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

Jonathan C. F. Lee, Carlos Barajas-López and Jan D. Huizinga

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 ( < 100 ms) and another with a slow rate of I_to inactivating ( > 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 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; (fast I_to) = 11 ± 4 ms compared to (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 of 2 ± 1 mV ms^-1 with maximal activation (95 % complete) at 5 mV ms^-1. The slow I_to had a 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.

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