Interactions between vasoconstrictors and vasodilators in regulating hemodynamics of distinct vascular beds

We examined whether interactions between angiotensin II (Ang II), endothelin (ET), nitric oxide (NO), and prostaglandins (PGs) differentially regulate perfusion to distinct vascular beds. For this, we blocked either angiotensin AT1 or ET receptors or both and then sequentially inhibited NO and PG synthesis in anesthetized dogs. Blocking Ang II or ET had similar effects on systemic hemodynamics: Mean arterial pressure fell slightly without altering cardiac output. Blocking both caused a synergistic fall in mean arterial pressure and increased cardiac output. Pulmonary vascular resistance was not altered by blocking Ang II, ET, or both but progressively increased during NO and PG blockade in group 2 (which had unblocked ET receptors), suggesting that endogenous ET exerts pulmonary vasoconstriction that is tempered by NO and PGs. In the kidney, blocking Ang II increased regional blood flow (RBF), glomerular filtration rate (GFR), and fractional excretion of sodium (FENa). In contrast, blocking ET did not alter RBF, and it decreased GFR and FENa. Combined Ang II and ET blockade markedly increased RBF without altering GFR, and FENa was maintained at the levels as when only ET was blocked. Sequentially inhibiting NO and PGs decreased RBF when Ang II or ET were blocked but had little effect when both were blocked. Finally, Ang II or ET blockade did not alter iliac blood flow. Inhibiting NO and PGs decreased iliac blood flow when Ang II or ET but not both were blocked. These results suggest that regional differences in the interactions between endogenous Ang II, ET, NO, and PGs are important determinants in systemic, pulmonary, and regional hemodynamics.

Deep circumflex iliac cutaneous free flap in cats

OBJECTIVE

To develop and assess the survival of a microvascular cutaneous free flap based on the ventral branch of the deep circumflex iliac (DCI) artery and vein in cats.

STUDY DESIGN

Experimental study.

ANIMALS

Phase 1: 6 feline cadavers; Phase 2: 2 adult cats; Phase 3: 10 adult cats.

METHODS

Phase 1: Selective angiographic study of the deep circumflex iliac artery was completed in 6 feline cadavers. After injection of the DCI artery with barium, high-detail radiographs were made of skin flaps harvested from the lateral flank and thigh region. The extent of the cutaneous angiosome was mapped with regard to the underlying anatomical landmarks. Phase 2: An island flap based on anatomic boundaries of the DCI angiosome derived from phase 1 of the study was elevated in 2 cats. Flaps were observed for 3 weeks for survival. Phase 3: Free skin flaps based on the DCI vessels were harvested in 10 cats and transferred to the dorsal interscapular region. Flaps were evaluated for 2 weeks for survival. Tissue samples were collected for histopathology, and angiograms of the flaps were completed.

RESULTS

Phase 1: Angiograms revealed a large primary cutaneous angiosome of the DCI artery located over the lateral femoral region, which extended from the iliac crest to the level of the patella. Phase 2: All island flaps survived for 3 weeks. Phase 3: Six free flaps survived for 2 weeks, and 4 flaps failed completely. Failure of 1 flap occurred because of avulsion of the venous and arterial anastomosis postoperatively. Another cat had intraoperative hemorrhage, which resulted in anemia and hypovolemia and likely caused the flap to fail. The other 2 flaps that failed had poor perfusion intraoperatively and had the longest ischemia times.

CONCLUSIONS

The cutaneous DCI free flap in cats may be clinically useful in reconstruction of large cutaneous wounds. The length of ischemia time for successful cutaneous free flap transfer in the cat may be shorter than in other species.

CLINICAL RELEVANCE

Large wounds created by trauma or oncologic ablative surgery in cats could be reconstructed with cutaneous microvascular free flap. Additional studies assessing the critical ischemia time of cutaneous flaps in cats and evaluating the use of this flap clinically are needed.

Effects of chronic systemic hypoxia on contraction evoked by noradrenaline in the rat iliac artery

Comparisons were made between responses evoked by noradrenaline (NA) in iliac artery rings from normoxic (N) rats and chronically hypoxic (CH) rats kept in 12 % O(2) for 3-4 weeks. At P(O(2)) of 100 mmHg, cumulative concentration-response curves (CCRC) to NA were greatly depressed in endothelium-intact (E+) rings, but not endothelium-denuded (E-) rings, of CH rats relative to N rats. However, CCRCs evoked by NA in E+ and E- rings during nitric oxide (NO) synthase inhibition were similar in N and CH rats. Reducing P(O(2)) to 55 mmHg depressed CCRCs to NA in E+ and E- rings of CH and N rats in the absence and presence of NO synthase inhibition. At P(O(2)) of 100 mmHg, CCRCs evoked by phenylephrine were comparable in E+ and E- rings of N and CH rats as were CCRCs for the relaxation evoked by isoprenaline, which were similarly rightward shifted by NO synthase inhibition. However, CCRCs evoked by the NO donor sodium nitroprusside were leftward shifted in E- rings of CH rats relative to N rats. Further, in the presence of the alpha(2) adrenoceptor inhibitor rauwolscine, CCRCs to NA were comparable in E+ rings of CH and N rats. Thus, the depressive effects of chronic hypoxia on NA-evoked contractions of iliac artery are additional to those of acute hypoxia. We propose that they reflect a facilitation of the contribution of NO to alpha(2) adrenoceptor-evoked relaxation that includes an increased sensitivity of the vascular smooth muscle of arteries from CH rats to NO.

