CHARACTERIZATION OF CONTRACTILE PROPERTIES OF PORCINE MESENTERIC AND TRACHEOBRONCHIAL LYMPHATIC SMOOTH-MUSCLE

We performed morphometric and length-tension analyses comparing mesenteric and tracheobronchial lymph vessel segments to determine the potential of the latter tissue to regulate pulmonary and mediastinal lymph flow via alterations in smooth muscle tone. Fresh porcine lymph vessel rings were prepared for 1) in vitro assessment of length-tension relationships and 2) histologic preparation and measurement of smooth muscle cross-sectional area (SMA). Mesenteric and tracheobronchial optimal vessel ring lengths were 3.1+/-0.2 and 3.5+/-0.2 mm, and maximum active tensions were 1518+/-25 and 1703+/-162 mg. Smooth muscle formed indistinct layers in each tissue, and only 30% of the smooth muscle was oriented circumferentially Stress generated by the circular smooth muscle was similar to that generated by other types of vascular smooth muscle. In 75% of mesenteric vessel rings spontaneous contractions were observed that had a mean contraction frequency of 1.7+/-0.2 min(-1) and a mean contraction amplitude of 349+/-35 mg, while only 40% of tracheobronchial vessels exhibited spontaneous contractions (p<0.001) that had a mean frequency of 0.6+/-0.2 min(-1) (p=0.0021) and a mean contraction amplitude of 118+/-10 mg (p<0.0001). We conclude that tracheobronchial lymphatic vascular smooth muscle is capable of developing stress similar to that generated by mesenteric lymph vessels, and that spontaneous rhythmic contractile activity is qualitatively and quantitatively different in tracheobronchial than in mesenteric porcine lymph vessels. The data suggest that tracheobronchial lymph vessels are capable of regulating pulmonary and mediastinal lymph flow through intrinsic mechanisms. Such regulation may occur by alterations in vascular resistance rather than via spontaneous pumping activity.