Ion channels are effector proteins that mediate uterine excitability throughout gestation. This review will focus primarily on the role of potassium channels in regulating myometrial tone in pregnancy and labor contractions. During gestation, potassium channels maintain the uterus in a state of quiescence by contributing to the resting membrane potential and counteracting contractile stimuli. This review summarizes the current knowledge about the significance of the potassium channels in maintaining a normal gestational period and initiating labor contractions at term.

-K channels localize to caveolae in human myometrium: a role for an actin-channel-caveolin complex in the regulation of myometrial smooth muscle K ϩ current.

Large conductance Ca2+ -and voltage-activated K + (maxi-K) channels modulate human myometrial smooth muscle cell (hMSMC) excitability; however, the role of individual alternatively spliced isoforms remains unclear. We have previously shown that the transcript of a human maxi-K channel isoform (mK44) is expressed predominantly in myometrial and aortic smooth muscle and forms a functional channel in heterologous expression systems. The mK44 isoform contains unique consensus motifs for both endoproteolytic cleavage and N -myristoylation, although the function of these post-translational modifications is unknown. The goal of these studies was to determine the role of post-translational modifications in regulating mK44 channel function in hMSMCs. An mK44-specific antibody indicated that this channel is localized intracellularly in hMSMCs and translocates to the cell membrane in response to increases in intracellular Ca 2+ . Immunological analyses using an N-terminally myc-tagged mK44 construct demonstrated endoproteolytical cleavage of mK44 in hMSMCs resulting in membrane localization of the mK44 N-termini and intracellular retention of the pore-forming C-termini. Caffeine-induced Ca 2+ release from intracellular stores resulted in translocation of the C-termini of mK44 to the cell membrane and co-localization with its N-termini. Translocation of mK44 channels to the cell membrane was concomitant with repolarization of the hMSMCs. Endoproteolytic digest of mK44 did not occur in HEK293 cells or mouse fibroblasts. MK44 truncated at a putative N -myristoylation site did not produce current when expressed alone, but formed a functional channel when co-expressed with the N-terminus. These findings provide novel insight into cell-specific regulation of maxi-K channel function.

Myometrial maxi-K channels are modulated by L L subunits. We aimed to determine whether L L subunits are modulated to a¡ect uterine excitability during gestation. RNase protection analyses revealed that mouse L L1 subunit transcripts are regulated during gestation with peak expression at day 14 of pregnancy. Immunohistochemical analysis indicates an increase of this subunit during gestation. Upregulation of the L L1 transcript occurs with 4-day exposure to 17L L-estradiol but not progesterone, and acute estradiol exposure has no e¡ect on L L1 transcript expression. These ¢ndings verify that L L1 subunit transcript is regulated in mouse myometrium during gestation and estrogens may contribute to this increase. ß

Estrogens exert their biological action via both genomic and non-genomic mechanisms. Proteins different from classical estradiol receptors are believed to mediate the latter effects. Here we demonstrate that the maxi-K channel functions as an estrogen-binding protein in transfected HEK293 cells. Whole-cell maxi-K channel currents and protein expression were attenuated by exposure to either 17␣-or 17␤-estradiol. This effect was dose-dependent for 17␤-estradiol at concentrations ranging from 10 nM to 1 M, while 17␣-estradiol inhibited channel expression only at 1 M. These effects were mediated by direct low affinity binding of estradiol to the maxi-K channel but not to its accessory ␤1-subunit, as revealed by cell membrane estradiol binding assays. However, specific binding of estradiol to the channel was facilitated by the presence of the ␤1 subunit. Addition of MG-132, a blocker of proteasomal degradation, stabilized channel expression. These data suggest that channel down-regulation is mediated by estrogen-induced proteasomal degradation, similar to the pathway used for estrogen receptor degradation. Membrane expression of endogenous maxi-K channels in cultured vascular smooth muscle cells was also attenuated by prolonged exposure to 17␣-and 17␤-estradiol. Thus our studies demonstrate that estrogen binds to maxi-K channels and may directly regulate channel expression and function. These results will have important implications in understanding estradiol-induced effects in multiple tissues including vascular smooth muscle.Existing epidemiological data suggest a beneficial role for estrogens in maintaining cardiovascular protection in pre-menopausal women. However, other data show that estrogen-containing preparations can negatively affect the health of premenopausal as well as post-menopausal women (1-6). These results indicate the existence of complex estradiol-signaling mechanisms, possibly mediated by proteins other than classical intracellular estradiol receptors (7).One protein that has been hypothesized to be a potential effector of the action of estrogen is the large conductance Ca 2ϩ -and voltage-activated K ϩ channel (maxi-K channel). This channel provides a buffering repolarizing current in response to various physiological stimuli (8 -11). The mechanism by which estrogen interacts with this channel is unknown. However, both 17␤-estradiol and tamoxifen, a partial agonist of nuclear estradiol receptors, have been shown to activate maxi-K channels in both heterologous expression systems and smooth muscle cells, resulting in greater repolarizing currents (12, 13). Because acute incubation with either 17␤-estradiol or membrane-impermeable estrogen derivatives activate maxi-K channels, signal transduction pathways other than those mediated by classical nuclear estradiol receptors ␣ (ER␣) 1 or ␤ (ER␤) are likely to be involved. Valverde et al. (14) showed that activation of maxi-K channels by 17␤-estradiol occurs when the channel is associated with its accessory ␤1-subunit. This effect was not observed in t...