Passive biaxial mechanical response of aged human iliac arteries

Inflation and extension tests of arteries are essential for the understanding of arterial wall mechanics. Data for such tests of human arteries are rare. At autopsy we harvested 10 non-diseased external iliac arteries of aged subjects (52-87 yrs). Structural homogeneity was ensured by means of ultrasound imaging, and anamneses of patients were recorded. We measured the axial in situ stretches, load-free geometries and opening angles. Passive biaxial mechanical responses of preconditioned cylindrical specimens were studied in 37 degrees C calcium-free Tyrode solution under quasistatic loading conditions. Specimens were subjected to pressure cycles varying from 0 to 33.3 kPa (250 mmHg) at nine fixed axial loads, varying from 0 to 9.90N. For the description of the load-deformation behavior we employed five "two-dimensional" orthotropic strain-energy functions frequently used in arterial wall mechanics. The associated constitutive models were compared in regard to their ability of representing the experimental data. Histology showed that the arteries were of the muscular type. In contrast to animal arteries they exhibited intimal layers of considerable thickness. The average ratio of wall thickness to outer diameter was 7.7, which is much less than observed for common animal arteries. We found a clear correlation between age and the axial in situ stretch lambda is (r = -0.72, P = 0.03), and between age and distensibility of specimens, i.e. aged specimens are less distensible. Axial in situ stretches were clearly smaller (1.07 +/- 0.09, mean +/- SD) than in animal arteries. For one specimen lambda is was even smaller than 1.0, i.e. the vessel elongated axially upon excision. The nonlinear and anisotropic load-deformation behavior showed small hystereses. For the majority of specimens we observed axial stretches smaller than 1.3 and circumferential stretches smaller than 1.1 for the investigated loading range. Data from in situ inflation tests showed a significant increase of the axial stretch with intraluminal pressure. Thus, for this type of artery the axial in situ stretch of a non-pressurized vessel is not representative of the axial in vivo stretch. None of the constitutive models were able to represent the deformation behavior of the entire loading range. For the physiological loading range, however, some of the models achieved good agreement with the experimental data.

Remote preconditioning lessens the deterioration of pulmonary function after repeated coronary artery occlusion and reperfusion in sheep

PURPOSE

We investigated whether remote organ preconditioning (RPC) can preserve pulmonary function following repeated myocardial ischemia/reperfusion in a model mimicking multi-vessel off-pump coronary artery bypass (OPCAB) revascularization.

METHODS

Nine sheep (Group-RPC) underwent RPC by three episodes of five-minute occlusion and five-minute reperfusion of the iliac artery. Five sheep (Group-C) were time-matched controls. Afterwards, ten-minute occlusion and reperfusion of the left anterior descending, the first diagonal and the left circumflex coronary arteries were performed consecutively. Hemodynamic and respiratory parameters and arterial blood gases were measured until 120 min after the final coronary reperfusion. Anesthesia was maintained with halothane in oxygen and nitrous oxide. Animals were ventilated with a tidal volume of 15-20 mL.kg(-1) in a non-rebreathing system, and a respiratory rate 14-16 min, with 5-cm H(2)O positive end expiratory pressure after thoracotomy.

RESULTS

Repeated coronary occlusion and reperfusion was associated in this experimental model with an increase in pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) and a decrease in PaO(2) and PaO(2)/FIO(2) in Group-C. After 120 min reperfusion, PaO(2) and PaO(2)/FIO(2) in Group-RPC were higher (192 +/- 69 mmHg and 241 +/- 78 vs 115 +/- 54 mmHg and 129 +/- 64, P < 0.05), while PVR and PAP were lower than in Group-C. At 120 min of reperfusion, PaO(2) and PaO(2)/FIO(2) were inversely correlated with PVR (P < 0.01).

CONCLUSIONS

RPC by transient occlusion of the iliac artery improves lung gas exchange after repeated coronary artery occlusion and reperfusion mimicking OPCAB surgery, and preserves low PVR in sheep.

