Effects of increasing perfusion flow rate on perfused capillary surface area of the rat lung

Increasing blood flow through the rabbit, dog or human lung results in a proportional increase in dynamically perfused capillary surface area (DPCSA). We have now tested the hypothesis that the rat lung responds similarly to increased flow. Male Wistar rats (350-400g, N=6) were anesthetized with sodium pentobarbital, the chest was opened and catheters were placed into the main pulmonary artery and left atrium. The lungs were thus perfused with 5%dextran (70KD) in Kreb's and ventilated with 95%02/5%CO2 at peak pressure of 11cm water. DPCSA was estimated from the single pass hydrolysis of the synthetic angiotensin converting enzyme (ACE) substrate BPAP, as previously reported and reflected in the parameter Amax/Km (=E*kcat/Km, where E is perfused capillary ACE mass). As perfusion flow increased from 11.5+/-0.5 to 44+/-2 ml/min, pulmonary vascular resistance decreased proportionally. Conversely, Amax/Km increased from 18+/-2 to 68+/-8 ml/min reflecting capillary recruitment in response to increasing flow. Percent BPAP hydrolysis remained statistically unchanged, around 80%, suggesting unaltered capillary transit times of the substrate. These findings indicate that over the range of flows studied, the rat may respond to increased flow by capillary recruitment.