Distinct effects of omega-toxins and various groups of Ca(2+)-entry inhibitors on nicotinic acetylcholine receptor and Ca2+ channels of chromaffin cells.
The effects of omega-toxins and various Ca2+ antagonist subtypes on the 45Ca2+ entry into bovine adrenal medullary chromaffin cells stimulated via nicotinic acetylcholine receptors or via direct depolarization with K+, have been compared. The conditions selected to stimulate the 45Ca2+ entry consisted of a 60-s period of exposure of cells to 100 microM of the nicotinic acetylcholine receptor agonist dimethylphenylpiperazinium or to 70 mM K+. The N-type voltage-dependent Ca2+ channel blockers omega-conotoxin GVIA and MVIIA (1 microM) inhibited 45Ca2+ entry stimulated by dimethylphenylpiperazinium or K+ by around 25-30%. The P-type Ca2+ channel blocker omega-agatoxin IVA (10 nM) did not affect the dimethylphenylpiperazinium nor the K+ responses; 1 microM (Q-channel blockade) inhibited both responses by around 50%. The N/P/Q-type Ca2+ channel blocker omega-contoxin MVIIC (1 microM) inhibited the K+ evoked 45Ca2+ entry by 70%, while dimethylphenylpiperazinium was blocked by 50% (P < 0.001). The L-type Ca2+ channel blockers nifedipine, furnidipine, diltiazem or verapamil (3 microM each) inhibited much more the dimethylphenylpiperazinium than the K+ response. The dimethylphenylpiperazinium signal was blocked 71, 88, 89, and 53%, respectively, by nifedipine, furnidipine, diltiazem and verapamil, and the K+ response by 38, 29, 22, and 10%. Combined omega-conotoxin MVIIC (1 microM) and furnidipine (3 microM) blocked 100% of the K+ evoked 45Ca2+ entry. However, combined omega-conotoxin GVIA (1 microM), and furnidipine left unblocked 50% of the K+ response. The "wide spectrum' Ca2+ channel antagonists flunarizine or dotarizine (3 microM each) blocked the dimethylphenylpiperazinium and the K+ responses to a similar extent (50%); cinnarizine (3 microM) inhibited more the dimethylphenylpiperazinium (82%) than the K+ response (21%). At 3 microM, the highly lipophilic beta-adrenoceptor antagonist (+/-)-propranolol, reduced by 68% the dimethylphenylpiperazinium signal and by 23% the K+ signal. Other high lipophilic beta-adrenoceptor antagonists such as metoprolol and labetalol, reduced little the dimethylphenylpiperazinium and the K+ responses. The highly lipophilic agent penfluridol blocked the dimethylpiperazinium response by 30% and the K+ response by 50%. One of the least lipophilic compounds tested, (+)-lubeluzole, blocked by 40% the dimethylphenylpiperazinium and the K+ responses. These data are compatible with the idea that the various omega-toxin peptides used to separate pharmacologically the different voltage-dependent Ca2+ channels expressed by neurones, do not block the neuronal nicotinic acetylcholine receptor ion channel. In contrast the L-type Ca2+ channel blockers do block the nicotinic acetylcholine receptor ionophore. Lipophilicity of the compounds is not a requirement for Ca2+ channel or nicotinic acetylcholine receptor blockade.
Villarroya
M
M
Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Spain.
De la Fuente
M T
MT
López
M G
MG
Gandía
L
L
García
A G
AG
D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002132:Calcium Radioisotopes; D002417:Cattle; D019439:Chromaffin Cells / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure*; D004246:Dimethylphenylpiperazinium Iodide; D010455:Peptides / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000187:drug effects*; D013111:Spider Venoms / Q000494:pharmacology*; D013268:Stimulation, Chemical; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002132:Calcium Radioisotopes; D010455:Peptides; D011978:Receptors, Nicotinic; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020960:omega-Conotoxins; D004246:Dimethylphenylpiperazinium Iodide; C078452:ziconotide; D020866:omega-Conotoxin GVIA; D011188:Potassium; D002118:Calcium