Endothelium-dependent nitric oxide and hyperpolarization-mediated venous relaxation pathways in rat inferior vena cava

IntroductionThe vascular endothelium plays a major role in the control of arterial tone; however, its role in venous tissues is less clear. The purpose of this study was to determine the role of endothelium in the control of venous function and the relaxation pathways involved.MethodsCircular segments of inferior vena cava (IVC) from male Sprague–Dawley rats were suspended between two wires and isometric contraction to phenylephrine (Phe; 10−5M) and 96 mM KCl was measured. Acetylcholine (Ach; 10−10 to 10−5M) was added and the percentage of venous relaxation was measured. To determine the role of nitric oxide (NO) and prostacyclin (PGI2), vein relaxation was measured in the presence of the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME; 3 × 10−4 M) and the cyclooxygenase inhibitor indomethacin (10−5 M). To measure the role of hyperpolarization, vein relaxation was measured in the presence of K+ channel activator cromakalim (10−11 to 10−6 M), and the nonselective K+ channel blocker tetraethylammonium (TEA; 10−3 M). To test for the contribution of a specific K+ channel, the effects of K+ channel blockers: glibenclamide (adenosine triphosphate [ATP]-sensitive KATP, 10−5 M), 4-aminopyridine (4-AP; voltage-dependent Kv, 10−3 M), apamin (small conductance Ca2+-dependent SKCa, 10−7 M), and iberiotoxin (large conductance Ca2+-dependent BKCa, 10−8 M) on Ach-induced relaxation were tested.ResultsAch caused concentration-dependent relaxation of Phe contraction (maximum 49.9 ± 4.9%). Removal of endothelium abolished Ach-induced relaxation. IVC treatment with L-NAME partially reduced Ach relaxation (32.8 ± 4.9%). In IVC treated with L-NAME plus indomethacin, significant Ach-induced relaxation (33.6 ± 3.2%) could still be observed, suggesting a role of endothelium-derived hyperpolarizing factor (EDHF). In IVC treated with L-NAME, indomethacin and TEA, Ach relaxation was abolished, supporting a role of EDHF. In veins stimulated with high KCl, Ach caused relaxation (maximum 59.5 ± 3.5%) that was abolished in the presence of L-NAME and indomethacin suggesting that any Ach-induced EDHF is blocked in the presence of high KCl depolarizing solution, which does not favor outward movement of K+ ion and membrane hyperpolarization. Cromakalim, an activator of KATP, caused significant IVC relaxation when applied alone or on top of maximal Ach-induced relaxation, suggesting that the Ach response may not involve KATP. Ach-induced relaxation was not inhibited by glibenclamide, 4-AP, or apamin, suggesting little role of KATP, Kv or SKCa, respectively. In contrast, iberiotoxin significantly inhibited Ach-induced relaxation, suggesting a role of BKCa.ConclusionsThus, endothelium-dependent venous relaxation plays a major role in the control of venous function. In addition to NO, an EDHF pathway involving BKCa may play a role in endothelium-dependent venous relaxation.

Clinical RelevanceEndothelial dysfunction plays a major role in the pathogenesis of arterial disease and could also affect the course of venous disease. Although the endothelium-derived mediators have been well characterized in the arterial wall, the mechanisms of venous dilation are poorly understood. The present study in rat inferior vena cava, demonstrates that the mechanisms of venous relaxation partly involve the nitric oxide (NO) pathway, as well as a significant portion involving the hyperpolarization pathway. Although activation of ATP-sensitive K+ channels (KATP) could cause venous relaxation, Ach-induced endothelium-dependent relaxation seems to involve the large conductance Ca2+-dependent K+ channel (BKCa). The identification of the mechanisms of venous relaxation could be important in the management of venous disease. Pharmacologic activators of the NO pathway and K+ channels could be useful in reducing vein restenosis and graft failure. On the other hand, pharmacologic therapy using specific blockers of the NO pathway and K+ channels could be useful in the management of primary and recurrent varicose veins.