A combination of extrinsic (passive) and intrinsic (active) forces move lymph against a hydrostatic pressure gradient in most regions of the body. The effectiveness of the lymph pump system impacts not only interstitial fluid balance but other aspects of overall homeostasis. This review focuses on the mechanisms that regulate the intrinsic, active contractions of collecting lymphatic vessels in relation to their ability to actively transport lymph. Lymph propulsion requires not only robust contractions of lymphatic muscle cells, but contraction waves that are synchronized over the length of a lymphangion as well as properly functioning intraluminal valves. Normal lymphatic pump function is determined by the intrinsic properties of lymphatic muscle and the regulation of pumping by lymphatic preload, afterload, spontaneous contraction rate, contractility and neural influences. Lymphatic contractile dysfunction, barrier dysfunction and valve defects are common themes among pathologies that directly involve the lymphatic system, such as inherited and acquired forms of lymphoedema, and pathologies that indirectly involve the lymphatic system, such as inflammation, obesity and metabolic syndrome, and inflammatory bowel disease.

We identified a regional dichotomy in murine lymphatic contractile function with regard to vessel location within the periphery or visceral cavity. All vessels isolated from peripheral regions [cervical, popliteal, inguinal, axillary, and internodal inguinal axillary (Ing-Ax)] developed robust contractions with maximal ejection fractions (EFs) of 50-80% in our ex vivo isobaric myograph experiments. Conversely, vessels isolated from the visceral cavity (mesenteric, thoracic duct, and iliac) demonstrated maximal EFs of ≤10%. Using pressure myography, sharp electrode membrane potential recordings, and Ca imaging, we assessed the role of L-type Ca channels in this contractile dichotomy. Ing-Ax membrane potential revealed a ~2-s action potential (AP) cycle (resting -35 mV, spike -5 mV, and plateau -11 mV) with a plateau phase that was significantly lengthened by the L-type Ca channel agonist Bay K8644 (BayK; 100 nM). APs recorded from mesenteric vessels, however, displayed a slower upstroke and an elongated time over threshold. BayK (100 nM) increased the mesenteric AP upstroke velocity and plateau duration but also significantly hyperpolarized the vessel. Contractions of vessels from both regions were preceded by Ca flashes, detected with a smooth muscle-specific endogenous Ca reporter, that typically were coordinated over the length of the vessel. Similar to the membrane potential recordings, Ca flashes in mesenteric vessels were weaker and had a slower rise time but were longer lasting than those in Ing-Ax vessels. BayK (100 nM) significantly increased the Ca transient amplitude and duration in both vessels and decreased time to peak Ca in mesenteric vessels. However, a higher concentration (1 μM) of BayK was required to produce even a modest increase in EF in visceral lymphatics, which remained at <20%. NEW & NOTEWORTHY Lymphatic collecting vessels isolated from murine peripheral tissues, but not from the visceral cavities, display robust contractile behavior similar to lymphatic vessels from other animal models and humans. These differences are partially explained by L-type Ca channel activity as revealed by the first measurements of murine lymphatic action potentials and contraction-associated Ca transients.

Rationale: Mutations in GJC2 and GJA1, encoding connexins (Cxs) 47 and 43 respectively, are linked to lymphedema, but the underlying mechanisms are unknown. Because efficient lymph transport relies on the coordinated contractions of lymphatic muscle cells (LMCs) and the electrical coupling of those through Cxs, Cx-related lymphedema is proposed to result from dyssynchronous contractions of lymphatic vessels. Objective: Determine which Cx isoforms in LMCs and/or lymphatic endothelial cells (LECs) are required for the entrainment of lymphatic contraction waves and efficient lymph transport. Methods and Results: We developed novel methods to quantify the spatiotemporal entrainment of lymphatic contraction waves and used optogenetic techniques to analyze calcium signaling within and between the LMC and LEC layers. Genetic deletion of the major LEC Cxs (Cx43, Cx47, or Cx37) revealed that none were necessary for the synchronization of the global calcium events that triggered propagating contraction waves. We identified Cx45 in human and mouse LMCs as the critical Cx mediating the conduction of pacemaking signals and entrained contractions. Smooth muscle-specific Cx45 deficiency resulted in 10–18-fold reduction in conduction speed, partial-to-severe loss of contractile coordination, and impaired lymph pump function ex vivo and in vivo. Cx45-deficiency resulted in profound inhibition of lymph transport in vivo, but only under an imposed gravitational load. Conclusions: Our results: 1) identify Cx45 as the Cx isoform mediating the entrainment of the contraction waves in LMCs; 2) show that major endothelial Cxs are dispensable for the entrainment of contractions; 3) reveal a lack of coupling between LECs and LMCs, in contrast to arterioles; 4) point to lymphatic valve defects, rather than contraction dyssynchrony, as the mechanism underlying GJC2- or GJA1-related lymphedema; and 5) show that a gravitational load exacerbates lymphatic contractile defects in the intact mouse hindlimb, which is likely critical for the development of lymphedema in the adult mouse.

Biocatalytic electrodes made of buckypaper were modified with PQQ‐dependent glucose dehydrogenase on the anode and with laccase on the cathode. The enzyme modified electrodes were assembled in a biofuel cell which was first characterized in human serum solution and then the electrodes were placed onto exposed rat cremaster tissue. Glucose and oxygen dissolved in blood were used as the fuel and oxidizer, respectively, for the implanted biofuel cell operation. The steady‐state open circuitry voltage of 140±30 mV and short circuitry current of 10±3 µA (current density ca. 5 µA cm−2 based on the geometrical electrode area of 2 cm2) were achieved in the in vivo operating biofuel cell. Future applications of implanted biofuel cells for powering of biomedical and sensor devices are discussed.

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