Activation of the renin-angiotensin system contributes to the peripheral vasoconstriction reflexly caused by stomach distension in anaesthetized pigs

Gastric distension in anaesthetized pigs reflexly elicits peripheral vasoconstriction and an increase in plasma renin activity (PRA), with vagal afferent and sympathetic efferent limbs. The aim of the present study was to quantify the contribution of the renin-angiotensin system to the peripheral vasoconstriction. In pigs anaesthetized with alpha-chloralose, changes in anterior descending coronary, superior mesenteric and left external iliac blood flow caused by stomach distension before and after blockade of angiotensin II receptors with losartan were assessed using electromagnetic flowmeters. Gastric distension for periods of 30 min was performed by injecting 0.8 l warm Ringer solution into balloons positioned within the viscus. Changes in heart rate and renal blood flow were prevented by atrial pacing and injection of phentolamine into the renal arteries, and changes in regional perfusion pressure and in baroreceptor activity were minimized by aortic constriction and denervation of the carotid sinuses. PRA was assessed by radioimmunoassay of angiotensin I. Before blockade of angiotensin II receptors by administration of losartan, stomach distension decreased coronary blood flow by 14.2 % in six pigs and mesenteric and iliac blood flow by 11 % and 17.3 %, respectively, in another six pigs. After administration of losartan, these decreases were significantly reduced to 7.4 %, 6.8 % and 8.7 %, respectively. The above responses were abolished by bilateral section of the subdiaphragmatic vagal nerves. These results show that the peripheral vasoconstriction reflexly caused by stomach distension was significantly contributed to by the concomitant activation of the renin-angiotensin system.

A description of arterial wall mechanics using limiting chain extensibility constitutive models

Certain aspects of the mechanical response of arterial walls can be described using nonlinear elasticity theory. Uniaxial tests on vascular walls reveal nonlinear stress-strain behavior, with higher extensibility in the low stretch range and progressively lower extensibility with increasing stretch. This phenomenon is well known in the framework of rubber-like materials where it is called a strain-hardening or strain-stiffening effect. Constitutive models of incompressible hyperelasticity that take this into account include power-law models and limiting chain extensibility models. Our purpose in this paper is to bring to the attention of the biomechanics community some essential features of one such model of the latter type due to Gent. This model is compared with isotropic versions of biomechanical constitutive models by Takamizawa-Hayashi and Fung; the latter is a limiting version of a power-law material. Two particular problems are considered for which experimental data on arterial wall deformations are available. The first concerns small oscillations superposed on a large static stretch of a vertical string of arterial tissue. It is shown that the exponential model of Fung and the Gent model match well with the experimental data. The second problem is the extension of an internally pressurized circular cylindrical tube. It is shown that an inversion phenomenon observed experimentally for the human iliac artery can be described within a membrane theory by the Gent model whereas this cannot be described using the exponential model. The foregoing considerations are carried out for isotropic elastic materials in the absence of residual stress. Extensions to include anisotropy are also indicated.

Altered structure and reduced distensibility of arteries in Dahl salt-sensitive rats

OBJECTIVES

The role of salt sensitivity in arterial stiffening and the structural basis of reduced arterial distensibility were investigated in Dahl salt-sensitive (DS) rats.

METHODS

Three-month-old male DS rats received a normal (0.7% NaCl) or a high-sodium (2% NaCl) diet for 3 months. Dahl salt-resistant (DR) rats were controls. Pressure-volume (distensibility) relationships were measured in in-vitro-perfused segments of right carotid and iliac arteries, in the presence and absence of extracellular calcium. The left carotid and iliac arteries were perfusion-fixed at 100 mmHg for morphometric measurements.

RESULTS

The average monthly tail systolic blood pressure (SBP) of DS rats on normal and high-sodium diets were increased compared to that of DR rats. Compared to controls, carotid and iliac artery pressure-volume curves of DS rats on normal and high-sodium diets were shifted toward the pressure axis, without a change in elastic moduli. In DS rats, reduced distensibility of the carotid artery was accompanied by increased lumen diameter and increased thickness of media and elastic lamellae, the wall to lumen ratio being unchanged; wall thickness was increased and lumen diameter unchanged in the iliac artery. The high-sodium diet had no effect on either distensibility or dimensions of carotid and iliac arteries in DS or DR rats.

CONCLUSION

Geometry (increased or unchanged lumen and increased wall thickness), rather than increased stiffness of wall components, appears to be the cause of reduced distensibility of arteries in DS rats. Structural and functional adaptation to salt sensitivity may occur on what is considered a 'normal' sodium diet.

Pharmacological characterization of alpha1-adrenoceptor in mouse iliac artery

Subtypes of alpha(1)-adrenoceptor-mediated contraction to noradrenaline in the mouse iliac artery were determined (pharmaco-mechanically). Prazosin, 2-[2,6-dimethoxyphenoxyethyl]aminomethyl-1,4-benzodioxane hydrochloride (WB 4101) and 5-methylurapidil shifted the concentration-response curve for noradrenaline to the right, giving the pA(2) values of 9.30, 9.55 and 8.71, respectively. 8-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]-ethyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride (BMY 7378) shifted the concentration-response curve for noradrenaline to the right and the pA(2) value was 6.62. These results indicate that the contractile response to noradrenaline in the mouse iliac artery is predominantly mediated by the alpha(1A) -adrenoceptor subtype.

