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Low- and high-voltage-activated calcium channel currents and their modulation in the dorsal root ganglion cell line
E M Kobrinsky1, H A Pearson, A C Dolphin
1Department of Pharmacology, Royal Free Hospital School of Medicine, London, U.K.
Abstract:
The dorsal root ganglion-neuroblastoma cell line ND7-23 expresses low-voltage-activated calcium channel currents, and also expresses high-voltage-activated currents in about 50% of differentiated cells. Calcium channel currents were recorded with Ba2+ as the charge carrier. Low-voltage-activated currents were maximally activated at -30 mV and completely inactivated at holding potentials of -60 to -50 mV. omega-Conotoxin GVIA produced a reversible inhibition of low-voltage-activated currents, whereas the inhibition of high-voltage-activated current was largely irreversible. Dihydropyridine antagonists did not inhibit low-voltage-activated currents, whereas they inhibited a sustained, high-voltage-activated current. Low-voltage-activated currents were inhibited to a greater extent than high-voltage-activated currents by Ni2+ (100 microM) and by phenytoin (10 microM). Bradykinin (0.1 microM), baclofen (2 microM) and internal guanosine-5'-O-3-thiotriphosphate (100 microM) inhibited low-voltage-activated currents without affecting their kinetics of activation. Two classes of low-voltage-activated current were distinguished by their kinetics of inactivation. In the majority of cells, currents were slowly inactivating with a time-constant of inactivation of about 50 ms. They also exhibited a sustained component to the current, representing about 20% of the peak current. This component could be distinguished pharmacologically from high-voltage-activated current. The remainder of cells expressed a rapidly and completely inactivating current, with a time-constant of inactivation of about 20 ms. Two distinct single channel currents were observed in these cells, from cell-attached patch measurements, one had a single channel conductance of 7.9 pS, and the ensemble average current showed some inactivation. It is likely that this channel subtype underlies the low-voltage-activated current. The other showed long openings in the presence of a dihydropyridine agonist, had a conductance of 23.1 pS, and was non-inactivating.