Oxytocin-Induced Ca2+ Responses in Human Myometrial Cells1

Complex spatiotemporal changes in intracellular Ca2+ were monitored in an immortalized human myometrial cell line (PHM1-41) and first-passage human myometrial cells after oxytocin stimulation (1. 0-1000 nM). Laser cytometry revealed intracellular Ca2+ oscillations in both culture systems starting at 1.0 nM, which were followed by repetitive Ca2+ transients by 10-15 min that lasted for at least 90 min. The amplitude of the initial Ca2+ spike was dose dependent, while the frequency of Ca2+ oscillations identified by Fast Fourier Transform (FFT) tended to increase with dose. Removal of oxytocin resulted in termination of oscillations. Analysis of the sources of the Ca2+ involved in oscillations indicated that the major contribution to oscillation frequencies of </= 6 mHz in cells was from the inositol 1,4,5-trisphosphate-sensitive pool, accounting for about 60% of the frequencies. Most of the remaining frequencies were attributable to extracellular Ca2+, which presumably comes from plasma membrane channels other than L-type channels. When oscillation frequencies exceeded 6 mHz, a significant contribution from a ryanodine-sensitive Ca2+ pool was detected. Eight-bromo-cAMP suppressed both the initial Ca2+ spike and the long-term oscillations. Prostaglandin E1 and E2 caused a significant increase in the frequency of oxytocin-induced Ca2+ oscillations. FFT analysis may be of considerable value for study of the mechanisms of rhythmic Ca2+ transients and their function in myometrial cells, as well as the mechanisms by which uterotonins and tocolytic agents impact myometrial Ca2+ regulation.