Removal of adenosine from the rabbit pulmonary circulation, in vivo and in vitro

The contributions of pulmonary endothelial and blood cells to the removal of adenosine during a single passage through the rabbit pulmonary circulation were investigated. In the isolated, blood-free perfused rabbit lung in situ, single pass pulmonary removal of [3H]-adenosine injected into the pulmonary artery was accomplished by a low-affinity, saturable process yielding apparent Michaelis-Menten constants of Km = 498 +/- 36 microM and Vmax = 39 +/- 4 mumol/min per lung. Similar experiments in the intact, anesthetized animal, in vivo revealed a rapid, high-affinity removal of [3H]adenosine from plasma with apparent Michaelis-Menten constants of Km = 3.3 +/- 0.5 microM and Vmax = 6.5 +/- 2.4 mumol/min per lung. However, complete recovery of the injected adenosine was achieved upon lysing of cells from blood collected during a single transpulmonary passage of the substrate, indicating that blood cells were responsible for adenosine removal in vivo. The rate of disappearance of adenosine from plasma, observed by incubating the substrate with whole rabbit blood in vitro, was comparable to that found in vivo. We conclude that although, in the absence of circulating blood, rabbit lung is able to extract adenosine in vitro, this mechanism is of little significance in vivo where blood cells appear to be primarily responsible for such removal.