Intravascular adenosine: the endothelial mediators of its negative dromotropic effects

Intravascular adenosine may exert its negative dromotropic effect via activation of luminal coronary endothelial receptors, which suggests the presence of transcellular dromotropic mediators of endothelial origin, perhaps nitric oxide (NO) and prostaglandins. We decided to test this hypothesis in isolated guinea pig hearts retrogradely perfused with Krebs–Henseleit solution. A pair of stimulating electrodes were placed in the right atria and the auricular–ventricular (A-V) delay recorded by means of a recording electrode placed on the left atria and an electrode placed on the tip of the ventricle. Hearts were paced at a rate of 3.8±0.2 Hz and perfused at a coronary flow of 9±0.25 ml/min. To obtain dose–response curves, single doses (as boluses) of different concentrations of adenosine were infused and the maximal increase in A-V delay induced by each dose was determined. Agents that inhibit NO accumulation, such as N G -nitro- l -arginine methyl ester ( l -NAME) and oxyhemoglobin, diminished the effect of adenosine while NO-sparing agents, such as superoxide dismutase and dithiotreitol, enhanced the adenosine effect. Infusion of NO and the NO donor morpholinosydnonimine increased the A-V delay in a dose-dependent manner. In addition, the dose–response curve for adenosine was displaced downward and to the right by indomethacin, indicating also the involvement of prostaglandins. Infusion of l -NAME in addition to indomethacin further diminished the effects of adenosine, indicating that NO and prostaglandins acted simultaneously. To selectively activate intravascular endothelial adenosine receptors, adenosine amino congener (ADAC), an adenosine A 1 receptor agonist, was covalently coupled to 2×10 6 Da dextran. When intracoronarily infused, the dextran–ADAC complex remains in the blood vessel lumen because it is too large to diffuse to the interstitium. On intracoronary administration, the dextran–ADAC complex caused a negative dromotropic effect which was diminished by l -NAME and indomethacin. Our data indicate that the dromotropic effect caused by intracoronarily administered adenosine is the result solely of activation of intravascular endothelial adenosine receptors, possibly type A 1 , and that NO and prostaglandins are synergistic endothelial mediators of this effect.