Calcium Currents in Bullfrog Sympathetic Neurons L Activation Kinetics and Pharmacology

The calcium current o f bullfrog sympathetic neurons activates and deactivates rapidly (7 < 3 ms). For brief depolarizations, the current can be fit reasonably well by a Hodgkin-Huxley-type model with a single gating particle o f charge + 3. With 2 mM Ca 2+ as the charge carrier, half-maximal activation occurs at ~ 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 Ba ~+ is blocked by Cd ~+ at micromolar concentrations, and by 100 nM w-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. Nifedlpine at least partially reverses the effect o f Bay K 8644, but has little effect on its own. Muscarinic agonists and other ligands that inhibit the M-type potassium current o f frog sympathetic neurons have weak inhibitory effects on the calcium current as well. One interpretation o f these results is that the N-type calcium current predominates in these cells, with a minor contribution of L-type current. I N T R O D U C T I O N Calcium currents are o f part icular interest due to the pivotal role o f intracellular calcium ions as second messengers. This and the accompanying pape r ( Jones and Marks, 1989) describe the kinetic and pharmacological proper t ies o f voltage-dependent calcium cur ren t in bul l f rog sympathetic neurons , based on whole-cell recording f rom isolated cells. A prel iminary communica t ion o f some o f these results has appeared (Marks and Jones , 1988). M A T E R I A L S AND M E T H O D S Neurons were isolated from paravertebral sympathetic ganglia of adult bullfrogs (Rana catesbe/ana) and maintained at 4~ as described previously (Kuffier and Sejnowski, 1983; Jones, Address reprint requests to Dr. Stephen W. Jones, Department of Physiology and Biophysics, Case Western Reserve University, 2119 Abington Road, Cleveland, OH 44106. J. GEN. PHYSIOL. C) The Rockefeller University Press 9 0022-1295/89/07/0151/17 $2.00 Volume 94 July 1989 151-167 151 on A ril 4, 2017 D ow nladed fom Published July 1, 1989