European Journal of Clinical Investigation (1979) 9,257-260
Hepatic disposal of endogenous growth hormone and prolactin in man PAUL BRATUSCH-MARRAIN, OLA BJORKMAN, WERNER WALDHAUSL & JOHN WAHREN, I. Medizinische Universitatsklinik, Division of Clinical Endocrinology and Diabetes Mellitus, Vienna, Austria, and Department of Clinical Physiology, Karolinska Institute, Huddinge University Hospital, Huddinge, Sweden Received 8 January 1979
Abstract. In order to examine the role of the liver in the disposal of endogenous growth hormone and prolactin, the hepatic uptake of both pituitary hormones was determined directly in healthy men in the basal state (n= 12) and after stimulation of growth hormone and prolactin production by an intravenous arginine load (30 g/30 min; n = 5) and exercise (n = 3). The hepatic venous catheter technique was employed. Uptake of growth hormone from the splanchnic bed in the basal state was 0.16 0.07 pg/min and increased to 2.37 k0.81 (SE) pg/min following arginine infusion in parallel with a rise in arterial serum growth hormone concentrations from basal, 0.9 0.2 ng/ml, to 13.8 3.5 ng/ml. Splanchnic fractional extraction and clearance rate exhibited an increase from 20 + 3% and 1.97k 0.36 ml/kg.min in the basal state to 32+ 10% and 3.63 f0.61 ml/kg.min, respectively, following arginine loading. Similar data were obtained in the studies employing exercise as a growth hormone stimulant (splanchnic uptake, 1.99k 0.89 pg/min; fractional extraction, 40 f2%; splanchnic clearance rate, 3.42 + 0.06 ml/kg.min). In contrast, no significant uptake of endogenous prolactin by the liver was demonstrated, either in the basal state or during the arginine induced rise in serum prolactin concentrations from a basal (6.8 k0.5 ng/ml) to a stimulated concentration (20.2 k 5.1 ng/ml). These results indicate a major role for the liver in growth hormone metabolism, accounting for approximately 50% of total growth hormone clearance, but not in prolactin disposal. This indicates that the hormones have different sites of action and different metabolic fates.
+
Key words. Growth hormone, prolactin, liver, splanchnic clearance, man. Introduction
The human liver contains receptors which are specific Correspondence: Dr P. Bratusch-Marrain, I. Medizinische Universitatsklinik, Lazarettgasse 14, A-1090 Vienna, Austria. 0014-2972/79/0800-0257$02.00 0 1979 Blackwell Scientific Publications
for growth hormone (GH) and which do not bind prolactin (PRL) [ 11. This is of interest, because the liver may be a primary target organ in which G H leads to somatomedin production [2] as well as a major site of G H degradation in addition to other organs such as the kidney [3]. The splanchnic uptake of endogenous G H was examined directly in healthy males employing the hepatic venous catheter technique in order to estimate quantitatively the hepatic disposal of GH. The hepatic uptake of endogenous PRL was also measured because of the chemical and biological similarities between G H and PRL and because of the possibility of differentiating between their biologic sites of action. Stimuli known to induce enhanced secretion of both pituitary hormones such as arginine loading [4] and exercise [5] were applied to assess the effect of elevated serum G H and PRL concentrations on their hepatic uptake, fractional extraction, and splanchnic clearance rate. Procedures and Methods
Subjects. The subjects were twelve healthy adult males aged 2 1 4 2 years. None was obese or had diabetes mellitus or liver disease. They were informed of the nature and possible risks involved in the study before giving their voluntary consent to participate. The protocol was reviewed and approved by the Ethical Committee of the Karolinska Institute, Stockholm.
Procedures. All subjects were studied after an overnight fast. Catheters were inserted percutaneously into a peripheral vein, a brachial artery, and into a rightsided hepatic vein under fluoroscopic control as described previously [6, 71. Blood samples were removed simultaneously for the determination of hormone concentrations from the arterial and hepatic venous catheters. In the basal state samples were drawn in duplicate 10-30 min apart. Arterial and hepatic serum samples for determination of G H and PRL were also obtained after stimulation of pituitary hormone secretion. After the basal period, arginine (30 g) was infused intravenously for 30 min in five subjects. Blood sampling was performed at timed intervals during the 257
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PAUL BRATUSCH-MARRAIN et al.
