Evidence continues to implicate reduced placental perfusion as the cause of preeclampsia, initiating a sequence of events leading to altered vascular function and hypertension. The present study was designed to determine the influence of reduced uteroplacental perfusion pressure (RUPP) on the responsiveness of uterine arcuate resistance arteries. A condition of RUPP was surgically induced in pregnant Sprague-Dawley rats on Gestational Day 14. On Gestational Day 20, uterine arcuate arteries were mounted on a small-vessel wire myograph and challenged with incremental concentrations of vasoconstrictors and vasorelaxants for measurement of isometric tension. Compared to the sham-operated controls, uterine arteries from the RUPP group demonstrated an increased maximal tension in response to phenylephrine (P < 0.01); potassium chloride at 30 mM (P < 0.05), 60 mM (P < 0.01), and 120 mM (P < 0.01); and angiotensin II (P < 0.05). In arteries from the RUPP and sham-operated control groups, endothelium-dependent relaxation in response to acetylcholine (P < 0.05) and calcium ionophore (A23187; P < 0.05) was significantly reduced in the RUPP group compared to the sham-operated controls. Fetal growth indices, including litter size, fetal weight, and placental weight, were significantly reduced in the RUPP group compared to sham-operated controls, which is consistent with significant growth restriction. Data suggest that RUPP promotes hyperresponsiveness and impaired endothelium-dependent relaxation in uterine arcuate arteries, leading to intrauterine fetal growth restriction.

. Effects of chronic portal hypertension on agonist-induced actin polymerization in small mesenteric arteries. Am J Physiol Heart Circ Physiol 290: H1915-H1921, 2006. First published December 9, 2005 doi:10.1152/ajpheart.00643.2005.-The ability of arterial smooth muscle to respond to vasoconstrictor stimuli is reduced in chronic portal hypertension (PHT). Additional evidence supports the existence of a postreceptor defect in vascular smooth muscle excitation contraction coupling. However, the nature of this defect is unclear. Recent studies have shown that vasoconstrictor stimuli induce actin polymerization in smooth muscle and that the associated increase in F-actin is necessary for force development. In the present study we have tested the hypothesis that impaired actin polymerization contributes to reduced vasoconstrictor function in small mesenteric arteries derived from rats with chronic prehepatic PHT. In vitro studies were conducted on small mesenteric artery vessel rings isolated from normal and PHT rats. Isometric tension responses to incremental concentrations of phenylephrine were significantly reduced in PHT arteries. The ability to polymerize actin in portal hypertensive mesenteric arteries stimulated by phenylephrine was attenuated compared with control. Inhibition of cAMP-dependent protein kinase (PKA) restored agonist-induced actin polymerization of arteries from PHT rats to normal levels. Depolymerization of actin in arteries from normal rats reduced maximal contractile force but not myosin phosphorylation, suggesting a key role for the dynamic regulation of actin polymerization in the maintenance of vascular smooth muscle contraction. We conclude that reductions in agonistinduced maximal force development of PHT vascular smooth muscle is due, in part, to impaired actin polymerization, and prolonged PKA activation may underlie these changes. vascular smooth muscle; protein kinase A; myosin; phenylephrine; isometric tension ONE OF THE MOST INTRIGUING vascular consequences of chronic portal hypertension is the decreased ability of blood vessels to respond to vasoconstrictor stimuli (2, 3). The observed vasoconstrictor dysfunction is systemic in nature in that it is not limited to the splanchnic vascular territory (13,18,19). This finding has been attributed to a postreceptor defect in vascular smooth muscle excitation contraction coupling (21, 24). Additional evidence supports the contention that prolonged elevation in cyclic nucleotide-dependent vasodilators plays an important role in this defect inasmuch as vasoconstrictor effectiveness in portal hypertension can be restored by protein kinase A (PKA) inhibition (31). However, the cellular mechanism whereby cyclic nucleotide-dependent pathways impair vasoconstriction in chronic portal hypertension is still unclear.Most previous investigations suggested that cyclic nucleotide-dependent relaxation occurs through inhibition or reversal of the Ca 2ϩ -dependent phosphorylation of the regulatory myosin light chain (MLC 20 ). However, prolonged activat...

Reduced perfusion to the placenta in early pregnancy is believed to be the initiating factor in the development of preeclampsia, triggering local ischemia and systemic vascular hyperresponsiveness. This sequence of events creates a predisposition to the development of altered vascular function and hypertension. This study was designed to determine the influence of placental insufficiency on the responsiveness of mesenteric resistance arteries in an animal model of preeclampsia. Placental insufficiency was induced by reduction in uteroplacental perfusion pressure (RUPP) in experimental Sprague-Dawley rat dams. The uterine branches of the ovarian arteries and the abdominal aortae of pregnant rats were surgically constricted on gestational Day 14. Dams in the control group underwent a sham procedure. Rats were euthanized on gestational Day 20, followed by removal of the small intestine and adjacent mesentery. First-order mesenteric resistance arteries were mounted on a small vessel wire myograph and challenged with incremental concentrations of vasoconstrictors and vasorelaxants. Mesenteric arteries in dams with placental insufficiency demonstrated an increased maximal tension to phenylephrine (7.15 +/- 0.15 vs. 5.4 +/- 0.27 mN/mm, P < 0.001); potassium chloride at 60 mM (3.43 +/- 0.11 vs. 2.77 +/- 0.14 mN/mm, P < 0.01) and 120 mM (3.92 +/- 0.18 vs. 2.97 +/- 0.16 mN/mm, P < 0.01); and angiotensin II (2.59 +/- 0.42 vs. 1.51 +/- 0.22 mN/mm, P < 0.05). Maximal relaxation to endothelium-dependent relaxants acetylcholine and calcium ionophore (A23187) was not significantly reduced. Data suggest that placental insufficiency leads to hyperresponsiveness to vasoconstrictor stimuli in mesenteric arteries.

A variety of contractile stimuli increases actin polymerization, which is essential for smooth muscle contraction. However, the mechanism(s) of actin polymerization associated with smooth muscle contraction is not fully understood. We tested the hypothesis that phosphorylated myosin triggers actin polymerization. The present study was conducted in isolated intact or beta-escin-permeabilized rat small mesenteric arteries. Reductions in the 20-kDa myosin regulatory light chain (MLC20) phosphorylation were achieved by inhibiting MLC kinase with ML-7. Increases in MLC20 phosphorylation were achieved by inhibiting myosin light chain phosphatase with microcystin. Isometric force, the degree of actin polymerization as indicated by the F-actin-to-G-actin ratio, and MLC20 phosphorylation were determined. Reductions in MLC20 phosphorylation were associated with a decreased force development and actin polymerization. Increased MLC20 phosphorylation was associated with an increased force generation and actin polymerization. We also found that a heptapeptide that mimics the actin-binding motif of myosin II enhanced microcystin-induced force generation and actin polymerization without affecting MLC20 phosphorylation in beta-escin-permeabilized vessels. Collectively, our data demonstrate that MLC20 phosphorylation is capable of triggering actin polymerization. We further suggest that the binding of myosin to actin triggers actin polymerization and enhances the force development in arterial smooth muscle.

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