Effect of Pregnancy on Vascular Cgmp Production and Vasorelaxation in the Rat

Endothelin mediates renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Profound vasodilation of the kidneys and other nonreproductive organs transpires during early pregnancy. Because nitric oxide (NO) was found to mediate renal vasodilation and hyperfiltration in conscious pregnant rats, and endogenous endothelin (ET) was suggested to be vasodilatory in the renal circulation of nonpregnant rats, we tested whether endothelin mediates the NO-dependent changes in the renal circulation during pregnancy. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious pregnant and virgin rats before and during infusion of 30 micrograms/min RES-701-1 (a selective ETB receptor subtype antagonist). Baseline GFR and ERPF were significantly increased by 35% in gravid rats relative to virgin controls. During infusion of RES-701-1, the pregnant rats responded more robustly, showing a greater decline in both GFR and ERPF such that renal function converged in the two groups of rats. ERPF also converged in pregnant and virgin rats during infusion of SB-209760, a nonselective ETA/B receptor subtype antagonist. Combined infusion of Nomega-nitro-L-arginine methyl ester [L-NAME, an NO synthase (NOS) inhibitor] and RES-701-1 reduced GFR and ERPF to levels comparable to those reached with either agent given alone, suggesting inhibition of a common vasodilatory pathway. RES-701-1 and SB-209670 significantly lowered the cGMP content of small renal arteries from gravid and virgin rats in vitro, strengthening the link between the renal endothelial ETB receptor subtype and NO. Importantly, we showed that RES-701-1 is not a direct inhibitor of NOS. We conclude that endothelin mediates the NO-dependent changes in the renal circulation of conscious rats during pregnancy.

Possible involvement of endothelium-derived hyperpolarizing factor in vascular responses of abdominal aorta from pregnant rats

Increased relaxant response to acetylcholine during pregnancy is proposed to be due to an estrogen-mediated increase in nitric oxide release. We studied acetylcholine-induced pathways of relaxation in the thoracic and abdominal aortic rings from pregnant and nonpregnant Wistar-Kyoto rats and measured basal and stimulated release of nitrites in these vessels. Endothelium-dependent relaxation was significantly greater in pregnant than in nonpregnant rats. Acetylcholine provoked a concentration-dependent relaxation on thoracic and abdominal aortic rings from nonpregnant and pregnant rats. After N118-nitro-L-arginine methyl ester pretreatment, the relaxation was significantly inhibited in the two preparations of nonpregnant and pregnant rodents. The relaxation was not inhibited by indomethacin in any of the aortic segments from pregnant and nonpregnant rats. After cytochrome P450 arachidonic acid metabolism inhibitor clotrimazole, a nonsignificant decrease in the Emax to acetylcholine-induced relaxation was observed in the thoracic segments of pregnant and nonpregnant rats. On the other hand, in abdominal aorta, clotrimazole decreased maximal relaxation in rings from pregnant rats (P<.05) but did not change the acetylcholine-induced relaxation from nonpregnant rats. Our results show an increase in the acetylcholine-stimulated release of nitrites in thoracic aortic rings from pregnant rats compared with rings from nonpregnant rats, which cannot be evidenced in abdominal aortic rings. These results suggest that acetylcholine-induced vasodilation in the abdominal segment from pregnant rats is mediated only in part by nitric oxide, the remainder apparently due to an endothelium-derived vasodilator, cytochrome P450-dependent, which may be endothelium-derived hyperpolarizing factor/epoxyeicosatrienoic acid.

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.

The effect of induced reflections on human arterial waveforms

1. External compression of the human brachial artery results in waveform reflection. 2. Resonant waves are found superimposed on the original blood pressure signal. 3. The mechanism may be likened to the operation of an organ pipe with reflections at both ends of an arterial segment. 4. This has implications for measurements in arteries where external pressure is applied.

The role of angiotensin converting enzyme and nitric oxide in the enhanced systemic depressor responses to bradykinin in pregnant rats

1. In the present study we have examined the effects of angiotensin I converting enzyme (ACE) inhibition and nitric oxide (NO) synthesis inhibition on bradykinin (BK) depressor responses in pregnancy in both Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). 2. Ramipril (10 mg/kg, i.v.) significantly reduced mean blood pressure (MBP) in all animals. Further administration of L-nitro-arginine (L-NOARG, 20 mg/kg, i.v.) increased MBP to a less extent in pregnant WKY and to a similar level in pregnant SHR compared with their non-pregnant controls (controls), respectively. 3. Systemic depressor responses to BK were increased in pregnant WKY and were unchanged in pregnant SHR as compared with their controls. Ramipril (10 mg/kg, i.v.) potentiated BK responses in all groups, and abolished the differences between pregnant and non-pregnant WKY. Further administration of L-NOARG (20 mg/kg, i.v.) did not further influence BK responses in all groups. 4. The results suggest that systemic depressor responses to BK are enhanced in pregnant WKY and unchanged in pregnant SHR. Decreased ACE activity may contribute to enhanced systemic depressor responses to BK in pregnant WKY.