Cardiac endothelial transport and metabolism of adenosine and inosine

Abstract

The influence of transmembrane flux limitations on cellular metabolism of purine nucleosides was assessed in whole organ studies. Transcapillary transport of the purine nucleosides adenosine (Ado) and inosine (Ino) via paracellular diffusion through interendothelial clefts in parallel with carrier-mediated transendothelial fluxes was studied in isolated, Krebs-Henseleit-perfused rabbit and guinea pig hearts. After injection into coronary inflow, multiple-indicator dilution curves were obtained from coronary outflow for 90 s for 131I-labeled albumin (intravascular reference tracer), [3H]arabinofuranosyl hypoxanthine (AraH; extracellular reference tracer and nonreactive adenosine analog), and either [14C]Ado or [14C]Ino. Ado or Ino was separated from their degradative products, hypoxanthine, xanthine, and uric acid, in each outflow sample by HPLC and radioisotope counting. Ado and Ino, but not AraH, permeate the luminal membrane of endothelial cells via a saturable transporter with permeability-surface area product PS(ecl) and also diffuse passively through interendothelial clefts with the same conductance (PSg) as AraH. These parallel conductances were estimated via fitting with an axially distributed, multi-pathway, four-region blood-tissue exchange model. PSg for AraH were approximately 4 and 2.5 ml. g(-1). min(-1) in rabbits and guinea pigs, respectively. In contrast, transplasmalemmal conductances (endothelial PS(ecl)) were approximately 0.2 ml. g(-1). min(-1) for both Ado and Ino in rabbit hearts but approximately 2 ml. g(-1). min(-1) in guinea pig hearts, an order of magnitude different. Purine nucleoside metabolism also differs between guinea pig and rabbit cardiac endothelium. In guinea pig heart, 50% of the tracer Ado bolus was retained, 35% was washed out as Ado, and 15% was lost as effluent metabolites; 25% of Ino was retained, 50% washed out, and 25% was lost as metabolites. In rabbit heart, 45% of Ado was retained and 5% lost as metabolites, and 7% of Ino was retained and 3% lost as metabolites. We conclude that endothelial transport of Ado and Ino is a prime determinant of their metabolic fates: where transport rates are high, metabolic transformation is high.

Fig. 1

Schematic of 4-region, 3-barrier model for blood-tissue exchange used for analysis of indicator dilution curves. Parameter symbols are defined in Glossary. The analysis used a set of such modules in parallel to account for flow heterogeneity.

Fig. 2

Coronary sinus outflow dilution curves for [¹⁴C]inosine ([¹⁴C]Ino), [³H]arabinofuranosyl hypoxanthine ([³H]AraH), and ¹³¹I-albumin; model solutions are continuous lines. Extractions and recoveries are listed in Tables 1 and 2. Parameters for model fits are in Table 4; each estimate is given with its individual standard deviation. Top: data from an isolated, perfused rabbit heart. F_p = 3.89 ml · g⁻¹ · min⁻¹. Bottom: data from an isolated, perfused guinea pig heart. F_p = 3.57 ml · g⁻¹ · min⁻¹. Note that Ino curve is lower than AraH curve, indicating higher values of PS_ecl in guinea pig compared with rabbit.

Fig. 3

Adenosine (Ado) transport and transformation in isolated, perfused rabbit (left) and guinea pig (right) heart. Top panels: normalized outflow dilution curves for [¹⁴C]Ado and the 2 reference tracers, ¹³¹I-albumin (intravascular) and [³H]AraH (extracellular); model solutions are shown by lines. Bottom panels: semilog plot of Ado and its metabolites, Ino, hypoxanthine (Hx), xanthine (Xa), and uric acid (UA), in the outflow. The Ado curve is the same as in panel above. Linesjoin data points and are not model solutions. Note striking difference in shapes of effluent curves for metabolites in the 2 species. Curves labeled UA and Xa from the rabbit (bottom left) are not quantifiable, isotope count rates being 10⁻⁵ times those of albumin curve.