Effect of N-benzoyl-d-phenylalanine and metformin on insulin receptors in neonatal streptozotocin-induced diabetic rats: Studies on insulin binding to erythrocytes

Abstract

In the present study, we focused on the insulin-receptor binding in circulating erythrocytes of N-benzoyl-d-phenylalanine (NBDP) and metformin in neonatal streptozotocin (nSTZ)-induced male Wistar rats. We measured blood levels of glucose and plasma insulin and the binding of insulin to cell-membrane ER receptors in NBDP and metformin-treated diabetic rats. The mean specific binding of insulin to ER was significantly lower in diabetic control rats (DC) (53.0 ± 3.1%) than in NBDP (62.0 ± 3.1%), metformin (66.0 ± 3.3%) and NBDP and metformin combination-treated (72.0 ± 4.2%) diabetic rats, resulting in a significant decrease in plasma insulin. Scatchard plot analysis demonstrated that the decrease in insulin binding was accounted for by a lower number of insulin receptor sites per cell in DC rats when compared with NBDP and metformin-treated rats. High-affinity (Kd1), low-affinity (Kd2), and kinetic analysis revealed an increase in the average receptor affinity in ER from NBDP and metformin-treated diabetic rats having NBDP 2.0 ± 0.10×10−10 M−1 (Kd1); 12.0 ± 0.85×10−8 M−1 (Kd2), Metformin 2.1 ± 0.15×10−10 M−1 (Kd1); 15.0 ± 0.80×10−8 M−1 (Kd2), NBDP and metformin 2.7 ± 0.10×10−10 M−1 (Kd1); 20.0 ± 1.2×10−8 M−1 (Kd2) compared with 0.9 ± 0.06×10−10 M−1 (Kd1); 6.0 ± 0.30×10−8 M−1 (Kd2) in DC rats. The results suggest an acute alteration in the number of insulin receptors on ER membranes in nSTZ induced diabetic control rats. Treatment with NBDP along with metformin significantly improved specific insulin binding, with receptor number and affinity binding reaching almost normal non-diabetic levels. The data presented here show that NBDP along with metformin increase total ER membrane insulin binding sites with a concomitant significant increase in plasma insulin.