Thermal anomalies in acetylcholine release: evidence for control of membrane fusion by interfacial water

Thermal anomalies in the rate of acetylcholine release from electrically stimulated guinea-pig ileum have been demonstrated at 18 degrees C and 30 degrees C. Because similar anomalies occur in many aqueous systems where interfacial water plays a dominant role, it is concluded that the anomalies in acetylcholine secretion may indicate domination of the release process by interfacial water. Sharp breaks in the Arrhenius plots at the temperatures of the anomalies separate non-linear portions of positive gradient from a linear portion of negative gradient. The negative curve probably results from the rate of a competitive reaction (such as the rate of removal of calcium from the nerve terminal) while the positive portions may arise from cooperative H-bonding of water molecules. It is proposed that water-dominated phase transitions in membrane lipids may also mediate the effects of both temperature and calcium ions on neurotransmitter release in particular and membrane fusion in general.