Contraction-initiated NO-dependent lymphatic relaxation: a self-regulatory mechanism in rat thoracic duct

Abstract

The objectives of this study were to evaluate the physiological importance of the flow and shear generated by phasic contractions of lymphatic vessels and the mechanisms responsible for the influences of such shear on lymphatic pumping. Lymphatic segments of the rat thoracic duct were isolated, cannulated and pressurized. The diastolic diameters were measured in phasically non-active segments. The diastolic and systolic diameters, half-relaxation time (HRT), contraction frequency, ejection fraction and fractional pump flow were determined in phasically active segments. Since imposed flow was excluded, flow and shear occurred only as a result of the intrinsic contractions in phasically active segments whereas in phasically non-active segments contraction-generated flow and shear were absent. The influences of incrementally increased transmural pressure (from 1 to 5 cmH(2)O) were examined in control conditions and after NO synthase blockade (l-NAME 10(-4) m) or cyclooxygenase blockade (indomethacin 10(-5) m). The spontaneous phasic contractions produced a flow-dependent diastolic relaxation. This reduction of the lymphatic tone is a regulatory mechanism that maintains pumping in thoracic duct in an energy-saving/efficient mode: it improves diastolic filling (enhanced lusitropy - lowering HRT), makes lymphatic contractions stronger (enhanced inotropy - higher contraction amplitude) and propels more fluid forward during each contraction (elevated ejection fraction) while decreasing contraction frequency (reduced chronotropy). The findings also demonstrated that the NO pathway, not the cyclooxygenase pathway is responsible for this reduction of lymphatic tone and is the prevailing pathway responsible for the self-regulatory adjustment of thoracic duct pumping to changes in lymph flow pattern.

Figure 1. Typical diameter tracings of phasic contractions of the thoracic duct segment at a transmural pressure of 3 cmH₂O before (A) and after (B) administration of l-NAME (10⁻⁴m)

Yellow marker points were set at the positions that correspond to the end-systolic diameter. The yellow separator line and red marker points were set at the positions which correspond to the half-relaxed diameter. The time interval between the yellow marker point and the next red marker point corresponds to the half-relaxation time (HRT). The X-axis represents time in seconds; the Y-axis represents outer lymphatic diameter in micrometres.

Figure 2. Pressure-induced changes in the lymphatic tone index of rat thoracic duct after administration of l-NAME (10⁻⁴m; A) and indomethacin (10⁻⁵m; B)

Experimental conditions: A: Control C – control for contracting segments; l-NAME C –l-NAME administration in contracting segments; Control NC – control for non-contracting segments; l-NAME NC –l-NAME administration in non-contracting segments. B: Control C – control for contracting segments; INDO C – indomethacin administration in contracting segments; Control NC – control for non-contracting segments; INDO NC – indomethacin administration in non-contracting segments. *Significant differences (P ≤ 0.05) between lymphatic tone index in control phasically active segments and lymphatic tone indices for other experimental conditions at each level of transmural pressure; presented separately for A and B.

Figure 3. Pressure-induced changes in normalized systolic diameter (A) and half-relaxation time (B) in phasically active segments of rat thoracic duct after administration of l-NAME (10⁻⁴m)

*Significant differences (P ≤ 0.05) between half-relaxation times before and after l-NAME administration for each level of transmural pressure.

Figure 4. Pressure-induced changes in the active lymph pump parameters in phasically active segments of rat thoracic duct after administration of l-NAME (10⁻⁴m)

*Significant differences (P ≤ 0.05) between pumping parameters before and after l-NAME administration for each level of transmural pressure.

Figure 5. Pressure-induced changes in the active lymph pump parameters in phasically active segments of rat thoracic duct before and after administration of indomethacin (10⁻⁵m)