Action potentials and net membrane currents of isolated smooth muscle cells (urinary bladder of the guinea-pig)

1. Cells were isolated by incubating chunks of tissue from the urinary bladder of the guinea-pig in a high potassium, low chloride medium containing 0.2 mM calcium plus the enzymes collagenase and pronase. After isolation, the cells were superfused with a physiological salt solution (PSS) containing 150 mM NaCl, 3.6 mM CaCl₂ and 5.4 mM KCl (35°C). Patch electrodes filled with an isotonic KCl-solution were used for whole cell recordings. With a single electrode voltage clamp we measured a capacitance of 50±5 pF per cell, an input resistance of 200±25 kOhm ·cm² and a series resistance of 44±4 Ohm·cm².
2. The cells had resting potentials of ?52±2 mV. They did not beat spontaneously but responded to stimuli with single action potentials (APs) which rose from the threshold (?38 mV) with a maximal rate of 6.5±1.8 V/s to an overshoot of 22±3 mV. The AP lasted for 36±4 ms (measured between threshold and ?40 mV). Continuous cathodal current produced repetitive activity, a pacemaker depolarization followed the AP and preceded the next upstroke.
3. Net membrane currents evoked by clamp steps to positive potentials were composed of an inward and an outward component. The inward component generating the upstroke of the AP was carried by Ca ions (i _Ca, Klöckner and Isenberg 1985). The repolarization resulted from a potassium outward currenti _K. Ca-channel blockers (5 mM NiCl₂) reducedi _K suggesting that (part of)i _K was Ca-activated.
4. i _K rose within about 100 ms to a peak of 40–200 μA/cm² from which it inactivated slowly and incompletely. The inactivatingi _K followed a bell-shaped voltage-dependence, the noninactivatingi _K an outwardly rectifying one. Both parts had similar steady state inactivation curves with a half maximal inactivation potential at ?36 mV and a slope of 9 mV.
5. Repolarization to ?50 mV induced outward tail currents which reversed polarity at ?85 mV (the calculated potassium equilibrium potential). The amplitude and the time course of the envelope of the tail currents varied in proportion toi _K during the prestep. Thus, the tail current is suggested to reflect the turning off of a potassium conductance which had been activated during the prepulse.
6. i _K was largely reduced but not blocked by 20 or 150 mM tetraethylammonium (TEA). TEA did not significantly change the resting potential, but it prolonged the AP and facilitated upstroke and overshoot.i _K could be blocked by loading the cells with Cs released from Cs-filled patch electrodes.
7. We compare the results with the data from multicellular tissue (Creed 1971). The more negative resting potential and the absence of spontaneous APs are mainly attributed to the absence of transmitter release from nerve terminals. The isolated cell is suggested as a model of the postsynaptic membrane properties.