1 A study has been made of the modulation of calcium-activated potassium channels in cultured neurones of avian ciliary ganglia by sodium nitroprusside and L-arginine. 2 Sodium nitroprusside (100 muM) reduced the net outward current by 22 +/- 1% at 4.8 ms (mean +/- s.e.mean) and 25 +/- 1% at 350 ms during a test depolarization to +40 mV from a holding potential of -40 mV. The outward current remained reduced for the duration of the recording following a single application of sodium nitroprusside. These effects did not occur if the influx of calcium ions was first blocked with Cd2+ (500 muM). Application of ferrocyanide (100 muM) reduced the net outward current by only 6 +/- 3 % at 350 ms during a test depolarization to +40 mV. 3 L-Arginine (270 muM) reduced the net outward current on average by 19 +/- 2% at 4.8 ms and 22 +/- 2% at 350 ms during a test depolarization to + 40 mV. The current remained in this reduced state for the duration of the recording following a single application of L-arginine. These effects were reduced to 11 +/- 1% at 4.8 ms and 11 +/- 2% at 350 ms in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 muM). 4 In order to alleviate the dependence of calcium-activated potassium channels (I(k(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 muM CaCl2, and the Ca2+ in the bathing solution was replaced with EGTA. Under these conditions sodium nitroprusside reduced the total outward current during a depolarizing pulse of + 40 mV by 9 +/- 1% at 4.8 ms and by 36 +/- 3% at 350 ms. L-Arginine (270 muM) reduced this current under the same conditions by 9 +/- 1% at 4.8 ms and by 35 +/- 2% at 350 ms. 5 Calcium-activated potassium currents were sensitive to apamin (50 nm), as this reduced the outward current by 23 +/- 3% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 MV. L-Arginine still decreased the outward current in the presence of apamin (50 nm), by 5 +/- 1% at 4.8 ms and by 19 +/- 2% at 350 ms, indicating that L-arginine could reduce an apamin-insensitive I(k(Ca)). 6 Calcium-activated potassium currents were also sensitive to charybdotoxin (10 nm), as this reduced the outward current by 34 +/- 4% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to +40 MV. L-Arginine still decreased the outward current in the presence of charybdotoxin, by 6 +/- 1% at 4.8 ms and 12 +/- 4% at 350 ms, showing that L-arginine could reduce a charybdotoxin-insensitive I(k(Ca)). 7 The present results indicate that NO-synthase in ciliary ganglia can modulate I(k(Ca)) by a method which is independent of the action of NO on the calcium channels. The I(k(Ca)) is decreased significantly at 4.8 ms into a depolarizing pulse, at a time that would decrease the rate of repolarization of the action potential. I(k(Ca)) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.
