Calcium redistribution in individual cells correlated with ionophore action on motility

Intact bovine epididymal spermatozoa were rapidly frozen (but otherwise unfixed) and examined in the cryogenic stage of a scanning electron microscope equipped with an energy‐dispersive x‐ray analysis system. Distributions of calcium (also of P, S, K and Cl) were determined in 0.1 μm 2 areas from the acrosome, nucleus, midpiece and flagella of cells that were subjected to a variety of conditions experimental that altered sperm total calcium content. Spectrophotometric measurements using the metallochromic indicator, murexide, showed that a net accumulation of calcium followed in vitro incubation of sperm in the presence of this cation. Accumulated calcium was distributed homogenously amongst the cell population, but was apparently concentrated in the mitochondria contained within the sperm midpiece as determined by electron microanalysis of individual cells. A net loss of previously accumulated calcium followed treatment of sperm with low levels of the divalent cation ionophore, A23187. Electron microanalysis indicated a corresponding depletion of mitochondrial calcium content. In contrast, subsequent treatment of sperm with high levels of A23187 resulted in net calcium uptake as determined by murexide absorbance changes. Electron microanalysis indicated that this reaccumulated calcium was distributed throughout the intracellular space. No significant changes in the intracellular concentrations or distributions of other elements were observed following ionophore treatment. These results support previous suggestions that the mechanism of motility activation mediated by A23187 involves an action of increased intracellular Ca 2 + concentrations upon the contractile apparatus of the sperm flagella.