We hypothesize that actin polymerization within vascular smooth muscle (VSM) in response to increased intravascular pressure is a novel and previously unrecognized mechanism underlying arterial myogenic behavior. This hypothesis is based on the following observations. 1) Unlike skeletal or cardiac muscle, VSM contains a substantial pool of unpolymerized globular (G) actin whose function is not known. 2) The cytosolic concentration of G-actin is significantly reduced by an elevation in intravascular pressure, demonstrating the dynamic nature of actin within VSM and implying a shift in the F:G equilibrium in favor of F-actin. 3) Agents that inhibit actin polymerization and stabilize the cytoskeleton (cytochalasins and latrunculin) inhibit the development of myogenic tone and decrease the effectiveness of myogenic reactivity. 4) Depolymerization of F-actin with cytochalasin D causes VSM relaxation and increased G-actin content, whereas polymerization of F-actin with jasplakinolide causes VSM contraction and decreased G-actin content. These results are consistent with observations in other cell types in which actin dynamics have been implicated in contractility and/or motility. Actin filament formation in VSM may therefore underlie mechanotransduction and, by providing additional sites for interaction with myosin, enhance force production in response to pressure. Although the mechanism by which actin polymerization is stimulated by pressure is not known, it likely occurs via integrin-mediated activation of signal transduction pathways previously associated with VSM contraction (e.g., PKC activation, Rho A, and tyrosine phosphorylation).

Myogenic behavior, prevalent in resistance arteries and arterioles, involves arterial constriction in response to intravascular pressure. This process is often studied in vitro by using cannulated, pressurized arterial segments from different regional circulations. We propose a comprehensive model for myogenicity that consists of three interrelated but dissociable phases: 1) the initial development of myogenic tone (MT), 2) myogenic reactivity to subsequent changes in pressure (MR), and 3) forced dilatation at high transmural pressures (FD). The three phases span the physiological range of transmural pressures (e.g., MT, 40-60 mmHg; MR, 60-140 mmHg; FD, >140 mmHg in cerebral arteries) and are characterized by distinct changes in cytosolic calcium ([Ca(2+)](i)), which do not parallel arterial diameter or wall tension, and therefore suggest the existence of additional regulatory mechanisms. Specifically, the development of MT is accompanied by a substantial (200%) elevation in [Ca(2+)](i) and a reduction in lumen diameter and wall tension, whereas MR is associated with relatively small [Ca(2+)](i) increments (<20% over the entire pressure range) despite considerable increases in wall tension and force production but little or no change in diameter. FD is characterized by a significant additional elevation in [Ca(2+)](i) (>50%), complete loss of force production, and a rapid increase in wall tension. The utility of this model is that it provides a framework for comparing myogenic behavior of vessels of different size and anatomic origin and for investigating the underlying cellular mechanisms that govern vascular smooth muscle mechanotransduction and contribute to the regulation of peripheral resistance.

Several recent studies have implicated the RhoA-Rho kinase pathway in arterial myogenic behavior. The goal of this study was to determine the effects of Rho kinase inhibition (Y-27632) on cerebral artery calcium and diameter responses as a function of transmural pressure. Excised segments of rat posterior cerebral arteries (100-200 microm) were cannulated and pressurized in an arteriograph at 37 degrees C. Increasing pressure from 10 to 60 mmHg triggered an elevation of cytosolic calcium concentration ([Ca(2+)](i)) from 113 +/- 9 to 199 +/- 12 nM and development of myogenic tone. Further elevation of pressure to 120 mmHg induced only a minor additional increase in [Ca(2+)](i) and constriction. Y-27632 (0.3-10 microM) inhibited myogenic tone in a concentration-dependent manner at 60 and 120 mmHg with comparable efficacy; conversely, sensitivity was decreased at 120 vs. 60 mmHg (50% inhibitory concentration: 2.5 +/- 0.3 vs. 1.4 +/- 0.1 microM; P < 0.05). Dilation was accompanied by further increases in [Ca(2+)](i) and an enhancement of Ca(2+) oscillatory activity. Y-27632 also effectively dilated the vessels permeabilized with alpha-toxin in a concentration-dependent manner. However, dilator effects of Y-27632 at low concentrations were larger at 60 vs. 100 mmHg. In summary, the results support a significant role for RhoA-Rho kinase pathway in cerebral artery mechanotransduction of pressure into sustained vasoconstriction (myogenic tone and reactivity) via mechanisms that augment smooth muscle calcium sensitivity. Potential downstream events may involve inhibition of myosin phosphatase and/or stimulation of actin polymerization, both of which are associated with increased smooth muscle force production.

