Adenosine and adenosine receptors in the cardiovascular system: biochemistry, physiology, and pharmacology

Lack of adenosine causes myocardial refractoriness

OBJECTIVES

This study sought to investigate the myocardial mechanisms causing refractoriness to ischemic preconditioning in pigs.

BACKGROUND

Ischemic preconditioning in the pig vanishes after 60 min and cannot be reinstated by a second cycle of brief coronary occlusions at this time point. Ischemic preconditioning has been shown to be mediated by adenosine A1-receptors. Because myocardial adenosine production during ischemia ceases as the number of repeated brief ischemic episodes increases, we hypothesized that this lack of adenosine may cause this myocardial refractoriness.

METHODS

In open chest pigs, ischemic preconditioning was achieved by repeated brief coronary occlusions. Myocardial adenosine content was assessed by high performance liquid chromatographic analysis of serial myocardial biopsy samples; infarct size (percent infarcted area of the area at risk) was determined using tetrazolium salts.

RESULTS

Ischemic preconditioning by two cycles of occlusion of the left anterior descending coronary artery (10 min) and reperfusion (30 min) decreased infarct size ([mean +/- SEM] 40.4+/-2.9%; control: 76.9+/-1.8%, p < 0.001). Prolonging the second reperfusion period to 60 min caused ischemic preconditioning to vanish (79.0+/-0.5%) and caused refractoriness to a second cycle of preconditioning (70.0+/-2.0%). Myocardial adenosine content increased only during the first coronary occlusion (baseline: 110.9+/-42.0 nmol/g dry weight; first coronary occlusion: 1,686.2+/-244.1, p < 0.001) but not during subsequent coronary occlusions. In refractory myocardium, intramyocardial microinfusion of the adenosine A1-receptor agonist N6-cyclohexyladenosine (CHA [0.3 mmol/liter]) again decreased infarct size (27.4+/-7.0%, p < 0.001 vs. control).

CONCLUSIONS

Myocardial refractoriness may be caused by the inability to produce adenosine endogenously. In refractory myocardium, application of CHA reinduces cardioprotection.

Molecular cloning and characterization of a nitrobenzylthioinosine-insensitive (ei) equilibrative nucleoside transporter from human placenta

Mammalian equilibrative nucleoside transporters are typically divided into two classes, es and ei, based on their sensitivity or resistance respectively to inhibition by nitrobenzylthioinosine (NBMPR). Previously, we have reported the isolation of a cDNA clone encoding a prototypic es-type transporter, hENT1 (human equilibrative nucleoside transporter 1), from human placenta. We now report the molecular cloning and functional expression in Xenopus oocytes of a cDNA from the same tissue encoding a homologous ei-type transporter, which we designate hENT2. This 456-residue protein is 46% identical in amino acid sequence with hENT1 and corresponds to a full-length form of the delayed-early proliferative response gene product HNP36, a protein of unknown function previously cloned in a form bearing a sequence deletion. In addition to placenta, hENT2 is found in brain, heart and ovarian tissue. Like hENT1, hENT2 mediates saturable transport of the pyrimidine nucleoside uridine (Km 0.2+/-0.03 mM) and also transports the purine nucleoside adenosine. However, in contrast with hENT1, which is potently inhibited by NBMPR (Ki 2 nM), hENT2 is NBMPR-insensitive (IC50<1 microM). It is also much less sensitive to inhibition by the coronary vasoactive drugs dipyridamole and dilazep and to the lidoflazine analogue draflazine, properties that closely resemble those reported for classical ei-type transport in studies with intact cells.