ALAMETHICIN CHANNEL PERMEATION BY CA2+, MN2+ AND NI2+ IN BOVINE CHROMAFFIN CELLS

Alamethicin causes a concentration-dependent increase of [Ca2+]i in suspensions of bovine adrenal chromaffin cells loaded with fura-2. The basal levels of Ca(i)2+ (234 +/- 37 nM; n = 4) increased to a maximum of 2347 +/- 791 nM (n = 3) with 100-mu-g/ml alamethicin. In the presence of 1 mM Ca(e)2+ the increase reached a plateau within about 2-5 s. This increase was due to Ca2+ entry into chromafin cells, since in the absence of Ca(e)2+ alamethicin did not modify [Ca2+]i. This contrasts with ionomycin (1-mu-M) which produced a Ca(i)2+ transient even in the absence of Ca(e)2+. 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 mM) addition. This suggests that Mn2+ is able to permeate voltage-dependent Ca2+ channels. Ni2+ uptake into either resting or 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.