Abstract
Electron microscopic studies of perfused rat adrenals indicate that plasma lipoproteins become concentrated in a specialized cell surface compartment called microvillar channels. Closely associated plasma membranes of sinusoidal microvilli of zona fasciculata cells form channels that normally are filled with electron dense particles the size of high density lipoproteins (HDL). In rats made acutely deficient in plasma lipoproteins (by treatment with 4-aminopyrazolo[3,4-d]pyrimidine (4-APP) for 1 day), particles within the microvillar channels are decreased in number. When adrenal glands of these rats are perfused with media lacking plasma lipoproteins, many but not all of these HDL-like particles are washed out. However, when these adrenals are perfused with large amounts (100-500 micrograms protein/ml) of HDL, microvillar channels become packed with electron dense particles similar to those found in vivo. These microvillar channels become wider and filled with larger particles when low density lipoproteins (LDL) are perfused through the adrenals. Autoradiograms of 125I-labeled HDL-perfused adrenals show silver grains specifically associated with the cell surface microvillar channels, and confirm the notion that the particles filling the channels are exogenously delivered HDL. Physiologic data from similarly perfused adrenals in a parallel study show that the channel-refilling process is directly related to selective (i.e., nonendocytic) cholesterol uptake and that this cholesterol uptake is associated with corticosterone production. Together, these data suggest the hypothesis that plasma lipoprotein cholesterol utilized for corticosteroid synthesis in rat adrenal fasciculata cells may be derived from lipoproteins trapped in surface-associated microvillar channels. Although the mechanism responsible for the cholesterol transfer is not yet defined, it is clearly distinct from the classical process of receptor-mediated endocytosis and catabolism of lipoprotein particles.
Highlights
Electron microscopic studies of perfused rat adrenals indicate that plasma lipoproteins become concentrated in a specialized cell surface compartment called microvillar channels
Our studies reveal a complex network of extracellular channels on the sinusoidal face of zona fasciculata cells which contain endogenous high density lipoproteins (HDL)-lipoproteins in nonperfused animals, and trap great numbers of HDL or low density lipoproteins (LDL) lipoproteins when these lipoproteins are administered by perfusion
The adrenocortical cell microvilli fasciculata cells were totally covered with abnormal- shareanumber of features with those previously deappearinegndogenous spherical particles, largeinr scribed in the luteinized ovary, and this includes the average diameter (20.8 mp) than the rat HDL or LDL presence of channels between closely apposed microvilli used in perfusion studies
Summary
Electron microscopic studies of perfused rat adrenals indicate that plasma lipoproteins become concentrated in a specialized cell surface compartment called microvillar channels. It is quite clear that in another steroidogenic tissue, the luteinized ovary [9,10,11,12], most supplied HDL and LDL become associated with specialized sites of the cell plasma membrane known as microvillar channels. These are flat, open-ended spaces between closely associated microvilli that appear to trap great numbers of intact lipoprotein particles (both HDL and LDL) during perfusion experiments [11]. Given our favorable results using nonrecycling perfusion techniques to deliver lipoprotein cholesterol directly to adrenal glands of various rat models (l), we felt an indepth examination of the microvillar surface of the corticosterone-producing cells of this organ was in order
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