The effect of tezosentan, a non-selective endothelin receptor antagonist, on shear stress-induced changes in arterial diameter of the anaesthetized dog

The effects of changes in the mean (S(m)) and pulsatile (S(p)) components of arterial wall shear stress on arterial dilatation of the iliac artery of the anaesthetized dog were examined in the absence and presence of the endothelin receptor antagonist tezosentan (10 mg kg(-1) I.V.; Ro 61-0612; [5-isopropyl-pyridine-2-sulphonic acid 6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2-(2-1H-tetrazol-5-yl-pyridin-4-yl)-pyrimidin-4-ylamide]). Changes in shear stress were brought about by varying local peripheral resistance and stroke volume using a distal infusion of acetylcholine and stimulation of the left ansa subclavia. An increase in S(m) from 1.81 +/- 0.3 to 7.29 +/- 0.7 N m(-2) (means +/- S.E.M.) before tezosentan caused an endothelium-dependent arterial dilatation which was unaffected by administration of tezosentan for a similar increase in S(m) from 1.34 +/- 0.6 to 5.76 +/- 1.4 N m(-2) (means +/- S.E.M.). In contrast, increasing the S(p) from 7.1 +/- 0.8 to a maximum of 11.5 +/- 1.1 N m(-2) (means +/- S.E.M.) before tezosentan reduced arterial diameter significantly. Importantly, after administration of tezosentan subsequent increases in S(p) caused arterial dilatation for the same increase in S(p) achieved prior to tezosentan, increasing from a baseline of 4.23 +/- 0.4 to a maximum of 9.03 +/- 0.9 N m(-2) (means +/- S.E.M.; P < 0.001). In conclusion, the results of this study provide the first in vivo evidence that pulsatile shear stress is a stimulus for the release of endothelin from the vascular endothelium.

Reverse arterial wall shear stress causes nitric oxide-dependent vasodilatation in the anaesthetised dog

The effects of a maintained increase in mean arterial wall shear stress (SS(m)) caused by blood flow in the normal and reverse direction on dilatation of the iliac artery were examined in the anaesthetised dog. Blood flow in the left iliac artery was varied in both the forwards and reverse directions by a perfusion pump connecting the right and left femoral arteries. An increase in blood flow, and therefore SS(m) in either direction, caused an increase in arterial diameter. However, an increase in forwards SS(m) (control 4.1+/-0.11 mm) caused a significantly greater change in arterial diameter than an equivalent increase in the reverse direction (control 4.3+/-0.08), 0.198+/-0.02 mm vs. 0.132+/-0.02 mm (mean+/-SEM) respectively, for the same increase in SS(m) (3.23 N/m(2)). The increase in arterial diameter in response to an increase in forwards or reverse SS(m) was attenuated by L-NAME (80 mg/kg i.v.), indicating that the arterial dilatation was mediated by nitric oxide (NO). These findings confirm that endothelial NO release is dependent on the steady-state SS(m) and that the response occurs irrespective of the direction in which this force is applied, but is attenuated in the reverse direction.

The effect of testosterone on regional blood flow in prepubertal anaesthetized pigs

This work was undertaken to study the effects of testosterone on the coronary, mesenteric, renal and iliac circulations and to determine the mechanisms of action involved. In prepubertal pigs of both sexes anaesthetized with sodium pentobarbitone, changes in left circumflex or anterior descending coronary, superior mesenteric, left renal and left external iliac blood flow caused by intra-arterial infusion of testosterone were assessed using electromagnetic flowmeters. Changes in heart rate and arterial blood pressure were prevented by atrial pacing and by connecting the arterial system to a pressurized reservoir containing Ringer solution. In 12 pigs, intra-arterial infusion of testosterone for 5 min to achieve a stable intra-arterial concentration of 1 microg l(-1) increased coronary, mesenteric, renal and iliac blood flow without affecting the maximum rate of change of left ventricular systolic pressure (left ventricular dP/dt(max)) and filling pressures of the heart. In a further five pigs, a concentration-response curve was obtained by graded increases in the intra-arterial concentration of the hormone between 0.125 and 8 microg l(-1). The mechanisms of these responses were studied in the 12 pigs by repeating the experiment after haemodynamic variables had returned to the control values before infusions. In six pigs, blockade of muscarinic cholinoceptors and adrenoceptors with atropine, propranolol and phentolamine did not affect the responses caused by intra-arterial infusion of testosterone performed to achieve a stable intra-arterial concentration of 1 microg l(-1). In the same pigs and in the remaining six pigs, the increases in coronary, mesenteric, renal and iliac blood flow caused by intra-arterial infusion of testosterone performed to achieve a stable intra-arterial concentration of 1 microg l(-1) were prevented by intra-arterial injection of N(omega)-nitro-L-arginine methyl ester. The present study shows that intra-arterial infusion of testosterone dilated coronary, mesenteric, renal and iliac circulations. The mechanism of this response involved the release of nitric oxide.

