CORTICOSTEROID-INDUCED CHANGES IN UREA METABOUSM IN PATIENTS WITH HEPATOCELLULAR DISEASE

Transcription

1 GASTROENTEROLOGY Copyright 1972 by The Williams & Wilkins Co. Vol. 62, No.4 Printed in U.S.A. CORTICOSTEROID-INDUCED CHANGES IN UREA METABOUSM IN PATIENTS WITH HEPATOCELLULAR DISEASE E. A. JONES, M.D., M.R.C.P., G. D. CAIN, M.D., AND G. DICKINSON Department of Medicine, Royal Free Hospital, and Department of Biophysics, National Institute for Medical Research, Mill Hill, London, England Quantitative studies of urea metabolism have been made by a kinetic technique using 13C urea in 8 patients with hepatocellular disease before and after a 13-day course of prednisolone. After treatment the mean plasma urea concentration was higher in every case, the increases varying between 4 and 22 mg per 100 ml (P < 0.01). The initial mean absolute synthetic rate of urea (228 mg per kg per day) was 45% lower than that in 2 control subjects (413 mg per kg per day). Treatment with prednisolone was associated with a 109% increase in the mean absolute synthetic rate of urea (to 476 mg per kg per day) (0.1 > P> 0.05). It is inferred that the liver in patients with hepatocellular disease usually has the capacity to increase the synthetic rate of urea appreciably in response to an appropriate stimulus. The data can be adequately explained in terms of the known effects of corticosteroids on nitrogen metabolism including the catabolic effect of these drugs on protein metabolism in peripheral tissues. The results imply that corticosteroids increase the turnover of nonprotein nitrogenous compounds of small molecular weight, including ammonia, in patients with liver disease. Corticosteroids are frequently administered to patients with hepatocellular disease. However, little is known about the metabolic consequences of the action of these drugs in subjects with deficient hepatic function. It has recently been shown that the disturbance of nitrogen metabolism induced in patients with liver disease by the admin- Received July I, Accepted November 8, Address requests for reprints to: Dr. E. A. Jones, Department of Medicine, Royal Free Hospital, London, W. C. 1., England. This work was supported in part by a grant from the Medical Research Council, London, England. Dr. Cain was sponsored by a Mead Johnson Fellowship through the American College of Physicians and the Kelsey-Leary Foundation of Houston, Texas. Dr. Cain's present address is: The University of Texas Medical Branch, Galveston, Texas istration of corticosteroids IS usually associated with an increase in the synthetic rate of albumin. 1 Other aspects of the quantitative effects of corticosteroids on nitrogen metabolism in patients with liver disease have been studied little. In this paper the effects of a 13- day course of prednisolone on urea metabolism in 8 patients with hepatocellular disease are reported. The results indicate that corticosteroid therapy in these patients is associated consistently with increases in the plasma urea concentration and the body urea pool which are usually associated with a decrease in the half-life of labeled urea in the body and an increase in the absolute synthetic rate of urea. Materials and Methods Patients. Clinical data and initial serum biochemistry of the 8 patients studied are

