Pinocytosis in the rabbit corneal endothelium.
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The generation, flow, directionality and fusion of phagocytic and fluid-phase pinocytic vesicles in cultured macrophages and fibroblasts are reviewed. Specific plasma membrane (PM) receptors, receptor mobility, contractile cytoplasmic elements and lipid composition of the PM serve to regulate the flow of large phagosomes into the perinuclear zone. Fluid-phase vesicles are constitutively generated and carry large quantities of PM, fluid and solutes into the cytoplasm. Quantitative information is cited on the rates of vesicular generation, fusion with other members of the vacuolar system, fluid and solute uptake, and digestion and solute release. The nature and composition of fluid-phase vesicles, phagocytic vacuoles and PM are compared. Once interiorized, PM and its component polypeptides rapidly cycle back to the cell surface. The flow rates of both the centrifugal and the centripetal compartments as well as the fate of a minor degradation pool are illustrated and compared to the turnover of individual membrane polypeptides. Implications of membrane flow for cell shape, motility and new PM insertion are discussed.
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The role of tight junctions in permeability barriers is now well established. Other roles for this junction have been postulated but have not been demonstrated. In both adult and prepubertal dogs, stimulation with human chorionic gonadotropin (hCG) results in an increased number of pinocytotic vesicles on the membrane faces enclosed by tight junctions within the inter-Sertoli cell junctions of the testis. It is suggested that these membranous regions, protected from the normal flow of intercellular fluids, may act as regions of selective intercellular exchange.
The effects of cis-diamminedichloroplatinum(II) [cisplatin], a potential anticancer drug, were studied on pinocytotic functions in the cellular slime mould Dictyostelium discoideum by administering FITC-dextran as a fluid phase marker. Cisplatin treatment at a concentration of 100 and 200 micrograms/ml for 1 h causes inhibition in pinocytotic uptake in growing Dictyostelium cells in a dose-dependent manner. Cisplatin treatment induced the association of more actin with the cell cortex, thereby presumably restricting the flexibility of the cell membrane and inhibiting the formation of pinosomes. Ultrastructural analysis of cisplatin-treated cells showed a lower number of pinosomes. These results have been discussed in the light of cisplatin's known actions that affect various cellular functions.
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The fate of the extracellular marker horseradish peroxidase (HRP), following intense transmitter release was studied using identified muscle fibers from the frog sartorius nerve-muscle preparation. The muscle was stimulated indirectly via its nerve at 10 Hz or K+-depolarized for 15 min. Other preparations were also stimulated or K+-depolarized for 15 min and then rested for an additional 15 min. Endings from only identified muscle fibers were photographed with the electron microscope. It was found that in the paradigms studied above, less than 10% of the mean number of synaptic vesicle profiles per section contained the marker. Following electrical stimulation, there was a statistically significant decrease in the mean number of synaptic vesicle profiles per section. After a 15 min rest period, the vesicle profile number had returned to the control value. At this time point, the endplate potential was but 25% of the control. K+-depolarization caused no significant change in the mean number of synaptic vesicle profiles per section. Experiments were also performed to rule out any direct effect of the label on the number of coated and synaptic vesicle profiles. The mean number of labeled coated vesicle profiles increased during either electrical stimulation or K+-depolarization, and then fell during the subsequent rest period. Their numbers accounted for less than 2% of the total number vesicles/section. A suprisingly high number of coated vesicle profiles (as high as 41%) contained no label. This finding is inconsistent with the exclusive role of coated vesicles associated with synaptic vesicle membrane recycling. The low level of HRP labeling of synaptic vesicles is also inconsistent with synaptic vesicles undergoing exo- and endocytosis along the presynaptic plasma membrane.
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