Involvement of Alternative Calcium Conductance on Human Myometrial Contractile and Pharmacological Properties

Abstract

Objective: This study aimed to investigate the physiological role of alternative calcium conduct once contractions triggered by oxytocin and PGF2α in human myometrium. This conductance, supported by TRPC and TRPV channels, may provide alternative pathways to control either free intracellular and/or submembrane Ca 2+ -concentrations, which in turn will modulate membrane polarization and contractile responses. Study design:Uterine biopsies were obtained from consenting women undergoing elective caesarian delivery at term without labor (N = 29). Isometric tension measurements were performed on uterine strips (n = 174). Amplitudes, frequencies and areas under the curve (AUC) of phasic contractions as well as resting tone were measured under various experimental conditions. Norgestimate, which has been shown to inhibit TRPC isoforms, was added to isolated organ baths to delineate their putative functional involvement. In order to assess the role of TRPV4 channels, rhythmic activity triggered by uterotonic drugs was determined in the absence and presence of either 1 μM HC-067047 (TRPV4 antagonist) or 100 nM GSK1016790A (TRPV4 agonist). Addition of 50 nM iberiotoxin (IbTX) as well as of 10 µM NS-1619 was also used to assess the involvement of GKCa channels in controlling uterine reactivity and contractility.Results: Micromolar concentrations of norgestimate consistently decreased the resting tone, frequency and maximal amplitude of oxytocin -and PGF2α -induced contractions. In contrast, the TRPV4 agonist GSK1016790A abolished the rhythmic contractions, resulting in a strong and reversible tocolytic effect. Addition of iberiotoxin (a GKCa blocker) reversed the effects of GSK1016790A, while NS1619 mimicked the rapid tocolytic effects of the TRPV4 agonist. Conclusion:Acute pharmacological inhibition of TRPC channels by norgestimate had minor effects on contractile parameters although resting -tone was lowered. In contrast, selectiveTRPV4 activation led to GKCa activation, which in turn hyperpolarized the myometrial cell membrane, inactivating Ca 2+ channels and efficiently abrogated contractile activity. Collectively, these data suggest that alternative calcium conduct ance may play a physiological role in the modulation of myometrial reactivity prior to delivery. A rapid switch from phasic contractions to quiescence by this new class of tocolytics may potentially be of interest in delaying parturition in preterm labor.

Activation of TRPV4 channels leads to a consistent tocolytic effect on human myometrial tissues

“…Of the study population, 91 % did not smoke during pregnancy. All biopsy specimens were recovered from the surgery room by the obstetrician in duty and assessed for their functional and pharmacological properties as previously reported [ 16 , 17 ].…”

“…Interestingly, Fig. 5 B demonstrate that the tocolytic effect induce by nM concentration of GSK1016790A in the presence of PGF 2α and Oxytocin was abolished in the presence of 50 nM IbTX, a highly selective blocker of BKCa channels [ 17 ].

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“…Exclusion criteria included: 1) spontaneous or oxytocin-induced labor and 2) presence of infections (chorioamnionitis, HIV, genital herpes, hepatitis B and C, or 3 vaginal bleedings during the third trimester). During the cesarean section, immediately after delivery, all myometrium biopsies were excised from the upper lip of the lower uterine segment incision in the midline, as previously described [ 16 , 17 ]. Once collected, all tissue biopsies were transported in physiological salt solution and used within 1 h of collection, unless specified otherwise.…”

“…Myometrial strips were mounted for isometric recordings under 2 g of resting tension in an isolated organ bath system as previously described [ 16 , 17 ].…”

“…The amplitude and area under the curve (AUC) were calculated each recording [ 16 , 17 ] Contractile activities were quantified by calculating the amplitude and the area under the curve for each experimental condition using Sigma Plot 11.0 (SPSS-Science, Chicago, IL). Normal distributions were assessed and analyzed using a paired t -test (parametric test).…”