Heterogeneous expression of transient outward currents in smooth muscle cells of the mouse small intestine

The objective for this paper was to characterize the transient outward current (I(to)) present in smooth muscle cells of the intestinal external muscularis. Two populations of cells were identified, one with a fast rate of I(to) inactivation (tau < 100 ms) and another with a slow rate of I(to) inactivating (tau > 150 ms). The chord conductance for the fast I(to) was smaller than the chord conductance of the slow I(to) (0.5 +/- 0.1 vs. 1.3 +/- 0.1 nS pF(-1), respectively). The inactivation was fitted by mono-exponentials to give a tau for the fast and slow I(to) of 44 and 229 ms, respectively. Combined plots of voltage dependent activation and inactivation processes revealed voltage ranges where window currents were possible; a 20 mV range for the fast I(to) from -56 to -36 mV and a 47 mV range for the slow I(to) from -42 to +5 mV. The fast I(to) recovered more quickly from inactivation than the slow I(to); tau(fast I(to)) = 11 +/- 4 ms compared to tau(slow I(to)) = 42 +/- 16 ms. The effect of different rates of depolarization on I(to) activation was examined. The plots of peak currents evoked by different rates of depolarization were well fitted by inverse exponential functions. The fast I(to) had a larger response to fast rates of depolarization by having a tau of 2 +/- 1 mV ms(-1) with maximal activation (95 % complete) at 5 mV ms(-1). The slow I(to) had a tau of 14 +/- 9 mV ms(-1) with maximal activation (95 % complete) at 42 mV ms(-1). The properties of these currents suggest that the two transient outward currents may contribute differently to slow waves and action potentials generated by the smooth muscle cells.