Calcium-activated chloride currents (I_Cl-Ca) have been observed in several types of smooth muscle (Arnaudeau et al. 1994). These currents may be important in the generation of spontaneous electrical activity, which has obvious implications in pregnancy and parturition where the control of electrical activity is necessary. In uterine smooth muscle cells these currents can be activated by calcium release from the sarcoplasmic reticulum through the use of oxytocin. As L-type current (I_Ca) is the major source of activator Ca²⁺ in uterine cells, the aim of our work was to characterise the I_Cl-Ca and determine the role of L-type I_Ca in its activation.

Pregnant female Wistar rats (18Ð21 days gestation) were killed humanely and cells obtained from longitudinal myometrium via enzyme isolation. Whole-cell membrane currents were recorded using the conventional patch-clamp technique. Cells were perfused with Krebs solution and the pipettes contained 140 mM KCl or CsCl, 8 mM NaCl, 4 mM MgATP, 10 mM Hepes and 5 mM EGTA. Cells were maintained at a holding potential of -60 mV before application of depolarising voltage pulses.

With KCl pipette solution an initial inward Ca²⁺ current followed by a large outward K⁺ current was observed with depolarising voltage pulses. A tail current was observed upon repolarisation. In the majority of cells this tail current was a result of the outward K⁺ current deactivation. In approximately 30 % of the cells (28 out of 90), a long-lasting inward tail current was observed. CsCl pipette solution was used to remove interfering K⁺ currents and enable the characterisation of this inward tail current. Its reversal potential was 4 ± 6 mV, n = 4, i.e. very close to the expected Cl^– reversal potential under our conditions. Using Ba²⁺ as the charge carrier, or niflumic acid (an inhibitor of chloride channels) resulted in the loss of the inward tail current. The currentÐvoltage relationship of the peak tail current closely followed the currentÐvoltage relationship of the peak I_Ca but the rate of its decay was much faster compared with the decay of [Ca²⁺] (t = 39 ± 2.7 vs. 920 ± 46 ms, n = 7, respectively, P < 0.001, paired t test). Bay K, a Ca²⁺ channel agonist, increased peak I_Ca and peak tail current. There was a strong correlation between the amplitudes of the peak I_Ca and inward tail current (r = 0.92 ± 0.012, n = 66, P < 0.0001). The results expressed are the mean ± S.E.M.

Our data suggest a calcium-activated chloride current is present in a subpopulation of uterine myocytes. Ca²⁺ entering the cell through L-type channels can activate this current. These currents will contribute to the excitability of the uterus and possibly to pacemaker potentials.

This work was funded by The Wellcome Trust.

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