Alamethicin channel permeation by Ca2+, Mn2+ and Ni2+ in bovine chromaffin cells

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Abstract
Alamethicin causes a concentration‐dependent increase of (Ca2+), in suspensions of bovine adrenal chromaffin cells loaded with fura‐2. The basal levels of Ca2+ (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 Ca2+ the increase reached a plateau within about 2–5 s. This increase was due to Ca2+ entry into chromaffin cells, since in the absence of Ca2+ alamethicin did not modify [Ca2+]. This contrasts with ionomycin (1 μM) which produced a Ca2+ transient even in the absence of Ca2+, Mn2+ 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 Mn2+ which was drastically increased by cell depolarization by K+ (50 nM) addition. This suggests that Mn2+ is able to permeate voltage‐dependent Ca2+ channels. Ni2+ 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 Ni2+ was decreased by 50 mM K+. Overall, the results are compatible with the formation by alamethicin of ion channels in chromaffin cell plasma membranes.