Increased acetylcholine-induced vasodilation in pregnant rats: A role for gap junctional communication

Enhanced endothelin receptor type B-mediated vasodilation and underlying [Ca²⁺]i in mesenteric microvessels of pregnant rats

BACKGROUND AND PURPOSE

Normal pregnancy is associated with decreased vascular resistance and increased release of vasodilators. Endothelin-1 (ET-1) causes vasoconstriction via endothelin receptor type A (ET(A)R), but could activate ET(B)R in the endothelium and release vasodilator substances. However, the roles of ET(B)R in the regulation of vascular function during pregnancy and the vascular mediators involved are unclear.

EXPERIMENTAL APPROACH

Pressurized mesenteric microvessels from pregnant and virgin Sprague-Dawley rats were loaded with fura-2/AM for simultaneous measurement of diameter and [Ca²⁺]i.

KEY RESULTS

High KCl (51 mM) and phenylephrine (PHE) caused increases in vasoconstriction and [Ca²⁺]i that were similar in pregnant and virgin rats. ET-1 caused vasoconstriction that was less in pregnant than virgin rats, with small increases in [Ca²⁺]i. Pretreatment with the ET(B)R antagonist BQ-788 caused greater enhancement of ET-1-induced vasoconstriction in pregnant rats. ACh caused endothelium-dependent relaxation and decreased [Ca²⁺]i, and was more potent in pregnant than in virgin rats. ET-1 + ET(A)R antagonist BQ-123, and the ET(B)R agonists sarafotoxin 6c (S6c) and IRL-1620 caused greater vasodilation in pregnant than in virgin rats with no changes in [Ca²⁺]i, suggesting up-regulated ET(B)R-mediated relaxation pathways. ACh-, S6c- and IRL-1620-induced relaxation was reduced by the NO synthase inhibitor Nω-nitro-L-arginine methyl ester, and abolished by tetraethylammonium or endothelium removal. Western blots revealed greater amount of ET(B)R in intact microvessels of pregnant than virgin rats, but reduced levels in endothelium-denuded microvessels, supporting a role of endothelial ET(B)R.

CONCLUSIONS AND IMPLICATIONS

The enhanced ET(B)R-mediated microvascular relaxation may contribute to the decreased vasoconstriction and vascular resistance during pregnancy.

Arterial endothelium-derived hyperpolarization: potential role in pregnancy adaptations and complications

Pregnancy encompasses substantial changes in vascular function to accommodate dramatic increases in blood volume and uteroplacental blood flow to the growing fetus. Despite increased hemodynamics, decreased peripheral resistance results in a reduction in mean arterial blood pressure. Vascular tone, and hence peripheral resistance, is determined by a delicate balance of constrictor and dilator capacities. In the normal physiological response to pregnancy, endothelial-derived hyperpolarization (EDH) has been shown to be a major contributor; both EDH and nitric oxide (NO) are predominantly involved in providing an increased vascular capacity for vasodilation. The ability of EDH and NO to adequately accommodate increased blood volume is tested in pathological states such as placental insufficiency or diabetes and both EDH and NO-dependent mechanisms seem to be impacted in these situations. Pregnancy complications also have an impact on the cardiovascular health of the offspring. In adult offspring born from complicated pregnancies, the data suggest that EDH mechanisms are largely maintained, whereas NO is commonly reduced. A diversity of EDH mechanisms may be useful in providing many targets for potential therapeutic avenues for compromised pregnancies; however, further research delineating the mechanisms of EDH and the interactions of NO and EDH, in normal and pathological pregnancies is required.

Increased myoendothelial gap junctions mediate the enhanced response to epoxyeicosatrienoic acid and acetylcholine in mesenteric arterial vessels of cirrhotic rats

BACKGROUND

Cirrhotic portal hypertension is characterized by mesenteric arterial vasodilation and hyporeactivity to vasoconstrictors.

AIM

We evaluated the role of epoxyeicosatrienoic acid (EET) and of myoendothelial gap junctions (GJ) in the haemodynamic alterations of experimental cirrhosis.

