Effects of endothelial cell removal on constrictor and dilator responses of aortae of pregnant rats

Effects of Kaempferia parviflora rhizomes dichloromethane extract on vascular functions in middle-aged male rat

ETHNOPHARMACOLOGICAL RELEVANCE

In Thai traditional medicine, rhizomes of Kaempferia parviflora (KP) have been used for treating hypertension and for the promotion of longevity with good health and well being. Ageing is one of the most important risk factors for development of cardiovascular disease. To investigate whether a 6 weeks oral administration of a dichloromethane extract of fresh rhizomes of Kaempferia parviflora (KPD) had any effects on vascular functions, on the accumulation of lipid, as well as on any signs of gross organ toxicity in middle-aged rats.

MATERIALS AND METHODS

Fresh rhizomes of Kaempferia parviflora were first macerated twice with 95% ethanol to remove the dark color before extracting three times with 100% dichloromethane. The dichloromethane extract was evaporated under reduced pressure to obtain the dried Kaempferia parviflora dichloromethane extract (KPD). The rats were orally administered with the KPD at a dosage of 100mg/kg body weight, or with the same volume of the vehicle (tween 80, 0.2g: carboxy-methylcellulose sodium, 0.2g: distilled water 10 ml) once or twice a day for 6 weeks. Vascular functions were studied on isolated thoracic aorta and the mesenteric artery. The vascular eNOS enzyme was measured by Western blot analysis. Blood chemistry was measured by enzymatic methods. Liver cell lipid accumulation was measured using oil red O staining.

RESULTS

A 6 weeks treatment of KPD once a day had no significant effects on any of the studied parameters. When the KPD was given twice a day, the contractile responses to phenylephrine of the thoracic aorta and mesenteric artery were lower than the vehicle control group, and this effect was abolished by N(G)-nitro-l-arginine or by removal of the vascular endothelium. Vasorelaxation to acetylcholine, but not to glyceryl trinitrate, by the thoracic aortic and mesenteric ring precontracted with phenylephrine was higher from the KPD treated rats than those from the vehicle control groups. Western blot analysis showed a higher quantity of thoracic- and mesenteric-eNOS protein obtained from the KPD treated rats. In addition, the body weight, serum glucose and triglycerides levels, visceral and subcutaneous fat, as well as liver lipid accumulation were all significantly decreased in the KPD treated rats compared to those of the vehicle control. No differences were found between the KPD treated-, and the vehicle-control for animal food intake, internal organ weight, serum ALP, SGOT, SGPT, BUN and creatinine levels, serum cholesterol, HDL-C and LDL-C levels, nor total blood cell counts.

CONCLUSIONS

The chronic oral administration of KPD extract, to middle aged rats, caused a decrease in vascular responsiveness to phenylephrine with an increase in the acetylcholine induced vasorelaxation, due to an increase in nitric oxide production from their blood vessels. The extract also caused a decrease in visceral and subcutaneous fat, fasting serum glucose and triglyceride levels and liver lipid accumulation, with no changes to liver and kidney functions or to total blood cell counts. It is possible that these KPD extracts could be developed as a health product for mid-aged humans to reduce obesity, diabetes type II and cardiovascular disease.

Hypotensive activity of an n-butanol extract and their purified compounds from leaves of Phyllanthus acidus (L.) Skeels in rats

