Complex sentence: However, chronic dysregulated blood glucose in diabetes causes a marked toxicity toward those classes of vascular endothelium which passively assimilate glucose (i.e. in spite of low insulin), notably the pericytes of various microvasculatures.
Split into simpler sentences: However, chronic dysregulated blood glucose in diabetes is toxic to cells of the vascular endothelium which passively assimilate glucose. That is, cells in which insulin is not required for intercellular transport of glucose, notably the pericytes of various microvasculatures.