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 [ 125 I]ET-1 showed that pericytes contained high-affinity binding sites with /Q of 3 x 1O~1 0 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 IP 3 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 [ 3 H]glycerol labeling, and IP 3 level showed a 98% increase in [ 3 H]DAG at 10 min and a fourfold increase for IP 3 , respectively. Cells exposed to 25 mM glucose only had a 32% increase for DAG, and no increase for IP 3 was observed. Dose-response studies on the stimulation of [ 3 H]DAG increase showed the range of ET-1's effect to be between 10" 9 and 1 0 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