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We studied the role of the endothelium in diameter changes as a function of flow of the isolated femoral artery of the rabbit (n = 15) perfused and superfused with a physiological salt solution (37 °C). In 10 vessels, diameters were studied before and after exposure to gossypol, an agent that impairs the endothelial function pharmacologically. In 5 of these 10 vessels we added albumin (1.5%) to the perfusion solution. The mean external diameter (±SEM) after equilibration for 60 min at a transmural pressure of 50 cm H₂O (n = 10) was: 1,426 ± 34 µM Vessels were then constricted with norepinephrine (1.0–1.5 µM in the supervision solution) to 70% of the resting diameter, acetylcholine was used to check endothelial function. All vessels constricted as flow was increased (p < 0.001), irrespective of the impairment of the endothelial function by gossypol or the presence of albumin. It is therefore unlikely that the flow-induced constriction results from a ‘wash away’ effect of endothelium-derived relaxing factor (EDRF). To test whether EDRF could still play a role after gossypol, we used hemoglobin (n = 5) to bind EDRF. Flow-dependent constriction was still observed, although the mean diameter was decreased. We conclude that flow-dependent constriction is either mediated via the endothelial cells, but not via EDRF, or that the endothelial cells are not involved.

-27632). Slow acetylcholine-induced pulmonary vasodilation (262 Ϯ 5 s) was not due to the RT of endothelial NO release (45-55 s) and was always longer than RT in renal arteries (15 Ϯ 4 s). The rate-determining step is located in the smooth muscle cells. This was confirmed by the existing differences between the RT of the NO solution and KCl-induced renal and pulmonary vasoreactivity in endothelium-denuded arteries. We found that the pulmonary contractile amplitude increases and the RT decreases by L-NNA or LPA. In contrast, Y-27632 reduced the contractile amplitude and increased the RT in pulmonary arteries. These phenomena were dependent on the contractile stimulus (phenylephrine or KCl). In conclusion, slow pulmonary vasoreactivity is a smooth muscle cell characteristic that can be enhanced by RhoA and NO or endothelium removal. These effects were counteracted by Rho kinase inhibition. We show a role for RhoA/Rho kinase and NO in the modulation of pulmonary vascular reactivity. nitric oxide electrode; endothelium; amplitude of constriction; response time ISOLATED SYSTEMIC ARTERIES show large differences in the amplitude and speed of their responses to constrictors and dilators. These differences depend on the type and location of the vessel and differences in smooth muscle cell (SMC) characteristics (3,9,17,18,22). The characteristics of the vascular reactivity of pulmonary arteries, which form a special group in the circulation, are not well described in the literature.The two objectives of the present study were therefore as follows: First, do pulmonary arteries exhibit similar magnitudes of contraction and response times to agonists as renal arteries? Renal arteries are, like pulmonary arteries, organ conduit arteries that are sensitive to similar agonists as pulmonary arteries. Second, if the magnitudes of contraction and response times are different, is it possible to modify these characteristics so that the renal and pulmonary vascular responses become similar?With respect to the second objective, we investigated whether modification of the pulmonary vascular contractility and response time can be accomplished by changes in the sensitivity of the myosin light chain (MLC) for Ca 2ϩ . One of the ways in which the sensitivity of the contractile apparatus for Ca 2ϩ can be modified is via changing the activity of MLC phosphatase. Figure 1 shows the role of MLC phosphatase in the regulation of SMC contraction. MLC phosphatase is inactivated by Rho kinase, which is activated by a GTP-bound active form of Rho (RhoA), thereby increasing the SMC Ca 2ϩ sensitivity (6, 10). RhoA can be activated by the bioactive lipid lysophosphatidic acid (LPA) and inhibited by nitric oxide (NO) via a cGMP protein kinase, whereas Rho kinase can be inhibited by Y-27632 (13, 16, 18). Thus LPA and NO inhibition would both lead to increased RhoA activity and therefore increased Ca 2ϩ sensitivity, suggesting a similarity in their effects. To answer the second aim of our study in more detail, we therefore investigated whether RhoA activ...

