Pulmonary lymphatic filling is increased in spontaneously hypertensive rats

Extravascular lung liquid must rely on tissue-space pressure gradients to d rive it into the lymphatics because the fluid is outside the lymphatic cont ractile pumping and valve control. Focal tissue pressure changes could resu lt from muscular contraction in the blood vessel walls. Perivascular lympha tics usually lie within the adventitia of pulmonary blood vessels, and are generally more noticeable in veins than arteries. Spontaneously hypertensiv e rats have exaggerated focal pulmonary venous muscle (venous sphincters). These muscular tufts are often near initial lymphatics; if their contractio n was important for lymph transport, spontaneously hypertensive rats could have more lymphatic filling in the areas of the pulmonary venous sphincters than normotensive rats. Because the focal muscularity is found in pulmonar y veins more than arteries, veins may have more focal lymphatic filling tha n arteries. To test these hypotheses, lung histology and vascular and lymph atic casts of spontaneously hypertensive and normotensive rats were examine d. Contracted venous sphincters were found on 108 of 127 veins with lymphat ics in the spontaneously hypertensive rats and 5 of 41 in the normotensive rats P<0.01). The spontaneously hypertensive rats had deeper venous contrac tions and more lymphatic filling around both arteries and veins (P<0.01). I n the hypertensive rats, the venous was greater than the arterial lymphatic filling (P<0.01). On the pleural surface, hypertensive rats also had great er lymphatic filling than controls (P<0.01). This anatomic evidence suggest s that pulmonary venous sphinters are associated with focal lymphatic filli ng, and perivascular muscle action might be a component of the pulmonary ly mphatic system.