Endothelial adenosine transporter characterization in perfused guinea pig hearts

Adenosine (Ado), a smooth muscle vasodilator and modulator of cardiac funct ion, is taken up by many cell types via a saturable transporter, blockable by dipyridamole. To quantitate the influences of endothelial cells in gover ning the blood-tissue exchange of Ado and its concentration in the intersti tial fluid, one must define the permeability-surface area products (PS) for Ado via passive transport through interendothelial gaps [PSg (Ado)] and ac ross the endothelial cell luminal membrane (PSecl) in their normal in vivo setting. With the use of the multiple-indicator dilution (MID) technique in Krebs-Ringer perfused, isolated guinea pig hearts (preserving endothelial myocyte geometry) and by separating Ado metabolites by HPLC, we found perme ability-surface area products for an extracellular solute, sucrose, via pas sive transport through interendothelial gaps [PSg (Suc)] to be 1.9 +/- 0.6 ml . g(-1) . min(-1) (n = 16 MID curves in 4 hearts) and took PSg (Ado) to be 1.2 times PSg (Suc). MID curves were obtained with background nontracer Ado concentrations up to 800 mu m, partially saturating the transporter and reducing its effective PSecl for Ado. The estimated maximum value for PSec l in the absence of background adenosine was 1.1 +/- 0.1 ml . g(-1) . min(- 1) [maximum rate of transporter conformational change to move the substrate from one side of the membrane to the other (maximal velocity; V-max) times surface area of 125 +/- 11 nmol . g(-1) . min(-1)], and the Michaelis-Ment en constant (K-m) was 114 +/- 12 mu M, where +/- indicates 95% confidence l imits. Physiologically, only high Ado release with hypoxia or ischemia will partially saturate the transporter.