In myometrium control of electrical activity is essential for a successful pregnancy and parturition. Through understanding the mechanisms behind spontaneous activity we may better understand problems occurring in labour. Ca²⁺-activated Cl^– currents (I_Cl-Ca) occur in smooth muscles where they play a role in electrical activity and spontaneous activity. These currents have been observed in rat myometrium (Arnaudeau, 1994) although they are not well characterized. The aim of this work is to characterize these currents in rat myometrium to investigate their functional significance and to correlate the results to human data.

Pregnant rats (18-21 days gestation) were killed humanely and longitudinal myometrium strips dissected. Human biopsies of non-labouring myometrium was obtained from elective caesareans at term, with local ethics committee approval and written informed consent. Strips were either loaded with the calcium-sensitive indicator Indo-1 and used to measure force/calcium or single cells produced via enzyme digestion. Whole-cell membrane currents were recorded using the patch clamp technique. The results are expressed as means ± S.E.M. where appropriate; significance was tested using the appropriate Student’s paired t test with significance taken at P < 0.05, n = number of experiments.

Depolarisation of single cells produced an initial inward Ca²⁺ current (I_Ca). Chloride current (I_Cl-Ca) was observed upon repolarisation. In the isolated rat cells I_Ca was manipulated using Ba²⁺, which is a suitable charge carrier but is unable to activate calcium activated currents. Ba²⁺ abolished I_Cl-Ca (n = 4), despite producing an initial inward current. Bay K8644 (2 µM), an agonist for I_Ca increased I_Ca and I_Cl-Ca proportionally (2.2 ± 0.2-fold and 1.8 ± 0.1-fold, respectively, P < 0.05; n = 13). The I_Cl-Ca reversal potential was altered using chloride substitution; with almost equal [Cl^–] inside the pipette and outside, the reversal potential was -3 ± 1.5 mV (n = 3); reducing extracellular [Cl^–] by 75 mM through replacement with glutamate gave a reversal potential of 16.7 ± 4.4 mV (n = 3). These values are consistent with calculated values. Niflumic acid (10 µM) was used to inhibit the chloride channel in single cells resulting in a loss of tail current (n = 4). In rat tissue strips, which were spontaneously active, application of niflumic acid resulted in a decreased frequency or abolition of the Ca²⁺ transients and contractions (n = 5 and n = 4 respectively). Experiments are being performed on human cells to compare the data obtained to that from rats.

Calcium activated chloride channels are present in rat myometrium and do have functional significance.

This work was funded by the Wellcome Trust and the MRC.