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Interaction of equine spermatozoa with oviductal epithelial cells (OEC) prolongs sperm viability and maintains low intracellular calcium concentration (Ca2+i) in spermatozoa. Experiments were designed to investigate 1) whether release of spermatozoa from OEC in vitro is associated with elevated Ca2+i and 2) whether soluble products from OEC or direct membrane contact between spermatozoa and OEC mediates the effects of OEC on sperm Ca2+i. In the first experiment, changes in Ca2+i in spermatozoa loaded with indo-1 acetoxymethylester were determined in motile spermatozoa released from OEC monolayers after 4 h of culture compared to Ca2+i in spermatozoa still attached to OEC. In addition, Ca2+i was determined in spermatozoa incubated with OEC-conditioned medium for 6 h compared to that in spermatozoa incubated in control medium. Ca2+i was higher in motile spermatozoa released from OEC than in spermatozoa still attached to OEC after 4 h of incubation. Incubation in OEC-conditioned medium resulted in lower sperm Ca2+i only at 4 h of incubation, but not at 0.5, 2, or 6 h of incubation. In the second experiment, a suspension of apical plasma membrane vesicles (AMV) isolated from isthmic oviductal epithelium was used to study the specific effect of sperm contact with OEC membranes on sperm viability, capacitation, and Ca2+i. Direct membrane contact between spermatozoa and AMV prolonged sperm viability, delayed capacitation, and maintained low Ca2+i in spermatozoa. These results indicated that membrane contact between equine spermatozoa and OEC is required to maintain low Ca2+i, delay capacitation, and prolong viability of spermatozoa in vitro. Modulation of capacitation rate for spermatozoa stored in the isthmic sperm reservoir might ensure the availability of a competent sperm population at the time of fertilization.
Dobrinski et al. (Tue,) studied this question.