Adaptation of canine saphenous veins to grafting. Correlation of contractility and contractile protein content

Vascular Smooth Muscle Remodeling in Conductive and Resistance Arteries in Hypertension

Cardiovascular disease is a leading cause of death worldwide and accounts for >17.3 million deaths per year, with an estimated increase in incidence to 23.6 million by 2030. ¹ Cardiovascular death represents 31% of all global deaths ² -with stroke, heart attack, and ruptured aneurysms predominantly contributing to these high mortality rates. A key risk factor for cardiovascular disease is hypertension. Although treatment or reduction in hypertension can prevent the onset of cardiovascular events, existing therapies are only partially effective. A key pathological hallmark of hypertension is increased peripheral vascular resistance because of structural and functional changes in large (conductive) and small (resistance) arteries. In this review, we discuss the clinical implications of vascular remodeling, compare the differences between vascular smooth muscle cell remodeling in conductive and resistance arteries, discuss the genetic factors associated with vascular smooth muscle cell function in hypertensive patients, and provide a prospective assessment of current and future research and pharmacological targets for the treatment of hypertension.

Structural and Functional Differences Between Porcine Aorta and Vena Cava

Elastin and collagen fibers are the major load-bearing extracellular matrix (ECM) constituents of the vascular wall. Arteries function differently than veins in the circulatory system; however as a result from several treatment options, veins are subjected to sudden elevated arterial pressure. It is thus important to recognize the fundamental structure and function differences between a vein and an artery. Our research compared the relationship between biaxial mechanical function and ECM structure of porcine thoracic aorta and inferior vena cava. Our study suggests that aorta contains slightly more elastin than collagen due to the cyclical extensibility, but vena cava contains almost four times more collagen than elastin to maintain integrity. Furthermore, multiphoton imaging of vena cava showed longitudinally oriented elastin and circumferentially oriented collagen that is recruited at supraphysiologic stress, but low levels of strain. However in aorta, elastin is distributed uniformly, and the primarily circumferentially oriented collagen is recruited at higher levels of strain than vena cava. These structural observations support the functional finding that vena cava is highly anisotropic with the longitude being more compliant and the circumference stiffening substantially at low levels of strain. Overall, our research demonstrates that fiber distributions and recruitment should be considered in addition to relative collagen and elastin contents. Also, the importance of accounting for the structural and functional differences between arteries and veins should be taken into account when considering disease treatment options.

Distribution of orientation of smooth muscle bundles does not change along human great and small varicose veins

Wall remodeling in varicose veins is associated with hypertrophy of subendothelial tissue, increase in inner diameter, wrinkling and invagination of the endothelial layer. Due to structural alterations of the wall, the smooth muscle cells (SMCs) change their original circular and longitudinal orientations. Our aim was to quantify the volume fraction of circularly, longitudinally and obliquely oriented SMCs within both the inner and outer half of the wall of 11 great saphenous varicose veins and five small saphenous varicose veins. Using stereological methods applied on cross-sections of the vessels regularly gained each 5 cm along the vessel we determined the wall thickness (846 ± 319 μm, mean ± standard deviation), the volume fraction of circular SMCs in the inner (0.19 ± 0.13) and outer (0.06 ± 0.06) layers, the volume fraction of longitudinal SMCs in the inner (0.06 ± 0.05) and outer (0.05 ± 0.04) layers, the volume fraction of oblique SMCs in the inner (0.15 ± 0.08) and outer (0.09 ± 0.08) layers, and the total volume fraction of SMCs in the inner (0.4 ± 0.1) and outer (0.21 ± 0.09) layers. The volume fraction of SMCs with circular and oblique but not with longitudinal orientation was greater in the inner layer compared to the outer layer. The SMC orientation distribution was uniform along the varicose saphenous veins. With increasing wall thickness, the volume fraction of longitudinal and oblique SMC bundles increased in both layers at the expansion of circular SMC bundles. The main differences in the orientation of the SMCs in the inner and outer wall layers should be taken into account when computational modeling of varicose saphenous veins is attempted.

