A spin-label study of plasma membranes of adrenal chromaffin cells

Chromaffin-cell membranes were labeled with two nitroxide spin labels, one probing the interior of the membrane and one probing the interfacial region. Both spin labels indicate that the membrane undergoes a phase transition at about 26 degrees C. An Arrhenius plot of acetylcholinesterase activity exhibits a discontinuity at 26 degrees C, consistent with the existence of a phase transition at that temperature. Acetylcholine, which stimulates chromaffin cells to secrete catecholamines, and hexamethonium, a cholinergic blocker, do not affect the rotational correlation times of the spin labels. These results argue that cholinergic stimulation does not affect the fluidity of the chromaffin-cell membrane.