We aimed to investigate the effects, identify the active substances and establish the mechanisms involved in the hypotensive activity of an n-butanol extract from leaves of Phyllanthus acidus (PA extract). PA extract caused a decrease in blood pressure of anesthetized rats that was not modified by atropine or propranolol. PA extract caused a persistent dilatation of thoracic aortic rings preconstricted with either phenylephrine or KCl, and these effects were not modified by LNA or removal of the vascular endothelium. For phenylephrine-preconstricted aortic rings, the dilatory activity of the PA extract was not modified by atropine, propranolol or indomethacin. TEA, glybenclamide or ODQ significantly inhibited the dilatory activity of the PA extract on endothelium-denuded aortic rings. Nifedipine or a Ca(2+)-free medium depressed the aortic rings constrictor response to phenylephrine, and that was further augmented by the PA extract. Adenosine, 4-hydroxybenzoic acid, caffeic acid, hypogallic acid, and kaempferol were isolated from the PA extract. Each caused a decrease in blood pressure and dilatation of the aortic rings. LNA or removal of the endothelium reduced this activity. ODQ and TEA attenuated the vasodilatory activity of adenosine whereas glybenclamide and ODQ attenuated the effect of hypogallic acid. These results suggest that the hypotensive activities of the PA extract is likely the result of the direct action of these five compounds on the blood vessels by stimulating release of nitric oxide from the vascular endothelium, in part through stimulation of soluble guanylate cyclase, and opening of K(ATP) and K(Ca) channels in the vascular smooth muscle.

Increased alpha-1 adrenoceptor expression in pregnant rats with subrenal aortic coarctation

The progression of pregnancy is associated with attenuation in vasopressor response to adrenergic agonists. In pregnancy-induced hypertension this attenuation is reverted. It is not known if this reversion involves alpha-1 adrenoceptor expression.

OBJECTIVE

In this work we propose that in pregnant rats with subrenal aortic coarctation there are changes in the expression of alpha-1 adrenergic receptors in the thoracic and abdominal aorta during pregnancy.

METHODS

We used non-pregnant, normal pregnant and pregnant with subrenal aortic coarctation female Wistar rats. Pregnancy-induced hypertension indicators, systolic blood pressure, 24 hours proteinuria, pup weight and maternal weight were measured. Dose response curves to phenylephrine were carried out to determine vascular reactivity along pregnancy. Alpha 1-adrenoceptors were detected from thoracic and abdominal aorta using immunoblot.

RESULTS

Results show significant increases in arterial pressure and proteinuria in pregnant rats with SRAC at the end of the third week. Pregnancy reduces alpha-(1-A, -B) and (-D) adrenoceptor expression and this event is reverted by subrenal aortic coarctation. This phenomenon is more apparent in the abdominal segment of the aorta.

CONCLUSIONS

These findings suggest that subrenal aortic coarctation is a good animal model of pregnancy-induced hypertension and that alpha1-adrenoceptors participate in its physiopathology increasing their expression in a segment-dependent manner.

Cardiovascular effects of an n-butanol extract from fresh fruits of Randia siamensis

Randia siamensis is used in Thai folklore medicine for inducing abortion and controlling blood pressure. The present study investigated the cardiovascular effects of an R. siamensis fruit extract, and mechanisms involved in anesthetized normal and reserpinized rats. R. siamensis (0.4-12 mg/kg) i.v. increased the mean arterial pressure (MAP) and heart rate. Both effects were significantly inhibited by phentolamine (2 mg/kg, i.v.) or propranolol (0.6 mg/kg, i.v.). The combination of phentolamine and propranolol, or reserpine pretreatment, inhibited the positive chronotropic effect with a slight decrease in the MAP. In vitro, R. siamensis (0.001-0.3 mg/ml) increased the rate of beating of the right atrium and the strength of the electrical field-stimulated contraction of the left atrium, both effects were inhibited by propranolol, or with reserpine pretreated rats. R. siamensis (0.01-3 mg/ml) produced a contraction of isolated thoracic aorta, which was potentiated by N(G)-nitro-L-arginine (LNA), or by removal of the vascular endothelium, but inhibited by phentolamine, or reserpine. R. siamensis (0.3-3 mg/ml) caused a relaxation of phenylephrine-preconstricted aortic rings, which was potentiated with reserpine pretreatment, and abolished after removal of the vascular endothelium, or in the presence of LNA. These results suggest that R. siamensis extract exerts both hypertensive and positive chronotropic effects via the alpha- and beta-adrenergic receptors of blood vessels and the heart, due to release of endogenous catecholamines, likely from nerve ending and adrenal medulla. The hypotensive activity results from the release of nitric oxide causing dilatation of the blood vessels. The present data support the folklore therapeutic uses of this plant.

