Characterization of Ca2+ Uptake by Guinea Pig Epididymal Spermatozoa1

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Abstract
Comparative studies of Ca2+-uptake by guinea pig spermatozoa were performed with fresh epididymal sperm and with cells preincubated in a chemically defined, Ca2+-free medium for capacitation. Calcium uptake was negligible in fresh spermatozoa, but increased dramatically after the 20 min of incubation at 37.degree. C in the presence of pyruvate and lactate. Spermatozoa incubated in the absence of these substrates accumulated only 34% as much 45Ca2+ as was taken up by cells in complete medium. The monosaccharides glucose, fructose, and mannose and the nonmetabolizable sugars 2-deoxyglucose and sucrose inhibited the enhancement of Ca2+-permeability. In the presence of 6 mM sucrose 45Ca2+ uptake was not influenced by external sodium chloride concentration between 0 mM and 145 mM. The respiratory activity of the capacitated spermatozoa not only was higher than that of uncapacitated cells, but it was stimulated by Ca2+. No effect of Ca2+ on respiration of fresh spermatozoa was detected. An increase in calcium uptake was associated with increasing pH of the medium. It is possible that a regulatory mechanism through the calcium permeability of the plasma membrane of guinea pig spermatozoa exists and controls the development of physiological events related with the fertilization process. The sugar composition, the availability of the energy substrates lactate and pyruvate, and the pH of the reproductive tract fluids could play an important role in the accessibility of Ca2+ into the cells in vivo, as has been demonstrated in vitro. The enhancement of calcium permeability during the preincubation could be a useful indicator to verify if capacitation has occurred.