Dihydropyridine actions on calcium currents of frog sympathetic neurons.
Dihydropyridines (DHPs) generally have little effect on whole-cell calcium currents of neurons, even at concentrations far higher than those effective on muscle. Either neuronal calcium currents are much less sensitive to DHPs, or only a small proportion of the current is DHP-sensitive. We find that DHP agonists and antagonists act at low concentration on calcium currents in frog sympathetic neurons but that the effects are small even at optimal concentrations. The half-maximal dose (EC50) of the agonist Bay K 8644 is approximately 50 nM, and the effect of Bay K 8644 is blocked by 50% at approximately 300 nM nifedipine, from a holding potential of -80 mV. Nifedipine is more effective from a holding potential of -50 mV. These results suggest the presence of an L-type calcium current, with DHP sensitivity similar to L-currents in cardiac muscle. The predominant (greater than 90%) calcium current in frog sympathetic neurons is a DHP-resistant N-type current. However, high concentrations of DHPs (10 microM) partially block N-type calcium current, as well as voltage-dependent sodium and potassium currents.
Jones
S W
SW
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106.
Jacobs
L S
LS
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004095:Dihydropyridines / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology*; D011892:Rana catesbeiana; D011894:Rana pipiens; D013564:Sympathetic Nervous System / Q000502:physiology*
D015220:Calcium Channels; D004095:Dihydropyridines; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D009543:Nifedipine