Calcium Binding to Plasma Membrane Vesicles of Boar Spermatozoa

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Abstract
Purified plasma membrane vesicles from boar spermatozoa are shown to bind radioactive calcium with high affinity. Calcium binding to high affinity sites is enhanced by Na+ or K+. Binding is also markedly stimulated by low concentration of La+3 and Ho+3 but not by other large electrophilic cations. Increased calcium uptake in the presence of La+3 appears to be due to exposure of more binding sites deep within the plasma membrane vesicles. High concentrations of La+3 block calcium access to these sites. The ability of these vesicles to bind high concentrations of calcium suggests that the plasma membrane may play an important role in the storage and release of calcium in intact spermatozoa.