2 April 1972 LIVER PHYSIOLOGY AND DISEASE 613 given in table 1. Their ages varied between 25 and 71 years. There were five females and three males. All the patients had hepatocellular disease established by needle biopsy. Five had cirrhosis. Of these, 3 (patients 2, 6, and 7) had a previous history of chronic active hepatitis and 1 (no. 1) was an alcoholic. Two patients had chronic hepatitis (no. 3, chronic aggressive hepatitis, no. 4, chronic persistent hepatitis)? The duration of illness based on clinical history and biochemical tests of liver function varied between 0.3 and 15 years. Data on albumin and fibrinogen metabolism in these patients have been published elsewhere. ' In all cases serum biochemical changes were consistent with hepatocellular disease. Total serum bilirubin concentrations varied between < 0.5 and 26 mg per 100 m!. The serum-conjugated bilirubin varied between < 0.5 and 22 mg per 100 m!. Three patients (nos. 2, 3, and 5) were deeply jaundiced. The alkaline phosphatase ranged between 16 and 57 King-Armstrong units. The aspartate transaminase (serum glutamine oxaloacetic transaminase) varied between 20 and 520 international units per liter. At the beginning of the studies all patients were considered to be in a metabolically steady state with particular reference to body weight, hematocrit, and serum urea and albumin concentrations. Corticosteroids had not been administered to any of the patients during the previous 6 months. Material. 13C urea was prepared from barium 13C carbonate. The enrichment of the batches used was 51.1 and 65.5 atoms per 100 atoms excess and the purity was 90 and 96% respectively. Experimental design. The studies were carried out with the fully informed consent and cooperation of each patient. For 12 hr before and throughout the 10- to 12-hr period of a study all patients received a low protein diet (10 g of protein per day) to minimize postprandial fluctuations in the synthetic rate of urea. 3 At zero time an accurately weighed mass of 13C urea (50 to 100 mg) dissolved in 5 to 10 ml of sterile N saline was injected intravenously. Approximately 10 timed specimens of venous blood were subsequently taken into heparinized tubes and the plasma separated. The first specimen was taken at 5 min. The intervals between the remaining specimens varied between 15 min initially to about 2 hr towards the end of the period of sampling. The morning after the first study orally administered prednisolone was begun in a dose of 1 mg per kg of body weight per day except in patient 7 who received 0.75 mg per kg of body weight per day. On the 13th day of treatment a second study was conducted. Immediately prior to the second study a base line specimen of venous blood was taken to determine any residual excess 13C enrichment of plasma urea from the first study. After the second study the dose of prednisolone was reduced over 2 weeks to a maintenance dose of less than 15 mg per day. Laboratory procedures. Proteins were precipitated from plasma specimens with a citric acid-tungstate mixture and the carbon of urea in the supernatant was released quantitatively in the form of carbon dioxide by incubation with urease in vacuo. The plasma urea concentration of each specimen was determined from the volume of carbon dioxide released, measured as milligrams of urea carbon in a specially calibrated manometer Patient TABLE 1. Patients studied and serum biochemistry before and after prednisolone Bilirubin Alkaline Aspartate Clinical phosphatase transaminase Age Sex Diagnosis duration of Total Conjugated illness Before After Before After Before After Before After yr yr mg/ioo ml King.Armstron InterTUltional units units/liter 1 43 F Cirrhosis of alcoholic 4 <0.5 < 0.5 <0.5 < M Cryptogenic cirrhosis M Chronic hepatitis F Chronic hepatitis M Cholestatic hepatitis F Cryptogenic cirrhosis F Cryptogenic cirrhosis F Cryptogenic cirrhosis Normal range < 0.5 < 0.5 < 13 < 17

