Rearrangement of intramembranous particles and fusion promoted in chicken erythrocytes by intracellular Ca2+.

Ca2+ was introduced into fresh and ATP-depleted chicken erythrocytes through the aid of the inophore A23187. Intracellular Ca2+ (10-40 mM) induced fusion in ATP-depleted cells after 30-60 min incubation at 37 degrees C, but not in fresh cells. Fresh cells underwent a higher degree of haemolysis than ATP-depleted cells after accumulation of Ca2+. Uptake of Ca2+ was the same in these two systems. Intracellular Ca2+ induced rearrangement of intramembranous particles, as revealed by freeze-etching studies. The intramembranous particles in the protoplasmic face of fractured membranes obtained from fresh cells incubated with 1 mM of Ca2+ were more scattered and their density was lower than in control cells. Incubation with higher concentrations of Ca2+ (10-40 mM) induced transient changes in the intramembranous particles' density with the appearance of protrusions and depressions on the protoplasmic and exoplasmic faces of the fractured membranes, respectively. These effects were reversible upon removal of Ca2+ by washing the cells with ethyleneglycol bis(alpha-aminoethylether)-N,N'-tetraacetic acid; rearrangement of intramembranous particles was less evident after accumulation of Ca2+ in ATP-depleted cells, whose fractured membranes did not contain any protrusions or depressions. Transferring Ca2+-loaded cells to the cold caused the formation of large smooth areas devoid of intramembranous particles in the protoplasmic face of the fractured membranes. Cells containing Ca2+ appeared spherical, and removal of Ca2+ restored the normal oval shape of chicken erythrocytes.