Effect of insulin treatment on circulating insulin-like growth factor I and IGF-binding proteins in cats with diabetes mellitus

Background: Insulin-like growth factor-I (IGF-I) is used to screen for acromegaly in diabetic cats. In humans, most circulating IGF-I forms ternary complexes (TC) with IGF-binding protein (IGFBP-3) and an acid-labile subunit. Compared to humans, the amount of TC in cats is more variable. IGF-I concentrations are reported to increase during insulin treatment, more rapidly in cats achieving remission.

Objectives: To investigate (i) factors associated with circulating IGF-I concentrations, including IGF-binding protein profiles (ii) effect of insulin treatment on IGF-I concentrations and (ii) IGF-I as prognostic marker of DM remission.Animals: 31 privately-owned diabetic cats of which 24 were followed 1 year, and 13 healthy cats.

Methods: In this prospective study, insulin, IGF-I, glucose and fructosamine were measured. IGF-binding forms were determined by chromatography in 14 diabetic and 13 healthy cats. IGF- I, IGF-II, IGFBP-3 and IGFBP-5 was measured by mass spectrometry in 3 cats achieving remission.

Results: IGF-I median (interquartile range) before start of insulin treatment was 300 (160-556) ng/mL. IGF-I was positively associated with TC (p<0.0001) and endogenous insulin (p=0.005) and negatively associated with fructosamine (p<0.0001). IGF-I was 2-fold higher 2-4 weeks after start of insulin treatment compared with baseline (p=0.0001) and predicted future remission (p=0.046). In cats that went into remission, the amount of TC and IGFBP-3 increased, suggesting the increase in IGF-I is dependent on TC formation.

Conclusions: Insulin treatment should be accounted for when interpreting IGF-I in diabetic cats. IGF-I 2-4 weeks after initiation of insulin treatment shows promise as prognostic marker for remission in diabetic cats.