A comparative study of endothelium-derived relaxing factor-mediated relaxation and smooth muscle cell function in arterial and venous vein bypass grafts

The development of intimal hyperplasia in reversed vein grafts is associated with altered endothelial and vasomotor function. This study examines the effect of surgery on the morphology and vasomotor function of experimental arterial and venous vein bypass grafts. Twelve reversed vein grafts, 12 in situ vein grafts and 12 venovenous grafts were placed in 24 New Zealand White rabbits. All grafts remained patent and were harvested after 28 days. Isometric contraction to norepinephrine, histamine, bradykinin, serotonin and relaxation to acetylcholine and sodium nitroprusside following pre-contraction with prostaglandin F(2 alpha) were determined on the grafts and on contralateral jugular veins. Compared to the contralateral jugular veins, norepinephrine supersensitivity was induced in the reversed vein grafts, and venovenous vein grafts but not in the in situ vein grafts. Decrease in histamine sensitivity occurred in all grafted vessels. Bradykinin responses were significantly reduced in the in situ vein grafts and reversed vein grafts. Contractile responses to serotonin developed in the in situ vein grafts and reversed vein grafts only. Acetylcholine-induced endothelium-derived relaxing factor-mediated relaxation of the contralateral jugular veins was preserved in both venovenous grafts and in situ vein grafts but was lost in reversed vein grafts. All tissues relaxed to sodium nitroprusside in dose-dependent manner. The data suggest that norepinephrine supersensitivity in reversed vein grafts results from excision of the vessel. Attenuation of bradykinin responses and the enhanced contractile responses to serotonin appear predominantly to result from arterialization. Decreases in histamine sensitivity appear related both to excision and to arterialization. Neither the excision of the vein nor arterialization individually influences the alterations in endothelium-derived relaxing factor-mediated relaxation. However, a combination of excision and arterialization results in the altered endothelium-derived relaxing factor-mediated relaxation. This study suggests that the surgical preparation of the vein and the surgical procedure used have significantly different effects on endothelium-derived relaxing factor-mediated relaxation and smooth muscle contractility in vein grafts.

Effects of pulsatile perfusion on human saphenous vein vasoreactivity: a preliminary report

This study examined the effects of exposure to arterial blood pressure and flow on human saphenous vein catecholamine sensitivity. Unused portions of saphenous vein from eight patients undergoing peripheral bypass procedures were mounted parallel in a specially designed organ culture apparatus and perfused with tissue culture medium with 95% CO(2) at 37 degrees C. One segment was fixed between two cannulas while the medium was gently agitated (control) and the other was actively perfused via a pulsatile pump system at a rate of 60 beats/min, peak pressure of 100 mmHg and peak flow of 200 ml/min (pulsed; mean pressure 60 mmHg; mean flow 115 ml/min). After 48 h, vein segments were removed and tested for in vitro isometric contraction in response to KCI, norepinephrine and histamine, and relaxation in response to acetylcholine, calcium ionophore A23187, and sodium nitroprusside. There were no differences in mean(s.e.m.) maximal contraction in response to KCI (control 0.61(0.16) g versus pulsed 0.72(0.27)g; P = n.s.), norepinephrine (control 1.00(0.56) g versus pulsed 1.51(0.54) g; P= n.s.), or histamine (control 1.47(0.85) g versus pulsed 1.95(0.64) g; P= n.s.). However, pulsed veins exhibited increased sensitivity to both norepinephrine (control -logED50 6.20(0.23) versus pulsed mean(s.e.m.) 6.60(0.17); P< 0.05) and histamine (control -logED(50) 5.60(0.27) versus pulsed 6.24(0.20); P = 0.05). Pulsed veins exhibited slightly less acetylcholine-induced relaxation although the difference did not reach statistical significance (control mean(s.e.m.) relaxation at 1 x 10(6)M 9.2(14.0)% versus pulsed -13.3(6.4)%; P = n.s.). There were no differences in relaxation in response to either A23187 (control 1 x 10-(4)M 178(19)% versus pulsed 191(68)% or sodium nitroprusside (control 225(15)% versus pulsed 254(17)%; P = n.s.). The data presented herein indicate that exposure of human saphenous vein to the hemodynamics of the arterial environment for 48 h results in catecholamine supersensitivity while contractile and relaxant function are not affected.

