Ghrelin and growth hormone secretagogues, physiological and pharmacological aspect.

The first "growth hormone secretagogues" (GHSs) were discovered by Bowers et al. in 1977. In 1996 the GHSs receptor (GHS-R 1a) was cloned. The endogenous ligand for this receptor, ghrelin, was not identified until 1999. Synthetic molecules termed GHSs are substances that stimulate growth hormone (GH) release, via a separate pathway distinct from GH releasing hormone (GHRH)/somatostatin. Ghrelin displays strong GH-releasing activity through the activation of the GHS-R 1a. Apart from stimulating GH secretion, ghrelin and many synthetic GHSs: 1) stimulate prolactin and ACTH secretion; 2) negatively influence the pituitary-gonadal axis; 3) stimulate appetite and positive energy balance; 4) modulate pancreatic endocrine function and affect glucose levels; 5) have cardiovascular actions. The control of ghrelin secretion is not well established at present, although nutrition is an important regulator. Investigators have exploited the ability of GHSs and ghrelin to release GH by mechanisms different from GHRH as a diagnostic tool, which is the present main clinical use of some GHSs. As an alternative to GH, GH deficient conditions could be treated with any substance which would release endogenous GH, such as synthetic GHSs. It is likely that GHSs, acting as either agonists or antagonists on different pathophysiological processes, might have some other clinical impact and therapeutic potential. At least theoretically ghrelin receptor antagonists could be anti-obesity drugs, as blockers of the orexigenic signal from the gastrointestinal tract to the brain. Inverse agonists of the ghrelin receptor, by blocking the constitutive receptor activity, might lower the set-point for hunger between meals.

Metformin reduces thyrotropin levels in obese, diabetic women with primary hypothyroidism on thyroxine replacement therapy.

Context It has been reported that metformin might modify thyroid hormone economy. In two retrospective studies, initiation of treatment with metformin caused suppression of TSH to subnormal levels. Objective To prospectively evaluate if administration of metformin to obese, diabetic patients with primary hypothyroidism on stable thyroxine replacement doses modifies TSH levels. Patients and methods Eight obese, diabetic postmenopausal women with primary hypothyroidism participated in the study. They received 1,700 mg of metformin daily for 3 months. Weight, TSH, free T4, and free T3 levels were measured at baseline, 3 months after metformin initiation and 3 months after its withdrawal. Results After 3 months of on metformin, mean TSH was significantly lower than basal TSH (3.11 +/- 0.50 microUI/ml vs. 1.18 +/- 0.36 microUI/ml; P = 0.01). Mean TSH 3 months after metformin withdrawal was 2.21 +/- 0.37 microUI/ml, significantly higher than TSH after metformin (P = 0.05), but not different from basal TSH. Mean fT4 level increased during metformin administration (basal fT4: 1.23 +/- 0.06 ng/dl, fT4 after metformin: 1.32 +/- 0.04 ng/dl; P = ns), and decreased after its withdrawal (fT4 3 months after metformin withdrawal: 1.15 +/- 0.05 ng/dl; vs. 3 months after metformin, P = 0.04; vs. basal; P = ns). Conclusions In obese, diabetic patients with primary hypothyroidism on thyroxine replacement treatment, short-term metformin administration is associated with a significant fall in TSH.

Effect of oral glucose on acylated and total ghrelin secretion in acromegalic patients.

OBJECTIVE: The pathophysiology of ghrelin secretion in acromegaly is unclear. Our aim was to study circulating fasting ghrelin levels and their response to oral glucose in acromegalic patients and normal control subjects. DESIGN AND PATIENTS: 9 acromegalic patients (4 male, 5 female; 59.4+/-3.6 years; 28.6+/-1.0 kg/m2) and 9 age and BMI matched healthy control subjects (4 male, 5 female; 59.1+/-1.4 years; 26.5+/-0.8 kg/m2) were included. We obtained blood samples for glucose, insulin, GH, total ghrelin and acylated ghrelin at times 0, 30, 60, 90 and 120 minutes after 75 g of oral glucose. RESULTS: Fasting GH and IGF-I were statistically different between patients and controls: GH (microg/l): 6.7+/-1.4 vs. 0.8+/-0.4, p<0.01; IGF-I (ng/ml): 414+/-75 vs. 86+/-6, p<0.01. Fasting total ghrelin (pg/ml) were similar in the patient and in the control group, 916+/-132 vs. 844+/-169, p=ns. In both groups total ghrelin levels decreased during oral glucose, and nadir total ghrelin was lower than fasting ghrelin: patients: 916+/-132 vs. 747+/-95, p<0.05; controls: 844+/-169 vs. 625+/-90, p<0.05). The AUCs of total ghrelin (pg/ml*min) were not different between the two groups: 98953+/-13052 vs. 83773+/-13096, p=ns). Fasting acylated ghrelin (pg/ml) were similar in the patient and the control group 65+/-13 pg/ml vs.74+/-14 pg/ml, p=ns. In both groups acylated ghrelin levels decreased during oral glucose, and nadir acylated ghrelin levels were lower than basal acylated ghrelin levels: patients: 65+/-13 vs. 42+/-6, p<0.05; controls: 74+/-14 pg/ml vs. 37+/-4 pg/ml, p<0.05). The AUCs of acylated ghrelin (pg/ml*min) were not different between the two groups: patients: 6173+/-992 vs. controls 8648+/-2742, p=ns). In acromegalic patients there was a negative correlation between fasting, both total and acylated, ghrelin and both fasting and post oral glucose insulin levels. CONCLUSIONS: These data suggest that circulating total and acylated ghrelin in acromegaly is regulated by insulin and not by GH hypersecretion.