The study investigates the effects of hypertonic media and potassium depletion on receptor-mediated endocytosis in fibroblasts. Both treatments inhibit endocytosis by blocking the formation of clathrin-coated pits, leading to the appearance of empty clathrin "microcages" on the cell surface. These microcages, which are similar in size and structure to those formed in vitro by clathrin polymerization in acidic conditions, compete with normal clathrin for assembly into coated pits, leading to their inhibition. The changes in clathrin distribution and coated pit formation are reversible upon return to normal medium, suggesting that the inhibition is due to an abnormal formation of microcages rather than a permanent loss of clathrin. The study also examines the distribution of LDL receptors, which becomes dispersed on the cell surface during hypertonic treatment but reClusters upon recovery. The findings suggest that both treatments interfere with clathrin polymerization, leading to the inhibition of endocytosis.The study investigates the effects of hypertonic media and potassium depletion on receptor-mediated endocytosis in fibroblasts. Both treatments inhibit endocytosis by blocking the formation of clathrin-coated pits, leading to the appearance of empty clathrin "microcages" on the cell surface. These microcages, which are similar in size and structure to those formed in vitro by clathrin polymerization in acidic conditions, compete with normal clathrin for assembly into coated pits, leading to their inhibition. The changes in clathrin distribution and coated pit formation are reversible upon return to normal medium, suggesting that the inhibition is due to an abnormal formation of microcages rather than a permanent loss of clathrin. The study also examines the distribution of LDL receptors, which becomes dispersed on the cell surface during hypertonic treatment but reClusters upon recovery. The findings suggest that both treatments interfere with clathrin polymerization, leading to the inhibition of endocytosis.