Evidence for an Inhibitory Effect of Physiological Levels of Insulin on the Growth Hormone (GH) Response to GH-Releasing Hormone in Healthy Subjects

It has been previously reported that in healthy subjects, the acute reduction of free fatty acids (FFA) levels by acipimox enhances the GH response to GHRH. In the present study, the GH response to GHRH was evaluated during acute blockade of lipolysis obtained either by acipimox or by insulin at different infusion rates. Six healthy subjects (four men and two women, 25.8 ± 1.9 yrs old, mean± se) underwent three GHRH tests (50 μg iv, at 1300 h) during: 1) iv 0.9% NaCl infusion (1200–1500 h) after oral acipimox administration (250 mg) at 0700 h and at 1100 h; 2) 0.1 mU·kg−1·min−1 euglycemic insulin clamp (1200–1500 h) after oral acipimox administration (250 mg at 0700 h and at 1100 h); 3) 0.4 mU·kg−1·min−1 euglycemic insulin clamp (1200–1500 h) after oral placebo administration (at 0700 and 1100 h). Serum insulin (immunoreactive insulin) levels were significantly different in the three tests (12 ± 2, 100 ± 10, 194 ± 19 pmol/L, P < 0.05), plasma FFA were low and similar (0.04 ± 0.003, 0.02 ± 0.005, 0.02 ± 0....