Vascular endothelial cells release dilatory compounds like nitric oxide and prostacyclin, as well as contractile factors like endothelin-1 (ET-1). We investigated the interaction of ET-1 with nitric-oxide-mediated dilation in cannulated pressurized (75 mm Hg) arterioles from rat cremaster muscle (180 ± 3 µm). Arterioles constricted spontaneously to 101 ± 3 µm, while ET-1 (0.4 nM)increased constriction to 78 ± 3 µm. Acetylcholine, an endothelium-dependent nitric-oxide-mediated vasodilator induced a dose-dependent dilation during spontaneous tone. After addition of ET-1, the response to acetylcholine was significantly impaired. Nitroprusside, an endothelium-independent nitric oxide donor, induced a dose-dependent dilation that was almost completely inhibited by ET-1. In contrast, 8-Br-cGMP-induced dilation was unaffected. Thus, ET-1 appears to inhibit nitric-oxide-mediated dilation at the level of cGMP formation or degradation. The effect of ET-1 appears to be specific for nitric oxide as responses to a prostacyclin analogue were impaired at low doses only. The inhibitory effect of ET-1 on nitric-oxide-mediated dilation could be mimicked with high potassium (65 ± 6 µm), but not with phenylephrine (74 ± 8 µm)-induced constriction. These data show a direct inhibitory effect of ET-1 on nitric-oxide-mediated dilation in isolated skeletal muscle arterioles.

Kerkhof, Cornel J. M., Peter J. W. Van Der Linden, and Pieter Sipkema. Role of myocardium and endothelium in coronary vascular smooth muscle responses to hypoxia. Am J Physiol Heart Circ Physiol 282: H1296-H1303, 2002. First published November 15, 2001 10.1152/ajpheart.00179.2001.-Hypoxia triggers a mechanism that induces vasodilation in the whole heart but not necessarily in isolated coronary arteries. We therefore studied the role of cardiomyocytes (CM), smooth muscle cells (SMC), and endothelial cells (EC) in coronary responses to hypoxia (PO 2 of 5-10 mmHg). In an attempt to determine the factor(s) released in response to hypoxia, we inhibited the contribution of adenosine, ATP-sensitive K ϩ channels, prostaglandins, and nitric oxide. Isolated rat septal artery segments without (ϪT) and with a layer of cardiac tissue (ϩT) were mounted in a double wire myograph, and constriction was induced. Hypoxia induced a decrease in isometric force of 21% and 61% in ϪT and ϩT segments, respectively (P Ͻ 0.05). EC removal increased the relaxation to hypoxia in ϪT segments to 33% but had the same effect in ϩT segments (61%). Only one of the inhibitors, the adenosine antagonist in ϩT segments, partially affected the relaxation due to hypoxia. The role of adenosine is thus limited and other mechanisms have to contribute. We conclude that hypoxia induces a relaxation of SMC that is augmented by the presence of CM and blunted by the endothelium. A single mediator does not induce those effects. endothelial cells; smooth muscle cells; cardiomyocytes; ATPsensitive potassium channels; adenosine SEVERAL INVESTIGATORS (1,2,4,6,10,20,21,27) have shown in isolated whole heart preparations that hypoxia (lowering the PO 2 in the perfusion solution) induces vasodilation in the whole heart. However, it is not clear whether or not mediators of hypoxia-induced vasodilation originate from vascular or myocardial tissue.The results obtained from studies on the effect of hypoxia on isolated coronary artery tone are not consistent and range from dilation or decreases in isometric force (3,8,12,34) to constriction or increases in isometric force (9,18,22,26,30) or no effect at all (7). Efforts to understand oxygen-sensitive mechanisms by means of studying isolated cells complicated things even more. Several investigators have shown in isolated endothelial cells that hypoxia can increase the production of prostaglandins (17, 33) and nitric oxide (NO) (11, 24) or can decrease the production of prostaglandins (31) and NO (33). Gellai et al. (8) provided evidence for a direct effect of oxygen on coronary smooth muscle cells. These authors showed that contractile function was markedly depressed at a PO 2 below 5 mmHg. Myocytes are also sensitive to changes in oxygen tension. Mustafa (19) showed that isolated embryonic cardiac cells from chickens release adenosine and its degradation products in response to hypoxia. Furthermore, Kawaguchi et al. (14) showed that isolated heart tissue from neonatal rats released free fatty acid and prostacyclin durin...