Inhibition of myosin light chain phosphorylation decreases rat mesenteric lymphatic pump function

Abstract

Molecular mechanisms involved in the regulation of lymphatic muscle contraction are not well understood. We hypothesized that phosphorylation of myosin light chain (MLC) might play an essential role in lymphatic muscle contractility. Molecular analyses showed the presence of MLC20 mRNA and protein in lymphatics from mesenteric, femoral, cervical and thoracic duct. Interestingly differential expression of MLC kinase and phosphatase was found in these lymphatics. To understand the functional roles of MLC phosphorylation, the pumping activities of the isolated and cannulated rat mesenteric lymphatic vessels were measured, determined and detected over a range of transmural pressures (1~5 cm H2O) in the presence or absence of a MLCK specific inhibitor ML-7 (1–100 μM). Results showed that ML-7 produced a significant inhibition of lymph pump activity. The lymphatic systolic diameter was increased. The diastolic diameter was not altered. The contraction frequency, stroke volume, ejection fraction, and pump flow were completely inhibited by ML-7 at 100 μM. Immunohistochemical and Western blot analyses demonstrated a reduced staining of the phospho-MLC in the vessels treated with ML-7. These results are the first evidence that the phosphorylation of MLC involved in the regulation of lymphatic contractile activities. (NIH HL075199 and HL80526).