NMR studies of 5-hydroxytryptamine transport through large unilamellar vesicle membranes

Nuclear magnetic resonance techniques developed to study membrane permeability in closed membrane systems have been used to investigate transport of 5-hydroxytryptamine across the phospholipid membranes of large unilamellar vesicles. The vesicles, modeling the 5-hydroxytryptamine storage organelles of blood platelets, contained a high internal level of ATP buffered at a pH low relative to the external solution. The resultant pH gradient drove accumulation of 5-hydroxytryptamine to a level consistent with selective transport of the neutral amine. The upfield shifts of the 5-hydroxytryptamine resonances resulting from complexation with internally confined ATP were utilized to resolve and, simultaneously, to observe the internal and external amine. Simulation of the time evolution of the 5-hydroxytryptamine concentration allowed measurement of a permeability coefficient of 1.4 +/- 0.5 X 10(-5) cm/sec for the neutral amine.