Properties of the inactivating outward current in single smooth muscle cells isolated from the rat anococcygeus

The properties of the voltage- and time-dependent outward current in single smooth muscle cells isolated from the rat anococcygeus were studied. The outward current was activated by depolarizations to membrane potentials positive to –40 mV. Activation followed third order kinetics; at + 20 mV, the time for the current to reach half its maximal amplitude was around 55 ms. The current inactivated with a time course that could best be described by a single exponential with a time constant around 1500 ms. The steady-state inactivation curve was voltage dependent over the range –110 to –30 mV, with a half-inactivation point of –67 mV. Recovery from inactivation followed an exponential time course with a time constant of around 770 ms at –90 mV. Deactivating tail current analysis revealed that a 10-fold change in the extracellular potassium ion concentration resulted in a 42 mV change in the reversal potential of the current. The current was blocked by 4-aminopyridine, tetraethylammonium, quinine and verapamil with IC₅₀'s — the concentrations producing 50% inhibition of the peak current — of 2 mM, 4 mM, 12 M and 20 M respectively. The current was not blocked by Toxin I (100 nM) or glibenclamide (10 M). The current was still present in cells containing 5 mM EGTA; in these cells, replacing extracellular calcium with cadmium depressed the peak current by around 12%. This could be explained, at least in part, by a negative shift in the voltage dependence of inactivation.