Effects of long-term growth hormone (GH) and triiodothyronine (T 3) 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. The 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/m 2) 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 μg on even dates, 20 μg 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