Pregnancy-associated increase in rat systemic arteries endothelial nitric oxide production diminishes vasoconstrictor but does not enhance vasodilator responses

Late pregnancy in rats is characterized by a decrease in arterial pressure and in isolated arterial vessels response to vasoconstrictors. In uterine arteries the pregnancy-associated attenuation of the response to vasoconstrictors has been attributed to an increase in basal and agonist-induced endothelial NO production. However, the role of NO in pregnancy-associated changes of systemic arteries reactivity to vasoactive agents remains to be fully elucidated. We examined whether pregnancy influences the reactivity of systemic arteries to vasodilator or vasoconstrictor agents through NO-dependent mechanisms. Thoracic aortic rings and mesenteric arterial bed of late pregnant rats showed refractoriness to phenylephrine-induced vasoconstriction that was abolished by NO synthase inhibition. The potency of L-NNA to enhance tension of aortic rings preconstricted with phenylephrine (10-20% of their maximal response) was significantly lower in preparations from pregnant animals. In phenylephrine-contracted aortas and mesenteric bed, the effects of the endothelium-dependent vasodilators acetylcholine, A23187 and bradykinin, were not influenced by pregnancy. Similarly, pregnancy did not affect the vasodilator responses of adenosine, isoproterenol, capsaicin, nitroprusside, forskolin, and Hoe234 in the mesenteric bed. NO synthase activity measured by determining the conversion of L-[(3)H]-arginine to L-[(3)H]-citrulline in aorta and mesenteric arteries homogenates was not altered by pregnancy. These findings show that endothelial-dependent and -independent vasodilators action as well as NO synthase activity in systemic arteries is uninfluenced by pregnancy, whereas pregnancy-associated hyporeactivity of systemic arteries to vasoconstrictors is related to an enhanced endothelial NO production either spontaneous or elicited directly or indirectly by vasoconstrictor agents. This interpretation implies that the enhanced NO production observed in systemic arteries during late pregnancy involves cellular pathways other than the ones involved in the response to endothelium-dependent vasodilators such as acetylcholine.

Endothelium and aortic contraction to endothelin-1 in the pregnant rat

Endothelium-derived factors modulate tone and may be involved in hyporeactivity to vasoconstrictors, such as norepinephrine or angiotensin II, as has been previously described during gestation. The endothelium produces endothelin-1, a major vasoconstrictor peptide, therefore aortic contractions to endothelin-1 (10-10 to 3 ×10-7 M) were used to assess the role of the endothelium in pregnant Wistar rats (at 20 days of gestation). Late pregnancy is characterized by a significantly diminished systolic blood pressure in conscious rats (-17 mmHg, P < 0.001, n = 14). In pregnant and in age-matched nonpregnant female rats, endothelin-1 induced aortic contraction was greater when endothelium was present (at least P < 0.01). Indomethacin significantly reduced this contraction in aortic rings with intact endothelium in all groups. In aortic rings that had endothelium physically removed, contraction to endothelin-1 was greater in pregnant rats than in nonpregnant ones. Indomethacin decreased contraction of aortic rings in pregnant rats only. These results suggest an enhanced synthesis of vasoconstrictors by cyclooxygenases in vascular smooth muscle during pregnancy. In vessels with intact endothelium, we did not find hyporeactivity to endothelin-1 during late pregnancy. Contraction to endothelin-1 involved ETA receptors because it was decreased by BQ-123, an ETA receptor antagonist, whereas there was no significant change when using BQ-788, an ETB receptor antagonist.

Key words: endothelin-1, endothelium, contraction, aorta, gestation.

