Calcium currents in bullfrog sympathetic neurons. I. Activation kinetics and pharmacology.
The calcium current of bullfrog sympathetic neurons activates and deactivates rapidly (tau less than 3 ms). For brief depolarizations, the current can be fit reasonably well by a Hodgkin-Huxley-type model with a single gating particle of charge +3. With 2 mM Ca2+ as the charge carrier, half-maximal activation occurs at approximately -5 mV, near the voltage where activation and deactivation are slowest. When extracellular divalent ion concentrations are reduced, monovalent ions (e.g., Na+ and methylammonium) produce kinetically similar inward currents. Current carried by Ba2+ is blocked by Cd2+ at micromolar concentrations, and by 100 nM omega-conotoxin. Commercially available saxitoxin blocks the current, but different batches have quantitatively different potency. The dihydropyridine agonist Bay K 8644 induces a slight shift in activation kinetics to more negative voltages, with little effect on the peak current. Nifedipine at least partially reverses the effect of Bay K 8644, but has little effect on its own. Muscarinic agonists and other ligands that inhibit the M-type potassium current of frog sympathetic neurons have weak inhibitory effects on the calcium current as well. One interpretation of these results is that the N-type calcium current predominates in these cells, with a minor contribution of L-type current.
Jones
S W
SW
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106.
Marks
T N
TN
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002120:Calcium Channel Agonists / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004095:Dihydropyridines / Q000494:pharmacology; D005728:Ganglia, Sympathetic / Q000187:drug effects* / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D009543:Nifedipine / Q000494:pharmacology; D011892:Rana catesbeiana; D012530:Saxitoxin / Q000494:pharmacology; D015290:Second Messenger Systems / Q000187:drug effects
D002120:Calcium Channel Agonists; D015220:Calcium Channels; D004095:Dihydropyridines; D002104:Cadmium; D012530:Saxitoxin; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C038806:1,4-dihydropyridine; D000255:Adenosine Triphosphate; D009543:Nifedipine