3 614 LIVER PHYSIOLOGY AND DISEASE Vol. 62, No.4 attached to a high vacuum line. 3,. The l3c enrichment (atoms per 100 atoms excess) of plasma urea carbon. was measured by mass spectrometry of each specimen of carbon dioxide, using a 60 Nier Type Mass Spec trometer.5 Calculations. The mean and SEM of the individual estimates of plasma urea concentration during each study were determined. The value of plasma 13C urea enrichment measured immediately prior to the commencement of the second study was subtracted from all values of plasma 13C urea enrichment determined during the second study. As data on urinary excretion of l3c urea and l2c urea were not available it was not possible to estimate the total body urea pool by the isotopic dilution method of Walser and Bodenlos. 6 However, the highly significant correlation (r , P < 0.001) found between the body urea pool (milligrams per kilogram) estimated by this method and the plasma urea concentration determined independently in 13 studies of urea metabolism in a group of patients having a wide range of plasma urea concentrations 7 enables the following relationship to be derived: P = C where P = body urea pool (milligrams per kilogram) and C = plasma urea concentration (mg per 100 mi). This relationship was used to calculate the body urea pool from the values of mean plasma urea concentration obtained in these studies. All the curves of log plasma l3c urea (atoms per 100 atoms excess) against time were essentially linear after 140 min, suggesting that equilibration of the l3c urea throughout the body was complete by this time. The linear regression coefficients (K min - 1) of the log l3c urea enrichment time curves were calculated by the method of least squares from the experimental points obtained after 140 min. The changes in plasma urea concentration during each study were small and inconsistent. Under these conditions the plasma l3c urea disappearance curve, which is a direct measure of the rate of loss of urea, is also a valid index of the fractional rate of urea synthesis. Hence the fraction of the body urea pool turned over daily (F per cent of pool per day) and the absolute synthetic rate of urea (8 mg per kg per day) can be calculated from the following equations: F = 1440 K x logelo x 100 F 8= Px- 100 The half-life of l3c urea in the body (t \', hr) was calculated from the values of K: loge2 t --- \', - 60 K Paired Student t-tests were used to test the significance of the changes in urea metabolism observed. Results The course of corticosteroids was associated with an episode of hepatic coma in 1 patient (no. 2) and with clinical signs of hepatic precoma in 3 others (nos. 4, 7, and 8). Changes in the serum bilirubin, alkaline phosphatase, and aspartate transaminase were inconsistent (table 1). The changes in body weight, plasma urea concentration, body urea pool, halflife of labeled urea and fractional and absolute synthetic rates of urea before and after the course of corticosteroids are shown in table 2. Before treatment the mean half-life of labeled urea (27.0 hr) was longer and the mean absolute synthetic rate of urea (228 mg per kg per day) was lower than corresponding values obtained from similar data on 2 control subjects (8.0 hr and 413 mg per kg per day).7 The plasma urea concentration and body urea pool were higher after corticosteroids in all cases, the increases varying between 4 and 22 mg per 100 ml (P < 0.01) and between 22 and 79 mg per kg (P < 0.01) respectively. In 6 of the 8 patients (nos. 1, 2, 4, 5, 7, and 8) there were decreases in the half-life of labeled urea in the body and increases in the fractional and absolute synthetic rates. In 1 of the patients (no. 7) the synthetic rate of urea after corticosteroids was appreciably higher than values found in patients with uremia and the stagnant loop syndrome, two conditions associated with increased rates of urea formation. 7 Discussion Using data published elsewhere, 7 a highly significant correlation has been

4 April 1972 LIVER PHYSIOLOGY AND DISEASE 615 TABLE 2. Body weight and urea metabolism b efore and after prednisolone Body weight Synthesis Plasma urea concentration Ha lf life of Mean ± SEMiII Urea pool labeled urea Pa Fractional Absolute tient Before After Before After Before After Before After Before After B efore After kg mg/joo ml mg/kg hr % pool/day mg/kg/day ± 0.30(7) ± 0. 26(11) ± 0.41(9) ± 0.40(12) ± 0.58(10) 34.2 ± 0.80(11) ± 0.74(10) 24.0 ± 0.85(11) ± 0.92(10) 44.0 ± 2.46(11) ± 0.93(10) 18.7 ± 1.08(11) ± 1.66(10) 33.5 ± 2.47(11) ± 0.74(10) ± 0.61(11) P < 0.01 P < > P > 0.1 3> P > > P > Numbers III parentheses represent number of samples. found between estimates of the mean plasma urea concentration and independent estimates of the total body urea pool. The formula obtained, which relates these two variables, enables the total body urea pool to be calculated from estimates of the mean plasma urea concentration only, assuming steady state conditions. Thus the complex isotopic dilution method of Walser and Bodenlos 6 does not appear to be necessary to estimate the total body urea pool. However, estimates of the total body urea pool from the mean plasma urea concentration, using the formula quoted in this paper, nevertheless depend on the validity of assumptions inherent in the method of Walser and Bodenlos. 6 The method of calculating the absolute synthetic rate of urea used in these studies does not take into account the multicompartmental nature of urea metabolism in man. 8 However, the use of a single pool model of urea metabolism in these studies is likely to result in an underestimate of the true value by less than 7%. 8 This potential error is small in relation to the magnitude of the changes in urea synthesis observed and may well apply to a similar degree to the estimates made both before and after the administration of the drug. The studies reported here indicate that the administration of a high pharmacological dose of corticosteroids to patients with liver disease is associated consistently with appreciable increases in the plasma urea concentration. This could be due to increased synthesis and/or decreased renal excretion of urea. The finding that the absolute synthetic rates of urea were usually increased, although the increases did not quite reach the generally accepted level of statistical significance, suggests that increased synthesis of urea may well be an important factor responsible for the increased. plasma urea concentration. The data show that the diseased liver often has the capacity to increase the synthetic rate of urea, in some cases to high values. The 2 patients (nos. 3 and 6) in whom prednisolone therapy was not associated with a decrease in the half-life of labeled urea and an increase in the absolute synthetic rate of urea exhibited certain features not shared by the other 6 patients. Patient 3 had the highest serum bilirubin and aspartate transaminase values and the most active hepatocellular disease by histological criteria. Patient 6 had the highest serum alkaline phosphatase level and evidence of severely decompensated disease, which took the form of fluid retention requiring a strict low salt diet and diuretics acting on both the proximal and distal renal tubules to achieve adequate control. These features may have contributed in part to the apparent different responses of urea metabolism to prednisolone in these 2 patients. However, urea synthesis may be labile and fluctuate appreciably from day to day.

