Cardiac Myocyte and Coronary Endothelial Cell Adenosine Transport

Summary

Zero trans rates of adenosine flux into myocytes and endothelial cells isolated from hearts of adult rats were measured and compared. Results from these studies show that cardiac myocyte adenosine influx occurs by a saturable transport system with K_m = 6.2 μM and V_max = 0.96 pmol/mg/s and a nonsaturable process with a rate constant of 1.8 x 10⁻³/s (24°C). The saturable transporter is inhibited by nitrobenzylthioinosine and verapamil (K_i = 17 μM). Adenosine transported into the cells is rapidly phosphorylated and at extracellular adenosine concentrations below 32 μM there is no time-dependent increase in intracellular adenosine concentrations. Above 32 μM extracellular adenosine, intracellular adenosine progressively increases with time. Intracellular adenosine found at high extracellular adenosine is not bound to cell protein. Noncultured endothelial cells exhibit transport kinetics distinct from those of myocytes. At extracellular adenosine concentrations 50 μM or less, adenosine transport is solely dependent on a saturable transport system. Only at adenosine concentrations 100 μM or greater is there evidence for a nonsaturable component. Saturable transport K_m is about 0.4 μM and has a V_max of 74 pmol/mg protein⁻¹/s⁻¹. Thus, the maximum adenosine transport velocity of endothelial cells exceeds that of myocytes (per mg cell protein) by at least 20-fold, and the adenosine concentration at which inward transport is 50% active is about 15-fold less for endothelial cells than for myocytes. These properties of endothelial cell adenosine transport probably limit the amount of vascular adenosine which reaches smooth muscle cells and serve to reduce myocyte-derived adenosine diffusion into the vascular space.