Neomycin blocks dihydropyridine-insensitive Ca2+ influx in bovine adrenal chromaffin cells.
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.
Duarte
C B
CB
Center for Neurosciences, University of Coimbra, Portugal.
Tome
A R
AR
Forsberg
E
E
Carvalho
C A
CA
Carvalho
A P
AP
Santos
R M
RM
Rosario
L M
LM
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
D002121:Calcium Channel Blockers; D004095:Dihydropyridines; C038806:1,4-dihydropyridine; D009355:Neomycin; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine; D004837:Epinephrine