Author(s): A S Clanachan, F E Parkinson, T P Heaton
Nucleoside transport systems catalyse the movement of nucleoside molecules across plasma membranes. Inhibitors of transport potentiate many of the physiological and pharmacological effects of adenosine, an effect that seems to result from an impaired removal of adenosine from the vicinity of its receptors located on the extracellular surface of adenosine-responsive cells. Many classes of drugs have been reported to alter nucleoside transporter function and thus adenosine effects, but limitations in methodology have resulted in few valid quantitative assessments of drug affinities for transporters in adenosine-sensitive systems. We have measured the transport of adenosine and 2-chloroadenosine and the binding of the high-affinity probe, nitrobenzylthioinosine (NBMPR), in guinea pig and rat cardiac myocytes We also examined drug-induced inhibition of these processes. These studies have confirmed that NBMPR is a valid probe in these cells and inhibition of NBMPR binding is predictive of nucleoside transport inhibition. The transport system in guinea pig cardiac myocytes resembles that in human erythrocytes. Rat cardiac myocytes possess different transporters; these also possess high affinity for NBMPR, but can be distinguished by their low sensitivity to dipyridamole. NBMPR-binding studies in guinea pig cardiac membranes indicate that several adenosine receptor agonists possess affinities greater than adenosine for the transport system. 2-Chloroadenosine is a transporter substrate, but the nature of the interaction of other adenosine receptor agonists is unclear. Inosine, reported to have weak adenosine-receptor activity, has only a two fold lower affinity than adenosine for NBMPR sites. It potentiated the receptor-mediated actions of adenosine by competing with adenosine for the transporter. Quantitative assessments of drug interactions with nucleoside transport systems, either by direct flux studies or by correctly validated high-affinity binding probes, will extend our knowledge of transporter subtypes and the symmetry of transporters with respect to substrates and inhibitors.