Hidetaka Izumi

Gestational changes in L-arginine-induced relaxation of pregnant rat and human myometrial smooth muscle

OBJECTIVE We intended to demonstrate the presence of an L-arginine-nitric oxide system in human myometrium and to clarify the mechanisms of action of nitric oxide on rat myometrium during gestation. STUDY DESIGN By examining very small myometrial muscle strips (approximately 750 muscle cells), characteristic features of contraction of rat longitudinal muscle at the midstage of gestation (day 16) and during delivery at term were determined. RESULTS Spontaneous contractions were significantly different during delivery compared with the midstage of gestation of rat myometrium. L-Arginine relaxed spontaneous and carbachol-induced, but not potassium chloride-evoked, contractions at both stages. However, much higher concentrations of L-arginine were required during delivery, 8-Bromo-cyclic guanosine monophosphate inhibited spontaneous contractions from concentrations of 1 nmol/L in the midstage of gestation and from 0.1 mmol/L during delivery. In human myometrial tissues L-arginine also inhibited contractions during the late stages of gestation. CONCLUSION (1) The experimental model is sufficient to compare properties of longitudinal myometrial strips during gestation. (2) In rat and human myometrium an L-arginine-nitric oxide system has an important role in inhibiting uterine contractility and possibly maintaining pregnancy. (3) The relaxing effect of the nitric oxide system is largely because of the voltage-independent action of cyclic guanosine monophosphate systems.

Role of nitric oxide on vasorelaxation in human umbilical artery

OBJECTIVE Endothelium-derived relaxing factor (or nitric oxide) is thought to play an important role in control of blood flow in umbilical blood vessels at midgestation compared with term. Previous studies suggest that histamine releases endothelium-derived relaxing factor from umbilical arteries. In this study we intended to clarify the mechanism by which histamine releases endothelium-derived relaxing factor and causes vasorelaxation in human umbilical artery at the midstage (18 to 22 weeks) of gestation. STUDY DESIGN By means of very thin muscle strips that allow rapid diffusional access of applied drugs (in a few seconds), contractile properties of human umbilical artery were examined. Isometric tensions were measured in response to potassium chloride (39 mmol/L) or caffeine and inhibitory effects of histamine, A23187, glyceryl trinitrate, and 8-bromo-cyclic guanosine monophosphate on these contractions were also examined. RESULTS Histamine (0.01 to 0.1 mumol/L) did not inhibit 39 mmol/L K(+)-induced contractions of tissues taken at the terminal (38 to 41 weeks) stage of gestation. However, at midgestation histamine (0.01 to 0.1 mumol/L), A23187 (10 mumol/L), and 8-bromo-cyclic guanosine monophosphate (membrane-permeable analog of cyclic guanosine monophosphate, 0.1 mmol/L) inhibited 39 mmol/L K(+)-induced contractions. The inhibitory effects of histamine were antagonized by mepyramine (an H1 antagonist), L-NG-nitro arginine, methylene blue, and Ca++ depletion of the extracellular space but not by cimetidine (an H2 antagonist). Caffeine produced contractions both in the presence and absence of extracellular Ca++ possibly because of the release of Ca++ from intracellular storage sites. Glyceryl trinitrate and 8-bromo-cyclic guanosine monophosphate reduced the caffeine-induced contractions in Ca(++)-free solution. In addition, 10 mumol/L cyclic guanosine monophosphate did not attenuate the Ca++ sensitivity for contractile elements. CONCLUSION These results suggest that (1) histamine coupled to the histamine H1 receptor increases intracellular Ca++ concentration to stimulate nitric oxide synthase in human umbilical endothelial cells, (2) nitric oxide from endothelial cells activates guanylate cyclase to produce cyclic guanosine monophosphate in the umbilical smooth muscle cells, and (3) cyclic guanosine monophosphate relaxes the umbilical tissues, perhaps as a result of the activation of a Ca++ extrusion system.

Some mechanical properties of skinned fibres of pregnant human myometrium

The properties of contractile elements and intracellular Ca2+ storage sites of pregnant human myometrium were studied by recording the mechanical responses in skinned (saponin-treated and membrane-permeable) fibres. Calmodulin increased the amplitude of contractions induced by Ca2+ and the Ca2+ sensitivity for contractile elements in small myometrium strips, but PGF2 alpha, PGE2, oxytocin, or cyclic AMP failed to produce similar effects. After accumulation of Ca2+ in intracellular Ca2+ storage sites, 10 mumol/l PGF2 alpha, 10 mumol/l PGE2, 30 mmol/l caffeine, and 20 mumol/l InsP3 (inositol-trisphosphate) produced contractions by releasing Ca2+ from storage sites. However, 20 nmol/l oxytocin had no effects under the same conditions. The InsP3 sensitive Ca2+ store was much larger than those of PGs or caffeine. These results suggest that pregnant human myometrium contracts with low Ca2+ by a calmodulin sensitive system. The data also indicate that direct application of PGF2 alpha, or PGE2 into the cells discharges Ca2+ from Ca2+ storage sites and that oxytocin extricates Ca2+ via a pathway involving InsP3 by activation of phosphoinositide turnover. We suggest that these agents induce added contractile responses due to a Ca2+ release mechanism from store sites in addition to the influx of Ca2+ from the extracellular space.

Comparison of nitric oxide and prostacyclin in endothelium-dependent vasorelaxation of human umbilical artery at midgestation☆☆☆★

OBJECTIVE We intended to compare the relative importance of nitric oxide and prostacyclin as endothelium-derived vasorelaxing factors released by histamine in human umbilical artery at the midstage (18 to 22 weeks) of gestation. STUDY DESIGN By use of very thin muscle strips, which allows rapid diffusional access of applied drugs (in a few seconds), isometric tensions were recorded in response to histamine. The histamine-induced contractions and the relaxing effect of histamine on the potassium chloride (39 mmol/L) contractions were studied in relation to the existence of endothelium, L-NG-nitro arginine, and indomethacin. The relaxing effects of glyceryl trinitrate and prostacyclin on the potassium chloride contractions were also examined. RESULTS The contractile responses to histamine were more sensitive and the relation tensions of histamine contractions, compared with the 39 mmol/L K(+)-induced contractions, were large in endothelium-denuded strips than in endothelium-intact strips. Histamine contractions were enhanced in endothelium-intact strips in the presence of 10 mumol/L L-NG-nitro arginine (competitive inhibitor of nitric oxide synthase) but not in the presence of 10 mumol/L indomethacin (cyclooxygenase inhibitor). Histamine produced a concentration-dependent relaxation during the maintained contraction induced by 39 mmol/L K+. These histamine-induced relaxations were completely blocked by L-NG-nitro arginine but not by indomethacin. Glyceryl trinitrate and prostacyclin relaxed the sustained contractions induced by 39 mmol/L K+ in a dose-dependent manner; however, the degree of relaxation by glyceryl trinitrate was more prominent than that by prostacyclin. CONCLUSION These results suggest that nitric oxide is more potent than prostacyclin as a vasorelaxing substance released from the endothelium and that nitric oxide has an important role for controlling fetoplacental circulation at midgestation.