Axial stretch regulates rat tail collecting lymphatic vessel contractions

Abstract

Lymphatic contractions play a fundamental role in maintaining tissue and organ homeostasis. The lymphatic system relies on orchestrated contraction of collecting lymphatic vessels, via lymphatic muscle cells and one-way valves, to transport lymph from the interstitial space back to the great veins, against an adverse pressure gradient. Circumferential stretch is known to regulate contractile function in collecting lymphatic vessels; however, less is known about the role of axial stretch in regulating contraction. It is likely that collecting lymphatic vessels are under axial strain in vivo and that the opening and closing of lymphatic valves leads to significant changes in axial strain throughout the pumping cycle. The purpose of this paper is to quantify the responsiveness of lympatic pumping to altered axial stretch. In situ measurements suggest that rat tail collecting lymphatic vessels are under an axial stretch of ~1.24 under normal physiological loads. Ex vivo experiments on isolated rat tail collecting lymphatics showed that the contractile metrics such as contractile amplitude, frequency, ejection fraction, and fractional pump flow are sensitive to axial stretch. Multiphoton microscopy showed that the predominant orientation of collagen fibers is in the axial direction, while lymphatic muscle cell nuclei and actin fibers are oriented in both circumferential and longitudinal directions, suggesting an axial component to contraction. Taken together, these results demonstrate the significance of axial stretch in lymphatic contractile function, suggest that axial stretch may play an important role in regulating lymph transport, and demonstrate that changes in axial strains could be an important factor in disease progression.

Characterization of rat tail lymphatic contractility and biomechanics: incorporating nitric oxide-mediated vasoregulation

“…The estimated parameters for the axial and diagonal collagen fibres implies the axial fibre family plays a significant role in the load-bearing function. The significance of axial stretch in passive behaviour of lymphatic vessels is in agreement with our recent quantification of collagen fibre orientation in the rat tail lymphatic, where the vessels have a predominant organization of collagen fibres in the axial direction and experience axial prestretch of approximately 20% in situ [9]. Non-parametric bootstrapping, used to assess the uniqueness of the fitted parameter sets and to obtain the confidence intervals of estimated parameters, showed that the three-fibre family model yielded a unique set of material parameters.…”

“…The three-fibre family constitutive model yielded a good fit (R 2 = 0.90 ± 0.03) for the rat tail lymphatic vessels (figure 1 and table 1, using the geometric parameters in table 2). Supporting the previous experimental observation, our modelling results indicate that a three-fibre family model was sufficient to provide a good fit of biaxial data [9]. Based on the estimated parameters, the axial fibre family and two symmetric diagonal fibre families significantly contribute to the load-bearing function of lymphatic vessels; however, the isotropic term (namely, c(I C À 3)) had a much smaller contribution; i.e.…”

“…For inflation and extension of a materially symmetric thin-walled tube, [F] ¼ diag(l r ,l u ,l Z ), where the three stretch ratios are l r ¼ h=H, l u ¼ r=R, and l z ¼ '=L and (h, r, ℓ) and (H, R, L) are thickness, radius, and length for loaded, and unloaded vessel, respectively. We observed that collagen fibres in the rat tail lymphatic vessels are distributed preferentially in the axial direction [9]; thus, we chose a three-family stored energy function…”

“…the frequency of contractions changes in response to pressure and the active tension changes with stretch for lymphatic vessels [6][7][8]. We have used the rat tail to show that collecting lymphatic vessels experience axial prestretch in vivo and to show that axial stretch is a regulator of lymphatic contractility in an ex vivo setting [9]. Further, small molecules such as nitric oxide (NO), histamine and endothelin-1 (ET-1) can regulate lymphatic contractility [10][11][12][13][14][15].…”

“…Passive cylindrical biaxial mechanical testing was performed to quantify the passive biomechanical response. Motivated by our previous multiphoton observation of collagen fibres in rat tail lymphatic vessels [9], a three-fibre family constitutive model (longitudinally and diagonally organized collagen fibres) was used to estimate the passive mechanical response. A modified Rachev-Hayashi model was used to characterize the active contractile function of rat tail lymphatic vessels and their respective changes due to the administration of SNP doses.…”