Numerical simulation of steady flow fields in a model of abdominal aorta with its peripheral branches

In the present study, a numerical calculation procedure based on a finite volume method was developed to simulate steady flow fields in a model of abdominal aorta with its peripheral branches. The study focused on the steady baseline flow fields and the wall shear stress (WSS) distribution as well as the localization of the reversed flow regions and results were compared to those obtained by other investigators. In the case of resting conditions, the existence of a region of reversed flow of about one to two diameters in size and next to the renal arteries and along the posterior wall as observed by other researchers was confirmed. However, under the exercise conditions this region could be wiped out. The flow reversal along the lateral walls proximal to the bifurcation persisted in both rest and exercise conditions. The WSS distribution and the wall shear stress gradient distribution were obtained. The lowest WSS occurred near the ostia of the renal arteries and the lateral walls of the iliac arteries. And the highest is always at the turn to the branch. The results were generally consistent with those obtained experimentally and numerically by other investigators. It was also shown that the steady flow might be used to depict the averaged behavior of pulsatile flow. The present computer code provides a platform for the future more realistic simulations.

Myogenic response of rat femoral small arteries in relation to wall structure and [Ca(2+)](i)

The present study investigated the influence of media thickness on myogenic tone and intracellular calcium concentration ([Ca(2+)](i)) in rat skeletal muscle small arteries. A ligature was loosely tied around one external iliac artery of 5-wk-old spontaneously hypertensive rats. At 18 wk of age, femoral artery blood pressure was 102 +/- 11 mmHg (n = 15) on the ligated side and 164 +/- 6 mmHg (n = 15) on the contralateral side. Small arteries feeding the gracilis muscle had a reduced media cross-sectional area and a reduced media-to-lumen ratio on the ligated side, where also the range of myogenic constriction was shifted to lower pressures. However, when expressed as a function of wall stress, diameter responses were nearly identical. [Ca(2+)](i) was higher in vessels from the ligated hindlimb at pressures above 10 mmHg, but vasoconstriction was not accompanied by changes in [Ca(2+)](i). Thus the myogenic constriction here seems due primarily to changes in intracellular calcium sensitivity, which are determined mainly by the force per cross-sectional area of the wall and therefore altered by changes in vascular structure.

Attenuated vascular responsiveness to noradrenaline release during dynamic exercise in dogs

During dynamic exercise, there is reduced responsiveness to alpha(1)- and alpha(2)-adrenergic receptor agonists in skeletal muscle vasculature. However, it is desirable to examine the sympathetic responsiveness to endogenous release of neurotransmitter, since exogenous sympathomimetic agents are dependent upon their ability to reach the abluminal receptor. Therefore, to further our understanding of sympathetic control of vasomotor tone during exercise, we employed a technique that would elicit the release of endogenous noradrenaline (norepinephrine) during dynamic exercise. Mongrel dogs (n = 8, 19-24 kg) were instrumented chronically with transit time ultrasound flow probes on both external iliac arteries. A catheter was placed in a side branch of the femoral artery for intra-arterial administration of tyramine, an agent which displaces noradrenaline from the nerve terminal. Doses of 0.5, 1.0 and 3.0 microg ml(-1) min(-1) of iliac blood flow were infused for 1 min at rest and during graded intensities of exercise. Dose-related decreases in iliac vascular conductance were achieved with these concentrations of tyramine. The reductions in iliac vascular conductance (means +/- S.E.M.) were 45 +/- 6 %, 30 +/- 4 %, 26 +/- 3 % and 17 +/- 2 %, for the 1.0 microg ml(-1) min(-1) dose at rest, 3.0 miles h(-1), 6.0 miles h(-1) and 6.0 miles h(-1), 10 % gradient, respectively. At all doses, the magnitude of vasoconstriction caused by administration of tyramine was inversely related to workload. We conclude that there is a reduced vascular responsiveness to sympathoactivation in dynamically exercising skeletal muscle.

Prior exercise and the response to insulin-induced hypoglycemia in the dog

To test whether hepatic insulin action and the response to an insulin-induced decrement in blood glucose are enhanced in the immediate postexercise state as they are during exercise, dogs had sampling (artery, portal vein, and hepatic vein) catheters and flow probes (portal vein and hepatic artery) implanted 16 days before a study. After 150 min of moderate treadmill exercise or rest, dogs were studied during a 150-min hyperinsulinemic (1 mU.kg(-1).min(-1)) euglycemic (n = 5 exercised and n = 9 sedentary) or hypoglycemic (65 mg/dl; n = 8 exercised and n = 9 sedentary) clamp. Net hepatic glucose output (NHGO) and endogenous glucose appearance (R(a)) and utilization (R(d)) were assessed with arteriovenous and isotopic ([3-(3)H]glucose) methods. Results show that, immediately after prolonged, moderate exercise, in relation to sedentary controls: 1) the glucose infusion rate required to maintain euglycemia, but not hypoglycemia, was higher; 2) R(d) was greater under euglycemic, but not hypoglycemic conditions; 3) NHGO, but not R(a), was suppressed more by a hyperinsulinemic euglycemic clamp, suggesting that hepatic glucose uptake was increased; 4) a decrement in glucose completely reversed the enhanced suppression of NHGO by insulin that followed exercise; and 5) arterial glucagon and cortisol were transiently higher in the presence of a decrement in glucose. In summary, an increase in insulin action that was readily evident under euglycemic conditions after exercise was abolished by moderate hypoglycemia. The means by which the glucoregulatory system is able to overcome the increase in insulin action during moderate hypoglycemia is related not to an increase in R(a) but to a reduction in insulin-stimulated R(d). The primary site of this reduction is the liver.