The objectives of this study were to determine whether placental growth factor (PlGF) exerts a vasodilatory effect on rat uterine vessels (arcuate arteries and veins) and to examine regional differences in reactivity by comparing these responses to those of comparably sized mesenteric vessels. We also sought to examine and compare its effects on human uterine and subcutaneous vessels. All vessels were studied in vitro, under pressurized (rat) or isometric wire-mounted (human) conditions, and exposed to a range of PlGF concentrations. Inhibitors of nitric oxide and prostaglandin synthesis were included in an effort to understand the causal mechanism(s). In rat uterine arteries, the effects of receptor inhibition and activation using selective ligands for VEGFR-1 (PlGF) vs. VEGFR-2 (VEGF-E) were determined, and real-time RT-PCR was performed to evaluate the effect of pregnancy on relative abundance of VEGFR-1 and VEGFR-2 message in the vascular wall. PlGF was a potent vasodilator of all vessels studied, with greatest sensitivity observed in rat uterine arteries. Pregnancy significantly augmented dilator sensitivity to PlGF, and this effect was associated with selective upregulation of VEGFR-1 message in the pregnant state. The contribution of nitric oxide was appreciable in rat and human uterine arteries, with lesser effects in rat uterine veins and mesenteric arteries, and with no observable effect in human subcutaneous vessels. Based on these results, we conclude that PlGF is a potent vasodilator of several vessel types in both humans and rats. Its potency and mechanism vary with physiological state and vessel location and are mediated solely by the VEGFR-1 receptor subtype. Gestational changes in the uterine circulation suggest that this factor may play a role in modulating uterine vascular remodeling and blood flow during the pregnant state.

Background and Purpose Parenchymal arterioles (PAs) are high resistance vessels in the brain that connect pial vessels to the microcirculation. We previously showed that PAs have increased vasoconstriction after ischemia and reperfusion that could increase perfusion deficit. Here, we investigated underlying mechanisms by which early post-ischemic reperfusion causes increased vasoconstriction of PAs. Methods Isolated and pressurized PAs from within the MCA territory were studied in male Wistar rats that were either nonischemic control (CTL; n=34) or after exposure to transient MCAO by filament occlusion for 2 hours with 30 minutes of reperfusion (MCAO; n=38). The relationships between pressure-induced tone, smooth muscle calcium (using Fura 2) and membrane potential were determined. Sensitivity of the contractile apparatus to calcium was measured in permeabilized arterioles using Staphaloccus aureus α-toxin. Reactivity to inhibition of TRPM4 (9-phenanthrol), Rho kinase (Y27632) and protein kinase C (Gö6976) was also measured. Results After MCAO, PAs had increased myogenic tone compared to controls (47±2% vs. 35±2% at 40 mmHg; p<0.01), without an increase in smooth muscle calcium (177±21 vs. 201±16 nmol/L; p>0.05) or membrane depolarization (−38±4 vs. −36±1 mV;p>0.05). In α-toxin permeabilized vessels, MCAO caused increased sensitivity of the contractile apparatus to calcium. MCAO did not affect dilation to TRPM4- or PKC-inhibition, but diminished dilation to Rho kinase inhibition. Conclusions The increased vasoconstriction of PAs during early post-ischemic reperfusion appears to be due to calcium sensitization of smooth muscle and could contribute to infarct expansion and limit neuroprotective agents from reaching their target tissue.