AN ELECTRON MICROSCOPY STUDY OF PHOSPHATASE ACTIVITY IN THE ENDOTHELIAL CELLS OF RABBIT AORTA

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Abstract
The fine structural localization of phosphatase activity in the endothelial lining of rabbit aorta was studied using the electron microscope. The tissue was incubated in media containing various substrates at different pH values. Only a neutral phosphatase activity was localized with a preferential specificity for adenosine triphosphate (ATP), although some affinity was found with adenosine diphosphate, adenosine monophosphate, β-glycerophosphate and phenylphosphate. The reaction product (lead phosphate), with ATP and ADP as substrates, was localized primarily in cell membrane invaginations at the intercellular spaces and in neighboring pinocytotic vesicles, and only slightly in similar invaginations at the luminal and intimal surfaces. Enzyme localization was similar using the other substrates, but with a lower concentration of reaction product that was absent from the invaginations at the luminal surface. The significance of these observations in light of the role of cytopempsis in intimal nutrition and pathogenesis of atherosclerosis is briefly discussed.