Endothelin-1 and relaxation of the rat aorta during pregnancy in nitroarginine-induced hypertension

In pregnant rats during hypertension induced by NO synthase inhibition, endothelin (ET) plasma levels are increased as in some preeclamptic women. Previously, the enhanced vasodepressor effect of endothelin-1 (ET-1) has been observed in this model, thus we decided to study the relaxation induced by ET-1 on the aorta. Non-pregnant or pregnant Wistar rats (n = 7 by group) were fed for 7 days (day 13-day 20) on a nitroarginine-enriched diet (L-NNA, 0.063% i.e. 30 mg/kg/day) or a control diet. Systolic blood pressure, measured by the tail cuff method on conscious rats at day 20 of gestation, was raised by the chronic L-NNA treatment (mean +/- s.e.m., mmHg, p < 0.001: pregnant L-NNA treated, 145 +/- 1.84 vs. pregnant control, 101 +/- 2.00 and non-pregnant L-NNA treated, 148 +/- 3.11 vs. non-pregnant control, 119 +/- 1.80). On day 20 ex vivo aortic ring relaxation was produced by ET-1 in vessels previously precontracted with norepinephrine only when endothelium was present. In control rats, ET-1 (10(-8) to 5 x 10(-8) M) produced a short but significant relaxation (mean value between 4 to 19%) followed by a long-lasting contracting phase, and a higher ET-1 concentration (10(-7) M) only produced contraction. Chronic L-NNA treatment decreased the level of relaxation (at least p < 0.05, in non-pregnant and pregnant rats) and with a 30 min L-NAME (10(-4) M) preincubation, relaxation was completely inhibited in non-pregnant and pregnant rats. BQ-123, an ETA receptor antagonist, did not produce any effect on ET-1 induced relaxation. BQ-788, an ETB receptor antagonist, significantly decreased it. In conclusion, in female rats, as in male rats, ET-1 induces a transient relaxation in the preconstricted aorta which involves endothelial ETB receptors. Despite a decrease in the systemic vascular reactivity during late gestation, the vasodilating and vasoconstricting properties of ET-1 on the aorta are not changed.

Pregnancy-induced elevation in aortic vascular smooth muscle actin in the Sprague-Dawley rat

A component of hemodynamic responses during pregnancy may include structural changes in the arterial tree. Smooth muscle alpha-actin is a major structural-contractile protein of muscle cells. We observed a marked increase. In expression of actin during pregnancy in the rat aorta, suggesting a structural alteration of conduit arteries.

Effects of pregnancy on femoral microvascular responses in the rat

OBJECTIVE

Our purpose was to test the hypothesis that augmented basal and vasodilator agonist-stimulated synthesis of endothelium-derived nitric oxide would blunt vasoconstrictor-induced contraction and enhance relaxation in isolated femoral resistance vessels from pregnant rats.

STUDY DESIGN

Potassium chloride-, phenylephrine-, and angiotensin II-induced contraction and acetylcholine-induced relaxation were assessed in small (approximately 240 microns diameter) femoral resistance arteries from gravid and virgin animals studied under isometric conditions; the effects of potassium chloride, phenylephrine, and acetylcholine were also studied in the presence of nitroarginine.

RESULTS

Maximal vasoconstriction was similar in vessels from virgin and pregnant rats, whereas phenylephrine potency was slightly enhanced in vessels from gravid animals. Nitroarginine augmented contraction in both groups but with greater effect on phenylephrine potency and on submaximal potassium chloride contractions in virgins. Acetylcholine-induced relaxation did not differ between groups and was similarly, but only partially, inhibited by nitroarginine.

CONCLUSIONS

Pregnancy does not result in vasoconstrictor resistance, increased basal nitric oxide synthesis, or augmented agonist-stimulated endothelium-dependent relaxation in this particular isolated rat microvascular preparation. Further, endothelium-dependent relaxation of these vessels depends only partly on nitric oxide.

