Preconcentration of Catecholamines into Liposomes with Imposed pH Gradients

The preconcentrations of epinephrine (EP) and norepinephrine (NE) into phosphatidylcholine liposomes with imposed pH gradient across the membrane were investigated by adding the catecholamine (CA) to the external medium of liposomes. To determine the CAs entrapped into liposomes, a simple method was also developed, based on the adsorption of CAs onto alumina and chemiluminescence detection. The uptake of the CAs into the liposomes was examined over a wide (delta)pH range of 0-5 units (internal pH 5.0). The maximum uptakes of EP and NE were 70% and 88%, respectively, around an external pH of 9 ((delta)pH 4), where zwitterionic species of EP and NE are dominant. The final concentrations of EP and NE in internal volumes were 34- and 43-fold greater than the initial concentrations of EP and NE, respectively. The uptake of the CAs was studied by using a mathematical model, which indicates that the uptake is contributed by the molar fraction of the zwitterion species in the external volume as well as the pH gradient. The uptake of EP was found to suffer from a steric hindrance due to a methyl group substituted for the hydrogen of the amino group. This study proves the capability of the liposomes for accumulation media of CAs.