FDRNa). See Fig. 2 legend for explanation of bars and symbols.

Excretion of water, sodium, potassium, and albumin. Urine flow rates were high (12-14 ml/min) because of the water load but increased compared with placebo at the two highest doses (Fig. 5). During and after infusion of 20 and 40 ng · kg¹ · min¹, the urine flow rate followed a biphasic pattern. In the 20 ng · kg¹ · min¹ series, the urine flow rate increased in the 2nd h of infusion to 15.8 ± 1.3 ml/min but decreased in the recovery period to 8.2 ± 1.9 ml/min. In the 1st h of infusion of 40 ng · kg¹ · min¹, urine flow rate increased to 15.9 ± 1.0 ml/min, but thereafter it fell markedly in the 2nd h of infusion to 6.4 ± 0.8 ml/min and particularly in the recovery period to 1.5 ± 0.6 ml/min, despite the sustained overhydration.

View larger version (38K): [in this window] [in a new window]   Fig. 5.   Renal effects of urodilatin infusion. A: urine flow rate. B: sodium excretion rate. C: potassium excretion rate. See Fig. 2 legend for explanation of bars and symbols.

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View larger version (16K): [in this window] [in a new window]   Fig. 6.   Albumin excretion rate: effect of urodilatin infusion. Values are means ± SE; n = 8, except at 40 ng · kg1 · min1, where n = 6. Open bars, baseline; hatched bars, end of infusion; crosshatched bars, recovery. * Significantly different from baseline (P < 0.05). § Significantly different from placebo at same time (P < 0.05).

Other parameters. Hematocrit increased compared with baseline and placebo in the 2nd h of infusion and the recovery period of the three highest doses (e.g., at 40 ng · kg¹ · min¹ from 41 ± 1 to 46 ± 1%; Table 2). Hb concentration and plasma concentrations of albumin and protein showed a similar pattern (Table 2). Plasma sodium concentration increased compared with baseline at the three highest doses, whereas plasma potassium concentrations decreased in these series (Table 2).

Unscheduled events. Unexpected effects were observed during the 2nd h of urodilatin infusion or later. Dizziness and/or nausea could appear up to 90 min after the infusion. One or the other was experienced by all six subjects receiving 40 ng · kg¹ · min¹, by six of eight subjects receiving 20 ng · kg¹ · min¹, and by two of eight subjects receiving 10 ng · kg¹ · min¹. In some cases, dizziness was experienced only when the subjects stood to void; in others it appeared while they sat in the bed, preventing voiding in the upright position. Vomiting occurred in one subject. Syncope was elicited in one subject at 20 ng · kg¹ · min¹ and in two subjects at 40 ng · kg¹ · min¹, after which further investigation of this dose was discontinued. When syncope occurred, the infusion was stopped and the subject was placed in the Trendelenburg position. Consciousness was regained within 10 s, and full recovery took place within 1 h in all subjects.

	DISCUSSION

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The present results show that infusions of urodilatin to normal humans at doses on the order of 1-10 pmol · min¹ · kg body wt¹ elicit pronounced cardiovascular, endocrine, and renal effects. The receptor responsible for the effects of ANP (ANP-R_A) is well described. This receptor has been identified by the presence of its corresponding mRNA, most abundantly in the kidney and the adrenal glands, but also in the heart, aorta, and cerebellum (25). ANP and urodilatin appear to have equal affinities to ANP-R_A and are apparently equally potent at generating cGMP, the second messenger of these peptides (44). Therefore, the effects of exogenous urodilatin are likely to be elicited by binding to ANP receptors, and it could be expected that the effects are similar to those of ANP. However, as mentioned in the introduction, previous investigations suggested that, compared with ANP, urodilatin is relatively resistant to degradation by peptidase in the kidney tubules (1, 22). This difference may be responsible for quantitative differences between the effects of the two peptides.

