Cholinergic stimulants and excess potassium ion increase the fluidity of plasma membranes isolated from adrenal chromaffin cells.

Chromaffin cell membranes from the bovine adrenal medulla were labelled with the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, and the fluorescence polarization (P) of the membrane suspensions was measured as a function of temperature. The P versus t profiles, between 20 and 37 degrees C, showed two linear regions separated by a break in the vicinity of 30 degrees C, reflecting a change in the phase behaviour of the constitutent lipids. Decreases in P values at higher temperature indicated progressive fluidization of the lipid bilayer. Previous incubation with either acetylcholine (0.5 mM) or nicotine (50 microM) produced further fluidization, the extent of which depended on the presence of added Ca2+ (2.2 mM). Thus, the flow activation energy, delta E, between approx. 30 and 37 degrees C was 9.1 kcal/mol for acetylcholine and 8.8 kcal/mol for acetylcholine plus Ca2+, as compared to 7.9 kcal/mol in the absence of acetylcholine and Ca2+. In the presence of nicotine, delta E was 11.4 kcal/mol when Ca2+ was absent and 9.5 kcal/mol when it was present. The cholinergic blocker, hexamethonium (0.5 mM), abolished the acetylcholine- or nicotine-induced changes. 65 mM K+ produced a similar fluidization, which was reversed by addition of Ca2+. An additive effect was observed when the membranes were incubated with both nicotine and K+, with delta E = 16.6 kcal/mol in the presence of Cas2+. These results indicate a receptor-mediated modulation of the lipid distribution between rigid and fluid regions in the membrane, which could be of importance for stimulated catecholamine secretion in the intact cell.