Equivalent In Vivo Biological Activity of Insulin Analogues and Human Insulin Despite Different In Vitro Potencies

In vivo biological potency of two human insulin analogues, Asp^B9,Glu^B27 insulin and Asp^B10 insulin with low and high affinity to the insulin receptor, respectively, was assessed by intravenous infusion of equimolar amounts in pigs, with the euglycemic clamp technique. Human insulin and the low- and high-affinity analogues showed equivalent glucose utilization rates in the steady state (mean ± SE 14.7 ± 1.4, 12.7 ± 1.5, and 12.2 ± 1.2 mg · kg⁻¹ · min⁻¹, respectively; n = 7). The corresponding plasma insulin levels, however, were markedly different (329 ± 25 and 856 ± 46 pM, P < 0.05; 197 ± 19 pM, P < 0.05). There was an inverse relationship between the insulin levels and the in vitro activities measured by binding to human hepatoma cells (HepG2; 100, 20, and 308%) or by incorporation of glucose into lipids in mouse free fat cells (100, 31, and 207%). The total amount of glucose infused during and after insulin infusion was equal for the three insulins, whereas glucose utilization as a function of time was somewhat different. By describing the individual plasma concentration courses with an open two-compartment model with elimination from the receptor compartment, the time courses for binding and elimination of the three insulins in the receptor compartment were estimated. The effect seems closely linked to the elimination of insulin from the receptors rather than to the amount of insulin bound to the receptors. In conclusion, the total effect of equimolar amounts of human insulin and the two insulin analogues on glucose utilization is equal regardless of the different receptor affinities of the insulins.