Effect of swimming on vascular reactivity to phenylephrine and KC1 in male rats

1. The present study aimed to examine whether there is any change in vascular responsiveness to phenylephrine and KC1 during exercise, and whether the vascular endothelium plays a role in these changes. 2. Adult male rats were subjected to a swimming schedule every day for 5-6 weeks. Studies were performed in vitro on thoracic aortae. 3. Maximum contractile response to phenylephrine of endothelium-intact thoracic aortic rings (passive tension 1.0 g) obtained from swimming rats (1.2 +/- 0.2 g, n = 8) was lower than of sedentary control rats (2.1 +/- 0.2 g, n = 8). When the endothelium was removed, however, the dose-response curves of both groups of rats were shifted to the left with an increase in maximum responses and they were no longer significantly different (max. tension, swimming rats: 3.2 +/- 0.3 g, n = 6, control rats: 3.4 +/- 0.4 g, n = 5). 4. Indomethacin did not significantly alter the dose-response curves. A similar effect to that obtained by removal of the endothelium was observed when methylene blue and indomethacin were both added. 5. Passive tension in the range of 2.5-3.0 g, caused a significant increase in active tension developed to phenylephrine (1 microM for endothelium-intact and 0.1 microM for endothelium-denuded) of thoracic aortic rings of both swimming and sedentary control rats compared to their corresponding groups when using passive tension of 1.0-1.5 g. 6. The reduction in responses to phenylephrine of endothelium-intact thoracic aortic rings of swimming rats persisted with the use of a passive tension of 3.0 g. The presence of 300 microM N0-nitro-L-arginine (LNOARG)caused a significant leftward shift of the curve with an increase in maximum responses when a passive tension of either 1.0 or 3.0 g was applied to the rings. However, for the rings with a passive tension of 1.0 g, L-NOARG caused a smaller increase in maximal contractile responses to phenylephrine of the rings of sedentary controls than those of swimming rats.7. There was no difference in the dose-response curves to depolarizing concentrations of KCl (20, 40, 80 and 120 mM) of endothelium-intact thoracic aortic rings from swimming and sedentary control rats.When the endothelium was removed, however, the dose-response curves of both groups of animals were shifted to the left with an increase in maximum responses. Moreover, the responses to KCl of endothelium-denuded thoracic aortic rings of swimming rats were greater than those of sedentary control rats.8. These results suggest that there were changes in vascular responsiveness to phenylephrine and KCl during exercise. The fall in sensitivity to phenylephrine with no change in KCl responses, and the increase in maximum responses to phenylephrine in the presence of L-NOARG in endothelium-intact aortae (passive tension 1.0 g) from swimming rats, were due to an increase in spontaneous release and upregulation of phenylephrine-stimulated release of EDRF/NO, and may not be a consequence of an increase in prostaglandins or a decrease in the production of endothelial constrictors by vascular endothelium. EDRF/NO may play an important role in modulating local vasodilatation.

Nitric oxide biosynthesis during pregnancy: implications for circulatory changes

1. The biosynthesis of NO and its second messenger, cGMP, increases from pre-pregnant levels during rat gestation. An increase in plasma level and urinary excretion of cGMP is also evident during human pregnancy. However, the relative contribution of the maternal vasculature and other tissues to increased NO and cGMP biosynthesis during gestation is uncertain. Consensus is lacking about the contribution of NO to reduced maternal vascular tone and reactivity during gestation in various organ beds; clearly, further investigation is still needed. That NO may also regulate vascular smooth muscle behaviour during pregnancy by altering membrane potential is another intriguing possibility. 2. The syncytiotrophoblast of the human placenta expresses significant NO synthase activity, and along with the fetoplacental endothelium undoubtedly contributes to NO production during pregnancy. 3. Finally, it should be emphasized that vascular studies in gravid animal models need to be extended to pregnant women.

Characteristics of vascular smooth muscle in the maternal resistance circulation during pregnancy in the rat

OBJECTIVE

Our purpose was to determine if pregnancy results in a decrease in arterial sensitivity to receptor-independent stimuli and a change in vascular smooth muscle membrane potential.

STUDY DESIGN

Mesenteric resistance arteries from late pregnant (n = 19) and age-matched virgin control (n = 20) Sprague-Dawley rats were studied in a pressurized arteriograph system or isometric myograph.

RESULTS

Arteries from pregnant rats were less sensitive to membrane depolarization by K+ than were those from nonpregnant rats (mean effective concentration that produced a 50% response 49 vs 39 mmol/L, pregnant vs nonpregnant, p < 0.05). Arterial basal tone and the myogenic response to increasing pressure steps were also reduced in arteries from pregnant rats compared with nonpregnant controls. The vascular smooth muscle membrane of the arteries from the pregnant rats was hyperpolarized compared with that from the control rats (-64 mV from pregnant rats vs -57 mV from nonpregnant rats, p < 0.01). This was associated with a reduction in vasomotion in the arteries from the pregnant rats (10% for pregnant rats vs 45% from nonpregnant rats, p < 0.01).