5 616 LIVER PHYSIOLOGY AND DISEASE Vol. 62, No.4 Although the net effect of prednisolone treatment may have been to increase urea synthesis during the 2-week period in all patients, it could be that on the particular day chosen for the second study, the half-life of labeled urea was longer and the absolute synthetic rate of urea smaller than the corresponding pretreatment values in patients 3 and 6. The actions of corticosteroids on nitrogen metabolism in vivo are complex. The net effect of these drugs on the metabolism of one particular nitrogenous substance, urea, probably depends on the summation of a number of stimulatory and inhibitory effects of corticosteroids on various metabolic pathways of nitrogenous substances, both within the liver cell and in other tissues, which affect urea metabolism either directly or indirectly It has been demonstrated in rats that the administration of corticosteroids, in doses relatively much higher than those used in the current studies, was associated with increased levels of the activities of hepatic urea cycle enzymes. 11 Changes in urea metabolism could arise secondary to corticosteroid induced changes in protein metabolism within the liver cell. Such changes include alterations in messenger ribonucleic acid profiles, 12 increased synthesis of ribonucleic acid and protein, increased amino acid incorporation into ribosomes,16-18 and enzyme induction Corticosteroids also stimulate protein catabolism in peripheral tissues, particularly skeletal muscle, with the result that increased quantities of small molecular weight substances, such as ammonia and amino acids, are delivered into the peripheral blood These substances are taken up by the liver and other visceral organs The availability of additional amino acid and ammonia nitrogen from peripheral sources for incorporation into the Krebs-Hensleit urea cycle in the liver could have contributed to an increase in urea synthesis and hence the plasma urea concentration_ A higher plasma urea concentration per se implies that a greater concentration of substrate urea is available for degradation to ammonia by urease of the intestinal bacterial flora. 7 It follows that the results of these studies are consistent with corticosteroid therapy being associated with an increased enterohepatic circulation of urea nitrogen and hence with an increase in the concentration of ammonia in portal vein blood. It would appear that measures which would tend to lower the plasma urea concentration in patients with liver disease, such as cessation of diuretic or corticosteroid therapy, are likely to be associated with a decrease in the ammonia concentration of portal vein blood also. In subjects with normal fiver function the fate of normal or even supranormal quantities of ammonia delivered to the liver can probably be accounted for by incorporation into urea. 7 However, the incorporation of the nitrogen of ammonia and other substances into urea may be less efficient in patients with liver disease, as the initial mean absolute synthetic rate of urea for the group of patients with liver disease was appreciably lower than that of controls. This finding could be due, at least in part, to patients with liver disease tending to have reduced activity of the enzymes of the hepatic urea cycle. However, data on the activity of these enzymes in patients with liver disease are not currently available. Although the data presented indicate that the diseased liver can, at least in some cases, increase urea synthesis appreciably, the urea synthetic rates in these studies after corticosteroids, may nevertheless by suboptimal in relation to an appreciably increased load of nonprotein nitrogenous substances, including ammonia, being delivered to the liver from peripheral tissues and from the intestine. In this context it would be of interest to determine the effect of corticosteroids on the ammonia concentration of peripheral venous blood in patients with liver disease. REFERENCES 1. Cain GD, Mayer G, Jones EA: Augmentation of albumin but not fibrinogen synthesis by corticosteroids in patients with hepatocellular disease. J Clin Invest 49: , 1970

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