Changes in Prostaglandin Binding Capacity to Single Oviductal Smooth Muscle Cells after Ovulation in the Rabbit*

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Abstract
Prostaglandins (PGs) are potent modulators of rabbit oviductal smooth muscle activity. PGE2 inhibits and PGF2α stimulates activity when administered either in vivo or in vitro. Previous studies have revealed that the sensitivity of the oviduct to exogenously administered PGs varies in the different anatomical regions of the oviduct and at certain times after ovulation corresponding to the time period of ovum transport in the rabbit. The results of the present studies indicate that these variations in sensitivity to the PGs are correlated with variations in the maximum PG binding capacities of the oviductal smooth muscle cells. Prostaglandin binding studies were performed in vitro on suspensions of single smooth muscle cells from the rabbit oviduct. Cell suspensions were prepared by incubating strips of the myosalpinx in a modified Hank's balanced salt solution containing collagenase and elastase. Cells were differentially isolated from the distal and proximal isthmic segments of the oviducts from estrous rabbits, 72 h pregnant rabbits, and from rabbits at 12, 24, 48 and 72 h after induction of ovulation. Separate binding studies for both PGE2 and PGF2α were then performed on each isthmic segment from each group. Significant variations in the maximum binding capacities (total sites per cell) but not in the receptor affinities (PGE2, Kdα 0.60 nM: PGF2α Kd = 0.17 nM) for both PGs were observed among certain experimental groups. Marked differences in total binding capacity were observed between estrous and pregnant animals for PGE2 binding in the proximal isthmus and PGF2α binding in the distal isthmus. The total number of PGF2= sites/cell in the distal isthmus changed from 8,500 in estrus to 5,000 in pregnancy. The total number of PGE2 sites per cell in the proximal isthmus increased from 12,700 in estrus to 26,700 in pregnancy. There were no significant differences in the total number of PGF2α sites per cell in the proximal isthmus among the experimental groups. In the distal isthmus, the number of PGE2 sites per cell was significantly increased at 48 h after induction of ovulation. These changes in PG binding are consistent with the previously reported increase in sensitivity of the proximal isthmus to PGE2 and the decrease in sensitivity of the distal isthmus to PGF2α in pregnant rabbits when compared to the sensitivity of the same oviductal segment in estrous rabbits.