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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Lymphatic vessels transition to state of summation above a critical contraction frequency

¹Department of Biomedical Engineering, Dwight Look College of Engineering, ²Michael E. DeBakey Institute, and ³Department of Physiology and Pharmacology, College of Veterinary Medicine, Texas A & M University, College Station, Texas

Submitted 5 July 2006 ; accepted in final form 8 March 2007

Although behavior of lymphatic vessels is analogous to that of ventricles, which completely relax between contractions, and blood vessels, which maintain a tonic constriction, the mixture of contractile properties can yield behavior unique to lymphatic vessels. In particular, because of their limited refractory period and slow rate of relaxation, lymphatic vessels lack the contractile properties that minimize summation in ventricles. We, therefore, hypothesized that lymphatic vessels transition to a state of summation when lymphatic vessel contraction frequency exceeds a critical value. We used an isovolumic, controlled-flow preparation to compare the time required for full relaxation with the time available to relax during diastole. We measured transmural pressure and diameter on segments of spontaneously contracting bovine mesenteric lymphatic vessels during 10 isovolumic volume steps. We found that beat-to-beat period (frequency^–1) decreased with increases in diameter and that total contraction time was constant or slightly increased with diameter. We further found that the convergence of beat-to-beat period and contraction cycle duration predicted a critical transition value, beyond which the vessel does not have time to fully relax. This incomplete relaxation and resulting mechanical summation significantly increase active tension in diastole. Because this transition occurs within a physiological range, contraction summation may represent a fundamental feature of lymphatic vessel function.

lymph flow; lymphatic wall tension; edema; mechanics; active lymph pump

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