Histamine, theophylline and tryptamine transport through lipid bilayer membranes

Diffusion of histamine, theophylline and tryptamine through planar lipid bilayer membranes was studied as a function of pH. Membranes were made of egg phosphatidylcholine plus cholesterol (1 : 1 mol ratio) in tetradecane. Tracer fluxes and electrical conductances were used to estimate the permeabilities to nonionic and ionic species. Only the nonionic forms crossed the membrane at a significant rate. The membrane permeabilities to the nonionic species were: histamine, 3.5 x 10(-5) cm x s-1; theophylline, 2.9 x 10(-4) cm x s-1; and tryptamine, 1.8 x 10(-1) cm x s-1. Chemical reactions in the unstirred layers are important in the transport of tryptamine and theophylline, but not histamine. For example, as pH decreased from 10.0 to 7.5 the ratio of nonionic (B) to ionic (BH+) tryptamine decreased by 300-fold, but the total tryptamine permeability decreased only 3-fold. The relative insensitivity of the total tryptamine permeability to the ratio, [B]/[BH+], is due to the rapid interconversion of B and BH+ in the instirred layers. Our model describing diffusion and reaction in the unstirred layers can explain some 'anomalous' relationships between pH and weak acid/base transport through lipid bilayer and biological membranes.