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Zusammenfassung:
The acidotropic agent ammonium chloride (NHCl) not only affects receptor metabolism by inhibiting lysosomal acidification, but can also affect the targeting of proteins to specific membranes in polarized cells, possibly through effects mediated by the cytoskeleton. The present study examines the effects of NHCl and perturbers of cytoskeleton structure on vasopressin V2 receptor expression in LLC-PK1 renal epithelial cells. Surprisingly, long-term pretreatment of cells with NHCl or short-term treatment with the actin perturber cytochalasin B resulted in an up to 70% increase in specific Arg-8-vasopressin binding compared to control cells, which was independent of the presence of NHCl in the binding test, and apparently the result of increased V2 receptor expression. Perturbers of microtubules such as colchicine and vinblastine had no such effect. A rhodamine-labeled analog of vasopressin was used to fluorescently label the V2 receptor of LLC-PK1 cells, and microscopic measurements of membrane-localized fluorescence confirmed the increased V2 receptor expression in the basal plasma membrane subsequent to NHCl pretreatment. Lateral mobility of the V2 receptor was measured in living cells using the technique of microphotolysis (photobleaching). The fraction of mobile receptors was 0.2 in cells pretreated with NHCl, markedly reduced compared to that of 0.9 in untreated cells. The apparent lateral diffusion coefficient D was about 3 × 10−10 cm2/s in both pretreated and untreated cells. Results for fluorescence labeling of the actin cytoskeleton indicate that NH4Cl pretreatment of LLC-PK1 cells results in pertubation of microfilament structure. All results imply that the cytoskeleton plays a central role in V2 receptor expression and lateral mobility.