CONCLUSION

Pregnancy results in alterations of the vascular smooth muscle, including changes in the regulation of membrane potential and a reduced sensitivity to receptor-independent stimuli.

Endogenous modulation of the blunted adrenergic response in resistance-sized mesenteric arteries from the pregnant rat

OBJECTIVE

We tested the hypothesis that during pregnancy the endothelium mediates the blunted response to adrenergic vasoconstriction.

STUDY DESIGN

Mesenteric resistance arteries from late pregnant (n = 6) and age-matched virgin control (n = 6) Sprague-Dawley rats were studied in a myograph.

RESULTS

Arteries from pregnant rats were 35% less sensitive to phenylephrine vasoconstriction than were those from nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.48 mumol/L, pregnant vs nonpregnant, p < 0.01). Meclofenamate had no effect on the vasoconstrictor response in arteries from either group. Inhibition of endothelium-derived relaxing factor with N(o)-nitro-L-arginine methyl ester or endothelial cell removal had a similar twofold increase in phenylephrine sensitivity in arteries from both the pregnant and nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.11 mumol/L for pregnant rats and 1.48 vs 0.72 mumol/L for nonpregnant rats, p < 0.01). However, methacholine relaxation response was potentiated in pregnant versus nonpregnant rats (mean effective concentration that produced a 50% response 0.030 vs 0.049 mumol/L, p < 0.01).

CONCLUSION

Although the potential for endothelium-dependent relaxation is augmented in mesenteric arteries of the pregnant rat, the decreased sensitivity to phenylephrine during pregnancy is not modulated acutely by endothelium-derived relaxing factor or by prostaglandin products of the cyclooxygenase pathway.

Prostaglandin- or endothelium-mediated vasodilation is not involved in the blunted responses of blood vessels to vasoconstrictors in pregnant rats

Pregnancy is associated with decreases of blood pressure and vascular sensitivity to vasopressor agents. We have hypothesized that the increased liberation of endogenous vasodilator(s) by the vascular endothelium or other structures could mediate these blunted responses. Thoracic aorta rings of nonpregnant, 21 days pregnant, and first day post partum rats respond similarly to acetylcholine, an endothelium-dependent vasorelaxant. In contrast, the potency of the response to sodium nitroprusside, an endothelium-independent vasorelaxant, is unchanged in tissues of pregnant rats and increased (p less than 0.05) in those of post partum animals. In the presence of indomethacin (10 mumol/L) the three groups of tissues show a decreased potency. The effects of phenylephrine on aortic rings of both nonpregnant and pregnant rats are markedly increased in the presence of Ng-monomethyl-L-arginine. Indeed, the concentration producing 50% of the maximum response of phenylephrine decreases (p less than 0.001) from 50.7 to 8.02, from 93.8 to 37.6, and from 60.4 to 5.97 nmol/L with the use of Ng-monomethyl-L-arginine (0.1 mmol/L) in rings from nonpregnant, pregnant, and postpartum rats, respectively. Simultaneously, the maximum response to phenylephrine increases markedly in the three groups of tissues. In the presence of Ng-monomethyl-L-arginine, indomethacin does not influence the response to phenylephrine. Our results do not support the possible involvement of an endogenous vasodilator (prostaglandin-like or endothelium-derived) in the blunted responses to vasoconstrictors during pregnancy.

