Hepatic disposal of endogenous growth hormone and prolactin in man

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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 &pM 0.07 μg/min and increased to 2.37 ± 0.81 (SE) μg/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 1.97 ± 0.36 ml/kg.min in the basal state to 32 ± 10% and 3.63 ± 0.61 ml/kg.min, respectively, following arginine loading. Similar data were obtained in the studies employing exercise as a growth hormone stimulant (splanchnic uptake, 1.99 ± 0.89 μg/min; fractional extraction, 40 ± 2%; 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 ± 0.5 ng/ml) to a stimulated concentration (20.2 ± 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.