Vector analysis of the wall shear rate at the human aortoiliac bifurcation using cine MR velocity mapping

OBJECTIVE

Small or oscillatory wall shear stress accelerates atherosclerosis. MR velocity mapping is feasible for vector analysis of wall shear rate (a spatial gradient of blood flow velocity at the vessel wall) in humans. A relationship between anatomic variations at the aortoiliac bifurcation and characteristics of wall shear rate was evaluated.

SUBJECTS AND METHODS

To obtain two components of wall shear rate vectors, an axial component along the vessel axis and a nonaxial component perpendicular to the former at the inner and outer walls of the common iliac arteries just distal to the aortoiliac bifurcation, we performed cine MR velocity mapping with three orthogonal velocity-encoded directions in seven volunteers.

RESULTS

The peak axial component at the outer wall (120.6 +/- 37.2 sec(-1)) was smaller than that at the inner wall (196.0 +/- 53.7 sec(-1)) (p < 0.01). Oscillation described by a time integral of the axial component in recessive blood flow direction over integrals in dominant and recessive directions at the outer wall was greater (0.24 +/- 0.11) than that at the inner wall (0.15 +/- 0.08) (p < 0.01). The intersecting angle between the extrapolation of the aortic axis and the direction of the axis of the common iliac artery correlated positively with the peak axial component (r = 0.577, p < 0.05) and inversely with oscillation (r = 0.603, p < 0.05).

CONCLUSION

Three-dimensional vector analysis with MR velocity mapping revealed that the outer wall at the aortoiliac bifurcation showed low and oscillatory shear rate, and this inclination was increased when the takeoff angle of the iliac artery was small.

Gap junction-dependent and -independent EDHF-type relaxations may involve smooth muscle cAMP accumulation

We have compared the mechanisms that contribute to endothelium-derived hyperpolarizing factor (EDHF)-type responses induced by ACh and the Ca(2+) ionophore A-23187 in the rabbit iliac artery. Relaxations to both agents were associated with ~1.5-fold elevations in smooth muscle cAMP levels and were attenuated by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (DDA) and potentiated by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Mechanical responses were inhibited by coadministration of the Ca(2+)-activated K(+) channel blockers apamin and charybdotoxin, both in the absence and presence of IBMX, but were unaffected by blockade of ATP-sensitive K(+) channels with the sulphonylurea glibenclamide. Relaxations and elevations in cAMP evoked by ACh were abolished by 18alpha-glycyrrhetinic acid, which disrupts gap junction plaques, whereas the corresponding responses to A-23187 were unaffected by this agent. Consistently, in "sandwich" bioassay experiments, A-23187, but not ACh, elicited extracellular release of a factor that evoked relaxations that were inhibited by DDA and potentiated by IBMX. These findings provide evidence that EDHF-type relaxations of rabbit iliac arteries evoked by ACh and A-23187 depend on cAMP accumulation in smooth muscle, but involve signaling via myoendothelial gap junctions and the extracellular space, respectively.

Intraoperative graft verification in renal transplants

The success of an arterial reconstruction is dependent on multiple factors, including patient selection, quality of the vessels, type of vascular conduit employed, and the surgical technique adopted. Transit time flow measurement is a technique that is now the standard of care for immediate graft patency verification following coronary artery bypass graft surgery. This study demonstrates that transit time flow measurement can be successfully used to evaluate renal transplant anastomoses: accurate intraoperative measurements provide a rapid objective assessment of renal graft patency.

Excessive length of iliac arteries in athletes with flow limitations measured by magnetic resonance angiography

PURPOSE

Kinking of the iliac arteries can cause flow limitations in endurance athletes. Such kinking may be treated by surgical release of the iliac arteries. However, when the length of the iliac artery is excessive, this may not be effective. Because threshold values of excessive length of the iliac arteries are unknown, normal values for endurance athletes were obtained and abnormalities encountered in these patients are reported.

METHODS

Forty-three endurance athletes (46 symptomatic legs) with flow limitations in the iliac arteries were examined using magnetic resonance angiography (MRA) with hips extended and flexed. The ratio of vessel length to straight-line distance was determined for the common and external iliac arteries. Sixteen national-level cyclists (32 reference legs) served as a control group.

RESULTS

For the common iliac artery, length ratios were significantly (P < 0.05) higher in the symptomatic legs than in the reference legs (symptomatic legs: 1.1 +/- 0.12, 1.22 +/- 0.19, reference legs 1.05 +/- 0.04, 1.11 +/- 0.05 with extended and flexed hips, respectively). For the external iliac artery, only in the position with hips flexed, the ratios in the symptomatic legs were significantly higher than in the reference legs (symptomatic legs: 1.11 +/- 0.09, 1.44 +/- 0.23, reference legs 1.08 +/- 0.05, 1.32 +/- 0.13 with extended and flexed hips, respectively). A small proportion of symptomatic legs had extremely high length ratios.