arginine infusion and for 60 min after the infusion. In another three subjects, moderate exercise loading according to half maximal oxygen consumption was prescribed for 30 min. Hepatic plasma flow (EHPF) was estimated by the constant infusion technique [8] using indocyanine green dye [9]. Splanchnic uptake of hormones was calculated as the product of the arterio-hepatic venous concentration differences and the EHPF. Fractional extraction of hormones was expressed as the percentage of arterio-hepatic venous differences from arterial concentration. The splanchnic clearance rate was calculated as the splanchnic uptake divided by the arterial concentration and expressed as the clearance rate per kg body weight. GH data below the sensitivity of the assay ( < 0 . 5 ng/ml) were omitted from splanchnic balance calculations; hence, the data presented in the text and table are smaller than the number of subjects studied. Analytical methods. Human GH was determined as reported previously [ 101. Levels of PRL were estimated by a commercial radioimmunoassay kit (Serono, Freiburg, FRG). The standard for the assay of GH was obtained from Dr Wilhelmi (HS-1032 B), and that for PRL was supplied by the Medical Research Council, London (MRC 7 11222). Doses up to 100 ng/ml of G H or PRL did not cross-react in the reciprocal assay. The lower limit of assay sensitivity was 0.5 ng/ml for GH, and 1.6 ng/ml for PRL. All hormone determinations for each study were run in the same assay. The intraassay coefficient of variation was 4x, for GH, and 5% for PRL, and the interassay coefficient 6% and 8% respectively. The Student's t-test was employed for statistical analysis. Data in the text, table and figures are given as mean SE.
Results Growth hormone
Mean values for arterial GH concentrations, splanchnic GH metabolism and EHPF in the basal state (n=9) are shown in Table 1. In the subjects who received an arginine load (n=5), mean arterial G H values were 0.9 k 0.2 ng/ml in the basal state, rising to
[
Arginine
*
0 - 30
1
I
I
I
I
0
15
30
45
60
Minutes
Figure 1. Arterial ( 0 ) and hepatic venous (*) concentrations of growth hormone and prolactin before, during. and after intravenous arginine infusion (30 g/30 min). Asterisks indicate significant differences between arterial and hepatic venous concentrations ("P
Hepatic disposal of endogenous growth hormone and prolactin in man PAUL BRATUSCH-MARRAIN, OLA BJORKMAN, WERNER WALDHAUSL & JOHN WAHREN, I. Medizinische Universitatsklinik, Division of Clinical Endocrinology and Diabetes Mellitus, Vienna, Austria, and Department of Clinical Physiology, Karolinska Institute, Huddinge University Hospital, Huddinge, Sweden Received 8 January 1979
Abstract. In order to examine the role of the liver in the disposal of endogenous growth hormone and prolactin, the hepatic uptake of both pituitary hormones was determined directly in healthy men in the basal state (n= 12) and after stimulation of growth hormone and prolactin production by an intravenous arginine load (30 g/30 min; n = 5) and exercise (n = 3). The hepatic venous catheter technique was employed. Uptake of growth hormone from the splanchnic bed in the basal state was 0.16 0.07 pg/min and increased to 2.37 k0.81 (SE) pg/min following arginine infusion in parallel with a rise in arterial serum growth hormone concentrations from basal, 0.9 0.2 ng/ml, to 13.8 3.5 ng/ml. Splanchnic fractional extraction and clearance rate exhibited an increase from 20 + 3% and 1.97k 0.36 ml/kg.min in the basal state to 32+ 10% and 3.63 f0.61 ml/kg.min, respectively, following arginine loading. Similar data were obtained in the studies employing exercise as a growth hormone stimulant (splanchnic uptake, 1.99k 0.89 pg/min; fractional extraction, 40 f2%; splanchnic clearance rate, 3.42 + 0.06 ml/kg.min). In contrast, no significant uptake of endogenous prolactin by the liver was demonstrated, either in the basal state or during the arginine induced rise in serum prolactin concentrations from a basal (6.8 k0.5 ng/ml) to a stimulated concentration (20.2 k 5.1 ng/ml). These results indicate a major role for the liver in growth hormone metabolism, accounting for approximately 50% of total growth hormone clearance, but not in prolactin disposal. This indicates that the hormones have different sites of action and different metabolic fates.