Reactivity of human saphenous veins at arterial perfusion pressures

Vasospasm of human saphenous vein grafts has been reported after aorta-coronary bypass operations. However, it is unknown whether veno-arterial grafts are inherently responsive to vasoconstrictor stimuli after implantation into the arterial circulation or whether their vasomotion is secondary to hemodynamic changes. Thus in this study we used in vitro methods to directly evaluate whether isolated human saphenous vein segments respond to vasoconstrictor agents at arterial pressure levels. External diameter and intraluminal flow were monitored in 12 human saphenous vein segments, which were perfused at 30 ml/min with physiologic salt solution at 90, 70, and 50 mm Hg. Increasing intraluminal pressure higher than 50 mm Hg or exposing the vein to Ca(2+)-free media did not increase vessel external diameter or intraluminal flow, which suggests that human saphenous veins were fully distended at pressures of 50 mm Hg or greater. However, all human saphenous veins were activated by a 1 mumol/L dose of norepinephrine at 50 mm Hg and dilated during subsequent intraluminal infusion of a 1 mumol/L dose of acetylcholine, showing intact vascular smooth muscle and endothelial cell function. In the same vessels, a 1 mumol/L concentration of 5-hydroxytryptamine constricted human saphenous veins by 19%, 22%, and 26% at intraluminal pressures of 90, 70, and 50 mm Hg, respectively, and reduced vessel flow by 6%, 24%, and 42% at the same pressure levels. Similarly, a 1 mumol/L concentration of norepinephrine constricted vessels pressurized at 90, 70, and 50 mm Hg by 9%, 12%, and 17%, respectively, and attenuated vessel flow by as much as 32%. We conclude that human saphenous vein segments are fully distended at perfusion pressures greater than 50 mm Hg, but can dynamically constrict to vasoactive agonists and regulate graft flow at intraluminal pressures as high as 90 mm Hg. Our findings in isolated human saphenous vein segments lend support to clinical observations that human saphenous vein grafts should be regarded as vasoactive conduits after implantation at arterial pressure levels.

Long-term human vein graft contractility and morphology: a functional and histopathological study of retrieved coronary vein grafts

Vasoreactivity of 11 coronary artery vein bypass grafts and 13 human saphenous veins was examined. Isometric tension studies were performed in response to potassium chloride (110 mmol/l), noradrenaline (10(-9)-10(-4) mol/l), serotonin (10(-9)-10(-4) mol/l) and histamine (10(-8)-10(-2) mol/l). After precontraction with noradrenaline (10(-5) mol/l), the response to acetylcholine (10(-8)-10(-4) mol/l) and the calcium ionophore A23187 (10(-8)-10(-4) mol/l) was also assessed. Results are given as mean(s.e.m.). Compared with saphenous veins, vein grafts showed decreased sensitivity to noradrenaline (1.7(0.5) versus 0.4(0.1) mumol/l, P = 0.01), no change in sensitivity to serotonin (55(18) versus 37(15) mumol/l, P > 0.05) and supersensitivity to histamine (3.2(0.9) versus 30.1(13.2) mumol/l, P = 0.01). Vein grafts had a decreased maximal contraction to potassium chloride (1.1(0.3) versus 5.5(0.8) g, P = 0.0001), noradrenaline (1.2(0.3) versus 4.1(0.8) g, P = 0.005), histamine (1.2(0.3) versus 4.5(0.8) g, P = 0.003) and serotonin (0.7(0.2) versus 5.7(0.6) g, P = 0.0002) compared with saphenous vein. Precontracted vein grafts did not relax in response to acetylcholine; in contrast, saphenous vein relaxed in a dose-dependent manner to a maximal relaxation of 22(3) per cent. Both saphenous vein and vein graft relaxed in response to A23187. Vein graft intimal thickness was approximately fourfold greater than that of saphenous vein (540(110) versus 136(30) microns). Scanning electron microscopy of vein and vein graft revealed an intact endothelium. Coronary artery vein grafts are capable of responding to various contractile agonists; these response are notably different from those of saphenous vein and there is a loss of endothelium-dependent relaxation. Even at a late stage vein grafts are not inert but are functional conduits with an abnormally responsive endothelium and a less potent, but significantly altered, smooth muscle contractile profile.

