Regional recruitment of rat diaphragmatic lymphatics in response to increased pleural or peritoneal fluid load

The specific role of the diaphragmatic tendinous and muscular tissues in sustaining lymph formation and propulsion in the diaphragm was studied in 24 anaesthetized spontaneously breathing supine rats. Three experimental protocols were used: (a) control; (b) peritoneal ascitis, induced through an intraperitoneal injection of 100 ml kg(-1) of iso-oncotic saline; and (c) pleural effusion, induced through an intrapleural injection of 6.6 ml kg(-1) saline solution. A group of animals (n = 12) was instrumented to measure the hydraulic transdiaphragmatic pressure gradient between the pleural and peritoneal cavities in the three protocols. In the other group (n = 12), the injected iso-oncotic saline was enriched with 2% fluorescent dextrans (molecular mass = 70 kDa); at 30 min from the injections these animals were suppressed and their diaphragm excised and processed for confocal microscopy analysis. In control conditions, in spite of a favourable peritoneal-to-pleural pressure gradient, the majority of the tracer absorbed into the diaphragmatic lymphatic system converges towards the deeper collecting lymphatic ducts. This suggests that diaphragmatic lymph formation mostly depends upon pressure gradients developing between the serosal cavities and the lymphatic vessel lumen. In addition, the tracer distributes to lymph vessels located in the muscular diaphragmatic tissue, suggesting that active muscle contraction, rather than passive tendon stretch, more efficiently enhances local diaphragmatic lymph flow. Vice versa, a prevailing recruitment of the lymphatics of the tendinous diaphragmatic regions was observed in peritoneal ascitis and pleural effusion, suggesting a functional adaptation of the diaphragmatic network to increased draining requirements.