1 Center for Applied Microcirculatory Research and 2 Department of Physiology, Health Sciences Center, A1115, University of Louisville School of Medicine, Louisville, KY 40292, USA
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: acetylcholine/mesenteric vascular bed/microcirculation/oestrous cycle/pregnancy
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The regulation of vascular tone and reactivity involves many factors. Endothelial cells modulate underlying vascular smooth muscle tone by releasing endothelium-derived relaxing factors such as nitric oxide (NO), prostacyclin (PGI2), and endothelium-derived hyperpolarizing factor (EDHF), and contracting factors including superoxide anion, thromboxane A2/prostaglandin H2 and endothelin 1 (Furchgott and Vanhoutte, 1989; Lévesque et al., 1994
). All endothelium-derived relaxing factors diffuse to the vascular smooth muscle after release by endothelial cells. In smooth muscle cells, NO and PGI2 initiate vascular relaxation by increasing cGMP and cAMP respectively. In contrast, EDHF activates K+ channels thereby causing K+ efflux, hyperpolarization of vascular smooth muscle cells and vascular relaxation. The K+ channel upon which this substance acts is tissue dependent, and the involvement of EDHF seems to be more significant in smaller resistance vessels than in larger vessels. This substance is still unidentified, but may be an epoxyeicosatetraenoic acid (Campbell et al., 1996
; Van de Voorde and Vanheel, 1997
; Suzuki et al., 1998
).
The reproductive hormones, in particular oestrogen, alter the production of the relaxing factors from the endothelium (Meyer et al., 1997). Oestrogen stimulates the synthesis of NO in the uterus and other tissues (Geary et al., 1998
; Kirsch et al., 1999
) and stimulates PGI2 production in vascular endothelial cells (Farhat et al., 1996
; Myers et al., 1996
; Jun et al., 1998
). The ability of oestrogen to stimulate the production of EDHF has not been studied, although EDHF production may be enhanced in pregnant rats (Bobadilla et al., 1997
) and guinea pigs (Keyes et al., 1998
). This suggests that basal, as well as stimulated, release of endothelium-derived relaxing factors may fluctuate during the reproductive cycle and pregnancy. In male and female rats, relaxation to acetylcholine or carbachol in the mesenteric vascular bed is mediated by endothelium-derived NO (Fortes et al., 1990
) and EDHF (Kamata et al., 1996
) with the relative contribution of EDHF greater in female than male rats (McCulloch and Randall, 1998
). Whether the relative contribution of the endothelial factors released by acetylcholine in this vascular bed is affected by the reproductive cycle or pregnancy is unknown.
The present study was designed to evaluate the dilatation to acetylcholine in the isolated, perfused mesenteric vascular bed from virgin female rats at various stages of the oestrous cycle (oestrus, dioestrus-1, dioestrus-2 and pro-oestrus), and in pregnant rats (gestation day 16). The contribution of the endothelial-derived productsNO, prostanoids and EDHFas mediators of acetylcholine-induced vascular relaxation in each group was also explored. In pregnant females, there is reduced contraction to vasoconstrictors which is thought to be secondary to increased synthesis of NO and PGI2 (Magness et al., 1992; Kim et al., 1994
) and possibly EDHF (Keyes et al., 1998
). Therefore, in the mesenteric vascular beds from cycling and pregnant rats, (i) the ability of a selective
1-adrenergic agonist, cirazoline, to induce tone, and (ii) the influence of NO, prostanoids, and EDHF on this cirazoline-induced tone were also assessed.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Animals
Virgin female SpragueDawley rats weighing 200225 g (Harlan, Indianapolis, IN, USA) were housed in a temperature- and humidity-controlled room of our Association for the Accreditation of Laboratory Animal Care (AALAC) -approved animal care facility with a 12 h light:12 h dark light cycle and access to distilled water and rat chow ad libitum. A vaginal smear was performed daily to determine the stage of oestrous cycle. Animals were monitored for at least two oestrous cycles and only those animals exhibiting regular 4 day oestrous cycles were used in this study. An animal was mated when her vaginal smear indicated that she was in pro-oestrous. The day that spermatozoa were present in the vaginal smear was considered day 0 of pregnancy.