METHODS

Thirty-five control rats and 35 rats with carbon tetrachloride (CCl(4))-induced cirrhosis were studied. Small resistance mesenteric arteries (diameter <350 μm) were connected to a pressure servo controller in a video-monitored perfusion system. Concentration-response curves to acetylcholine (ACh) were evaluated in mesenteric arteries pre-incubated with indomethacin, N(G)-nitro-L-arginine-methyl-ester and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one before and after the epoxygenase inhibitor miconazole or 18α-glycyrrhetinic acid (18α-GA) (GJ inhibitor). EC(50) was calculated. Concentration-response curves to 11,12-EET were also evaluated. mRNA and protein expression of connexins (Cxs) in the mesenteric arteries was evaluated by real-time PCR and immunohistochemistry.

RESULTS

The ACh response was increased in cirrhotic rats (EC(50): -6.55±0.10 vs. -6.01±0.10 log[M]; P<0.01) and was blunted by miconazole only in cirrhotic animals. 18α-GA blunted the response to ACh more in cirrhotic than that in control rats (P<0.05). Concentration-response curves to 11,12-EET showed an increased endothelium-dependent vasodilating response in cirrhotic rats (P<0.05); the BK(Ca) inhibitor Iberiotoxin (25 nM) blocked the response in normal rats but not in cirrhotic rats, while 18α-GA blunted the response in cirrhotic rats but not in control rats. An increased mRNA and protein expression of Cx40 and Cx43 in cirrhotic arteries was detected (P<0.05).

CONCLUSIONS

The increased nitric oxide/PGI(2)-independent vasodilation of mesenteric arterial circulation in cirrhosis is because of, at least in part, hyperreactivity to 11,12-EET through an increased expression of myoendothelial GJs.

Lipopolysaccharide reduces electrical coupling in microvascular endothelial cells by targeting connexin40 in a tyrosine-, ERK1/2-, PKA-, and PKC-dependent manner

Electrical coupling along the endothelium is central in the arteriolar conducted response and in control of vascular resistance. It has been shown that exposure of endothelium to lipopolysaccharide (LPS, an initiating factor in sepsis) reduces intercellular communication in vitro and in vivo. The molecular basis for this reduction is not known. We examined the effect of LPS on electrical coupling in monolayers of cultured mouse microvascular endothelial cells (MMEC) derived from the mouse hindlimb skeletal muscle. To assess coupling, we measured the spread of electrical current injected into the monolayer and computed the monolayer intercellular resistance (inverse measure of coupling). LPS (10 microg/ml, 1 h) reduced coupling (i.e., increased resistance) in MMEC isolated from wild-type, connexin37 (Cx37) null and Cx43(G60S) (nonfunctional mutant) mice, but not in MMEC derived from Cx40 null mice. LPS also activated JNK1/2, p38 and ERK1/2 MAP kinases. Pretreatment of WT monolayers with ERK1/2 inhibitor U0126 (20 microM, 1 h) prevented the LPS-induced decrease in coupling, while inhibition of JNK1/2 with SP600125 (20 microM, 1 h) and p38 with a p38 inhibitor (10 nM, 1 h) had no effect. Furthermore, inhibition of tyrosine kinases with PP-2 (10 nM, 1 h), activation of PKA by 8-bromo-cAMP (1 mM, 5 min), and activation of PKC by bryostatin-2 (10 nM, 1 h) also prevented the reduction in coupling. We propose that LPS reduces inter-endothelial electrical coupling via tyrosine-, ERK1/2-, PKA-, and PKC-dependent signaling that targets Cx40. We suggest that this mechanism contributes to compromised arteriolar function following LPS exposure.

Connexin 43 mediates endothelium-derived hyperpolarizing factor-induced vasodilatation in subcutaneous resistance arteries from healthy pregnant women

