Identification of the major membrane currents in freshly dispersed single smooth muscle cells of guinea‐pig ureter.

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Abstract
1. The passive and active electrical properties of freshly dispersed single cells of the guinea-pig ureter were investigated using standard patch-clamp techniques. 2. Action potentials, having a rapid rising phase and a prolonged plateau, were recorded on passing depolarizing currents through the patch pipette when ''near normal'' physiological gradients were established across the cell membrane (5.9 mM-K+, 1.5 mM-Ca2+ in the bath; 126 mM-K+ in the pipette) 3. Under voltage clamp, depolarization of potentials positive of -50 mV (from a holding potential of -70 or -80 mV) triggered a net inward current which reached a peak in 5-10 ms and then slowly inactivated. 4. The averaged membrane current to depolarization to potentials between -30 and 0 mV showed two distinct patterns after the peak of the inward current: the membrane current either moved slowly outward over 400 ms or there was one or more transient outward currents superimposed on the slowly decreasing inward current. Both outward currents were blocked when 5 mM-tetraethylammonium (TEA) was added to the bathing solution, resulting in an increased inward current at all potentials. 5. Replacing the extracellular Ca2+ with Co2+ (1.5-5 mM) blocked the inward current and the outward currents to reveal another transient outward current (voltage activated) which activated rapidly to reach a peak within 5 ms and which inactivated exponentially with a time constant of 10 ms. This voltage-activated outward current was inactivated if the membrane was held at -50 mV, but could be reactivated by short hyperpolarizing pre-pulses. The amplitude of this transient current in response to a fixed depolarization (to 0 mV) was half-maximum when the hyperpolarizing pre-pulse was to -66 mV. The voltage-activated outward current was reduced in amplitude when the extracellular potassium was raised to 46 mM or upon exposure to 1 mM-4-aminopyridine (4-AP), but was not affected by 4 mM-TEA. 6. Replacing K+ in the pipette and bathing solution with caesium (Cs) blocked all outward currents, revealing the time course and voltage dependence of the inward current, which could be carried by Ca2+ or Ba2+ with little effect on its rate of inactivation. 7. It was concluded that the inward current recorded in single ureter cells was due to the flow of current through voltage-activated Ca2+ channels. The TEA-sensitive outward currents, whether transient or slowly activating, are presumably K+ channels activated by Ca2+. The 4-AP sensitive transient K+ current recorded in the absence of Ca2+ has a number of properties similar toi a-type K+ currents recorded in neurones.

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