Adenosine modulation of potassium currents in postganglionic neurones of cultured avian ciliary ganglia

Nitric oxide modulation of calcium-activated potassium channels in postganglionic neurones of avian cultured ciliary ganglia

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 microM) 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 microM). Application of ferrocyanide (100 microM) reduced the net outward current by only 6 +/- 3% at 350 ms during a test depolarization to +40 mV. 3. L-Arginine (270 microM) 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 microM). 4. In order to alleviate the dependence of calcium-activated potassium channels (Ik(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 microM 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 microM) 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 Ik(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 Ik(Ca).7. The present results indicate that NO-synthase in ciliary ganglia can modulate Ik(Ca) by a method which is independent of the action of NO on the calcium channels. The Ik(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. Ik(Ca) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.

Adenosine modulation of potassium currents in preganglionic nerve terminals of avian ciliary ganglia

Potassium currents in calyciform nerve terminals of the avian ciliary ganglion were analyzed using a single microelectrode voltage clamp. The modulatory affects of adenosine on these currents was also determined. Intracellular CsCl (1 M) blocked the steady-state outward current, indicating that it is mostly carried by K+. Tetraethylammonium (TEA, 10 mM) blocked over 40% of the outward current at a command potential of -30 mV. If Ca2+ influx was blocked by CoCl2 (5 mM), the steady-state outward current was reduced by over 16% in the nerve terminals suggesting that about one fifth of the outward current passes through calcium-activated potassium channels IK(Ca). Adenosine (50 mM) decreased the outward steady-state current over a wide range of command potentials in most terminals studied but failed to decrease this current in the presence of CoCl2 (5 mM). It is concluded that adenosine blocks IK(Ca) in nerve terminals.