IDENTIFICATION OF NUCLEOSIDE TRANSPORT BINDING-SITES IN THE HUMAN MYOCARDIUM

The role of nucleoside transport in ischemia-reperfusion injury and ar rhythmias has been well documented in various animal models using sele ctive blockers. However, clinical application of nucleoside transport inhibitors remains to be demonstrated in humans. It is not known wheth er human heart has nucleoside transport similar to that of animals. Th e aim of this study is to pharmacologically identify the presence of n ucleoside transport binding sites in the human myocardium compared to animals. Myocardial tissue was obtained from guinea pig left and right ventricle, canine left ventricle, human intraoperative right atrium a nd human cadaveric right atrium and right and left ventricles. Myocard ial preparations were obtained from tissue samples after homogenized a nd a differential centrifugation. Equilibrium binding assays were perf ormed using [H-3]-p-nitrobenzylthioinosine (NBMPR) at room temperature in the presence or absence of non-radioactive NBMPR or other nucleosi de transport blockers such as p-nitrobenzylthioguanosine dipyridamole, lidoflazine, papaverin, adenosine and doxorubcine. From saturation cu rves and inhibition kinetics, we determined the relative maximal bindi ng (B-max) and dissociation constant (K-d) of [H-3]-NBMPR binding of h uman myocardial preparations. Results demonstrated that the fresh huma n myocardial preparations have a specific binding site for NBMPR with a B-max of 283 +/- 32 fmol/mg protein and K-d of 0.56 +/- 0.12 nM. The se values are lower than those obtained from guinea pigs (B-max = 1440 +/- 187 fmol/mg protein and K-d = 0.21 +/- 0.03 nM) and canine atrium (B-max 594 +/- 73 fmol/mg protein, and K-d = 1.12 +/- 0.22 nM). Displ acement kinetics studies revealed the relative potencies (of certain u nrelated drugs as follow: p-nitrobenzylthioguanosine > dipyridamole > lidoflazine > pavaverine > Diltazam > adenosine > doxyrubicin. It is c oncluded that human myocardium contains an active nucleoside transport site which may play a crucial role in post-ischemic reperfusion-media ted injury in a wide spectrum of ischemic syndromes.