Cell contact induces an increase in pinocytotic rate in cultured epithelial cells

Abstract

ENDOCYTOSIS is the process by which cells internalise part of the plasma membrane in association with extracellular components; they engulf large particles by phagocytosis and internalise components in the fluid phase by pinocytosis. Discrimination of phagocytosis from pinocytosis has been based on the size of the particle and the amount of energy required to internalise it, phagocytosis being inhibited more readily by metabolic poisons than pinocytosis^1–3. This is consistent with the view that more energy is required to internalise a large surface area than is needed for a small surface area. Vasiliev et al.⁴ described “contact inhibition of phagocytosis”: when cultured epithelial cells, but not fibroblasts, became confluent, the rate of phagocytosis decreased considerably. I report here that confluent cultured epithelial cells are not inhibited in their ability to undergo pinocytosis, but exhibit a greater rate of pinocytosis than do non-confluent cells. This difference is not due to diffusable factors, but is the result of cell contact.