title: Neomycin blocks dihydropyridine-insensitive Ca2+ influx in bovine adrenal chromaffin cells.
abstract: There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Although recent electrophysiological work indicates that the dihydropyridine-resistant pathway is partially mediated by omega-conotoxin-sensitive and -insensitive Ca2+ channels, the pharmacological sensitivity of the latter channels remains elusive. We have now found that combined incubations with nitrendipine (1 microM) and neomycin (0.5 mM) reduced high K+ (50 mM)-evoked intracellular Ca2+ concentration ([Ca2+]i) transients to a larger extent than each drug separately. [Ca2+]i was measured using the fluorescent intracellular Ca2+ indicator fura-2. Neomycin (0.05-2 mM) reduced high K(+)-evoked 45Ca2+ uptake in a dose-dependent manner (IC50 = 0.09 mM). In the presence of nitrendipine (1 microM), the minimal neomycin concentration necessary for total blockade of 45Ca2+ uptake was reduced to 0.3 mM. Moreover, in the absence of nitrendipine the 45Ca2+ uptake remaining in 0.3 mM neomycin (26% of maximum) was similar to the fractional inhibition by nitrendipine alone (29%). Neomycin (0.05-2 mM) inhibited the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) much more extensively at 2 mM than at 0.3 mM (percent inhibition = 59% and 15%, respectively). Neomycin (0.05-2 mM) blocked high K(+)-evoked noradrenaline and adrenaline release in a dose-dependent fashion (IC50 = 0.8-1.1 mM), the blockade efficiency being enhanced in the presence of 1 microM nitrendipine (IC50 = 0.17-0.19 mM). It is concluded that neomycin (< or = 0.3 mM) blocks preferentially the dihydropyridine-insensitive Ca2+ influx pathway of the chromaffin cell. Moreover, both the dihydropyridine-sensitive and the dihydropyridine-resistant, neomycin-sensitive Ca2+ influx pathways contribute strongly to depolarization-evoked catecholamine secretion.
authors.affiliation: Center for Neurosciences, University of Coimbra, Portugal.
authors.forename: C B
authors.initials: CB
authors.lastname: Duarte
authors.forename: A R
authors.initials: AR
authors.lastname: Tome
authors.forename: E
authors.initials: E
authors.lastname: Forsberg
authors.forename: C A
authors.initials: CA
authors.lastname: Carvalho
authors.forename: A P
authors.initials: AP
authors.lastname: Carvalho
authors.forename: R M
authors.initials: RM
authors.lastname: Santos
authors.forename: L M
authors.initials: LM
authors.lastname: Rosario
mesh_terms: D000313:Adrenal Medulla / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002417:Cattle; D002478:Cells, Cultured; D004095:Dihydropyridines / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000378:metabolism; D009355:Neomycin / Q000494:pharmacology*; D009638:Norepinephrine / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D004095:Dihydropyridines; C038806:1,4-dihydropyridine; D009355:Neomycin; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine; D004837:Epinephrine