Hepatic conversion of amino-nitrogen to urea in thyroid diseases. II. A study in hyperthyroid patients

Effects of long-term oral misoprostol administration on hepatic amino acid-nitrogen metabolism in patients with cirrhosis

BACKGROUND

The acute infusion of a Prostaglandin of E series 1 (PGE1) analogue results in nitrogen sparing in cirrhosis.

AIMS

To test the effects of long-term oral PGE1 on hepatic and whole-body nitrogen metabolism.

PATIENTS AND METHODS

Ten patients with advanced cirrhosis were studied in paired experiments, before and 30-50 days after oral misoprostol therapy. alpha-Amino-nitrogen levels and urea-nitrogen synthesis rate were measured in the post-absorptive state and in response to continuous alanine infusion (2 mmol/kg per hour for 4.5h). Data were used to compute the functional hepatic nitrogen clearance, i.e. the slope of the regression of alpha-amino-N levels to urea-N synthesis rate, and the apparent nitrogen exchange.

RESULTS

Misoprostol reduced urea-N synthesis rate (during fasting and in response to alanine), resulting in a positive nitrogen exchange. The functional hepatic nitrogen clearance slightly increased, and the regression line was rightwards shifted, indicating a reduced urea synthesis rate at any alpha-amino-N concentration. Amino acid- and ammonia-N did not accumulate in plasma. No systematic effects on insulin and glucagon were observed.

CONCLUSIONS

Data are consistent with a nitrogen sparing mechanism of misoprostol, not mediated by hormone levels. These effects may be beneficial in clinical hepatology, and need to be tested in controlled trials.

Effects of systemic prostaglandin E1 on hepatic amino acid-nitrogen metabolism in patients with cirrhosis

Prostaglandins of the E (PGE) series have long been considered "catabolic" hormones, but recent data suggest that they may be secreted in critically ill patients to counteract stress hormones, stimulating protein synthesis. Their use is under scrutiny to improve hepatic microcirculation and as cytoprotective agents. We tested the effects of PGE1 on hepatic and whole-body nitrogen metabolism in eight patients with cirrhosis. Urea-nitrogen synthesis rate, alpha-amino-nitrogen levels, and nitrogen exchange were measured in the basal, postabsorptive state and in response to continuous alanine infusion, in paired experiments, during superinfusion of PGE1 or saline. Splanchnic and systemic hemodynamics were assessed by echo-Doppler at the beginning and at the end of each experiment. PGE1 produced a rapid fall in plasma amino acids and in urea-nitrogen synthesis rate, as well as a positive nitrogen exchange. The slope of the regression of alpha-amino-nitrogen levels on urea-nitrogen synthesis rate, a measure of liver cell metabolic activity, was not affected, but the regression line was shifted rightward, suggesting a nitrogen-sparing effect of PGE1. Mesenteric artery and portal flow were unchanged, whereas femoral artery flow increased by 30%. Insulin and glucagon levels were not systematically different. We conclude that PGE1 reduces hepatic urea synthesis rate, independent of hormones and/or hepatic flow, possibly acting at the peripheral level on amino acid transport, thus reducing amino acid supply to the liver. The resulting net nitrogen sparing might be the basis for the beneficial effect of PGE1 in clinical hepatology.

Effects of long-term growth hormone (GH) and triiodothyronine (T3) administration on functional hepatic nitrogen clearance in normal man

BACKGROUND/AIMS

A decline in urea excretion is seen following long-term growth hormone administration, reflecting overall protein anabolism. Conversely, hyperthyroidism is characterized by increased urea synthesis and negative nitrogen metabolism. These seemingly opposite effects are presumed to reflect different actions on peripheral protein metabolism. The extent to which these hormonal systems have different direct effects on hepatic urea genesis has not been fully characterized.

METHODS

We measured urea nitrogen synthesis rates and blood alanine levels concomitantly before, during, and after a 4-h constant intravenous infusion of alanine (2 mmol.kg bw-1.h-1). Urea nitrogen synthesis rate was estimated hourly as urinary excretion corrected for gut hydrolysis and accumulation in body water. The slope of the linear relationship between urea nitrogen synthesis rate and alanine concentration represents the liver function as to conversion of amino-N, and is denoted the functional hepatic nitrogen clearance. Eight normal male subjects (age 21-27 years; body mass index 22.4-27.0 kg/m2) were randomly studied four times: 1) after 10 days of subcutaneous saline injections, 2) after 10 days of subcutaneous growth hormone injections (0.1 IU/kg per day), 3) after 10 days of triiodothyronine administration (40 micrograms on even dates, 20 micrograms on uneven dates) and 4) after 10 days given 2)+3). All injections were given at 20 00 h.

RESULTS

Growth hormone decreased functional hepatic nitrogen clearance (l/h) by 30% (from 33.8 +/- 3.2 l/h (control) to 23.8 +/- 1.5 l/h (10 days growth hormone) (mean +/- SE) (ANOVA; p < 0.01)). Triiodothyronine did not change functional hepatic nitrogen clearance (36.7 +/- 3.2 l/h), but triiodothyronine given together with growth hormone abolished the effect of growth hormone functional hepatic nitrogen clearance (38.8 +/- 4.8 l/h).

CONCLUSIONS

The results show that long-term growth hormone administration acts on liver by decreasing functional hepatic nitrogen clearance, thereby retaining amino-N in the body. Triiodothyronine has no effect on functional hepatic nitrogen clearance, but given together with growth hormone, it abolishes the effect of growth hormone on functional hepatic nitrogen clearance. A possible mechanism is the known effect of thyroid hormones in reducing the bioavailability of insulin-like growth factor-I. Thus, the effects of growth hormone and triiodothyronine on amino-N homeostasis are interdependent and to some extent exerted via interplay in their regulation of liver function as to amino-N conversion.

Effects of beta-blockade on hepatic conversion of amino acid nitrogen and on urea synthesis in cirrhosis

beta-Blockers are widely used to prevent gastrointestinal hemorrhage in cirrhosis. The metabolic effects of treatment are scarcely studied: hepatic function reportedly does not change significantly, but beta-adrenoceptors have been reported to regulate protein and amino acid metabolism. We studied hepatic nitrogen metabolism in response to constant alanine infusion in seven patients with cirrhosis before and 7 to 10 days after treatment with oral propranolol (60 to 100 mg/d). Beta-blockade was effective: it decreased heart rate by 25%, abolished orthostatic tachycardia, and reduced portal blood flow by 20%. Alanine-stimulated urea nitrogen synthesis rate (UNSR) was higher in patients with propranolol treatment, without any difference in aminonitrogen concentration. The kinetics of hepatic conversion of amino acid nitrogen into urea--ie, functional hepatic nitrogen clearance (FHNC)--increased by 30%, from (mean +/- SD) 17.0 +/- 4.1 to 22.0 +/- 6.6 L/h (P < .01). Increased urea production during alanine infusion resulted in negative nitrogen exchange even at the peak of alpha-aminonitrogen concentration. Basal insulin level was only slightly reduced during propranolol treatment, whereas the insulin response to alanine was significantly blunted. No differences in glucagon and cortisol were demonstrated. Epinephrine and norepinephrine levels were high-normal and did not vary after treatment. Increased urea production and stimulation of hepatic nitrogen clearance during beta-blockade may be mediated by relative hypoinsulinemia or by direct involvement of beta-adrenoceptors in the control of nitrogen metabolism, possibly by regulation of amino acid uptake and release in peripheral tissues.