T-type and L-type calcium currents in freshly dispersed human uterine smooth muscle cells

A dihydropyridine-sensitive T-type Ca2+ current is the main Ca2+ current carrier in mouse primary spermatocytes

Ca2+ entry through Ca2+ channels is likely to play an important role in the differentiation of male germ cells as well as in fertilization by mature sperm. Here we present a detailed analysis of Ca2+ currents expressed in acutely dissociated mouse primary spermatocytes. Patch-clamp recordings demonstrated that the only voltage-gated Ca2+ channels present belong to the family of T-type Ca2+ currents. Accordingly, Ni2+ (200 microM) and amiloride (500 microM) reduced current amplitude by 75 and 62%, respectively. To our knowledge, this is the first report of a system where T-type Ca2+ channels are expressed in isolation. Unexpectedly, 5 and 10 microM nifedipine also reduced peak currents by 38 and 53%, respectively significant inhibition of the Ca2+ current occurred at concentrations as low as 2 microM. Because mature sperm cells are unable to synthesize new proteins, these Ca2+ channels are also likely to be present in these cells, where they may contribute to the Ca2+ influx required to trigger the acrosome reaction. This notion is supported by the fact that concentrations of Ni2+ and nifedipine, which block these Ca2+ currents, also inhibit the acrosome reaction. Because these channels represent the primary pathway for voltage-gated Ca2+ entry in mouse spermatocytes, they may also participate in regulating meiotic cell division and sperm differentiation.

Human fetal membranes inhibit calcium L-channel activated uterine contractions

OBJECTIVE

Paracrine signals among fetal membranes, decidua, and uterus play an important role in the initiation of parturition in women. In previous work we demonstrated that fetal membranes inhibit uterine contractions. In the current study we test the hypothesis that the fetal membranes decrease uterine contractions by inhibition of the uterine calcium L-channel.

STUDY DESIGN

Our dual-chamber fetal membrane-uterine muscle in vitro model was used in this study. Rat uterine muscle strips were anchored into the maternal sides of the chambers. Fetal membranes (or Parafilm controls) were added to the chamber in a removable cassette. Uterine contractions were stimulated with the specific calcium L-channel agonist Bay K 8644.

RESULTS

When uterine muscle was exposed to full-thickness fetal membranes (amnion-chorion with attached decidua) or to the intact fetal components (chorion-amnion) or to chorion alone, the Bay K 8644 dose-response curve was significantly shifted to the right. When uterine muscle was exposed to amnion alone or to the decidua alone, the Bay K 8644 dose-response curve was not shifted. Fetal membranes, did not cause a shift in the ionomycin (a calcium ionophore) dose-response curve.

CONCLUSION

These results support the hypothesis and provide evidence that human fetal membranes, most likely chorion, release an endogenous calcium L-channel inhibitor.

Characterization of an oxytocin-induced rise in [Ca2+]i in single human myometrium smooth muscle cells

The effect of the uterotonic pituitary hormone oxytocin on the regulation of intracellular calcium concentration ([Ca2+]i) was studied in single cells of a smooth muscle cell line derived from human non-pregnant myometrium. [Ca2+]i was measured with fluorescence microscopy, and by recording the activity of Ca(2+)-activated potassium currents (IK(Ca)) on the whole cell and single channel level. Oxytocin induced a rapid and transient increase in [Ca2+]i that was paralleled by a significant increase in IK(Ca) activity. After removal of extracellular Ca2+, repetitive stimulation with oxytocin did not alter the [Ca2+]i transients initially; however, their amplitude became progressively smaller and the response was eventually abolished completely, indicating that oxytocin increased [Ca2+]i by release of Ca2+ from intracellular stores. Nifedipine did not alter the oxytocin-induced [Ca2+]i-transients suggesting that oxytocin failed to activate Ca2+ entry through voltage-operated Ca2+ channels. Thapsigargin abolished the oxytocin-induced [Ca2+]i transient. Caffeine alone had no effect on [Ca2+]i, however it diminished the oxytocin-induced [Ca2+]i transients. Ryanodine did not affect the oxytocin response indicating that these cells lack release of Ca2+ from the ryanodine receptor release channel. These results demonstrate that oxytocin elicited [Ca2+]i transients predominantly through Ca2+ release from thapsigargin-sensitive stores, presumably by activating an inositol 1,4,5-trisphosphate dependent pathway.

Ion channels and the control of myometrial electrical activity

Understanding the role of ion channels in the generation of slow waves and action potentials in the myometrium is critical in designing strategies to regulate uterine contractile activity. The development of the patch clamp technique has allowed the identification of specific types of channels in the myometrium and provided insights into their regulation by hormones and drugs. Specifically, new studies suggest that KATP and KCa channel openers could be important tools in the management of inappropriate uterine contractions, but peripheral effects will have to be controlled. Conversely, blockers of these same channels may have some effects on dystocia. The study of contractant-operated channels in the myometrium is still in its infancy, but promises new insights into possible modes of regulation as well. Myometrial activity is controlled at a number of levels. The regulation of ion channels is an important aspect, but receptor-mediated actions that do not appear to be voltage- or ion-dependent presumably are also important contributors and hence are sites of potential modulation as well. Clearly, future multifaceted approaches to tocolysis, and perhaps also dystocia, may well include agents targeting the activity of ion channels.