Plasma membrane: substructural changes correlated with electrical resistance and pinocytosis.

Inducers of pinocytosis in amoeba cause as much as a 50-fold decrease in the electrical resistance of the plasma membrane prior to the formation of the typical tunnels and vacuoles. In this state the thickness of the electron-transparent core or lamella of the unit membrane is at least twice as thick as that of the control. The changes in structure and resistance as well as the induction of pinocytosis are dependent on the initial external concentration of calcium. These changes are rapidly reversed when the concentration of calcium in the external medium is increased.