Intravascular adenosine: the endothelial mediators of its negative dromotropic effects

Abstract

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 A₁ receptor agonist, was covalently coupled to 2×10⁶ 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 A₁, and that NO and prostaglandins are synergistic endothelial mediators of this effect.

Introduction

Adenosine has well-defined cardiovascular actions which include coronary vasodilatation (Olsson et al., 1976; Schrader et al., 1977; Rubio et al., 1983; Kroll et al., 1987), a negative chronotropic/dromotropic effect (Clemo, 1986; Belardinelli et al., 1987; Balcells et al., 1992, Balcells et al., 1993), and an anti-adrenergic response (Dobson, 1983). While it is now known that most of these actions are mediated via membrane bound receptors (Olsson et al., 1976; Schrader et al., 1977; Jacobson et al., 1985, Jacobson et al., 1987), the precise mechanisms by which this metabolite exerts its cardiovascular effects have yet to be defined. Difficulties arise secondary to the numerous influences involved in its formation and metabolism. Within the myocardium, adenosine can be produced and metabolized by both the endothelium (Nees et al., 1985a, Nees et al., 1985b; Deussen et al., 1986; Dendorfer et al., 1987) and cardiac myocytes (Deussen et al., 1986; Dendorfer et al., 1987) and therefore is subject to the influence of both cell types. In addition, the relative importance of the endothelium becomes evident if one considers the following: adenosine remains confined to the intravascular compartment during its intracoronary infusion, in up to micromolar concentrations, because of the impermeable metabolic barrier imposed by the endothelium (Nees et al., 1985a, Nees et al., 1985b; Tietjan et al., 1990), and yet the vasodilatory and negative dromotropic effects of adenosine are observable at these concentrations (Nees et al., 1985a, Nees et al., 1985b; Clemo, 1986); the pharmacokinetics of macromolecular adenosine analogs are similar to those of their low molecular weight counterparts during intracoronary infusion, suggesting an intravascular site of action (Nees et al., 1985a, Nees et al., 1985b; Balcells et al., 1992, Balcells et al., 1993; Smits et al., 1995); and intracoronarily infused adenosine deaminase alters the vasodilator and dromotropic responses attributed to adenosine (Belardinelli et al., 1987; Rubio et al., 1987), but yet does not reach adequate interstitial concentrations to account for its pharmacologic effects (Tietjan et al., 1990). These observations require one to consider whether the adenosine-mediated cardiovascular effects have an endothelial component. Indeed, we have shown (Balcells et al., 1992, Balcells et al., 1993) that the selective activation or blockade of intravascular adenosine receptors, via adenosine conjugates or adenosine receptor antagonist bound to latex microspheres of 0.07 μm diameter, caused the same cardiovascular effects as free adenosine or the free adenosine antagonist (Balcells et al., 1992, Balcells et al., 1993), and yet these particles are confined to the vascular compartment because of their size. These data suggest that some of the effects of intravascular adenosine could result from the release of transcellular bioactive messengers by the endothelium.

The purpose of this study was to establish whether nitric oxide (NO) and prostaglandins are mediators of the negative dromotropic effects of intracoronarily administered adenosine as a result of selective activation of intravascular endothelial receptors.

Our results for the isolated perfused guinea pig heart show that: (1) Intracoronarily administered adenosine causes a negative dromotropic effect through activation of intravascularly located endothelial receptors, (2) this effect is mediated by the release of at least two mediators: NO and a derivative of the arachidonic acid metabolic pathway.