Calcium and potassium currents in the fast coxal depressor motor neuron of the cockroach Periplaneta americana

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1. Membrane currents have been examined in the cell body of the fast coxal depressor motor neuron (Df) of the cockroach Periplaneta americana with the use of two-electrode voltage clamp. 2. Most of the outward current induced by membrane depolarizations to between -40 and +80 mV was carried by K+ because it was blocked by external tetraethylammonium+ (TEA+; 20 mM) and internal Cs+. 3. Over the potential range -20 to +80 mV, a large proportion of this TEA+/Cs(+)-sensitive K+ current consisted of two temporal components, a transient outward current (IKtrans) and a sustained outward current (IKsus). IKtrans and a large proportion of IKsus appeared to be calcium-activated potassium currents (IK,Ca,trans and IK,Ca,sus, respectively) because these were suppressed by injecting ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), removing Ca2+ from the saline or replacing Ca2+ with Ba2+. After suppression of IK,Ca by internal EGTA or Ca(2+)-free saline, membrane depolarizations positive to -40 mV induced voltage-dependent outward currents (IK,V), which consisted of single-component outward relaxations. 4. When outward currents were blocked by external TEA+/internal Cs+, a voltage-dependent inward current consisting of a transient and a sustained component was observed over the potential range -40 to +40 mV. Both components of this inward current appeared to be carried by Ca2+ because they were blocked by external Cd2+ (1 mM), verapamil (0.1 mM), nifedipine (0.1 mM), or diltiazem (0.1 mM). 5. Both the transient component of the calcium current (ICa,trans) and the sustained component (ICa,sus) were maximal at 0 mV and present when Ca2+ in the saline were replaced by Ba2+. The inactivation of ICa,trans is voltage dependent, the rate of inactivation increasing with membrane depolarization. 6. The current-voltage relationships of Ca2+ currents differed from those of calcium-activated K+ currents. It is proposed that the discrepancy between these current-voltage relationships arises from the rapidity with which IK,Ca is saturated by Ca2+ entering through voltage-dependent channels and because the apparent reversal potential for ICa is not at ECa. 7. Although the similarity in the shape of IK,Ca and ICa might suggest that the time course of IK,Ca is determined by the kinetics of ICa, this appears unlikely in view of the rapid saturation of IK,Ca by Ca2+, which considerably outlasts the period of Ca2+ influx.