Depletion of intracellular potassium arrests coated pit formation and receptor-mediated endocytosis in fibroblasts

Depletion of intracellular potassium (K⁺) caused a marked reduction in the rate of endocytosis of receptor-bound low density lipoprotein (LDL) and epidermal growth factor (EGF) in human fibroblasts. K⁺ could be depleted slowly by a 3-hr incubation of cells in isotonic K⁺-free buffer. Rapid K⁺ depletion was induced by incubation of cells for 5 min with hypotonic medium, followed by transfer to isotonic K⁺-free buffer. Within 30 min of this treatment, cellular K⁺ levels fell by more than 60%. When the K⁺ level fell below a threshold of 40% of normal, the number of coated pits declined by 80% and the rate of endocytosis of ¹²⁵I-LDL decreased by 70 to 95% despite normal to increased receptor binding. Similar results were obtained with ¹²⁵I-epidermal growth factor. Addition of KCI to the culture medium up to 2 hr after K⁺ depletion restored cellular K⁺ levels and returned endocytosis of ¹²⁵I-LDL promptly to normal. RbCl was as effective as KCl, but CsCl, LiCI, and (CH₃)₄NCI had no effect. Restoration by KCI was blocked by ouabain, indicating that uptake via the Na⁺/K⁺ ATPase was required. These data demonstrate that depletion of intracellular K⁺ reversibly arrests coated pit formation and receptor-mediated endocytosis in human fibroblasts.