CONCLUSION

MRA is effective for determining vessel length. The ratio of vessel length to straight-line distance with extended and flexed hips is a good measure for excessive vessel length and achieves extreme values in a small subgroup of patients. Further prospective study is warranted to define maximal vessel length ratios, which still allow benefit from surgical release of the iliac arteries.

Analysis of the effects of graded levels of hypoxia on noradrenaline-evoked contraction in the rat iliac artery in vitro

In rings of rat iliac artery, contractions were evoked by noradrenaline (NA), the selective alpha(1) adrenoceptor agonist phenylephrine (PE), and K(+), which causes depolarisation-induced contraction. There was no evidence of alpha(2) adrenoceptor-evoked contraction. Hypoxia, induced by reducing P(O(2)) in the bath from 100 mmHg to 70, 55 or 40 mmHg, had similar effects on rings with (E+) and without (E-) endothelium. In E- rings, the NA concentration-response curve was biphasic, whereas that for PE was monophasic. Hypoxia reduced maximum contractions in response to NA and PE (NA(max) and PE(max), respectively) without affecting the concentrations that evoked 50 % of maximum contraction (EC(50)). At P(O(2)) of 70 mmHg, NA(max) of the high affinity alpha(1) receptor for NA (NA(maxh)) and PE(max) were reduced by approximately 15 %, but at P(O(2)) of 55 and 40 mmHg, NA(maxh) was severely attenuated while PE(max) fell by 45 and 75 %, respectively. Similarly, the Ca(2+) channel blocker nicardipine depressed NA(maxh) and PE(max), but P(O(2)) of 55 mmHg further reduced NA(max) and PE(max). Hypoxia also reduced contractions evoked by NA, PE or K(+) at the concentrations required to produce 80 % of the maximum contraction (EC(80)), receptor-mediated contractions being more affected. Ca(2+)-free conditions reduced the contractions evoked by NA and PE, at the EC(80), to approximately 10 % of control. The K(+) channel inhibitors glibenclamide and tetraethylammonium did not prevent hypoxia-induced depression of PE-evoked contraction. Thus, contractions evoked in iliac artery by the high affinity subtype of alpha(1) adrenoceptor for NA, which may respond to circulating levels of NA, and by the single alpha(1) adrenoceptor subtype for PE, are especially vulnerable to P(O(2)) levels less-than-or-equal 55 mmHg. We propose that this reflects hypoxia-induced inhibition of Ca(2+) influx through L-type and receptor-operated Ca(2+) channels; K(+) channel opening makes little contribution.

Nitric oxide regulates local arterial distensibility in vivo

BACKGROUND

Arterial stiffness is an important determinant of cardiovascular risk. Several lines of evidence support a role for the endothelium in regulating arterial stiffness by release of vasoactive mediators. We hypothesized that nitric oxide (NO) acting locally regulates arterial stiffness in vivo, and the aim of this experiment was to test this hypothesis in an ovine hind-limb preparation.

METHODS AND RESULTS

All studies were conducted in anesthetized sheep. Pulse wave velocity (PWV) was calculated by the foot-to-foot methodology from 2 pressure waveforms recorded simultaneously with a high-fidelity dual pressure-sensing catheter placed in the common iliac artery. Intra-arterial infusion of N(G)-monomethyl-L-arginine (L-NMMA) increased iliac PWV significantly, by 3+/-2% (P<0.01). Infusion of acetylcholine and glyceryl trinitrate reduced PWV significantly, by 6+/-4% (P=0.03) and 5+/-2% (P<0.01), respectively. Only the effect of acetylcholine, however, was significantly inhibited during coinfusion of L-NMMA (P=0.03). There was no change in systemic arterial pressure throughout the studies. Importantly, infusion of L-NMMA or acetylcholine distal to the common iliac artery (via the sheath) did not affect PWV.

CONCLUSIONS

These results demonstrate, for the first time, that basal NO production influences large-artery distensibility. In addition, exogenous acetylcholine and glyceryl trinitrate both increase arterial distensibility, the former mainly through NO production. This may help explain why conditions that exhibit endothelial dysfunction are also associated with increased arterial stiffness. Therefore, reversal of endothelial dysfunction or drugs that are large-artery vasorelaxants may be effective in reducing large-artery stiffness in humans, and thus cardiovascular risk.

