Differential half-maximal effects of human insulin and its analogs for in situ glucose transport and protein synthesis in rat soleus muscle

Analogs of human insulin have been used to discriminate between responses of metabolic and mitogenic (growth-related) pathways. This study compared the stimulatory effects of human insulin (HI) and 2 analogs (X2, B-Asp⁹, B-Glu²⁷ and H2, A-His⁸, B-His⁴, B-Glu¹⁰, B-His²⁷) on glucose uptake and protein synthesis in rat soleus muscle in situ. Glucose uptake, estimated by intramuscular (IM) injection of 2-deoxy[1, 2⁻³H]glucose with or without insulin, was maximally increased at 10⁻⁶ mol/L for HI and X2 and 10⁻⁷ mol/L for H2. HI had a larger effect (318%) than either X2 (156%) or H2 (124%). The half-maximal effect (ED₅₀) values for HI, X2, and H2 were 3.3 × 10⁻⁸ mol/L, 1.7 × 10⁻⁷ mol/L, and 1.6 × 10⁻⁹ mol/L, respectively. Protein synthesis, estimated by protein incorporation of [³H]phenylalanine injected into muscles with or without insulin, was maximally increased at 10⁻⁵ mol/L for HI and 10⁻⁶ for X2 and H2. HI had a larger effect in stimulating protein synthesis (34%) than either X2 (25%) or H2 (19.8%). The ED₅₀ values for HI, X2, and H2 were 3.0 × 10⁻⁷ mol/L, 3.2 × 10⁻⁷ mol/L, and 1.0 × 10⁻⁹ mol/L, respectively. The biological potency of each analog (ED₅₀insulin/ED₅₀analog) showed X2 to be less potent than HI for both glucose uptake (0.2) and protein synthesis (0.9), whereas H2 is more potent than HI with ratios of 20 and 300, respectively. These data suggest that this approach for studying insulin responsiveness in a single muscle in situ may be a useful tool for investigating insulin signaling in muscle in vivo.