Effects of pregnancy and endothelial cell removal on alpha-adrenoceptor-mediated responses of rat thoracic aortae

1. The possibility was examined that changes in sensitivity of the aorta to alpha-adrenoceptor agonists during pregnancy in the rat are due to changes in smooth muscle receptor-mediated responses and/or endothelial cell-mediated responses. 2. Maximum constrictor responses to phenylephrine (PE) of both endothelium-intact and denuded thoracic aortic rings were greater when tissues were obtained from 20-day pregnant rats compared with non-pregnant ones. For endothelium-denuded thoracic aortic rings, the pA2 value for phentolamine as an antagonist of PE was not significantly different for rings from 20-day pregnant rats compared with rings from non-pregnant rats. 3. Prazosin (1-50 nM) markedly depressed maximum contractile responses to PE of endothelium-intact (but not denuded) thoracic aortic rings from both non-pregnant and 20-day pregnant rats. However, this marked depression of maximum responses by prazosin did not occur in the additional presence of yohimbine (10 nM), and yohimbine alone (10-500 mM) did not affect maximum responses to PE. There was no significant difference between the pA2 value for prazosin against PE-induced contractions of endothelium-denuded aortic rings obtained from either non-pregnant or 20-day pregnant rats (in the presence of yohimbine, 10 nM). 4. These results provide no evidence for a change in aortic smooth muscle alpha-adrenoceptor affinity during pregnancy, although maximum responsiveness of aortic smooth muscle to PE is increased on day 20 of pregnancy (c.f. non-pregnant controls).

Effects of endothelial cell removal on alpha-adrenoceptor-mediated responses of aortae of pregnant rats

1. The possibility that changes in sensitivity of the aorta occurring during pregnancy in the rat are due to changes in uptake mechanisms, alpha- and/or beta-adrenoceptor-mediated responses was investigated. 2. Thoracic aortic rings from 20 day pregnant rats showed increased sensitivity to the constrictor effects of phenylephrine, with increased maximum responses when compared with those from non-pregnant animals. Removal of endothelium caused leftward shifts of the log concentration-response curves with further increases in maxima to the same extent in rings of both non-pregnant and 20 day pregnant rats. 3. Propranolol, beta-oestradiol, and nisoxetine did not significantly alter the CR-curves to phenylephrine of endothelium-intact or denuded thoracic aortic rings obtained from either non-pregnant or 20 day pregnant rats. 4. BHT-920 failed to cause marked constriction of endothelium-intact rings. After removal of endothelium, significant constrictor responses to BHT-920 occurred which were of similar magnitude for rings from both non-pregnant and 20 day pregnant rats. 5. Relaxant responses to BHT-920 of endothelium-intact rings preconstricted with phenylephrine were not significantly different between those from non-pregnant and 20 day pregnant rats. Removal of endothelium resulted in rightward shifts of the curves together with decreased maximum responses. 6. These results support earlier suggestions that the endothelium plays a role in controlling vascular reactivity to phenylephrine. The increased maximum response of thoracic aortic rings to phenylephrine seen during pregnancy does not appear to be due to marked changes in amine uptake mechanisms or beta- or alpha 2-adrenoceptor-mediated responses.

Pressor responsiveness to endothelin is not attenuated in gravid rats

The aim of this study was to evaluate the effect of synthetic human/porcine endothelin (ET-1) on mean arterial blood pressure (MAP) in pregnant and non-pregnant rats and to compare this to the effects of two well characterized agonists, angiotensin II (AII), and vasopressin (VP). On day 14 of gestation (parturition day 22) polyethylene catheters were chronically implanted in the abdominal aorta for monitoring of MAP and in the vena cava for administration of drugs. Pressor responsiveness was measured in conscious freely moving animals on day 20 and again on the 7th day post-partum. All three agonists increased MAP in a dose related manner. However, whereas the sensitivity of pregnant rats (P) to AII and VP was significantly blunted compared to postpartal (PP) measurements, the MAP responses to ET-1 were the same in both groups. Moreover, the combined administration of ET-1 at a subpressor dose (0.05 pmol/100 g bw) and AII or VP at effective doses significantly potentiated (particularly in P) the pressor effects of AII and VP. These results demonstrate that ET-1 and possibly other vasoactive substances of endothelial origin, override the compensatory mechanism of normal pregnancy with respect to the blunted responsiveness to AII and VP. Such a mechanism may be of particular relevance in the evolution of pregnancy-induced hypertension.