The effect of cyclic nucleotides and protein phosphorylation on the permeability of human erythrocyte ghosts to certain cations
Preparations of human erythrocyte membranes have been made which are in the form of sealed vesicles and which behave as osmometers on suspension in solutions of simple inorganic salts. Using these preparations the permeability of the membranes to Na+, K+, Mg2+ and Ca2+ was measured. Cyclic AMP (but not cyclic GMP) increased the permeability of the membranes to Ca2+ with a half maximal effect at a concentration of 25 μM but did not affect the permeability to the other ions tested. Phosphorylation of proteins in the erythrocyte membrane lowered the permeability to Ca2+ without affecting the permeability to the other ions tested and there was a good correlation between the time course of protein phosphorylation and decrease in Ca2+ permeability. It is postulated that the system through which cyclic AMP causes an initial rapid rise in Ca2+ permeability followed by increased phosphorylation of membrane proteins and reduced Ca2+ permeability may have a widespread occurrence in biological systems and serve to control the concentration of Ca2+ in the cytoplasm.