The effect of streptozotocin-induced diabetes on the early steps of glucagon action in isolated rat liver cells

Abstract

This study was undertaken to investigate the effect of experimental diabetes on the early steps of glucagon action. The binding of glucagon and glucagon-stimulated cyclic AMP accumulation in the presence of a potent phosphodiesterase inhibitor (IBMX, 0.1 mmol/l) were studied in liver cells isolated from control and streptozotocin-induced (65 mg/kg) diabetic rats. Comparative studies of insulin binding indicated that hepatocytes of diabetic rats bound twice as much ¹²⁵I-insulin (10.8±2.0%) as those of control rats (5.7±1.3%). Scatchard analysis and the competition plots of the data suggested that this was due to an increased number of receptors rather a change in their affinity. No significant change was observed in ¹²⁵I-glucagon binding of diabetic liver cells (5.8±0.5%) as compared to controls (6.8±0.4%). The number of molecules of glucagon bound to high and low affinity binding sites of control liver cells was (51±2)×10³ and (1300+134)×10³ sites/cell, respectively. The corresponding numbers in streptozotocin-treated rats were (45±5) ×10³ and (1000±167)×10³ sites/cell, respectively. Cyclic AMP response to concentrations of glucagon below 1 nmol/l was significantly lower in diabetics than in normals: for 0.3 nmol/l and 0.6 nmol/l of glucagon, cyclic AMP production was 48±7 pmol/ 10⁶ cells and 78±8 pmol/10⁶ cells in diabetics, as compared to 72±9 and 110±9 pmol/10⁶ cells in normals. At concentrations of glucagon that are maximally efficient (⩾7 nmol/l) cyclic AMP production was higher in diabetic (202±20 pmol/10⁶ cells) than in normal rats (156±7 pmol/10⁶ cells). Thus, diabetes seems to increase the quantity of adenylate cyclase and decrease its affinity for glucagon. Those changes are not related to a modification of the glucagon binding sites and are associated to an increase of insulin receptors.