The role of gap junctions in endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation of human arteries was assessed using connexin mimetic peptides (CMPs) designated (37,43)Gap27, (40)Gap27, and (43)Gap26 according to homology with the major vascular connexins (Cx37, Cx40, and Cx43). Resistance arteries were obtained from subcutaneous fat biopsies of healthy pregnant women undergoing elective cesarean section. Endothelium-dependent vasodilatation to bradykinin (BK) was assessed using wire myography. N(omega)-nitro-l-arginine methyl ester (l-NAME) and indomethacin (nitric oxide synthase and cyclooxygenase inhibitors, respectively) attenuated maximal relaxation to BK (R(max)) by approximately 50%. Coincubation with l-NAME, indomethacin, and the combined CMPs ((37,43)Gap27, (40)Gap27, and (43)Gap26) almost abolished relaxation to BK (R(max) = 12.2 +/- 3.7%). In arteries incubated with l-NAME and indomethacin, the addition of either (37,43)Gap27 or (40)Gap27 had no significant effect on R(max), whereas (43)Gap26 caused marked inhibition (R(max) = 21 +/- 6.4%, P = 0.005 vs. l-NAME plus indomethacin alone) that was similar to that of the triple combination. Endothelium-independent vasorelaxation was unaffected by CMPs, l-NAME, or indomethacin. Immunohistochemistry demonstrated Cx37, Cx40, and Cx43 expression in the endothelium and vascular smooth muscle. In pregnant women, EDHF-mediated vasorelaxation of subcutaneous resistance arteries is dependent on Cx43 and gap junctions.

The mechanism of EDHF-mediated responses in subcutaneous small arteries from healthy pregnant women

We studied the importance of endothelium-derived hyperpolarizing factor (EDHF) vs. nitric oxide (NO) and prostacyclin (PGI(2)) in bradykinin (BK)-induced relaxation in isolated small subcutaneous arteries from normal pregnant women. We also explored the contribution of cytochrome P-450 (CYP450) product of arachidonic acid (AA) metabolism, hydrogen peroxide (H(2)O(2)), and gap junctions that have been suggested to be involved in EDHF-mediated responses. Isolated arteries obtained from subcutaneous fat biopsies of normal pregnant women (n = 30) undergoing planned cesarean section were mounted in a wire-myography system. In norepinephrine-constricted vessels, incubation with N(G)-nitro-L-arginine methyl ester (L-NAME) resulted in a significant reduction in relaxation to BK. Simultaneous incubation with L-NAME and indomethacin failed to modify this response further. BK-mediated dilatation in the presence of K(+)-modified solution was decreased to similar level as obtained after incubation with L-NAME. Incubation with L-NAME abolished BK-induced responses in K(+)-modified solution. Sulfaphenazole, a specific inhibitor of CYP450 epoxygenase, and catalase (an enzyme that decomposes H(2)O(2)) did not affect the EDHF-mediated relaxation because concentration-response curves to BK were similar in arteries after incubation with L-NAME vs. L-NAME + sulfaphenazole and L-NAME + catalase. The inhibitor of gap junctions, 18 alpha-glycyrrhetinic acid, significantly reduced BK-mediated relaxation both without and with incubation with L-NAME. We found that both NO and EDHF, but not PGI(2), are involved in the endothelium-dependent dilatation to BK. BK-induced relaxation is almost equally mediated by NO and EDHF. CYP450 epoxygenase metabolites of AA or H(2)O(2) do not account for EDHF-mediated response; however, gap junctions are involved in the EDHF-mediated responses to BK in subcutaneous small arteries in normal pregnancy.

Glycine rectifies vascular dysfunction induced by dietary protein imbalance during pregnancy

Protein restriction in rat pregnancy programmes the development of elevated systolic blood pressure and vascular dysfunction in the offspring. A recent study has shown that hypertension is reversed by maternal glycine supplementation. Whether this protective effect is exerted directly on the embryo and fetus, or indirectly via effects on the mother, is unknown although we have previously shown abnormalities in the maternal vasculature. We tested the hypothesis that dietary glycine repletion would reverse endothelial dysfunction in protein-restricted pregnant rat dams using wire myography. Impaired acetylcholine- (P < 0.01) and isoprenaline-induced (P < 0.05) vasodilatation in isolated mesenteric arteries (MA) from protein-restricted pregnant dams was accompanied by reduced vascular nitric oxide (NO) release (P < 0.05). Dietary glycine supplementation reversed vascular dysfunction in MA (P < 0.05) and improved NO release thus potentially protecting the maternal circulation. The impaired NO release in the MA of low protein diet dams was not accompanied by reduced eNOS mRNA expression, suggesting that eNOS activity was altered. Protein restriction did not alter the vascular function of a conduit artery, the thoracic aorta. These results provide evidence that adequate provision of glycine, a conditionally essential amino acid in pregnancy, may play a role in the vascular adaptations to pregnancy, protecting the fetus from abnormal programming of the cardiovascular system.

