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

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.