Effect of i.p. insulin administration on IGF1 and IGFBP1 in type 1 diabetes

In type 1 diabetes mellitus (T1DM), low concentrations of IGF1 and high concentrations of IGF-binding protein 1 (IGFBP1) have been reported. It has been suggested that these abnormalities in the GH-IGF1 axis are due to low insulin concentrations in the portal vein. We hypothesized that the i.p. route of insulin administration increases IGF1 concentrations when compared with the s.c. route of insulin administration. IGF1 and IGFBP1 concentrations in samples derived from an open-label, randomized cross-over trial comparing the effects of s.c. and i.p. insulin delivery on glycaemia were determined. T1DM patients were randomized to receive either 6 months of continuous i.p. insulin infusion (CIPII) through an implantable pump (MIP 2007C, Medtronic) followed by 6 months of s.c. insulin infusion or vice versa with a washout phase in between. Data from 16 patients who had complete measurements during both treatment phases were analysed. The change in IGF1 concentrations during CIPII treatment was 10.4 μg/l (95% CI -0.94, 21.7 μg/l; P=0.06) and during s.c. insulin treatment was -2.2 μg/l (95% CI -13.5, 9.2 μg/l; P=0.69). When taking the effect of treatment order into account, the estimated change in IGF1 concentrations was found to be 12.6 μg/l (95% CI -3.1, 28.5 μg/l; P=0.11) with CIPII treatment compared with that with s.c. insulin treatment. IGFBP1 concentrations decreased to -100.7 μg/l (95% CI -143.0, -58.3 μg/l; P<0.01) with CIPII treatment. During CIPII treatment, parts of the GH-IGF1 axis changed compared with that observed during s.c. insulin treatment. This supports the hypothesis that the i.p. route of insulin administration is of importance in the IGF1 system.