Collecting lymphatic vessels share functional and biochemical characteristics with cardiac muscle; thus, we hypothesized that the lymphatic vessel pump would exhibit behavior analogous to homeometric regulation of the cardiac pump in its adaptation to elevated afterload, i.e., an increase in contractility. Single lymphangions containing two valves were isolated from the rat mesenteric microcirculation, cannulated, and pressurized for in vitro study. Pressures at either end of the lymphangion [input pressure (P(in)), preload; output pressure (P(out)), afterload] were set by a servo controller. Intralymphangion pressure (P(L)) was measured using a servo-null micropipette while internal diameter and valve positions were monitored using video methods. The responses to step- and ramp-wise increases in P(out) (at low, constant P(in)) were determined. P(L )and diameter data recorded during single contraction cycles were used to generate pressure-volume (P-V) relationships for the subsequent analysis of lymphangion pump behavior. Ramp-wise P(out) elevation led to progressive vessel constriction, a rise in end-systolic diameter, and an increase in contraction frequency. Step-wise P(out) elevation produced initial vessel distention followed by time-dependent declines in end-systolic and end-diastolic diameters. Significantly, a 30% leftward shift in the end-systolic P-V relationship accompanied an 84% increase in dP/dt after a step increase in P(out), consistent with an increase in contractility. Calculations of stroke work from the P-V loop area revealed that robust pumps produced net positive work to expel fluid throughout the entire afterload range, whereas weaker pumps exhibited progressively more negative work as gradual afterload elevation led to pump failure. We conclude that lymphatic muscle adapts to output pressure elevation with an intrinsic increase in contractility and that this compensatory mechanism facilitates the maintenance of lymph pump output in the face of edemagenic and/or gravitational loads.

Scallan JP, Wolpers JH, Muthuchamy M, Zawieja DC, Gashev AA, Davis MJ. Independent and interactive effects of preload and afterload on the pump function of the isolated lymphangion. Am J Physiol Heart Circ Physiol 303: H809 -H824, 2012. First published August 3, 2012; doi:10.1152/ajpheart.01098.2011We tested the responses of single, isolated lymphangions to selective changes in preload and the effects of changing preload on the response to an imposed afterload. The methods used were similar to those described in our companion paper.Step-wise increases in input pressure (Pin; preload) over a pressure range between 0.5 and 3 cmH2O, at constant output pressure (Pout), led to increases in end-diastolic diameter, decreases in endsystolic diameter, and increases in stroke volume. From a baseline of 1 cmH 2O, Pin elevation by 2-7 cmH2O consistently produced an immediate fall in stroke volume that subsequently recovered over a time course of 2-3 min. Surprisingly, this adaptation was associated with an increase in the slope of the end-systolic pressure-volume relationship, indicative of an increase in contractility. Lymphangions subjected to Pout levels exceeding their initial ejection limit would often accommodate by increasing diastolic filling to strengthen contraction sufficiently to match Pout. The lymphangion adaptation to various pressure combinations (Pin ramps with low or high levels of Pout, Pout ramps at low or intermediate levels of Pin, and combined Pin ϩ Pout ramps) were analyzed using pressure-volume data to calculate stroke work. Under relatively low imposed loads, stroke work was maximal at low preloads (Pin ϳ2 cmH2O), whereas at more elevated afterloads, the optimal preload for maximal work displayed a broad plateau over a Pin range of 5-11 cmH2O. These results provide new insights into the normal operation of the lymphatic pump, its comparison with the cardiac pump, and its potential capacity to adapt to increased loads during edemagenic and/or gravitational stress. pressure-volume loop; isolated vessel; edema; heterometric regulation; homeometric regulation; stroke work; contractility IN OUR COMPANION PAPER (10), we investigated the effects of selective afterload elevation on the pump function of single lymphangions isolated from the rat mesentery. Elevating output pressure (P out ), at constant preload [input pressure (P in )], led to an intrinsic increase in lymphatic muscle contractility (i.e., positive inotropy), as characterized by a leftward shift in the end systolic pressure-volume relationship (ESPVR), increased peak dP/dt, and the development of higher peak systolic pressure. We concluded that the enhancement in contractility enables the vessel to eject against the adverse pressure gradient that normally exists in most parts of the lymphatic system and that becomes substantially elevated in lymphatic vessels of dependent extremities, including those of humans (31).As in the heart, preload is a significant determinant of lymphatic pump function. Previous studies using isolated chains of seri...

Key points• Arterioles undergo a myogenic constriction, defined as a decrease in diameter in response to an increase in pressure, which serves to protect the downstream capillaries from changes in pressure and flow.• A lymphatic constriction was recently identified but it remained unknown whether it reflected a true myogenic constriction.• By selectively raising downstream pressure in isolated lymphatic vessels containing a single valve, we discovered that the upstream segment constricted, even when protected from increases in pressure by the closed valve. The constriction consisted of a myogenic component and a frequency component, which were blocked pharmacologically.• The lymphatic constriction facilitated proper closure of the intraluminal valves in the face of a hydrostatic gradient, preventing lymph backflow.• This work adds to our understanding of the lymphatic myogenic constriction by showing that it maintains a functioning valve system in lymphatic vessels and that it is mechanistically similar to the arteriolar myogenic constriction.Abstract Collecting lymphatic vessels generate pressure to transport lymph downstream to the subclavian vein against a significant pressure head. To investigate their response to elevated downstream pressure, collecting lymphatic vessels containing one valve (incomplete lymphangion) or two valves (complete lymphangion) were isolated from the rat mesentery and tied to glass cannulae capable of independent pressure control. Downstream pressure was selectively raised to various levels, either stepwise or ramp-wise, while keeping upstream pressure constant. Diameter and valve positions were tracked under video microscopy, while intralymphangion pressure was measured concurrently with a servo-null micropipette. Surprisingly, a potent lymphatic constriction occurred in response to the downstream pressure gradient due to (1) a pressure-dependent myogenic constriction and (2) a frequency-dependent decrease in diastolic diameter. The myogenic index of the lymphatic constriction (−3.3 ± 0.6, in mmHg) was greater than that of arterioles or collecting lymphatic vessels exposed to uniform increases in pressure (i.e. upstream and downstream pressures raised together). Additionally, the constriction was transmitted to the upstream lymphatic vessel segment even though it was protected from changes in pressure by a closed intraluminal valve; the conducted constriction was blocked by loading only the pressurized half of the vessel with either ML-7 (0.5 mM) to block contraction, or cromakalim (3 μM) to hyperpolarize the downstream muscle layer. Finally, we provide evidence that the lymphatic constriction is important to maintain normal intraluminal valve closure during each Abbreviations AMP, contraction amplitude; APSS, albumin-supplemented physiological saline solution; BSA, bovine serum albumin; EDD, end diastolic diameter; ESD, end systolic diameter; FREQ, contraction frequency; MaxD, maximum passive diameter; P f , final pressure after a step; P i , initial pressure preceding a step; P i...