CALCIUM AND POTASSIUM CURRENTS IN THE FAST COXAL DEPRESSOR MOTOR-NEURON OF THE COCKROACH PERIPLANETA-AMERICANA

1. Membrane currents have been examined in the cell body of the fast coral depressor motor neuron (D-f) 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 (I-K,I-Ca,I-trans and I-K,I-Ca,I-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 supression of I-K,I-Ca by internal EGTA or Ca2+-free saline, membrane depolarizations positive to -40 mV induced voltage-dependent outward currents (I-K,I-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 (I-Ca,I-trans) and the sustained component (I-Ca,I-sus) were maximal at 0 mV and present when Ca2+ in the saline were replaced by Ba2+. The inactivation of I-Ca,I-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 I-K,I-Ca is saturated by Ca2+ entering through voltage-dependent channels and because the apparent reversal potential for I-Ca is not at E(Ca). 7. Although the similarity in the shape of I-K,I-Ca and I-Ca might suggest that the time course of I-K,I-Ca is determined by the kinetics of I-Ca, this appears unlikely in view of the rapid saturation of I-K,I-Ca by Ca2+, which considerably outlasts the period of Ca2+ influx.