Heterogeneity among ovarian blood vessels: Endogenous hepatic lipase is concentrated in blood vessels of rat corpora lutea

We used indirect immunofluorescence and immunogold light microscopy to examine the distribution of hepatic lipase, an enzyme involved in lipoprotein metabolism, in ovaries of gonadotropin‐treated immature rats. Antibodies utilized were rabbit anti‐rat hepatic lipase IgG, anti‐rat von Willebrand factor (VWF, an endothelial cell marker), and goat anti‐rabbit IgG conjugated to gold particles or rhodamine. Immunoreagents were applied to fresh frozen sections of unfixed ovary or liver (positive control) or were delivered to ovaries by vascular perfusion before fixation in situ and silver‐enhancement of sections. Appropriate controls verified that the immunolocalizations were specific. Immunofluorescence implied that luteal but not stromal blood vessels of ovaries were positive for hepatic lipase, whereas luteal and stromal blood vessels bore VWF. The improved morphology gained by perfusing ovaries with antibodies allowed precise localization of the enzyme. Hepatic lipase was concentrated within thin‐walled vessels of corpora lutea but not those of stroma in ovaries at the time of peak steroidogenic activity. Quantification of hepatic lipase‐labeled vessels in stromal and luteal compartments confirmed our visual impression. Many images suggested that stromal vessels lacking hepatic lipase gained this enzyme upon contact with luteal tissue. Perfusion of ovaries with cationized ferritin labeled all ovarian vessels equally well, ruling out the possibility that the observed distribution of hepatic lipase was artifactual. These findings demonstrate that ovarian blood vessels are heterogeneous for hepatic lipase. Moreover, they imply that luteal tissue, perhaps luteal cells, may influence expression of hepatic lipase binding sites by endothelial cells.