Perfused mesenteric vascular bed
Female virgin rats on each day of the oestrous cycle (oestrus, dioestrus-1, diestrus-2, pro-oestrus) and pregnant rats on gestation day 16 were used in these studies. Following anaesthesia with an i.p. injection of ketamine (37.5 mg/kg) combined with xylazine (5.0 mg/kg), the abdominal cavity was opened and the superior mesenteric artery cannulated through an incision at its confluence with the dorsal aorta. The entire mesenteric vascular bed was then flushed with heparinized PSS, and the small intestinal borders were trimmed off. The bed was then transferred to a warmed (37°C) chamber where it was perfused with a gas mixture (95% O2:5% CO2)-saturated PSS maintained at 37°C. The perfusion rate was kept constant at 5ml/min using a peristaltic pump (Masterflex, Cole-Palmer Instruments Co., Vernon Hills, IL, USA). Changes in perfusion pressure were recorded via a Statham pressure transducer (model P23XL) coupled to a Grass polygraph recorder (model 7H).
The composition of PSS (in mmol/l) was: NaCl 118, KCl 4.7, CaCl2 2.5, KH2PO4 1.2, MgSO4 1.2, NaHCO3 12.5, glucose 11.1. The pH of the PSS was maintained at 7.4 after saturation with a 95% O2:5% CO2 gas mixture. Tissues were allowed to equilibrate for at least 30 min before the start of all experiments.
All experiments in this study involved measuring vasodilatation to ACh in the absence, and in the presence of inhibitors of nitric oxide synthase (NOS), cyclo-oxygenase, and K+ channels or combinations of these agents. Therefore, vascular tone in the isolated, perfused mesenteric vascular bed was generated by continuous infusion of CRZ, a synthetic imidazole derivative which is stable in aqueous medium at physiological pH and produces stable tone for many hours in this preparation (Adeagbo and Triggle, 1993).
Experimental protocols
Protocol 1
The purpose of this series of experiments was two-fold. The first was to determine the pressor effect of CRZ infusion and the relative modulation of this by nitric oxide synthesis inhibition with L-NA (100 µmol/l), cyclo-oxygenase inhibition with IBU (10 µmol/l) and K+ channel blockade with TBA (1 mmol/l). The second purpose of these experiments was to determine the influence of the oestrous cycle and pregnancy on ACh-induced dilatation and the relative contribution of NO, prostanoids and EDHF in mediating this response. The experiments were carried out in mesenteric vascular beds isolated from female rats at different stages of the oestrous cycle and late pregnancy.
Following the equilibration period, CRZ (1 µmol/l) was added to the perfusion medium and remained for the entire protocol. Once the developed tone plateaued, a concentrationresponse relationship to ACh was established by giving bolus injections (0.1 ml) of ACh into the perfusion tube (104 to 10 nmol). After the tissue had recovered from the highest concentration of ACh, L-NA was added to the perfusion solution (PSS) and co-infused with CRZ. After 20 min, the bolus injections of ACh were repeated. IBU was then added to the perfusion solution already containing CRZ and L-NA. After the tone reached a plateau (usually 20 min), ACh concentrations were given for a third time. Finally, TBA was added to the perfusion solution along with the CRZ, L-NA and IBU and again, after tone had levelled off (usually 40 min), the ACh injections were repeated. At the end of each experiment, sodium nitroprusside (SNP, 10 µmol/l) was administered to assess the ability of the vessel to dilate. The tissue was always allowed to return to its pre-injection pressure before another concentration of ACh or an inhibitor was given. One group of virgin rats on dioestrous day 1 and one group of pregnant rats were subjected to the protocol without the addition of L-NA, IBU or TBA to the PSS. These groups served as time controls.
The increase in perfusion pressure (tone) generated by CRZ and any change in tone produced by L-NA, IBU and TBA were compared for the different stages of oestrous cycle and the pregnant group. Concentrationresponse curves for the response to ACh were constructed in presence of CRZ (control), CRZ and L-NA (L-NA present), CRZ, L-NA and IBU (L-NA + IBU present) and CRZ, L-NA, IBU and TBA (L-NA, IBU and TBA present).
