Ca2+ inward currents evoked by membrane depolarization have been studied by the intracellular dialysis technique in the somatic membrane of isolated dorsal root ganglion neurones of new‐born rats. In about 20% of the investigated cells a hump has been detected on the descending branch of the current‐voltage curve, indicating the presence of two 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 to 1 ms at potentials between ‐60 and ‐25 mV ('fast' Ca2+ current). The second component of the Ca2+ current was activated at membrane depolarizations to between ‐55 and ‐50 mV. It could be recorded in all cells investigated 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 to 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 two 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.
The Journal of Physiology – Wiley
Published: Feb 1, 1985
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