Hyperpolarization-activated cyclic nucleotide-gated channels in peripheral diaphragmatic lymphatics

Abstract

Diaphragmatic lymphatic function is mainly sustained by pressure changes in the tissue and serosal cavities during cardiorespiratory cycles. The most peripheral diaphragmatic lymphatics are equipped with muscle cells (LMCs), which exhibit spontaneous contraction, whose molecular machinery is still undetermined. Hypothesizing that spontaneous contraction might involve hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in lymphatic LMCs, diaphragmatic specimens, including spontaneously contracting lymphatics, were excised from 33 anesthetized rats, moved to a perfusion chamber containing HEPES-Tyrode's solution, and treated with HCN channels inhibitors cesium chloride (CsCl), ivabradine, and ZD-7288. Compared with control, exposure to 10 mM CsCl reduced (-65%, n = 13, P < 0.01) the contraction frequency (F) and increased end-diastolic diameter (D, +7.3%, P < 0.01) without changes in end-systolic diameter (D). Ivabradine (300 μM) abolished contraction and increased D (-14%, n = 10, P < 0.01) or caused an incomplete inhibition of F (n = 3, P < 0.01), leaving D and D unaltered. ZD-7288 (200 μM) completely (n = 12, P < 0.01) abolished F, while D decreased to 90.9 ± 2.7% of control. HCN gene expression and immunostaining confirmed the presence of HCN1-4 channel isoforms, likely arranged in different configurations, in LMCs. Hence, all together, data suggest that HCN channels might play an important role in affecting contraction frequency of LMCs.

Lymphatic Vessel Network Structure and Physiology

“…Second, hyperpolarization-activated inward currents that are similar to the I f in the sinoatrial node of the heart were shown to influence CF of sheep mesenteric lymphatic vessels (667). In agreement, HCN channel inhibitors reduced CF of rat diaphragmatic lymphatics, and all four members of the HCN channel family were detected at the mRNA level and by immunofluorescence in these vessels (758). Third, T-type voltage-gated Ca 2+ channels, which have an established role in pacemaking in the heart (404), have been found in rat mesenteric lymphatic smooth muscle.…”

“…In the central part of the diaphragm, the pleural lymphatics typically lack smooth muscle and are thus non-contractile, but are tightly linked to the surrounding tissue and easily deformed by contraction of the skeletal muscle fibers during breathing (723). In contrast, the pleural lymphatics located in the more peripheral areas of the diaphragm, near the costal medium, do possess a smooth muscle layer and display intrinsic contractile activity (725, 758). Interestingly, the strength of contraction appears to be weak and unevenly distributed in these peripheral pleural lymphatic networks, possibly to optimize lymph flow through the network (724, 725).…”

Close down the lungs and keep them resting to minimize ventilator-induced lung injury

“…Such active lymph propulsion is attained by means of a complex interplay among sites and is able to organize lymph flow in an ordered way. More recently, it has been shown that spontaneous contraction of lymphatics located in the extreme diaphragmatic periphery might involve hyperpolarization-activated cyclic nucleotide-gated channels in lymphatics equipped with muscle cells [ 37 ]. Hence, the three-dimensional arrangement of the diaphragmatic lymphatic network seems to be finalized to efficiently exploit the stresses exerted by muscle fibers during the contracting inspiratory phase to promote lymph formation in superficial submesothelial lymphatics and its further propulsion in deeper intramuscular vessels [ 38 ].…”

Fluid Osmolarity Acutely and Differentially Modulates Lymphatic Vessels Intrinsic Contractions and Lymph Flow

“…These processes, with the aid of unidirectional parietal and intraluminal valves, give rise to hydraulic pressure gradients between the interstitial space and the lymphatic lumen, and between adjacent lymphangions in collecting lymphatics (Moriondo et al, 2015 ) promoting, respectively, fluid entry into and progression along lymphatic vessels. Two different, not mutually exclusive, mechanisms have been proposed for the rhythmic generation of contraction: one based on spontaneous transient depolarizations (STDs) due to calcium-dependent chloride currents (in LMCs the equilibrium potential of chloride is more positive than membrane resting potential due to its accumulation inside the cells), and the other based on I f -like currents (Van Helden, 1993 ; McCloskey et al, 1999 ; Van Helden and Zhao, 2000 ; Negrini et al, 2016 ).…”