Transport in isolated plasma membranes.

Plasma membrane vesicles constitute a simpler experimental system for studying transport compared to cells or intact tissue. The principal advantages of the vesicle approach are the elimination of metabolism as a complicating factor and the ability to control the composition of the solutions on both sides of the membrane. The major disadvantage is vesicle heterogeneity. However, techniques are available to avoid the kinetic artifacts that are due to the heterogeneity. Results of transport studies using membrane vesicles have conclusively shown that D-glucose and amino acids are co-transported with Na+ and that transport against a concentration gradient is driven by an electrochemical Na+ gradient. As a result of coupling between Na+ and the nonelectrolytes, estimates of the kinetic parameters of transport, Km and Vmax, require that the load on the Na+ gradient be taken into account. This has rarely been done. Although electrolyte transport is a major function of the plasma membrane, knowledge of the mechanisms involved is limited. Future investigations employing specific ionophores should contribute much to our understanding of the mechanisms underlying ATP-independent ion transport. Examples of the application of membrane vesicles for studying transport-related aspects of diseases are discussed.