Fibroblasts incubated in potassium-free medium lose about 80% of their intracellular potassium within 10 min and show disrupted clathrin-coated pits. When the K+ level reaches a critical threshold of 40% below normal, the number of coated pits declines by more than 80%, and the rate of endocytosis is decreased by 70-95%, despite normal receptor binding. However, the morphology of cells changes rapidly, suggesting that other important cellular functions may be affected (Altankov and Grinnell 1993; Larkin et al. 1983). Why K+ depletion affects endocytosis is unknown. Also, it is unclear to which extent other clathrin-independent internalization pathways are affected.

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