The effect of acetylcholine on the isolated human uterine artery rings was investigated. Acetylcholine (10(-10) M to 6 x 10(-5) M) induced concentration- and endothelium-dependent relaxation (pD2 = 7.4 +/- 0.02, maximal response was 77.5 +/- 6.3% of relaxation induced by papaverine at 3 x 10(-4) M) of the pre-contracted arterial segments. Indomethacin (10(-5) M), diethylcarbamazine (10(-4) M) and tetra-ethylammonium (3 x 10(-4) M) had no effects on acetylcholine-evoked relaxation. Methylene blue (10(-5) M) and NG-monomethyl-L-arginine (L-NMMA) (3 x 10(-6) to 3 x 10(-5) M) antagonized relaxation induced by acetylcholine. The inhibition of endothelium-dependent relaxation by L-NMMA (10(-5) M) was reversed by L-arginine (10(-5) M) but not by D-arginine (10(-4) M). It is concluded that in uterine artery acetylcholine induces endothelium-dependent relaxation of isolated uterine artery is probably mediated via endothelial nitric oxide formation.
It has been previously shown that vasoactive intestinal polypeptide (VIP) induces endothelium-dependent relaxation of the human uterine artery. However, the nature of the mediator of the VIP-induced endothelium-dependent relaxation of the human uterine artery has not yet been determined. Therefore these experiments were undertaken to examine the effects of VIP on human uterine arteries and to establish the role of various endothelial factors on the relaxation induced by VIP. The experiments were performed on isolated human uterine arterial rings. VIP (0.3-100 nM) induced a concentration-dependent relaxation of human uterine arteries with intact endothelium (pEC50 = 8.06+/-0.14, n = 28). After the removal of the endothelium this relaxation was abolished (n = 6). Indomethacin (10 microM), a cyclooxygenase inhibitor, and diethylcarbamazine (100 microM), a lipoxygenase blocker, had no effects on VIP-induced relaxation. In contrast, methylene blue (10 microM), a blocker of guanylate cyclase, NG-monomethyl-L-arginine (10 microM), an inhibitor of nitric oxide (NO) synthase, and 4-aminopyridine (1 mM), a non-selective blocker of K+ channels, antagonized the effect of VIP with suppression of maximal VIP-induced relaxation. Non-competitive antagonism with methylene blue revealed that the pKa value for VIP-receptor complex was 8.10+/-0.10 (n = 6) and the receptor reserve expressed as KA/EC50 was 0.89+/-0.11, where pKa = log10KA, and KA is the dissociation constant of VIP-receptor complex. Therefore, on the basis of the results presented, we can conclude that VIP induces endothelium-dependent relaxation in human uterine arteries, acting as a partial agonist on this blood vessel. It appears that endothelium-dependent relaxation induced by VIP in human uterine artery can be entirely explained by the release of NO from endothelial cells.
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