Establishment of heterogeneity among blood vessels: hormone-influenced appearance of hepatic lipase in specific subsets of the ovarian microvasculature.

We used biochemical and structural approaches to analyze the influence of gonadotropic hormones on the association of hepatic lipase with specific subsets of ovarian blood vessels. Western blotting was used to detect this enzyme in effluent collected from heparin-perfused ovaries of nonhormone-treated immature rats and those primed with pregnant mare's serum gonadotropin (PMSG) alone or in combination with human chorionic gonadotropin (hCG). The effects of these hormones on hepatic lipase distribution among ovarian blood vessels was assessed before and after hCG and/or PMSG treatment by immunofluorescence and immunogold cytochemistry. For the latter, immunoreagents and fixative were delivered directly to chilled, unfixed ovaries by in situ vascular perfusion. Data from biochemical and structural analyses indicated that hepatic lipase was absent from nonhormone-treated ovaries. As shown by Western blotting of ovarian effluent, the enzyme appeared following treatment with PMSG and PMSG-hCG; it increased in amount in a time-dependent manner, with a transient decline in the early hours after hCG injection. Enzyme levels paralleled growth and vascularization of follicles and corpora lutea; the fall tended to coincide with early events in luteal angiogenesis. Immunogold microscopy showed that hepatic lipase was abundant in thin-walled blood vessels of theca interna of follicles, corpora lutea, and interstitial cells but sparse in those of the stroma. Moreover, during neovascularization of differentiating corpora lutea, vascular sprouts arising from hepatic lipase-laden thecal vessels appeared to lose, then regain, the enzyme as development progressed. Our findings thus suggest 1) that hormones influence the establishment of endothelial cell heterogeneity within the microvasculature of a single organ and 2) that development of novel endothelial cell properties in specific subsets of blood vessels underlies compartmentalization of function within a tissue.