Metabolic regulation of the growth hormone independent insulin-like growth factor binding protein in human plasma

This study examines the regulation of circulating GH-independent insulin-like growth factor binding protein, BP-28. Commencing at 22.00 h, BP-28 in 5 normal adults rose 11-fold to peak values of 120 +/- 12 micrograms/l, remained elevated between 01.00 and 08.00 h, then fell rapidly following a meal. If meals were omitted, BP-28 remained at peak levels throughout the day. The fasting BP-28 level was higher in women (141 +/- 22 micrograms/l, N = 5) than men (59 +/- 14 micrograms/l, N = 7), and pregnancy caused a further 2-fold elevation. Oral glucose rapidly lowered BP-28 in diabetic and nondiabetic pregnant women, nonpregnant women, and men. In a heterogeneous group of 18 subjects, insulin (0.1 U/kg iv), with or without simultaneous administration of GnRH and TRH, elicited a 3- to 4-fold rise in BP-28, commencing 60 min after the nadir of plasma glucose, and independent of the response in GH, PRL, TSH, LH or cortisol. We conclude that BP-28 levels in adults are metabolically regulated, and postulate a role for this protein in the maintenance of glucose homeostasis.