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

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Abstract
The objective for this paper was to characterize the transient outward current (Ito) present in smooth muscle cells of the intestinal external muscularis. Two populations of cells were identified, one with a fast rate of Ito inactivation (τ < 100 ms) and another with a slow rate of Ito inactivating (τ > 150 ms). The chord conductance for the fast Ito was smaller than the chord conductance of the slow Ito (0.5 ± 0.1 vs. 1.3 ± 0.1 nS pF−1, respectively). The inactivation was fitted by mono‐exponentials to give a τ for the fast and slow Ito 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 Ito from −56 to −36 mV and a 47 mV range for the slow Ito from −42 to +5 mV. The fast Ito recovered more quickly from inactivation than the slow Ito; τ(fast Ito) = 11 ± 4 ms compared to τ(slow Ito) = 42 ± 16 ms. The effect of different rates of depolarization on Ito activation was examined. The plots of peak currents evoked by different rates of depolarization were well fitted by inverse exponential functions. The fast Ito had a larger response to fast rates of depolarization by having a τ of 2 ± 1 mV ms−1 with maximal activation (95 % complete) at 5 mV ms−1. The slow Ito had a τ 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.