Transport Systems for Adenosine in Mammalian Cell Membranes

The passage of adenosine and other nucleosides across plasma membranes may occur by simple diffusion, or may be mediated by nucleoside-specific transport systems that are intrinsic components of the plasma membrane. Both facilitated diffusion and sodium-dependent nucleoside transport (NT) systems have been described and each of these classes may possess several subtypes. It should be noted that, in some cells, adenosine fluxes are rapid. Consequently rapid sampling assays are required to measure initial rates of uptake that are representative of transport rates (see reviews by Paterson et al., 1985; Gati et al., 1989a). Interpretation of inhibition of nucleoside entry as transport inhibition requires that uptake rates be demonstrably initial rates. Thus, accurate assessment of NT and its inhibition can best be obtained using cells in culture or in fresh cells that have been dispersed enzymatically. This report will review mechanisms of adenosine transport and summarize the evidence in support of NT system heterogeneity, with particular emphasis on NT systems in cells and tissues likely to be involved in the modulation of the physiological and pharmacological actions of adenosine.