Protocol 2
The purpose of this protocol was to determine if the order in which inhibitors were added to the perfusion solution affected the vascular response to ACh. In these experiments, TBA was added to the perfusion solution prior to L-NA. For this protocol, pregnant rats and virgin rats during dioestrous-1 were used since they represent the two groups with the largest difference in reproductive hormone concentrations. In isolated, perfused mesenteries from pregnant and dioestrous-1 virgin animals, CRZ (1 µmol/l) was added to the perfusion solution to induce vascular tone. Bolus injections of ACh were given at increasing concentrations (104 to 10 nmol) into the perfusion tube to determine the control concentrationresponse relationship. The K+-channel blocker TBA (1 mmol/l) was then introduced into the perfusion solution and, after a 40 min equilibration period, ACh responsiveness was re-determined. Next, the NOS inhibitor L-NA (100 µmol/l) was introduced into the suffusion solution, in addition to CRZ and TBA. After at least a 20 min equilibration period, the vascular responsiveness to ACh was tested again. SNP (10 µmol/l) was injected at the end of each experiment to assess the ability of the vessel to dilate. The tissue was always allowed to return to its pre-injection pressure before another concentration of ACh or an inhibitor was given.
Concentrationresponse curves for ACh were constructed in presence of CRZ (control), CRZ and TBA (TBA present) and CRZ, TBA and L-NA (TBA + L-NA present).
Statistical analysis
Change in tone due to the addition of CRZ, L-NA, IBU or TBA to the perfusion solution was expressed in mmHg. Concentrationresponse curves were obtained for ACh alone (control), in the presence of CRZ and L-NA (L-NA present), CRZ, L-NA and IBU (L-NA+IBU present), and CRZ, L-NA, IBU and TBA (L-NA+IBU+TBA present). Relaxation to ACh at each concentration was expressed as a percentage of the total developed tone. Each concentrationresponse curve was used to determine the concentration of ACh that gave 50% of the maximal response (EC50) and the maximal response. Change in perfusion pressure, EC50s, and responses at each concentration of ACh were separately compared by one-way analysis of variance (ANOVA) and NewmanKeul's multiple range tests when a significant difference existed between groups. A P value <0.05 was taken as significant. Group data are reported as mean ± SEM.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
Effect of L-NA and/or IBU on ACh-induced relaxation
Bolus injections of ACh (0.00011 nmol) initiated concentration-dependent decreases in perfusion pressure of CRZ pre-constricted mesenteric vascular beds of all experimental groups. The response to ACh was not affected by the day of the oestrous cycle or by pregnancy (Table I).
|
|
When TBA was introduced into the perfusion solution containing CRZ prior to L-NA (protocol 2), it caused a significant reduction in ACh-induced dilatation of the mesenteric vascular beds from both pregnant and virgin dioestrous-1 rats (Figure 4). The addition of L-NA to the TBA- and CRZ-containing perfusion medium did not cause any further attenuation of the response to ACh (Figure 4
).
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Normal human gestation consistently leads to hyporesponsiveness to the pressor effects of angiotensin II, and perhaps also other pressure stimuli (Pan et al., 1990; Lindheimer et al., 1992). The refractoriness of the mesentery from pregnant rats to cirazoline observed in the present study is consistent with those findings. Since this effect was also seen in animals in pro-oestrus, this could be related to the increased hormone concentrations, presumably oestrogen, during pregnancy and pro-oestrus. It was reported recently (Chapman et al., 1997
) that women during the luteal phase of the menstrual cycle exhibited a lower systemic arterial pressure compared with women in the follicular phase. Aortic rings from rats in pro-oestrus showed a reduction in the maximal contraction to noradrenalin and increased synthesis of dilator prostaglandins compared with aortas from rats in oestrus, dioestrus or met oestrus (Zamorano et al., 1994
). Both these studies were performed when oestrogen concentrations were elevated, suggesting that oestrogen could be responsible for these vascular alterations.
Although the cardiovascular benefits of oestrogen therapy have been well documented (Mikkola et al., 1998), the mechanism by which oestrogen exerts its effects on the vasculature is not completely understood. Chronic administration of oestrogen has been associated with an enhanced release of NO and prostaglandins from the endothelium in both humans and experimental animals (Farhat et al., 1996
). In addition, oestrogen therapy has been associated with coronary blood flow regulation in part by up-regulating NOS activity (Gorodeski et al., 1995
). Furthermore, resistance vessels from the mesentery of ovariectomized female rats receiving oestrogen replacement, exhibited an increase in basal, but not ACh-stimulated, release of endothelium-derived relaxing factors (Meyer et al., 1997
).
