MAXIMAL SECRETORY CAPACITY OF SOMATOTROPE CELLS IN OBESITY - COMPARISON WITH GH DEFICIENCY

OBJECTIVE: To evaluate the maximal secretory capacity of somatotrope c ells in obesity and to compare it with that in hypopituitaric patients with GH deficiency. DESIGN: Stimulation with GHRH. (1 mu g/kg iv) com bined with arginine (ARG, 0.5 g/kg iv), which strongly potentiates the GH response to the neurohormone, likely inhibiting hypothalamic somat ostatin. The reproducibility of the GH response to GHRH + ARG was eval uated in a second session. SUBJECTS: Forty-five patients with simple o besity (OB 11 male and 34 female, age 40.5+/-1.8y, BMI 38.8 +/- 1.1 kg /m(2)), 49 patients with hypopituitarism (GHD, 23 male and 26 female, 43.6 +/- 2.4y, 24.7+/-0.7 kg/m(2)) and 44 normal young volunteers (NS, 25 male and 19 female, 33.8+/-1.0y, 21.6+/-0.3 kg/m(2)) were studied, MEASUREMENTS: GH levels were assayed by IRMA method, basally at - 60 and 0 min, and than every 15 min up to + 120 min. Basal IGF-I levels w ere assayed by RIA method, after acid-ethanol extraction. RESULTS: IGF -I levels in OB were lower (P<0.005) than those in NS but higher (P<0. 005) than those in GHD. Mean peak GH response to GHRH+ARG in OB was cl early lower than that in NS (P<0.005) and higher (P<0.005) than that i n GHD. Sixty-percent OB and 100% GHD showed peak GH responses lower th an the minimum normal limit in NS (16.5 mu g/l) while 4% OB and only 5 3% GHD with GH responses lower than 3 mu g/l, the limit under which GH replacement therapy of severe deficiency is allowed. Good intraindivi dual reproducibility of the GH response to GHRH + arginine test was pr esent in all groups (OB: r=0.78, P<0.0001; GHD: r=0.57, P<0.003; NS: r =0.74, P<0.0001;. CONCLUSIONS: The maximal secretory capacity of somat otrope cells is clearly less than normal in the obese but still more t han is seen in GHD subjects. However, in about 50% of obese patients, the pituitary GH releasable pool overlaps with that of hypopituitaric patients with GH deficiency. Thus, even when the maximal secretory cap acity of somatotrope cells is evaluated by a potent and reproducible p rovocative tests such as GHRH + arginine, overweight has to be taken i n a great account as the cause of severely impaired GH response in pat ients with suspected GH deficiency.