Multiple calcium channels in synaptosomes: voltage dependence of 1,4-dihydropyridine binding and effects on function.
The voltage dependence of binding of the calcium channel antagonist, (+)-[3H]PN200-110, to rat brain synaptosomes and the effects of dihydropyridines on 45Ca2+ uptake have been investigated. Under nondepolarizing conditions (+)-[3H]PN200-110 binds to a single class of sites with a Kd of 0.07 nM and a binding capacity of 182 fmol/mg of protein. When the synaptosomal membrane potential was dissipated either by osmotic lysis of the synaptosomes or by depolarization induced by raising the external K+ concentration, there was a decrease in affinity (approximately 7-fold) with no change in the number of sites. The effects of calcium channel ligands on 45Ca2+ uptake by synaptosomes have been measured as a function of external potassium concentration, i.e., membrane potential. Depolarization led to a rapid influx of 45Ca2+ whose magnitude was voltage-dependent. Verapamil (100 microM) almost completely inhibited calcium uptake at all potassium concentrations studied. In contrast, the effects of dihydropyridines (2 microM) appear to be voltage-sensitive. At relatively low levels of depolarization (10-25 mM K+) nitrendipine and PN200-110 completely inhibited 45Ca2+ influx, whereas the agonist Bay K8644 slightly potentiated the response. At higher K+ concentrations an additional dihydropyridine-insensitive component of calcium uptake was observed. These results provide evidence for the presence of dihydropyridine-sensitive calcium channels in synaptosomes which may be activated under conditions of partial depolarization.
Dunn
S M
SM
Department of Physiology and Biophysics, University of Iowa, Iowa City 52242.
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004095:Dihydropyridines / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D017275:Isradipine; D007700:Kinetics; D008297:Male; D009568:Nitrendipine / Q000494:pharmacology; D010069:Oxadiazoles / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013050:Spectrometry, Fluorescence; D013574:Synaptosomes / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
D004095:Dihydropyridines; D007473:Ion Channels; D010069:Oxadiazoles; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C038806:1,4-dihydropyridine; D009568:Nitrendipine; D014700:Verapamil; D002118:Calcium; D017275:Isradipine