Influence of perioperative storage solutions on long-term vein graft function and morphology

It has been shown that suboptimal preparation of a vein graft prior to its insertion results in immediate morphologic and functional damage to the endothelial cells but not to the underlying smooth muscle cells. However, little is known about whether such perioperative injury to the vein grafts influences the subsequent development of intimal hyperplasia and smooth muscle cell contractility. This study examines the influence of storage in saline solution or Ringer's lactate on the development of intimal hyperplasia and vasomotor function in experimental vein grafts. Twenty-six New Zealand white rabbits had a carotid vein bypass graft performed after the veins had been immersed (15 minutes) in either heparinized saline solution (Sal; n = 13) or Ringer's lactate (RL; n = 13), and each group was harvested after 28 days for either histologic (n = 8) or functional studies (n = 5; four 5 mm rings/graft). Saline storage of the vein graft resulted in a 38% increase in the thickness of the intimal hyperplasia (113 +/- 2 vs. 83 +/- 2 microns, Sal vs. RL; mean +/- SEM; p < 0.05) without a change in medial thickness (87 +/- 5 vs. 86 +/- 8 microns, Sal vs. RL; p > 0.05). The two sets of vein grafts showed no difference in sensitivity to norepinephrine, serotonin, and bradykinin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of long-term portal hypertension on structure, active force and content of contractile and structural proteins in smooth muscle of the rat portal vein

Growth of the smooth muscle cells in the rat portal vein was induced by a partial ligation of the vessel. The ligation caused an increase in the transmural pressure and segments of the portal vein were investigated 6 weeks after the ligation. The spontaneous contractile activity of the ligated veins was similar to that of the control veins. In the ligated vessels the active force at optimal length for force development was doubled, 22.8 +/- 1.3 compared with 12.5 +/- 1.4 mN for the controls. The cross-sectional area of the media in the ligated veins, determined on transverse sections, increased from the control value of 0.10 +/- 0.01 to 0.19 +/- 0.01 mm2. Electron microscopy revealed that the mean cross-sectional area of the smooth muscle cells in the ligated portal vein was doubled (controls: 6.4 +/- 0.6, hypertrophic: 13.6 +/- 1.8 microns2). This suggests hypertrophy of the smooth muscle cells in the vessel wall as the cause for the increase in cross-sectional area of the ligated veins. An increase in the number of intermediate filaments was observed in the hypertrophied smooth muscle. The relative contents of contractile (myosin and actin) and structural (desmin and vimentin) proteins were determined with SDS-polyacrylamide gel electrophoresis. The actin/myosin and vimentin/actin ratios were unaltered by hypertrophy. The hypertrophied veins showed an increase in the desmin/actin ratio (control: 0.20 +/- 0.01, hypertrophied: 0.27 +/- 0.03). The increased amounts of desmin correlates with the increased number of intermediate filaments observed by electron microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactate dehydrogenase activity and isoform distribution in the rat urinary bladder: effects of outlet obstruction and its removal

