TRUE INSULIN AND INTACT PROINSULIN LEVELS IN ACROMEGALIC PATIENTS

To determine whether proinsulin (PT) contributes significantly to the immunoreactive insulin (IRI) concentrations in acromegalics, we measur e PI, ''true insulin'' and IRI in a group of acromegalics compared wit h a control group. Serum PI was determined by the immunofluorimetric a ssay (IFMA). Insulin was also determined by an IFMA that measures true insulin and by a radioimmunoassay (RIA). We performed an oral glucose tolerance test (OGTT) in a total group of 46 subjects: 10 controls wi th normal OGTT and body mass index < 25 kg/m(2) (control group I), 10 controls with normal OGTT and body mass index > 25 kg/m(2) (control gr oup II), 15 patients with active acromegaly and normal OGTT and 11 pat ients with active acromegaly and IGT. Plasma glucose, serum GH, insuli n and proinsulin were measured in all OGTT samples. Basal levels of in sulin-like growth factor I (IGF-I) were measured in acromegalics. Mean body mass index in acromegalics with normal and impaired glucose tole rance were significantly higher compared with control group I and simi lar when compared with control group II. Proinsulin increased during O GTT in acromegalics with impaired glucose tolerance compared to contro l group I, and only fasting proinsulin compared to control group II, I n normal OGTT acromegalics, only fasting proinsulin was increased. The RIA insulin during OGTT was significantly higher for both acromegalic groups compared to control group I and only at fasting when compared with control group II. This difference was not evident when insulin wa s measured by IFMA, These results suggest that in acromegalics, hyperi nsulinism measured by RIA was at least in part due to hyperproinsulini sm.