Two types of calcium channels in the somatic membrane of new-born rat dorsal root ganglion neurones.
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.
Fedulova
S A
SA
Kostyuk
P G
PG
Veselovsky
N S
NS
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology; D005727:Ganglia, Spinal / Q000502:physiology*; D007473:Ion Channels / Q000502:physiology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000502:physiology*; D051381:Rats; D013324:Strontium / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D007473:Ion Channels; D001464:Barium; D000255:Adenosine Triphosphate; D014700:Verapamil; D000242:Cyclic AMP; D008274:Magnesium; D002118:Calcium; D013324:Strontium