The rat urinary bladder responded to infravesical obstruction by an increased weight. The weight began to increase after 3 days of obstruction. After 10 days the weight gain had become significant, but increased further with time. At 90 days the bladders weighed 12 times the initial weight. Lactate dehydrogenase (LDH) activity per unit bladder weight increased with the duration of the obstruction. The increase was significant in the 90 day group. The isoform pattern of LDH was determined. For the control group the M3H form was dominant, with M4 second; M4 became the dominant isoform after 3 days of obstruction. The relative amount of M4 had increased by 80% after 90 days. In one group, which was first obstructed for 10 days and then had the obstruction removed for 4 weeks, bladder weight and LDH isoform pattern were normal. We suggest that the changes in isoform pattern in the obstructed bladder reflect a metabolic adaptation to the prolonged voiding time and the increased detrusor pressure, with a consequently compromised oxygen supply, and that the changes are reversible if the outlet conditions are normalized.

Contractile and cytoskeletal proteins in smooth muscle during hypertrophy and its reversal

Hypertrophy of rat urinary bladder smooth muscle was induced by partial urethral obstruction. Bladder weight increased from 70 to 240 mg after 10 days and to 700 mg after 7 wk. Removal of the obstruction after 10 days caused a regression of bladder weight to 130 mg. The relative volume of smooth muscle in the bladder wall increased during hypertrophy. The concentration of myosin in the smooth muscle cells decreased in 10-day hypertrophied bladders, whereas the concentration of actin was unchanged. The actin-myosin ratio was 2.3 in controls, 3.3 in 10-day obstructed bladders, and 2.9 in 7-wk obstructed bladders. After removal of obstruction, the ratio was normalized. Two isoforms of myosin heavy chains were identified (SM1 and SM2). The relative amount of SM2 decreased during hypertrophy. The relative proportion of actin isoforms (alpha, beta, and gamma) was altered toward more gamma and less alpha. These changes were reversible upon removal of the obstruction. Desmin was the dominating intermediate filament protein. The concentration of desmin and filamin increased in the hypertrophic bladders. The increased desmin-actin and filamin-actin ratios in obstructed bladders were normalized after removal of the obstruction. The results suggest that the turnover of contractile and cytoskeletal proteins is fast and can be regulated in response to changes in the functional demands in smooth muscle.

Sequential changes of noradrenaline content of vein grafts in rats: quantitative estimation by high-performance liquid chromatography using electrochemical detection

Supradiaphragmatic inferior vena cava was transplanted end-to-end into the abdominal aorta of 11-week-old rats of the same inbred strain and same litter using microvascular technique. The grafts were removed 3 days, 4, 8, 16, and 35 weeks postoperatively, and their noradrenaline (NA) content was estimated by high-performance liquid chromatography using electrochemical detection. The amount of NA was significantly lower (P less than .001) in all vein grafts as compared to nontransplanted vena cava. The substantial decrease of NA in the vein grafts throughout the observation period indicates a persistent denervation of the transplant.

[Mechanism and significance of arteriolar media hypertrophy/ hyperplasia in arterial hypertension. Role of the Na+/H+ antiport]

The most common haemodynamic abnormality in human essential hypertension is an increase in systemic vascular resistance. Morphologic substrate for increased flow resistance is a narrowing of the lumen of arteriolar resistance vessels. During the course of essential hypertension, this is associated with an increase in wall (mainly media) thickness due to hypertrophy and hyperplasia of vascular smooth muscle cells. In contrast to concepts interpreting media thickening strictly as structural adaptation to increased perfusion pressure, various lines of evidence also point to pressure independent factors. In this context, extracellular factors such as "growth factors" as well as alterations in the activity of intracellular messenger systems must be considered. Recent studies suggest that substances generally known to act as vasoconstrictors such as angiotensin II, noradrenaline and arginine-vasopressin may also stimulate vascular smooth muscle cell growth and proliferation. Intracellular messenger systems with possible significance in the response to trophins and/or mitogens of vascular smooth muscle cells are phospholipase C, protein kinase C and the Na+/H+-antiport. These systems have been demonstrated to be altered in hypertension supporting the concept that one endogenous factor in human essential hypertension with pathophysiological significance, at least in a subgroup of patients, may be an enhanced reactivity of vascular smooth muscle cells to trophic and mitogenic stimuli. In this context, intracellular messenger systems such as phospholipase C, protein kinase C and/or the Na+/H+-antiport may play an important pathophysiological role.

