CALCIUM AND POTASSIUM CURRENTS IN THE FAST COXAL DEPRESSOR MOTONEURONE OF THE COCKROACH PERIPLANETA AMERICANA. DAVID, JONATHAN A. AND ROBERT M. PITMAN. School of Biological and Medical Sciences, Gatty Marine Laboratory, University of St. Andrews, Fife, KY16 8LB, U.K.
APStracts 2:0209N, 1995.
SUMMARY AND CONCLUSIONS
1. Membrane currents have been examined in the cell body of the fast coxal depressor motoneurone (Df) of the cockroach Periplaneta americana using two- electrode voltage-clamp. 2. Most of the outward current induced by membrane depolarizations to between -40 and +80 mV was carried by K+ since it was blocked by external 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) since these were suppressed by injecting EGTA, removing Ca2+ from the saline or replacing Ca2+ with Ba2+. Following supression of IK,Ca by internal EGTA or Ca2+-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+ since 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. 

Received 25 October 1995; accepted in final form 27 June 1995.
APS Manuscript Number J671-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.