What Is the Mechanism Underlying Pressure‐induced Chronotropy in Collecting Lymphatic Vessels?

Abstract

The rhythmic contractions of lymphatic smooth muscle (LSM) cells enable lymph propulsion against adverse pressure gradients. These contractions are initiated by LSM action potentials (APs), generated by an ionic pacemaker, whose rate is sensitive to the transmural pressure difference. We recently showed that SM‐specific deletion of the Ca2+‐activated Cl− channel, anoctamin1 (Ano1), reduced the basal AP firing rate of mouse inguinal‐axillary lymphatic (IALs) vessels to ~25% of normal and abrogated pressure‐induced chronotopy. However, IALs are efferent vessels that show only a modest (2‐fold) increase in contraction frequency over the presumed physiological pressure range 0.5 – 8 cmH2O. In contrast, popliteal afferent lymphatics (PLs) exhibit an 8‐fold increase in chronotropy from 0.5 – 5 cmH2O and SM‐specific deletion of Ano1 attenuates but does not abolish their pressure‐induced chronotropy. Here we asked: 1) what other ionic mechanisms participate in pressure‐induced chronotropy of PLs? 2) what accounts for the residual, pressure‐independent pacemaking in IALs and PLs after Ano1 deletion?To address these questions we measured the frequency‐pressure relationship (F‐P) of cannulated, pressurized PLs from control mice and mice deficient in key mechanosensitive and second‐messenger‐gated ion channels implicated in arterial pressure sensing. SM‐specific deletion of the mechanosensitive cation channel Piezo1 had no significant effect on F‐P. Global deletion of the osmosensitive cation channel TRPV4, or the DAG‐gated cation channels TRPC3 or TRPC6, or both, had no significant effect on F‐P. TRPM4−/− mice were not available, but the selective TRPM4 inhibitors, CBA and NBA, had minimal effects on F‐P. SM‐specific deletion of the cation channel TRPP1 (Pkd2) had no significant effect on F‐P. Deletion of T‐type Ca2+ channels Cav3.1 and/or Cav3.2 (previously implicated in lymphatic pacemaking), had no significant effect on F‐P. However, SM‐specific deletion of IP3R1, the only IP3R isoform expressed in mouse LSM, produced a phenotype similar to Ano1 deletion: a reduction in basal frequency and blunting of the F‐P relationship, suggesting a pressure‐induced IP3 generation downstream from mechanosensitive G‐protein(s). We then tested the prominent arterial SM mechanosensitive GPCR, AT1R. The inverse agonist losartan had no effect on F‐P nor did global knock out of AT1bR, the only isoform expressed in mouse LSM. In conclusion, our results point to a GPCR mechanosensing mechanism, independent of AT1R, that generates IP3 in a pressure‐sensitive manner to activate Ano1 and regulate the cycling rate of the ionic pacemaker in lymphatic smooth muscle.Support or Funding InformationNIH‐HL‐122578