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 muM) 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 (Ca2+)-Ca-45 uptake in a dose-dependent manner (IC50 = 0.09 mM). In the presence of nitrendipine (1 muM), the minimal neomycin concentration necessary for total blockade of (Ca2+)-Ca-45 uptake was reduced to 0.3 mM. Moreover, in the absence of nitrendipine the (Ca2+)-Ca-45 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 muM) 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 muM nitrendipine (IC50 = 0.17-0.19 mM). It is concluded that neomycin (less-than-or-equal-to 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.