Calcium homeostasis is regulated by spermine in mammalian spermatozoa

Calcium uptake by ejaculated ram spermatozoa is enhanced approximately two-fold by 1-2 nM spermine. However, there is a concentration-dependent reversal of this stimulation when the polyamine concentration is raised to 10 nM. Spermine decreases the Km for Ca²⁺ from 2.5 mM to 0.5 mM. These experiments were performed in the presence of lactate, conditions under which 90% of the cell calcium is in the mitochondria. However, calcium uptake into extramitochondrial regions (in absence of lactate) induced by Ca²⁺-ionophore, is 50% inhibited by 10 mM spermine. This concentration of spermine causes also 50% inhibition in acrosomal exocytosis induced by Ca²⁺-ionophore. The data indicate that spermine controls acrosomal exocytosis by modulating Ca²⁺ transport into spermatozoa.