Cardiovascular effects. It is demonstrated for the first time that even small doses of urodilatin may decrease stroke volume and cardiac output without changes in plasma volume or blood pressure. Stroke volume decreased by all doses of urodilatin. Cardiac output decreased at 5 and 10 ng · kg¹ · min¹, but not at 20 and 40 ng · kg¹ · min¹, probably because of the concomitant increases in HR. The decrease in stroke volume in response to urodilatin infusion may be explained by a negative inotropic effect and/or a reduction in preload. Direct investigations of human papillary muscles and of isolated guinea pig atria did not demonstrate decreases in contractility in response to ANP (5, 9). Under the present conditions, it cannot be excluded that a negative inotropic action contributed to the decrease in stroke volume, but it seems unlikely in view of the previous results. More likely, a reduction in preload occurred secondary to a reduction in plasma volume due to egression of fluid from the intravascular compartment or by venodilation. At 5 ng · kg¹ · min¹, stroke volume and cardiac output decreased without concomitant changes in any of the variables used as markers of plasma volume, suggesting that venodilation was the immediate cause of these decreases. In contrast, infusions of 10, 20, and 40 ng · kg¹ · min¹ caused significant increases in hematocrit, Hb, and plasma concentrations of protein and albumin, indicating decreases in plasma volume at these doses, consistent with the results of numerous studies on ANP (3). However, the cellular mechanism responsible for this transcapillary fluid shift is unknown (35). Lee et al. (31) found that ANP infusion to ganglion-blocked dogs during ANG II-induced hypertension caused a decrease in MAP and cardiac output secondary to venodilation. In addition, Ando et al. (2) found that ANP increased venous distensibility and capillary filtration in human forearms. It seems likely, therefore, that in the present study the decreases in stroke volume and cardiac output at 5 ng · kg¹ · min¹ were caused mainly by venodilation. With higher doses, venodilation and reduction of plasma volume were probably the primary reasons for the reduction in stroke volume.

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Metabolism of urodilatin. The antibody used in the ANP assay was unable to distinguish between ANP and urodilatin. Provided that infusion of urodilatin does not change the secretion rate of ANP, calculations of the steady-state concentrations of urodilatin provided values of 21-539 pg/ml during infusions of 5-40 ng · kg¹ · min¹, respectively (see Results). The corresponding apparent metabolic clearance rates of 405 to 85 ml · kg¹ · min¹ indicate dose-dependent kinetics. However, if the rate of endogenous secretion of ANP was attenuated secondary to the hypothesized decrease in preload, these clearance rates would be erroneously high. For example, if it is assumed that all the urodilatin infusions via venodilation reduced circulating levels of ANP by 50%, then the calculated plasma concentrations of urodilatin would be 40 pg/ml higher than those estimated on the basis of constant ANP secretion. Consequently, the hypothetical metabolic clearance rates can be recalculated to be 87 ± 7, 86 ± 6, 92 ± 13, and 78 ± 14 ml · kg¹ · min¹, respectively. Under this new assumption, the metabolism and thus the elimination half-life of exogenous urodilatin would appear to be independent of the infusion rate. Nonetheless, the metabolic clearance rate clearly exceeds cardiac output of plasma. One of the possible explanations is that a substantial part of the infused urodilatin was degraded within the circulation. The very low value of renal fractional excretion of urodilatin (<0.1%) indicates that most of the filtered urodilatin was removed from the renal tubules. Thus the present results are not compatible with the notion that urodilatin is relatively resistant to enzymatic degradation (1, 22).

Endocrine effects. Plasma cGMP concentrations and the urinary excretion rates of cGMP increased in response to urodilatin infusion in accordance with the notion that this substance is the second messenger conveying the effects of stimulation of ANP receptors. The delayed decline of cGMP concentration can be explained by the half-life of cGMP of ~20 min (10) and possibly also by a sustained intracellular production of cGMP.

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Renal hemodynamics. ERPF was decreased by all doses of urodilatin. It cannot be excluded that this was due to a urodilatin-induced inhibition of hippuran secretion. However, renal venous catheterization was used in humans to demonstrate that ANP infusion (1 µg/min to ~14 ng · kg¹ · min¹) caused an increase in the renal extraction of hippuran in the presence of a reduction in ERPF and renal blood flow (26). Thus it is assumed that our results reflect decreases in renal plasma flow. At low doses (5 and 10 ng · kg¹ · min¹) the decrease (by 13 and 23%) occurred without concomitant reductions in MAP. The systemic ANG II levels were suppressed and thus not the cause of renal vascular constriction. The most likely explanation is that augmented activity of renal sympathetic nerves increased the vascular resistance of the kidney as in other vascular beds. It has often been demonstrated that high doses of ANP increase GFR and filtration fraction concomitantly with an unchanged or decreased renal blood flow (3). At least two mechanisms might be involved in this response. First, dilation of afferent arterioles and constriction of efferent arterioles may increase the glomerular hydrostatic pressure (32). Second, GFR may increase because of an increase in the glomerular ultrafiltration coefficient due to relaxation of glomerular mesangial cells (42). It has recently been reported that urodilatin at 20 ng · kg¹ · min¹ may increase GFR and reduce renal plasma flow in healthy men (12). It is, therefore, remarkable that GFR did not increase measurably in any of the present experiments. It is possible, however, that, despite all efforts, small changes in GFR were not detected because of the inherent imprecision of the clearance method.

