ENDOTHELIUM-DEPENDENT MODULATION OF PACEMAKING IN LYMPHATIC VESSELS OF THE GUINEA-PIG MESENTERY

1. Endothelial control of the rate of constrictions and the underlying pacemaker potentials has been studied in vitro in guinea-pig mesenter ic lymphatic vessels. 2. ACh stimulated 60% of intraluminally perfused vessels to slow or abolish lymphatic constrictions. This action was i nhibited by atropine and was likely to be due to the release of endoth elium-derived nitric oxide (EDNO) as the effect was absent after endot helial lysis, mimicked by sodium nitroprusside (SNP), blocked by N-ome ga-nitro L-arginine (NOLA) and partially inhibited by Metylene Blue (M B). 3. The remaining 40% of perfused vessels did not mechanically resp ond to ACh or SNP. In four of seven such vessels this appeared to be d ue to excessive perfusion-associated release of EDNO, as incubation wi th NOLA restored the response to SNP. 4. Application of NOLA or MB in perfused vessels significantly increased constriction frequency, furth er indicating perfusion-associated release of EDNO. 5. ACh induced a m arked increase in endothelial [Ca2+](i) of both mechanically respondin g and non-responding vessels. This ACh-induced increase could be repet itively induced when Ca2+ was present in the perfusate, but rapidly ra n down when a Ca2+-free EGTA perfusate was used. 6. Intracellular reco rdings from the smooth muscle of non-perfused vessel segments demonstr ated an ACh-induced hyperpolarization and decrease in membrane resista nce, changes which were prevented by atr opine, NOLA, MB and endotheli al lysis and mimicked by SNP. 7. ACh directly reduced the size of the underlying pacemaker potentials termed spontaneous transient depolariz ations (STDs). 8. NOLA and MB enhanced STDs in non-perfused vessel seg ments indicating an endogenous release of EDNO. 9. It is concluded tha t the lymphatic endothelium produces and releases EDNO endogenously, d uring perfusion or after stimulation with ACh, to decrease the efficac y of STDs to generate action potentials and resultant constrictions.