The present study also reported a relatively higher increase in perfusion pressure in the mesenteric vascular bed from pregnant rats after addition of L-NA into the perfusion solution. This agrees with previous studies (Weiner et al., 1994; Nathan et al., 1995
) reporting an increase in basal vascular NO synthesis during pregnancy. If basal release of NO is increased in pregnancy, this could explain the refractoriness to CRZ observed in the pregnant group (Molnár and Hertelendy, 1992
).
The data presented here also revealed that in the isolated perfused mesenteric vascular bed pre-constricted with CRZ, ACh-induced relaxation did not differ among pregnant and non-pregnant cycling rats. Similarly, it was reported (Yamasaki et al., 1996) that neither ACh-induced relaxation nor basal NO biosynthesis is increased in the late pregnant rat femoral resistance vessels under isometric conditions. However, others report that ACh-induced relaxation is enhanced in mesenteric arteries from pregnant rats (Kim et al., 1994
; Gerber et al., 1998
), and in carotid arteries from the pregnant guinea pig (Weiner et al., 1989
). Taken together, these results suggest a divergent responsiveness to ACh between resistance and conductance vessels during pregnancy.
In this study, L-NA did not change the vascular reactivity of the mesenteric vascular bed to ACh in the pregnant group although it caused a small but statistically significant inhibition in the cycling virgin groups. This suggests two possibilities: (i) that during late pregnancy in rats, there is a shift to some mediator other than NO for the ACh vasodilator response in the mesentery; or (ii) that there is an up-regulation of NOS in the mesenteric vessels from the pregnant rat. If the latter were true, the concentration of L-NA used in the present study may not have been sufficient to inhibit the up-regulated NOS.
Thus, in the isolated, perfused mesenteric vascular bed, NO activity appeared to be up-regulated in pregnancy and possibly on pro-oestrus. This raises the question of why we did not observe an increase in the relaxation to ACh in these preparations. The lack of effect of pregnancy on ACh-induced dilation has been observed in other vessels such as the renal and uterine arteries (Jovanovi et al., 1994a
; Kim et al., 1994
). One possible reason for this seeming dichotomy is that the apparent increase in NO activity in the pregnant and pro-oestrous groups is caused by an increase in vascular smooth muscle NO activity rather than endothelium-released NO. In the human uterine artery, L-arginine-induced relaxation is mediated by non-endothelial NO production (Jovanovi
et al., 1994b
). Another possible reason is that since the portion of the ACh-induced dilation in this vascular bed attributable to NO is so small, it may not have been possible to detect any increase in the relaxation to ACh with the number of preparations reported here. In fact, although not statistically significant, at two points of the ACh concentrationresponse curve (0.001 and 0.01 nmol), the response of the pregnant group does appear larger than the virgin groups (data not shown).
In an earlier study (Gant et al., 1987), it was found that inhibition of cyclo-oxygenase restored the vascular refractoriness to angiotensin II in pregnant women, suggesting that prostanoids are a pivotal mediator of vascular reactivity during pregnancy. In our study, ibuprofen did not alter ACh-induced relaxation in the presence of L-NA, demonstrating that cyclo-oxygenase products are not major contributors to ACh-induced dilatation in the female mesenteric vascular bed. This agrees with previous findings (Meyer et al., 1997
) in which ACh-induced dilatation was not affected by cyclo-oxygenase inhibition in isolated mesenteric arterioles from ovariectomized or oestrogen-treated rats. Similarly, in recent studies using a double-perfused mesentery preparation from Wistar Kyoto and spontaneously hypertensive male rats, it was shown that the cyclo-oxygenase pathway was not implicated in the vasodilatation induced by ACh (Le Marquer-Domagala and Finet, 1997
). These studies all indicate that, at least in the mesenteric vascular bed, prostanoids are not involved in the vascular response to ACh.
