MULTIPLE COMPONENTS OF BOTH TRANSIENT AND SUSTAINED BARIUM CURRENTS IN A RAT DORSAL-ROOT GANGLION-CELL LINE

1. Currents through voltage‐activated Ca2+ channels in rat dorsal root ganglion (DRG) x mouse neuroblastoma hybrid (F‐11) cells were studied using the whole‐cell patch clamp technique with 30 mM‐Ba2+ as charge carrier. Two components of the inward Ba2+ current were distinguished on the basis of voltage dependence and time course. Each component could be further subdivided based on pharmacology. 2. A transient inward current activated at test potentials positive to ‐40 mV, peaked within 20 ms and then decayed during a 200 ms depolarization. The peak amplitude of the transient current occurred between ‐10 and +10 mV. With a 300 ms conditioning pulse, half‐inactivation of the transient current occurred at ‐30 mV. A sustained inward current activated at test potentials positive to ‐30 mV and reached a maximum at +20 to +30 mV. The sustained current showed little voltage‐dependent inactivation over 200 ms. The amplitudes of both the transient and sustained currents were increased by perfusing with Ba2+ instead of Ca2+. 3. Most F‐11 cells had both the transient and sustained Ba2+ currents although the relative amount of the two currents varied with culture conditions. The transient current was more prominent in cells fed with a ‘growth’ medium (15‐20% serum) whereas the sustained current was increased in cells fed with a ‘differentiation’ medium (1% serum plus growth factors). F‐11 cells can be used to study transient current in relative isolation from sustained Ca2+ current under certain culture conditions. The neuroblastoma parent of the F‐11 cell line, N18TG‐2 cells, exhibited little or no voltage‐dependent Ba2+ current. 4. Brief application of omega‐conotoxin fraction GVIA (10 microM) produced a long‐lasting block of 81% of the sustained current and 27% of the transient current. 5. The transient and sustained Ba2+ currents in F‐11 cells were reversibly blocked by brief exposure to Cd2+ or Ni2+. Block of the sustained current was evident with 100 nM‐Cd2+ whereas the threshold concentration for Ni2+ block was 1 microM. Cd2+ and Ni2+ were equipotent blockers of the transient current. Dose‐response curves for Cd2+ and Ni2+ block of both sustained and transient currents had shallow slopes suggesting that the block was more complex than a simple bimolecular interaction between blocker and one blocking site. Dose‐response curves were fitted by a model that included two binding sites for each divalent blocker.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society