MEMBRANE CONTACT WITH OVIDUCTAL EPITHELIUM MODULATES THE INTRACELLULAR CALCIUM-CONCENTRATION OF EQUINE SPERMATOZOA IN-VITRO

Interaction of equine spermatozoa with oviductal epithelial cells (OEC ) prolongs sperm viability and maintains low intracellular calcium con centration ([Ca2+](i)) in spermatozoa. Experiments were designed to in vestigate 1) whether release of spermatozoa from OEC in vitro is assoc iated with elevated [Ca2+](i) and 2) whether soluble products from OEC or direct membrane contact between spermatozoa and OEC mediates the e ffects of OEC on sperm [Ca2+](i). In the first experiment, changes in [Ca2+](i) in spermatozoa loaded with indo-1 acetoxymethylester were de termined 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 O EC-conditioned medium for 6 h compared to that in spermatozoa incubate d in control medium. [Ca2+](i) was higher in motile spermatozoa releas ed from OEC than in spermatozoa still attached to OEC after 4 h of inc ubation. 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 incu bation. In the second experiment, a suspension of apical plasma membra ne vesicles (AMV) isolated from isthmic oviductal epithelium was used to study the specific effect of sperm contact with OEC membranes on sp erm viability, capacitation, and [Ca2+](i). Direct membrane contact be tween spermatozoa and AMV prolonged sperm viability, delayed capacitat ion, and maintained low [Ca2+](i) in spermatozoa. These results indica ted that membrane contact between equine spermatozoa and OEC is requir ed to maintain low [Ca2+](i), delay capacitation, and prolong viabilit y of spermatozoa in vitro. Modulation of capacitation rate for spermat ozoa stored in the isthmic sperm reservoir might ensure the availabili ty of a competent sperm population at the time of fertilization.