INTRACELLULAR CALCIUM-CONCENTRATION IN EQUINE SPERMATOZOA ATTACHED TOOVIDUCTAL EPITHELIAL-CELLS IN-VITRO

Interaction of spermatozoa with oviductal epithelial cells (OEC) in th e oviductal isthmus prolongs the life span of spermatozoa. The hypothe sis that the interaction of equine spermatozoa with OEC affects their intracellular calcium concentration ([Ca2+](i)) was tested in a sperm- OEC coculture model. Changes in [Ca2+](i) in spermatozoa loaded with t he fluorescent calcium indicator indo-1 acetoxymethylester (AM) were d etermined for spermatozoa attached to OEC or to Matrigel, as well as f or free-swimming spermatozoa incubated without oviductal epithelium. [ Ca2+](i) was determined before incubation and at 0.5, 2, 4, and 6 h of incubation by ratio image analysis of fluorescent images captured at 405 nm and 490 nm. At each time point, [Ca2+](i) was lower in motile s permatozoa attached to OEC than in free-swimming spermatozoa, [Ca2+](i ) in spermatozoa attached to Matrigel was lower than in free-swimming spermatozoa and was comparable to [Ca2+](i) in spermatozoa attached to OEC only at 0.5 h of incubation. Beyond 0.5 h of incubation, [Ca2+](i ) was higher in spermatozoa attached to Matrigel than in spermatozoa a ttached to OEC. These results indicate that spermatozoa with low [Ca2](i) might preferentially attach to OEC and to Matrigel, but that [Ca2 +](i) is maintained at this basal level only in spermatozoa attached t o OEC. The reduced [Ca2+](i) in spermatozoa associated with OEC may pr event premature capacitation and acrosomal exocytosis of spermatozoa s tored in the isthmic sperm reservoir.