The inhibitory effect of glucose on growth hormone secretion is lost in obesity but not in hypertension

In obesity there is a clear reduction of both spontaneous and stimulated GH secretion. Furthermore, in obese patients the somatotrope responsiveness to provocative stimulation is selectively refractory to the inhibitory effect of glucose load. It has been hypothesized that hyperinsulinism of obese patients could play a role in the pathogenesis of these alterations. Aim of the present study was to verify the GH response to GHRH and the ability of glucose load to inhibit it in patients with essential hypertension in whom hyperinsulinism and insulin resistance are frequently present. To this goal, 7 patients with essential hypertension (HP, age, mean±SE: 29.6±2.4 yr, 3 females and 4 males, BMI: 21.7±1.2 kg/m2), 7 obese (OB, 4 females and 3 males, 31.9±4.1 yr, 35.6±2.0 kg/m2) and 7 normal subjects (NS, 4 females and 3 males, 28.3±3.9 yr, 21.0±1.6 kg/m2) underwent the following tests: GHRH (1 pg/kg iv at time 0) alone and preceded by oral glucose load (OGTT, 100 g po at — 45 min). Basal insulin levels were similar in HP and OB (11.3±0.5 and 12.7±2.2 µU/ml, respectively); these, in turn, were higher (p<0.005) than those in NS (6.8±0.8 µU/ml). Basal plasma glucose levels in HP were similar to those in OB and NS (80.3±3.6, 86.9±6.7 and 84.4±1.7 mg/dl, respectively). In HP and OB and NS basal GH (1.0±0.5, 1.0±0.6 and 0.3±0.1 eg/l, respectively) and IGF-I levels (132.6±14.8, 137.3±13.2 and 138.8±12.2 eg/l, respectively) were similar. In HP the GH response to GHRH (AUC: 1058.8±347.8 eg/l/min) was similar to that observed in NS (959.0±167.8 eg/l/min) and higher than that in OB (344.8±67.2 eg/l/min, p<0.01). OGTT clearly blunted (p<0.01) the GHRH-induced GH response in HP as well as in NS (401.8±104.4 and 521.6±76.6 (g/l/min, respectively) but not in OB (387.4±78.8 (g/l/min). The OGTT-induced insulin levels in HP did not differ from those of OB, both being higher (p<0.05) than those recorded in NS. Glucose levels after OGTT were similar in the three groups. In conclusion, this study demonstrates that, like in normal subjects but differently from in obese patients the GH response to GHRH is normal in patients with essential hypertension and it is normally inhibited by oral glucose load even when these patients show high insulin levels. Thus, it is unlikely that the low somatotrope secretion and its refractoriness to inhibition by glucose load in obesity is due to hyperinsulinism.