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We used intracellular microelectrodes to investigate the effects of the antiprogesterone (AP) compounds RU 486 and ZK 299 on cell-to-cell coupling in the guinea pig myometrium during pregnancy. The input resistance (Ro) of myometrial cells was high in nonpregnant tissues (44.6 +/- 6.39 M omega), but decreased by midgestation (Day 44 or 45 of gestation; 22.9 +/- 3.17 M omega), and was lowest at term (between 17.7 +/- 2.90 m omega and 13.1 +/- 4.34 M omega on Days 59-69). Treatment with the AP RU 486 or ZK 299 in three groups of midgestational animals reduced Ro to a similar level within 24 h. Lucifer Yellow (LY) was injected into smooth muscle cells as a direct but qualitative measure of metabolic coupling. In term and AP-treated animals, LY spread rapidly to neighboring cells within 60 sec, but little spread occurred in midgestational control tissues and no spread was seen even after 10 min in nonpregnant tissues. This correlation of decreased Ro (implying increased electrical coupling) with the development of extensive spread of LY indicates increased electrical and metabolical coupling between myometrial cells during labor. These data show that myometrial smooth muscle cells of guinea pigs are moderately well coupled before the onset of labor, and the coupling increases further, just prior to spontaneous delivery or due to treatment with APs. These events may be required for synchronizing and coordinating the electrical, metabolic, and contractile activity of labor.

Effects of Antiprogesterones on Myometrial Cell-to-Cell Coupling in Pregnant Guinea Pigs1