We have previously reported that there are three distinct K⁺ channels in the neonatal uterus (Li et al. 2002), and that the predominant K⁺ current was A type. A-type currents are rapidly activating and inactivating K⁺ currents, and it is considered that they can regulate the firing frequency of action potentials. The aim of this work was to characterize the A-type current present in these cells, and to determine their functional role. Therefore, we report here electrophysiological, force and imaging data.

Female rats, 13-15 days old, were humanely killed by cervical dislocation following CO₂ anaesthesia. The whole-cell patch clamp technique was used to measure outward K⁺ currents in enzymatically isolated uterine single cells. Force measurements in intact tissue were made to study their physiological significance and blockers of A-type channels were applied externally. Ca²⁺ wave activity was examined in whole uterine horns using confocal microscopy in fluo-4 AM loaded preparations. The data are given as means ± S.E.M., and n refers to the number of cells or animals.

Overall K⁺ currents were evoked by using depolarising steps varied from -40 mV to +70 mV in 10 mV increments at a holding potential of -80 mV. A-type currents were obtained by using digital subtraction from -90 mV conditioning pulse to -40 mV, and were present in 90 % of cells (n = 50). At +70 mV, the time to peak was 4.2 ± 1.5 ms (n = 12) and peak current densities averaged 0.06 nA pF^-1 (n = 12). A-type K⁺ currents were activated at -60 ± 5 mV and the half-inactivation potential was -65 ± 15 mV (n = 12). At +70 mV, the time constant of A-type current inactivation was best fitted by two exponentials and revealed two types of cells, one with slow inactivation time constants of 24 ± 1.9 ms and 3.3 ± 0.5 ms (n = 7), another with fast inactivation constants of 14.1 ± 2.2 ms and 1.1 ± 0.2 ms (n = 7). As reported previously, the A-type currents were sensitive to 4-aminopyridine (4-AP) in a dose-dependent manner. Force measurements of intact tissue from neonatal and adult uteri showed that the spontaneous contractions of neonate were more frequent than adult (8 ± 1 vs. 3 ± 1 (5 min)^-1, n = 5). Application of 0.5 mM 4-AP increased the amplitude and frequency of spontaneous contraction in neonates 15 ± 5 % and 7 ± 2 (5 min)^-1 (n = 5), respectively, but not in adults. Intracellular Ca²⁺ waves were observed in the neonatal tissue. They were asynchronous in the muscle bundles and not associated with contractions (n = 6).

In conclusion, these data confirm and characterize the presence of heterogeneous A-type K⁺ channels in neonatal uterus. These are functionally important as their inhibition produced an increase in force amplitude and frequency, suggesting they act to reduce excitability. It is suggested that they may play a role in keeping the spontaneous Ca²⁺ waves asynchronous and thereby not producing contraction.

We would like to thank ORS committee for funding this research.