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

Abstract

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 μM and 20 μM respectively. The current was not blocked by Toxin I (100 nM) or glibenclamide (10 μM). 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.