Comparison of functional responses of canine coronary artery and saphenous vein

Functional responses of canine circumflex coronary arteries and saphenous veins before and after grafting surgery were assessed following physiologic and pharmacologic interventions. Developed tension of ringed segments of vessels was recorded isometrically. Dose-dependent responses to norepinephrine revealed significantly greater responses of saphenous veins than coronary arteries (maximal responses were 2.79 +/- 0.45 and 0.44 +/- 0.34 g, respectively; p less than 0.001). Sensitivity to norepinephrine [as evaluated by 50% effective dose (ED50) value] was 3.3 times greater in the saphenous veins (p less than 0.001). STA2, a synthetic thromboxane A2 analog (see Methods), produced similar contractions of both vessels [maximal responses were 2.13 +/- 0.37 g in saphenous vein and 1.64 +/- 0.85 g in coronary artery; p less than not significant (NS)], while sensitivity to STA2 of saphenous veins was 3.1 times greater than that of coronary arteries (p less than 0.001). In contrast to the foregoing responses, coronary arteries demonstrated significantly greater responses to potassium than saphenous veins (maximal responses were 2.16 +/- 0.71 g and 1.40 +/- 0.56 g, respectively; p less than 0.001). Moreover, coronary arteries revealed 1.6 times greater sensitivity than saphenous veins (p less than 0.001). When saphenous veins were transplanted into the femoral artery, the segments of the grafts (2 weeks or 3 months after surgery) revealed 4.0 and 1.7 times greater sensitivity (denervation supersensitivity) to norepinephrine and potassium than those of the control veins (p less than 0.01, respectively). Contractile tension response, however, decreased by at least 50% 3 months after surgery, most probably due to a thickened vessel wall and a reduced compliance.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile properties during development of hypertrophy of the smooth muscle in the rat portal vein

Structural and mechanical alterations during hypertrophy of the rat portal vein were investigated. Growth of the vessel was induced by a partial ligature of the vessel causing an increased transmural pressure. Vessel segments from animals kept with ligature for 1, 3, 5 and 7 days, were compared with vessels from sham-operated animals. Maximal active force and vessel cross-sectional area increased with time in the ligated group. On day 7, force and cross-sectional area at the optimal length, were markedly increased in the ligated group (21.1 +/- 1.0 mN, 0.55 +/- 0.04 mm2, n = 9) compared with the control vessels (11.7 +/- 1.0 mN, 0.30 +/- 0.02 mm2, n = 7). Light and electron microscopy of preparations fixed at optimal length showed that the amount of smooth muscle and the cross-sectional area of cell profiles were almost doubled in the ligated group on day 7, consistent with hypertrophy of the smooth muscle. The force per smooth muscle cell area was similar in the two groups (ligated: 132 +/- 15; control: 145 +/- 16 mN mm-2, n = 4-5). The maximal shortening velocity was significantly lower in the hypertrophied group (ligated: 0.28 +/- 0.02; control: 0.41 +/- 0.01 optimal length s-1, n = 6). In chemically skinned preparations, activated by maximal thiophosphorylation of the myosin light chains, force was higher in the ligated group compared to the controls but no difference in maximal shortening velocity was observed. In conclusion, the increased transmural pressure is associated with a rapid increase in the amount of smooth muscle in the portal vein. The mechanical data show that after 7 days the force generating ability of the contractile system has increased in proportion to the smooth muscle cell mass. The unaltered maximal shortening velocity in the skinned hypertrophied preparations suggests that the kinetic properties of the maximally activated contractile system are unaltered. The decreased maximal shortening velocity in the intact hypertrophied preparations may reflect alterations in the excitation-contraction coupling.

