Perfusion technique determines alveolar fluid resorption rate in the isolated perfused rat lung

The isolated perfused lung (IPL) preparation is a well-established model for the study of alveolar epithelial sodium transport. We noted that preparations of normal fluid-filled rat lungs with recirculated perfusate reproducibly lost weight, whereas preparations in which the perfusate was discarded after a single pass through the lungs had a variable and lesser weight change. To confirm this, we performed IPL experiments by using male Sprague-Dawley specific-pathogen-free rats (175-225 g). In 10 IPLs, perfusate initially was discarded after passing through the lungs and then was recirculated continuously. During the single-pass period, the rate of weight change was +0.7 +/- 2.0 mg/min compared with -9.0 +/- 1.3 mg/min for the recirculating period. Adenosine 3',5'-cyclic monophosphate (cAMP) accumulated during recirculation. The weight loss induced by recirculation was reproduced by perfusion with 8-bromoadenosine 3',5'-cyclic monophosphate or terbutaline in single-pass fashion and blocked when the kinase inhibitor H-8 or phosphodiesterase was present in the recirculating perfusate. In summary, perfusate recirculation in the IPL stimulates fluid resorption at least partially via cAMP. This should be factored into the design and interpretation of IPL experiments.