The dopaminergic regulation of plasma growth hormone secretion in normal subjects.

Abstract

The role of endogenous dopamine (DA) on plasma GH secretion was studied using domperidone (DA receptor blocker which does not cross blood brain barrier) in 16 normal subjects. After a bolus injection of domperidone (10 mg, iv.), plasma PRL in 11 cases rose quickly and markedly from the basal level of 9.5+ 1.2 ng/ml (Mean+s.E.) to a maximum of 76.3+ 10.6 ng/ml at 30 min. In contrast, plasma GH in these cases showed a delayed and slight increases to domperidone injection where the values at 90 min and 120 min (3.5+0.8 ng/ml and 3.7+1.0 ng/ml) were significantly higher than those in control study (1.2±0.2 ng/ml and 1.0 ±0.1 ng/ml ; p <0.05; n=8). Domperidone infusion (0.22 mg/min/3 hr) was performed in the remaining 5 subjects. The plasma PRL responses were similar to those in the bolus injection of domperidone. These PRL responses were not modified when L-dopa was administered 30 min after the start of iv infusion of domperidone. Plasma GII showed slight but significant increases 135 min after the infusion compared to control study (4.3± 1.2 ng/ml vs. 1.0+0.1 ng/ml ; p <0.05). By the prior infusion of domperidone plasma GH responses to L-dopa was delayed and blunted, i. e., the occurrence of elevation and peak value of GH delayed by 15 min, and the values at 135 min and 150 min (2.3 ±0.5 ng/ml and 1.5 ±0.2 ng/ml) were significantly lower compared to those in the single L-dopa test (6.3+ 1.2 ng/ml and 5.0+ 1.2 ng/ml ; p <0.02 and 0.05, respectively). These results indicate that in normal subjects L-dopa has stimulatory role on GH secretion mainly at the level of CNS (hypothalamus) and partly at the level of median eminence. It is not plausible that endogenous DA has direct tonic inhibitory effects on pituitary somatotrophs.GH ; domperidone ; L-dopa ; dopamine ; median eminence Human growth hormone (OH) secretion is mainly requlated by hypothalamic OH-releasing hormone (GHRH) and OH-release inhibiting hormone (somatostatin) (Arimura and Culler 1985; Jansson et al. 1985). As plasma OH secretion is impaired in cases with hypothalamic lesion (Chihara et al. 1985), GHRH is thought to be more important than somatostatin in the regulation of OH secretion.

Pituitary Function and the Somatotrophic System in Patients with Idiopathic Parkinson’s Disease Under Chronic Dopaminergic Therapy

“…Dopaminergic influences on the lactotrophic system are inhibitory: basal prolactin concentrations are suppressed by DA, dopaminergic agonists and l ‐dopa in healthy individuals (14–22) and in patients with IPD (3, 10, 23–25). Dopaminergic agonists are able to abolish the TRH‐induced prolactin rise (24, 26, 27), which also might be diminished due to IPD itself (5).…”

Effects of hypothalamic dopamine on growth hormone‐releasing hormone‐induced growth hormone secretion and thyrotropin‐releasing hormone‐induced prolactin secretion in goats

“…The oral administration of L‐dopa alone (Mars & Genuth ; Hanew et al . ), an AADC inhibitor (MK‐486) plus L‐dopa (Mars & Genuth ) and carbi plus L‐dopa (Bansal et al . ) was shown to increase basal plasma GH concentrations in humans.…”

“…The oral administration of L-dopa alone (Mars & Genuth 1973;Hanew et al 1986), an AADC inhibitor (MK-486) plus L-dopa (Mars & Genuth 1973) and carbi plus L-dopa (Bansal et al 1981) was shown to increase basal plasma GH concentrations in humans. In these studies, the GH peaks induced by L-dopa appeared 60-90 min after its administration.…”

“…; Hanew et al . , ; Jaffe et al . ) because DA cannot directly cross the blood‐brain barrier (BBB) (Yen ).…”

“…L-dopa (a precursor of DA) is often used in in vivo studies on humans in order to clarify the effects of hypothalamic DA on the secretion of GH (Mars & Genuth 1973;Camanni et al 1978;Bansal et al 1981;Hanew et al 1986Hanew et al , 1996Jaffe et al 1996) because DA cannot directly cross the blood-brain barrier (BBB) (Yen 1991). L-dopa can cross the BBB and convert DA in the hypothalamus by aromatic L-amino acid decarboxylase (AADC) (Yen 1991;Tóth et al 2002).…”