Plasma and platelets potentiate a hypoxic vascular response of the isolated lung

To determine whether plasma and/or platelets play a role in the hypoxic response of pulmonary circulation, we perfused isolated rat lungs with different concentrations of plasma and varying numbers of platelets. As the lungs were perfused with a constant flow (8-13 ml/min), a change in the mean pulmonary artery perfusion pressure (PApP) during acute alveolar hypoxia was a measure of increased tone of the pulmonary circulation. It was found that the presence of plasma in the perfusate (both 20% and 100%) significantly (P < 0.05) potentiated the PApP response during repeated acute alveolar hypoxia, compared with the response to artificial perfusate without plasma. An isolated rat lung perfused with 6.5% dextran or 3% albumin (both in a physiological salt solution) showed only minimal pressure response during acute alveolar hypoxia. In the second part of the experiment, isolated lungs were perfused with a fresh 100% plasma containing different numbers of platelets. A dose-response-like relationship was observed between the number of platelets in the perfusate and the response of the pulmonary vasculature to acute hypoxia. The highest hypoxic response of the pulmonary vessels, as indicated by the increase in PApP, was shown when the perfusate contained a nearly normal in vivo number of platelets. It can be concluded that both plasma and platelets are important factors that can significantly alter the hypoxic response of the pulmonary vasculature in isolated rat lungs.