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

Abstract

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.

Lymphatics and lymph in acute lung injury

“…We know from animal models based on the micropuncture technique and intravital fluorescent labeling that lymphatics have unidirectional valves promoting centripetal flow among the diaphragmatic fibers [32] and interesting data suggest that the diaphragmatic lymphatic network can present a functional adaptation to pathological conditions (ascites and/or hydrothorax) [33 ].…”

Lymphatic Vessel Network Structure and Physiology

“…The networks draining the pleural and peritoneal sides are connected, yet under physiological conditions act in a functionally distinct manner, i.e. the lymph formed on pleural side does not typically appear within the network originating on the peritoneal side of the muscle, and vice versa (721). On the pleural side (Fig.…”

“…Under normal conditions, the initial lymphatics within the muscular region of the diaphragm drain both the pleural space and peritoneum. However, during pleural or peritoneal effusions, lymphatics in the tendinous region of the diaphragm are also recruited (721). …”

Lymph flow pattern in pleural diaphragmatic lymphatics during intrinsic and extrinsic isotonic contraction

“…Given the strategic role played by pleural lymphatics in setting the correct pleural fluid volume and subatmospheric pressure required to maintain the normal lung chest wall coupling (29), much effort has been spent in the study of the inner regulatory mechanisms of lymph drainage and propulsion in this particular lymphatic network. Diaphragmatic lymph formation is sustained by net hydraulic pressure gradients (1, 37) driving the entrance of fluid from the diaphragmatic interstitial space and from the pleural cavity into, respectively, the initial lymphatics and mesothelial stomata (28, 31) in continuity with submesothelial lacunae (13,20). Subsequent lymph propulsion relies on the presence of hydraulic pressure gradients developing between two consecutive tracts of the lymphatic vessel, separated by unidirectional intraluminal valves.…”