C_Li. In humans with normal and high dietary sodium intake, C_Li can be used as a directional marker of changes in end-proximal fluid delivery (43). The present increase in C_Li may have been caused by an increased filtered load of lithium and/or a decrease in proximal tubular reabsorption rate. Because the proximal tubular reabsorption of sodium and fluid is not load dependent, APR should be better suited than FPR to reflect changes in proximal reabsorption. In the present study, urodilatin did not change calculated APR. This is in agreement with the notion that the proximal tubules are without ANP receptors (14) and with micropuncture studies in rats showing that ANP did not change proximal tubular sodium transport (4). In the presence of an unchanged APR, even small statistically nonsignificant increases in GFR of 3-7 ml/min (~3-6%) would be sufficient to explain the present increases in C_Li and decreases in FPR because of the numeric difference between GFR and C_Li. Even with the use of technically optimal renal clearance studies, changes in GFR of 3-7 ml/min may easily escape detection. Taken together, the increases in C_Li in this study were most likely due to small but undetectable increases in filtration pressure and/or the ultrafiltration coefficient.

Excretion of sodium and water. Infusion of urodilatin at 5, 10, 20 and 40 ng · kg¹ · min¹ increased sodium excretion by 61, 92, 141, and 127%, respectively, demonstrating that urodilatin even at the lowest dose is a natriuretic substrate. Infusion of urodilatin at 40 ng · kg¹ · min¹ elicited a biphasic pattern in urine flow and sodium excretion rate. The attenuation of the natriuresis occurring during infusion of this high dose of urodilatin may have been caused by the concomitant fall in blood pressure and the subsequent activation of the RAAS. The attenuated diuresis corresponds well to the observed increases in plasma AVP concentration probably elicited by hypotension and nausea. A biphasic response in sodium excretion and urine flow rate has previously been demonstrated with infusion of high doses of urodilatin and ANP. One-hour infusions of urodilatin in doses of 10, 20 and 40 ng · kg¹ · min¹ to healthy men showed that the largest natriuretic response was obtained with 20 ng · kg¹ · min¹ (12). During a 4-h infusion of ANP to healthy volunteers (5 µg/min, ~70 ng · kg¹ · min¹), urine flow rate peaked in the 1st h of infusion and sodium excretion in the 2nd h (8).

Albumin excretion. Infusion of urodilatin at 20 and 40 ng · kg¹ · min¹ increased urinary albumin excretion rate ~5- and 15-fold, despite concomitant decreases in MAP. The mechanisms responsible for this effect may include an increase in the glomerular permeability to albumin and/or decreased tubular reabsorption. However, these mechanisms were not investigated in this study. In experimental animals, 98% of the filtered albumin seems to be reabsorbed in the proximal tubule by a process normally working near maximal capacity (45). Thus even a small increase in the filtered load of albumin may enhance the urinary excretion rate markedly. It has been demonstrated that ANP is able to increase the glomerular filtration coefficient by increasing glomerular capillary surface area and the hydraulic permeability, probably as a result of relaxation of glomerular mesangial cells (42). ANP infusion (400 ng · kg¹ · min¹ in 20 min) to normal men elicited a threefold increase in the urinary albumin excretion rate, whereas the renal excretion rate of ₂-microglobulin (used as an index of proximal tubular reabsorption of small peptides) remained unchanged, suggesting that tubular reabsorption of albumin was not affected (33). Further studies are necessary to establish whether the urodilatin-induced albuminuria is due to increased glomerular filtration or reduced tubular reabsorption.

Summary and conclusion. In summary, it is demonstrated for the first time that urodilatin may decrease stroke volume and cardiac output without changes in plasma volume or blood pressure. At higher doses, stroke volume is decreased further, possibly because of a decrease in plasma volume secondary to egression of fluid from the intravascular compartment. Blood pressure decreases with infusion of urodilatin at 20 ng · kg¹ · min¹, probably because of arteriolar vasodilation, as reflected by reductions in TPR. Using a new assay, we demonstrated that baseline urodilatin excretion rate under the present conditions is <10 pg/min. High metabolic clearance rates suggest that a substantial part of the infused urodilatin is degraded within the circulation. Most of the filtered urodilatin is reabsorbed and/or metabolized by the renal tubules, as indicated by the very low fractional excretion of urodilatin. In contrast to previous investigations, plasma levels of renin, ANG II, and aldosterone were depressed significantly by infusions of 5 and 10 ng · kg¹ · min¹. GFR did not change measurably, but ERPF was decreased and filtration fraction was increased by all doses of urodilatin by mechanisms probably involving constriction of the efferent arterioles. C_Li results confirm the notion that natriuresis occurs because of an increased end-proximal delivery of sodium and water and inhibition of distal tubular reabsorption of sodium. However, it was demonstrated that the natriuretic and diuretic effects of urodilatin at high doses are modified by concomitant falls in blood pressure, stimulation of the RAAS, and secretion of AVP. Unexpectedly, urodilatin markedly increased the urinary excretion rate of albumin.

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Perspectives