Inverse localization of Ca2+ and La3+ high affinity binding sites of the red cell membrane

Glutaraldehyde fixation in the presence of both 90 mM La3+ resulted in opaque deposits were also seen in specimens fixed in a glutaraldehyde-CaCl2 medium devoid of La3+. In that case only small amounts of a moderately opaque substance had accumulated or remained at the external surface of the erythrocyte membrane. Specimens previously fixed with Ca2+, La3+ supplemented glutaraldehyde lost their internal deposits completely during postifixation with OsO4. The findings provide evidence of Ca2+ high affinity binding sites at the internal surface of the erythrocyte membrane presumably identical with the filamentous matrix. Glycocalyx constituents are considered external Ca2+ low affinity binding sites, however, capable of accumulating high amounts of La3+ during glutaraldehyde fixation.