In all virgin and pregnant groups the majority of the ACh-induced dilatation was intact in spite of the presence of L-NA and IBU, suggesting that as in males, NO is not the major mediator of ACh-induced dilatation in the female mesenteric vascular bed (Adeagbo and Triggle, 1993; McCulloch and Randall, 1998
). In contrast to L-NA and IBU, the addition of the potassium channel blocker TBA profoundly attenuated the relaxation to ACh, although TBA did not significantly alter basal tone. Thus, the majority of the dilatation to ACh in this vascular bed appears to be mediated by the release of a non-NO, non-prostanoid factor, which dilates by opening K+ channels. Although the EDHF has yet to be identified, it is known to promote vasodilatation by activating K+-channels, resulting in smooth muscle cell membrane hyperpolarization. Therefore, the use of K+-channel blockers or varying extracellular K+ has been used consistently to demonstrate EDHF-mediated relaxation (Adeagbo and Triggle, 1993
; McCulloch and Randall, 1998
). TBA has been shown to block EDHF-mediated responses in many vessels, including coronary artery (Nakashima et al., 1997
; Popp et al., 1998
; Kaw and Hecker, 1999
), omental arteries (Ohlmann et al., 1997
) and carotid artery (Dong et al., 1997
).
This last finding agrees with that from a recent study (McCulloch and Randall, 1998) in which data from female rats on different oestrous cycle days were pooled and the responsiveness to carbachol in the isolated, perfused mesenteric vascular bed investigated. As in the present study, these workers found that EDHF is an important mediator of muscarinic receptor-induced dilatation in this vascular bed in the female. They and others (Kilpatrick and Cocks, 1994
; Hatake et al., 1995
) suggested that under normal conditions NO is the main mediator of ACh-induced relaxation in the female mesenteric vascular bed, but during NOS inhibition, there is an up-regulation of EDHF production that compensated for the lack of NO. This hypothesis was ruled out by the results from the second protocol of the present study (Figure 4
). It was found that perfusion of the female mesenteric vascular bed with TBA, a non-selective K+ channel blocker, in the presence of an intact NO system, dramatically inhibited relaxation to ACh. When L-NA was perfused along with TBA, there was no further inhibition of the ACh-induced relaxation of the mesenteric vascular bed of virgin rats or pregnant rats.
In conclusion, it was found that in cycling virgin and late-pregnant female SpragueDawley rats, the major portion of ACh-induced dilatation of the mesenteric vascular bed was probably mediated by EDHF. In addition, the present study suggested that NOS may be up-regulated in the mesenteric vascular bed from pregnant and possibly pro-oestrous virgin rats compared with virgin rats on other oestrous cycle days.
![]() |
Acknowledgments |
---|
![]() |
Notes |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bobadilla, R.A., Henkel, C.C., Henkel, E.C. et al. (1997) Possible involvement of endothelium-derived hyperpolarizing factor in vascular responses of abdominal aorta from pregnant rats. Hypertension, 30, 596602.
Campbell, W.B., Gebremedhin, D., Pratt, P.F. and Harder, D.R. (1996) Identification of epoxyeicosetetraenoic acids as endothelium-derived hyperpolarizing factors. Circ. Res., 78, 415423.
Chapman, A.B., Zamudio, S., Woodmansee, W. et al. (1997) Systemic and renal hemodynamic changes in the luteal phase of the menstrual cycle mimic early pregnancy. Am. J. Physiol., 273, F777F782.
Christensen, K.L. and Mulvany, M.J. (1993) Mesenteric arcade arteries contribute substantially to vascular resistance in conscious rats. J. Vasc. Res., 30, 7379.[ISI][Medline]
Dong, H., Waldron, G.J., Galipeau, D. et al. (1997) NO/PGI2-independent vasorelaxation and the cytochrome P450 pathway in rabbit carotid artery. Br. J. Pharmacol., 120, 695701.[Abstract]
Farhat, M.Y., Lavigne, M.C. and Ramwell, P.W. (1996) The vascular protective effects of estrogen. FASEB J., 10, 615624.
Fortes, Z.B., Oliveira, M.A., Scivoletto, R. et al. (1990) Nitric oxide release may be involved in the microcirculatory response to acetylcholine. Eur. J. Pharmacol., 182, 143147.[ISI][Medline]
Furchgott, R.F. and Vanhoutte, P.M. (1989) Endothelium-derived relaxing and contracting factors. FASEB J., 3, 20072018.
Gant, N.F., Whalley, P.J., Everett, R.B. et al. (1987) Control of vascular reactivity in pregnancy. Am. J. Kidney Dis., 9, 303307.[ISI][Medline]
Geary, G.G., Krause, D.N. and Duckles, S.P. (1998) Estrogen reduces myogenic tone through a nitric oxide-dependent mechanism in rat cerebral arteries. Am. J. Physiol., 275, H292H300.
