Ca2+ current and Ca2+ ‐activated chloride current in isolated smooth muscle cells of the sheep urethra

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Abstract
1 Isolated sheep urethral cells were studied using the perforated patch clamp technique (T= 37 °C). Depolarizing steps ranging from ‐40 to ‐10 mV evoked an inward current that peaked within 10 ms and a slower inward current. Stepping back to the holding potential of ‐80 mV evoked large inward tail currents. All three currents were abolished by nifedipine (1 μm). Substitution of external Ca2+ with Ba2+ resulted in potentiation of the fast inward current and blockade of the slow current and tails. 2 Changing the chloride equilibrium potential (ECl) from 0 to + 27 mV shifted the reversal potential of the tail currents from 1 ± 1 to 27 ± 1 mV (number of cells, n= 5). Chloride channel blockers, niflumic acid (10 μm) and anthracene‐9‐carboxylic acid (9AC, 1 mm), reduced the slow current and tails suggesting that these were Ca2+‐activated CF currents, ICl(Ca). 4 Caffeine (10 mm) induced currents that reversed at ECl and were blocked by niflumic acid (10 μm). 5 In current clamp mode, some cells developed spontaneous transient depolarizations (STDs) and action potentials. Short exposure to nifedipine blocked the action potentials and unmasked STDs. In contrast, 9AC and niflumic acid reduced the amplitude of the STDs and blocked the action potentials. 6 In conclusion, these cells have both L‐type ICa and ICl(Ca). The former appears to be responsible for the upstroke of the action potential, while the latter may act as a pacemaker current.