Two types of calcium channels in the somatic membrane of new‐born rat dorsal root ganglion neurones.

Abstract

Ca2+ inward currents evoked by membrane depolarization were studied by the intracellular dialysis technique in the somatic membrane of isolated dorsal root ganglion neurons of newborn rats. In .apprx. 20% of the investigated cells a hump was detected on the descending branch of the current-voltage curve, indicating the presence of 2 populations of Ca2+ channels differing in their potential-dependent characteristics. An initial less regular component of the Ca2+ current was activated at membrane potentials from -75 to -70 mV. Its amplitude reached 0.2-0.9 nA at 14.6 mM-extracellular Ca2+. The activation kinetics of this component could be approximated by the Hodgkin-Huxley equation using the square of the m variable. .tau.m varied in the range from 8-1 ms at potentials between -60 and -25 mV (fast Ca2+ current). The 2nd component of the Ca2+ current was activated at membrane depolarizations to between -55 and -50 mV. It could be recorded in all cells and reached a maximum value of 1-7 nA at the same extracellular Ca2+ concentration. This component decreased rapidly during cell dialysis with saline solutions. The decrease could be slowed down by cooling and accelerated by warming the extracellular solution. Intracellular introduction of 3'', 5''-cAMP together with ATP and Mg2+ not only prevented the decrease but often restored the maximal current amplitude to its initial level. The activation kinetics of this component could also be approximated by a square function .tau.m being in the range 16-2.5 ms at membrane potentials between -20 and +3 mV (slow Ca2+ current). The fast Ca2+ current inactivated exponentially at sustained depolarizations in a potential-dependent manner, .tau.h varying from 76-35 ms at potentials between -50 and -30 mV. The inactivation of the slow Ca2+ current studied in double-pulse experiments was current-dependent and developed very slowly (time constant of several hundreds of milliseconds). It slowed down even more at low temperature or after substitution of Ba2+ for Ca2+ in the extracellular solution. Both currents could also be carried by Ba2+ and Sr2+, although the ion-selecting properties of the 2 types of channels showed quantitative differences. Specific blockers of Ca2+ channels (Co2+, Mn2+, Cd2+, Ni2+ or verapamil) exerted similar effects on them. The existence of metabolically dependent and metabolically independent Ca2+ channels in the neuronal membrane and their possible functional role are discussed.