Endothelium-dependent responses in autogenous femoral veins grafted into the arterial circulation of the dog

Endothelium-dependent responses differ in arteries and veins of the dog. Experiments were performed to determine whether chronic grafting of veins into the arterial circulation would alter the endothelium-dependent responses of the veins. Segments of femoral veins were grafted to the femoral artery of the dog. 6 wk after surgery the venous grafts were removed from the dog, cut into rings, and suspended in organ chambers for isometric tension recording. In some rings the endothelial cells were removed. Acetylcholine and alpha 2-adrenergic agonists did not cause endothelium-dependent relaxations in venous grafts. The calcium ionophore (A23187) initiated such relaxations which were not mediated by prostanoids. Endothelium-dependent relaxations were also observed in venous grafts to ADP, thrombin, and arachidonic acid. In segments of graft where myo-intimal hyperplasia was prominent, relaxations to ADP, thrombin, and A23187 were blunted and in some segments contractions were observed. These results demonstrate the ability of the endothelium of venous grafts to initiate changes in tone of the smooth muscle.

Mechanical, morphological and pharmacological properties of oesophageal varices and small mesenteric veins in portal hypertensive rabbits

The properties of isolated small oesophageal and mesenteric veins were investigated in a rabbit model subjected to a 4-week period of portal vein stenosis. The animals developed oesophageal varices and these together with the small mesenteric veins were mounted on a myograph. Compared with sham-operated controls portal hypertension resulted in an increased normalized inner radius and media thickness of the oesophageal veins resulting in a non-significant increase in the media to lumen ratio, while the inner radius decreased as media thickness increased in mesenteric veins resulting in a significant (P less than 0.01) increase in the media to lumen ratio. When activated, the active wall tension of both the oesophageal and mesenteric veins was increased following portal hypertension, but the active media stress was unchanged. The EC50 for noradrenaline and angiotensin II did not change in portal hypertensive vessels compared with controls and was equal in oesophageal and mesenteric vessels. The ability of isoprenaline and serotonin to relax pre-contracted vessels of both types was reduced by the portal hypertension. The results of these studies suggest that the development of oesophageal varices is not only due to increased portal venous pressure.

Late increase in luminal diameter of aortocoronary venous bypass grafts associated with an increase in the vascular region under supply

In a previous study, a significant inverse relation was found between the luminal size of aortocoronary venous bypass grafts and the vascular resistance of the coronary region that was perfused by the bypass graft in late stages after bypass surgery. This observation suggested that changes in the graft-dependent vascular area could influence the luminal size of the vein graft, even when they occurred several years after operation. Whereas it is well established today that aortocoronary vein grafts often decrease in luminal diameter after implantation, an increase in the bypass lumen has so far not been reported. Therefore, changes in luminal diameter of 27 vein grafts in 21 patients who underwent at least two postoperative angiographic studies (first study 8 +/- 5 months after surgery, second study 58 +/- 32 months after surgery) were compared with the size of the vascular region supplied by the bypass. The graft diameter was found to be unchanged between the two studies (3.3 +/- 0.6 versus 3.4 +/- 0.7 mm, p = NS) when the dependent vascular area was unchanged. A significant increase in graft diameter from 2.8 +/- 0.8 to 3.9 +/- 0.9 mm (p less than 0.001) was observed in nine patients in whom the area of perfusion had increased between the two studies because of the development of occlusion or obstruction of major coronary branches that were now perfused from the grafted vessel by way of collateral vessels. These data support the contention that the luminal size of aortocoronary vein grafts can adapt to the needs of the dependent myocardial vascular region even late after operation rather than being the result of a nonreversible degenerative process as commonly assumed.