Metalloproteinase inhibition and the response to angioplasty and stenting in atherosclerotic primates

Determinants of restenosis after angioplasty include constrictive remodeling and intimal hyperplasia. Both processes require extensive matrix turnover, so matrix metalloproteinases (MMPs) have become potential targets of antirestenosis therapies. We studied the effects of RO113-2908, a broad-spectrum MMP inhibitor (MMPI), on the response to iliac artery angioplasty and stenting in atherosclerotic cynomolgus monkeys. Lumen diameter (LD) was measured angiographically, and artery wall geometry was assessed after perfusion-fixation at 4 weeks. Angiogenesis was measured in subcutaneous polyvinyl alcohol disks. Treatment provided significant, systemic MMP inhibitory activity (97+/-2.2% inhibition of 25 nmol/L MMP-12 by serum) and inhibited angiogenesis (P=0.007). In contrast, loss of gain in LD (P=0.73) and constrictive remodeling (external elastic lamina area ratio [injured/uninjured x 100]: MMPI, 106.3+/-9.6% vs control, 119.9+/-7.2%; P=0.27) were not substantially improved 4 weeks after angioplasty. Treatment also failed to reduce intimal hyperplasia after angioplasty (intimal area [mm(2)]: 1.4+/-0.3 vs 1.6+/-0.2, P=0.65) or stenting (2.4+/-0.2 vs 2.8+/-0.2, P=0.12). In summary, inhibition of MMP activity reduced angiogenesis but failed to prevent constrictive remodeling or intimal hyperplasia after angioplasty and stenting in atherosclerotic primates. Additional research is needed to define the spectrum of matrix-degrading proteases critical in healing atherosclerotic arteries after angioplasty.

Endothelial vasodilator production by uterine and systemic arteries. IX. eNOS gradients in cycling and pregnant ewes

The follicular phase (FOL) and pregnancy exhibit increases in uterine blood flow (UBF), estrogen levels, and uterine artery (UA) endothelial nitric oxide synthase (eNOS) expression. UA branching within the mesometrium increases the total vascular cross-sectional area, which reduces the vascular perfusion pressure gradient, thus locally decreasing the blood flow velocity. Shear stress (SS) activates eNOS and may be associated with UBF elevations during FOL and pregnancy. We hypothesized that regional differences in eNOS responses are observed with both decreases in vessel diameter and during the ovarian cycle and pregnancy. Endothelial isolated proteins were collected from renal (RA) and internal iliac arteries (II) as well as from primary (UA 1 degrees ), secondary (UA 2 degrees), and tertiary (UA 3 degrees) UA branches of nonpregnant luteal phase (LUT; n = 6) and FOL (n = 6) as well as midpregnant (MP; 82 +/- 1 days gestation, n = 6) and late pregnant (LP; 127 +/- 3 days gestation, n = 6) ewes (term = 145 +/- 3 days gestation) for Western blot analysis. LUT RA, II, and UA 1 degrees eNOS levels were similar. There was a 60.7 +/- 9.8% reduction in eNOS expression in UA 2 degrees and UA 3 degrees. A similar decreasing eNOS regional expression gradient was observed in LP ewes. No eNOS regional expression gradient was observed in FOL or MP ewes because eNOS increased in UA 2 degrees and UA 3 degrees. In UA 2 degrees and UA 3 degrees, MP > LP = FOL > LUT. Thus, with increasing UBF, FOL and pregnancy rises in SS may regulate eNOS protein expression in smaller diameter UAs. A decrease in LUT and LP UA 2 degrees and UA 3 degrees endothelial eNOS suggest a possible negative feedback mechanism due to downregulation of eNOS if SS is normalized.

Regional haemodynamic responses to activation of the medial prefrontal cortex depressor region

Electrical or chemical stimulation of the medial prefrontal cortex (MPFC) produces depressor and sympathoinhibitory responses. To characterise the MPFC depressor response more fully, we determined the regional haemodynamic changes which occurred in response to stimulation of the MPFC. In halothane-anaesthetised rats, we recorded arterial blood pressure and renal, superior mesenteric, and iliac arterial vascular conductance using miniaturised Doppler flow probes. Electrical stimulation of the MPFC (50-100 microA) was used to map the location of the depressor region. Increases in vascular conductance (or increases in blood flow) were recorded from the renal (+2.3+/-0.5 kHz/mmHgx10(3)), mesenteric (+4.4+/-0.4 kHz/mmHgx10(3)), and iliac (+8.3+/-1.0 kHz/mmHgx10(3)) vascular beds in response to stimulation of the MPFC depressor region coinciding with the ventral infralimbic (IL) and dorsal peduncular (DP) cortical areas. Similar responses were obtained after microinjection of the chemical excitant L-glutamate (n=3, 100 nl, 100 mM), indicating that the responses were due to excitation of cell bodies and not due to axons traversing the area. Administration of the nitric oxide synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 25 micromol/kg, i.v., n=5) significantly reduced the MPFC depressor response (51%, 12.5+/-1.2 to 6.1+/-2.5 mmHg). The increases in conductance in the hindquarter and mesenteric vascular beds were significantly reduced after L-NAME treatment (mesenteric by 77%, iliac by 70%), but there was no significant reduction of renal flow (35%). These observations indicate that the depressor region of the MPFC is localised to ventral regions (IL and DP) and that the depressor response is mediated by increased conductance in the hindquarters and mesenteric vascular beds. Furthermore, the depressor response may be mediated, in part, by release of nitric oxide in these vascular beds.