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Oxytocin-Induced Ca²⁺ Responses in Human Myometrial Cells¹

^a Departments of Veterinary Anatomy & Public Health, Texas A&M University, College Station, Texas 77843-4458 ^b Department of Biochemistry and Molecular Biology, and Obstetrics, Gynecology and Reproductive Sciences, University of Texas Medical School at Houston, Houston, Texas 77225-0708 ^c Department of Obstetrics, Gynecology and Reproductive Medicine, School of Medicine, SUNY at Stony Brook, Stony Brook, New York 11794-8091

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.

¹ Supported in part by Basic Research Grant No. 1-FY97–0426 from the March of Dimes Birth Defects Foundation and NIH P30-ES09106 ES (R.C.B.), NIH grant HD-09618 (B.M.S.), and NIH grant HD-30482 (J.A.).

² Correspondence. FAX: 409 847 8981; rburghardt{at}cvm.tamu.edu

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