Alamethicin channel permeation by Ca 2+, Mn 2+ and Ni 2+ in bovine chromaffin cells

Abstract

Alamethicin causes a concentration-dependent increase of (Ca 2+), in suspensions of bovine adrenal chromaffin cells loaded with fura-2. The basal levels of Ca 2+ (234 ± 37 nM; n=4) increased to a maximum of 2347±791 nM ( n=3) with 100 μg/ml alamethicin. In the presence of 1 nM Ca 2+ the increase reached a plateau within about 2–5 s. This increase was due to Ca 2+ entry into chromaffin cells, since in the absence of Ca 2+ alamethicin did not modify [Ca 2+]. This contrasts with ionomycin (1 μM) which produced a Ca 2+ transient even in the absence of Ca 2+, Mn 2+ ions also entered chromaffin cells in the presence of alamethicin, as measured by the quenching of fura-2-fluorescence following excitation at 360 nm. Resting chromaffin cells had a measurable permeability to Mn 2+ which was drastically increased by cell depolarization by K + (50 nM) addition. This suggests that Mn 2+ is able to permeate voltage-dependent Ca 2+ channels. Ni 2+ uptake into either resting of K +-stimulated chromaffin cells was undetectable, but addition of alamethicin induced rapid uptake of this cation. The alamethicin-induced entry of Ni 2+ was decreased by 50 mM K +. Overall, the results are compatible with the formation by alamethicin of ion channels in chromaffin cell plasma membranes.