Vascular adaptations to pregnancy in mice: effects on myogenic tone

The mechanisms underlying vascular adaptations in pregnancy remain to be fully elucidated. One of the contributory mechanisms for reduced vascular tone may be a reduction of myogenic tone. Myogenic tone was assessed as the difference between internal diameter in the presence and absence of external calcium at different intramural pressure steps (60-100 mmHg). Myogenic responses were reduced in resistance-sized mesenteric and main uterine arteries in late pregnant compared with nonpregnant C57BL/6J mice. In vessels from pregnant, but not nonpregnant mice, the myogenic response was enhanced by preincubation with nitric oxide (NO) synthase inhibitor N(G)-nitro-l-arginine methyl ester, was further elevated by the gap junction inhibitor 18-alpha glycyrrhetinic acid, but was unaltered by the prostaglandin H synthase inhibitor meclofenamate. Endothelium removal enhanced myogenic tone only in the vessels from pregnant animals, thus confirming the role of the endothelium in modulating myogenic tone in pregnancy. These results suggest that endothelium-derived NO as well as gap junction communications modulate myogenic tone in mouse pregnancy.

Paradoxical overexpression and translocation of connexin43 in homocysteine-treated endothelial cells

Hyperhomocysteinemia is an established cause of defective vasorelaxation. Gene expression screening of human umbilical vein endothelial cells (HUVEC) treated with homocysteine (Hcy) revealed that connexin43 (Cx43) was upregulated. Expression of Cx43 was increased more than twofold in Hcy-treated HUVEC. Gap junctional communication (Lucifer yellow and whole cell patch clamp) was not enhanced in Hcy-treated HUVEC. HUVEC expressing chimeric Cx43-green fluorescent protein exhibited it at cell-cell contacts in control but showed redistribution to the intracellular compartment(s) in Hcy-treated cells. Confocal microscopy of HUVEC stained with anti-Cx43, mitochondrial, and endoplasmic reticulum fluorescent markers showed the localization of Cx43 to the plasma membrane of control cells and its colocalization with the mitochondrial marker in Hcy-treated HUVEC. Studies of isolated mitochondria confirmed overexpression of Cx43 in the mitochondria of Hcy-treated HUVEC. Microdissected renal interlobar arteries, which normally exhibit endothelium-derived hyperpolarizing factor-induced vasorelaxation, showed reduced nitric oxide synthase- and cyclooxygenase-independent vasorelaxation to acetylcholine after pretreatment with Hcy. In summary, Hcy-induced upregulation of Cx43 transcript and protein expression are associated with unaltered intercellular communication, redistribution of Cx43 in HUVEC, and reduced nitric oxide- and prostanoid-independent vascular responses to acetylcholine in Hcy-treated arteries.

EDHF mediates flow-induced dilation in skeletal muscle arterioles of female eNOS-KO mice

Vasodilation to increases in flow was studied in isolated gracilis muscle arterioles of female endothelial nitric oxide synthase (eNOS)-knockout (KO) and female wild-type (WT) mice. Dilation to flow (0-10 microl/min) was similar in the two groups, yet calculated wall shear stress was significantly greater in arterioles of eNOS-KO than in arterioles of WT mice. Indomethacin, which inhibited flow-induced dilation in vessels of WT mice by approximately 40%, did not affect the responses of eNOS-KO mice, whereas miconazole and 6-(2-proparglyoxyphenyl)hexanoic acid (PPOH) abolished the responses. Basal release of epoxyeicosatrienonic acids from arterioles was inhibited by PPOH. Iberiotoxin eliminated flow-induced dilation in arterioles of eNOS-KO mice but had no effect on arterioles of WT mice. In WT mice, neither N(omega)-nitro-L-arginine methyl ester nor miconazole alone affected flow-induced dilation. Combination of both inhibitors inhibited the responses by approximately 50%. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) alone inhibited flow-induced dilation by approximately 49%. ODQ + indomethacin eliminated the responses. Thus, in arterioles of female WT mice, nitric oxide and prostaglandins mediate flow-induced dilation. When eNOS is inhibited, endothelium-derived hyperpolarizing factor substitutes for nitric oxide. In female eNOS-KO mice, metabolites of cytochrome P-450, via activation of large-conductance Ca2+-activated K+ channels of smooth muscle, mediate entirely the arteriolar dilation to flow.