Responsiveness of vein bypass grafts to stimulation with norepinephrine and 5-hydroxytryptamine

The in vitro reactivity of vein bypass grafts to norepinephrine (NE) and 5-hydroxytryptamine (5-HT) was studied in 20 rabbits undergoing bypass grafting. In these animals the right external jugular vein was grafted into the right carotid artery. The cumulative dose-response to NE and 5-HT of rings of vein grafts 2, 4, and 6 weeks after insertion was compared with that of rings from the normal contralateral jugular vein by means of an organ bath to measure changes in isometric tension. With NE there was no significant difference in the response of vein grafts harvested at 2 weeks and control veins. However, at 4 and 6 weeks there was a progressive decrease in the sensitivity of the grafts to NE. The difference in ED50 values (ED50 defined as concentration of agonist required to elicit 50% of the maximal response) between control veins and vein grafts at 4 weeks was twofold and at 6 weeks it was fivefold. None of the control veins responded to 5-HT stimulation. However, the vein grafts contracted with 5-HT, exhibiting sigmoid dose-response curves. The vein grafts showed intimal hyperplasia, which was maximal after 4 weeks. These results show that vein grafts undergo a progressive decrease in their sensitivity to norepinephrine and develop a marked, sustained increase in sensitivity to 5-HT. This finding is in contrast to previous observations in intimal hyperplastic rabbit aorta, suggesting a fundamental difference in the functional expression of arterial and venous intimal hyperplasia.

The evolution of morphologic and biomechanical changes in reversed and in-situ vein grafts

A comparative study of experimental reversed (RV) and in-situ (INS) vein grafts with respect to the evolution of morphologic and compliance characteristics was done in a canine model. In addition, the compliance characteristics in a series of human INS vein grafts were recorded as a function of time after operation. At 6 months after implantation, all experimental grafts displayed well-developed intimal hyperplasia. There was no significant difference in either absolute intimal thickness (INS 0.133 +/- 0.09 mm vs. RV 0.085 +/- 0.06 mm; NS) nor in the percentage of the total wall thickness occupied by the intima when experimental INS grafts were compared with RV grafts after 6 months. Similarly, compliance values of INS and RV vein grafts were similar at all time intervals examined up to 6 months after operation. Thirty-three human INS vein grafts had a mean compliance value of 1.74 +/- 0.72 (percent radial changes per mmHg X 10(-2) at a median postoperative interval of 14 weeks. This value did not differ significantly from those measured in the INS vein grafts. Although all vein grafts examined retained their native viscoelastic properties, this study suggests that functioning human INS vein grafts are less compliant than previously suspected on the basis of prior ex-vivo and clinical studies of RV saphenous vein grafts. The purported clinical superiority of the INS vein graft cannot be explained on the basis of superior biomechanical performance or failure to develop intimal hyperplasia.

The relaxant effect of glyceryltrinitrate on isolated human peripheral vein and its relation to cyclic GMP metabolism

The relaxant effect of glyceryltrinitrate (GTN) on human vena saphena magna was studied in vitro. Vessels contracted by serotonin (0.25 microM) and phenylephrine (0.1 mM) were relaxed to the same extent (EC50 = 10 microM) by GTN, whereas in 100 mM K+-depolarized vessels the relaxation was significantly lower. The relaxant effect produced by GTN was preceded by an elevation of cyclic guanosine-3',5'-monophosphate (cGMP). For 0.1 mM GTN there was a 3-fold increase in cGMP after 3 min. A correlation between relaxation and increase in cGMP was established. When GTN was combined with dipyridamole (5 microM) the relaxant effect of GTN was significantly greater (EC50 = 0.1 microM). Phosphodiesterase inhibition, as a possible mechanism behind the observed better relaxation for the combination (GTN+dipyridamole), is briefly discussed. In conclusion, the relaxant effect of GTN on isolated human vena saphena magna seems to be dependent on the contractile stimuli used, increased by the addition of DIP and to be mediated via cGMP.