Gerber, R.T., Anwar, M.A. and Poston, L. (1998) Enhanced acetylcholine induced relaxation in small mesenteric arteries from pregnant rats: an important role for endothelium-derived hyperpolarizing factor (EDHF). Br. J. Pharmacol., 125, 455460.[Abstract]
Gorodeski, G.I., Yang, T., Levy, M.N. et al. (1995) Effects of estrogen in vivo on coronary vascular resistance in perfused rabbit hearts. Am. J. Physiol., 269, R1333R1338.
Hatake, K., Wakabayashi, I. and Hishida, S. (1995) Endothelium-dependent relaxation resistant to NG-nitro-L-arginine in rat aorta. Eur. J. Pharmacol., 274, 2532.[ISI][Medline]
Jovanovi, S. and Jovanovi
, A. (1997) Remodelling of guinea-pig aorta during pregnancy: selective alteration of endothelial cells. Hum. Reprod., 12, 22972302.[Abstract]
Jovanovi, S. and Jovanovi
, A. (1998) Pregnancy is associated with hypertrophy of carotid artery endothelial and smooth muscle cells. Hum. Reprod., 13, 10741078.[Abstract]
Jovanovi, A., Grbovi
, L., Drekic, D. and Novakovi
, S. (1994a) Muscarinic receptor function in the guinea-pig uterine artery is not altered during pregnancy. Eur. J. Pharmacol., 257, 185194.
Jovanovi, A., Grbovi
, L. and Tuli
, I. (1994b) L-arginine induces relaxation of human uterine artery with both intact and denuded endothelium. Eur. J. Pharmacol., 256, 103107.[ISI][Medline]
Jun, S.S., Chen, Z., Pace, M.C. and Shaul, P.W. (1998) Estrogen upregulates cyclooxygenase-1 gene expression in ovine fetal pulmonary artery endothelium. J. Clin. Invest., 102, 176183.
Kamata, K., Numazawa, T. and Kasuya, Y. (1996) Characteristics of vasodilatation induced by acetylcholine and platelet-activating factor in the rat mesenteric arterial bed. Eur. J. Pharmacol., 298, 129136.[ISI][Medline]
Kaw, S. and Hecker, M. (1999) Endothelium-derived hyperpolarizing factor, but not nitric oxide or prostacyclin release, is resistant to menadione-induced oxidative stress in the bovine coronary artery. Naunyn-Schmiederberg's Arch. Pharmacol., 359, 133139.[ISI][Medline]
Keyes, L., Rodman, D.M., Curran-Everett, D. et al. (1998) Effect of K+ ATP channel on total and regional vascular resistance in guinea pig pregnancy. Am. J. Physiol., 275, H680H688.
Kilpatrick, E.V. and Cocks, T.M. (1994) Evidence for differential roles of nitric oxide (NO) and hyperpolarization in endothelium-dependent relaxation of pig isolated coronary artery. Br. J. Pharmacol., 112, 557565.[Abstract]
Kim, T.H., Weiner, C.P. and Thompson, L.P. (1994) Effect of pregnancy on contraction and endothelium-mediated relaxation of renal and mesenteric arteries. Am. J. Physiol., 267, H41H47.
Kirsch, E.A., Yuhanna, I.S., Chen, Z. et al. (1999) Estrogen acutely stimulates endothelial nitric oxide synthase in H441 human airway epithelial cells. Am. J. Respir. Cell. Mol. Biol., 20, 658666.
Le Marquer-Domagala, F. and Finet, M. (1997) Comparison of the nitric oxide and cyclooxygenase pathway in mesenteric resistance vessels of normotensive and spontaneously hypertensive rats. Br. J. Pharmacol., 121, 588594.[Abstract]
Lévesque, H., Cailleux, N. and Courtois, H. (1994) Vasoactive peptides of endothelial origin. Therapeutic perspectives. Presse Med., 23, 11721177.[ISI][Medline]
Lindheimer, M.D. and Katz, A.I. (1992) Renal physiology and disease in pregnancy. In Seldin, D.W. and Giebisch, G. (eds), The Kidney Physiology and Pathophysiology, 2nd edn. Raven Press, New York, pp. 33713431.
Magness, R.R., Rosenfeld, C.R., Faucher, D.J. and Mitchell, M.D. (1992) Uterine prostaglandin production in ovine pregnancy: effects of angiotensin II and indomethacin. Am. J. Physiol., 263, H188H197.
