A number of substances have been implicated in the regulation of oxytocin (OT) secretion from bovine corpus luteum in vivo. However, isolated bovine luteal cells cultured in a monolayer lose the ability to secrete OT in response to stimulatory substances. The present study investigated how cell-to-cell contact and the cytoskeleton affect OT secretion by isolated bovine luteal cells. In experiment 1, bovine midluteal cells (Days 8–12 of the estrous cycle) were stimulated with prostaglandin F2α (PGF2α; 1 μM), noradrenaline (NA; 10 μM), or growth hormone (GH; 5 nM) in two culture systems: In one system, cell monolayers were incubated in 24-well culture plates, and in the other system, aggregates of cells were incubated in glass tubes in a shaking water bath. The cells cultured in a monolayer underwent considerable spreading and showed a variety of shapes, whereas the cells cultured in glass tubes remained fully rounded during the experimental period and soon formed aggregates of cells. Although PGF2α, NA, and GH did not stimulate OT secretion by the monolayer cells, all tested substances stimulated OT secretion by the aggregated cells (P < 0.01). In experiment 2, the monolayer cells were pre-exposed for 1 h to an antimicrofilament agent (cytochalasin B; 1 μM) or two antimicrotubule agents (colchicine or vinblastine; 1 μM) before stimulation with PGF2α, NA, or GH. Although PGF2α, NA, and GH did not stimulate OT secretion by the monolayer cells in the presence of colchicine or vinblastine, they all stimulated OT secretion in the presence of cytochalasin B (P < 0.001). The overall results show that OT secretion by bovine luteal cells depends on microfilament function and cell shape. Moreover, the aggregate culture system that allows three-dimensional, cell-to-cell contact seems to be a good model for studying OT secretion by isolated bovine luteal cells.
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