+
Key words. Growth hormone, prolactin, liver, splanchnic clearance, man. Introduction
The human liver contains receptors which are specific Correspondence: Dr P. Bratusch-Marrain, I. Medizinische Universitatsklinik, Lazarettgasse 14, A-1090 Vienna, Austria. 0014-2972/79/0800-0257$02.00 0 1979 Blackwell Scientific Publications
for growth hormone (GH) and which do not bind prolactin (PRL) [ 11. This is of interest, because the liver may be a primary target organ in which G H leads to somatomedin production [2] as well as a major site of G H degradation in addition to other organs such as the kidney [3]. The splanchnic uptake of endogenous G H was examined directly in healthy males employing the hepatic venous catheter technique in order to estimate quantitatively the hepatic disposal of GH. The hepatic uptake of endogenous PRL was also measured because of the chemical and biological similarities between G H and PRL and because of the possibility of differentiating between their biologic sites of action. Stimuli known to induce enhanced secretion of both pituitary hormones such as arginine loading [4] and exercise [5] were applied to assess the effect of elevated serum G H and PRL concentrations on their hepatic uptake, fractional extraction, and splanchnic clearance rate. Procedures and Methods
Subjects. The subjects were twelve healthy adult males aged 2 1 4 2 years. None was obese or had diabetes mellitus or liver disease. They were informed of the nature and possible risks involved in the study before giving their voluntary consent to participate. The protocol was reviewed and approved by the Ethical Committee of the Karolinska Institute, Stockholm.
Procedures. All subjects were studied after an overnight fast. Catheters were inserted percutaneously into a peripheral vein, a brachial artery, and into a rightsided hepatic vein under fluoroscopic control as described previously [6, 71. Blood samples were removed simultaneously for the determination of hormone concentrations from the arterial and hepatic venous catheters. In the basal state samples were drawn in duplicate 10-30 min apart. Arterial and hepatic serum samples for determination of G H and PRL were also obtained after stimulation of pituitary hormone secretion. After the basal period, arginine (30 g) was infused intravenously for 30 min in five subjects. Blood sampling was performed at timed intervals during the 257
258
PAUL BRATUSCH-MARRAIN et al.
arginine infusion and for 60 min after the infusion. In another three subjects, moderate exercise loading according to half maximal oxygen consumption was prescribed for 30 min. Hepatic plasma flow (EHPF) was estimated by the constant infusion technique [8] using indocyanine green dye [9]. Splanchnic uptake of hormones was calculated as the product of the arterio-hepatic venous concentration differences and the EHPF. Fractional extraction of hormones was expressed as the percentage of arterio-hepatic venous differences from arterial concentration. The splanchnic clearance rate was calculated as the splanchnic uptake divided by the arterial concentration and expressed as the clearance rate per kg body weight. GH data below the sensitivity of the assay ( < 0 . 5 ng/ml) were omitted from splanchnic balance calculations; hence, the data presented in the text and table are smaller than the number of subjects studied. Analytical methods. Human GH was determined as reported previously [ 101. Levels of PRL were estimated by a commercial radioimmunoassay kit (Serono, Freiburg, FRG). The standard for the assay of GH was obtained from Dr Wilhelmi (HS-1032 B), and that for PRL was supplied by the Medical Research Council, London (MRC 7 11222). Doses up to 100 ng/ml of G H or PRL did not cross-react in the reciprocal assay. The lower limit of assay sensitivity was 0.5 ng/ml for GH, and 1.6 ng/ml for PRL. All hormone determinations for each study were run in the same assay. The intraassay coefficient of variation was 4x, for GH, and 5% for PRL, and the interassay coefficient 6% and 8% respectively. The Student's t-test was employed for statistical analysis. Data in the text, table and figures are given as mean SE.
Results Growth hormone
Mean values for arterial GH concentrations, splanchnic GH metabolism and EHPF in the basal state (n=9) are shown in Table 1. In the subjects who received an arginine load (n=5), mean arterial G H values were 0.9 k 0.2 ng/ml in the basal state, rising to
[
Arginine
*
0 - 30
1
I
I
I
I
0
15
30
45
60
Minutes
Figure 1. Arterial ( 0 ) and hepatic venous (*) concentrations of growth hormone and prolactin before, during. and after intravenous arginine infusion (30 g/30 min). Asterisks indicate significant differences between arterial and hepatic venous concentrations ("P