McCulloch, A.I. and Randall, M.D. (1998) Sex differences in the relative contributions of nitric oxide and EDHF to agonist-stimulated endothelium-dependent relaxations in the rat isolated mesenteric arterial bed. Br. J. Pharmacol., 123, 17001706.[Abstract]
Meyer, M.C., Cummings, K. and Osol, G. (1997) Estrogen replacement attenuates resistance artery adrenergic sensitivity via endothelial vasodilators. Am. J. Physiol., 272, H2264H2270.
Mikkola, T., Viinikka, L. and Ylikorkala, O. (1998) Estrogen and postmenopausal estrogen/progestin therapy: effect on endothelium-dependent prostacyclin, nitric oxide and endothelin-1 production. Eur. J. Obstet. Gynecol. Reprod. Biol., 79, 7582.[ISI][Medline]
Molnár, M. and Hertelendy, F. (1992) N-omega-nitro-L-arginine, an inhibitor of nitric oxide synthesis, increases blood pressure in rats and reverses the pregnancy-induced refractoriness to vasopressor agents. Am. J. Obstet. Gynecol., 166, 15601567.[ISI][Medline]
Myers, S.I., Turnage, R.H., Bartula, L. et al. (1996) Estrogen increases male rat aortic endothelial cell (RAEC) PGI2 release. Prostaglandins Leukotrienes Essent. Fatty Acids, 54, 403409.[ISI][Medline]
Nakashima, Y., Toki, Y., Fukami, Y. et al. (1997) Role of K+ channels in EDHF-dependent relaxation induced by acetylcholine in canine coronary artery. Heart & Vessels, 12, 287293.[ISI][Medline]
Nathan, L., Cuevas, J. and Chaudhuri, G. (1995) The role of nitric oxide in the altered vascular reactivity of pregnancy in the rat. Br. J. Pharmacol., 114, 955960.[Abstract]
Ohlmann, P., Martínez, M.C., Schneider, F. et al. (1997) Characterization of endothelium-derived relaxing factors released by bradykinin in human resistance arteries. Br. J. Pharmacol., 121, 567664.
Pan, Z.R., Lindheimer, M.D., Bailin, J. and Barron, W.M. (1990) Regulation of blood pressure in pregnancy: pressor system blockade and stimulation. Am. J. Physiol., 258, H1559H1572.
Pascoal, I.F., Lindheimer, M.D., Nalbantian-Brandt, C. and Umans, J.G. (1995) Contraction and endothelium-dependent relaxation in mesenteric microvessels from pregnant rats. Am. J. Physiol., 269, H1899H1904.
Popp, R., Fleming, I. and Busse, R. (1998) Pulsatile stretch in coronary arteries elicits release of endothelium-derived hyperpolarizing factor: a modulator of arterial compliance. Circ. Res., 82, 696703.
Suzuki, H., Yamamoto, Y. and Fukuta, H. (1998) Endothelium-derived hyperpolarizing factor and vasodilatation. Nippon Yakurigaku Zasshi, 112, 195202.[Medline]
Van de Voorde, J. and Vanheel, B. (1997) Influence of cytochrome P-450 inhibitors on endothelium-dependent nitro-L-arginine-resistant relaxation and cromakalim-induced relaxation in rat mesenteric arteries. J. Cardiovasc. Pharmacol., 29, 827832.[ISI][Medline]
Weiner, C., Martinez, E., Zhu, L.K. et al. (1989) In vitro release of endothelium-derived relaxing factor by acetylcholine is increased during the guinea pig pregnancy. Am. J. Obstet. Gynecol., 161, 15991605.[ISI][Medline]
Weiner, C.P., Knowles, R.G. and Moncada, S. (1994) Induction of nitric oxide synthases early in pregnancy. Am. J. Obstet. Gynecol., 171, 838843.[ISI][Medline]
Yamasaki, M., Lindheimer, M.D. and Umans, J.G. (1996) Effects of pregnancy on femoral microvascular responses in the rat. Am. J. Obstet. Gynecol., 175, 730736.[ISI][Medline]
Zamorano, B., Bruzzone, M.E. and Martinez, J.L. (1994) Influence of the estrous cycle on the norepinephrine-induced contraction of rat aorta: relationship to vascular prostanoid biosynthesis. Biol. Res., 27, 209215.[Medline]
Submitted on September 13, 1999; accepted on December 15, 1999.