Induction of Resistance to Endothelin-1's Biochemical Actions by Elevated Glucose Levels in Retinal Pericytes

Because retinal pericytes have contractile properties and are affected by diabetes, we have studied the responsiveness of pericytes to ET-1, a potent vasoconstrictor, in the presence of various concentrations of glucose. Cultured calf retinal pericytes were exposed to glucose levels of 5.5 or 25 mM for up to 8 days. Radioreceptor studies that used [¹²⁵I]ET-1 showed that pericytes contained high-affinity binding sites with K_d of 3 × 10⁻¹⁰ M, and these binding affinities were unaffected by glucose concentration. Receptor number appears to be elevated, but this increase was NS. Responsiveness of pericytes to ET-1 was studied with respect to stimulation of DAG and IP₃ levels and PKC activities. In contrast to receptor binding, exposure to 25 mM glucose for >6 days blunted pericyte responsiveness to ET-1. The time course of ET-1 stimulation as measured by [³H]glycerol labeling, and IP₃ level showed a 98% increase in [³H]DAG at 10 min and a fourfold increase for IP₃, respectively. Cells exposed to 25 mM glucose only had a 32% increase for DAG, and no increase for IP₃ was observed. Dose-response studies on the stimulation of [³H]DAG increase showed the range of ET-1's effect to be between 10⁻⁹ and 10⁻⁷ M. At maximum, cells exposed to 5.5 mM glucose had a 70% increase versus only a 30% increase in those exposed to 25 mM glucose. Similarly, ET-1 only increased the total DAG levels in pericytes exposed to 5.5 mM glucose by 41%. PKC activity also was measured because DAG is one of its cellular activators. In cells exposed to 5 mM glucose, ET-1 increased PKC-specific activity in the membraneous pool after 30 min by twoto fourfold without changes in the cytosol. In those cells exposed to 25 mM glucose, no increase in the membranous PKC activity was noted. These findings show that exposure of pericytes to elevated levels of glucose can cause resistance to some of ET-1's biochemical actions at the postreceptor sites. Possibly, the loss of pericyte function as observed in early diabetic retinopathy could be attributable to the developed resistance to ET-1 in the retinal microvessels.