Response of human umbilical artery to changes in transmural pressure

Vasorelaxant effect of levcromakalim on isolated umbilical arteries of preeclamptic women

OBJECTIVE

Potassium channel openers are revealed to be a new type of antihypertensive drug. We aimed to clarify the effects of levcromakalim, an ATP-sensitive potassium channel opener, on human isolated umbilical artery (UA) and to compare them with those of nifedipine and magnesium sulphate, which are currently used in the treatment of preeclampsia (PE).

STUDY DESIGN

A total of 52 umbilical arteries, isolated immediately after delivery from 27 healthy and 25 preeclamptic pregnant women, were placed into 10-ml organ baths filled with Kreb's solution at physiological pH and temperature. The concentration-dependent relaxations in response to levcromakalim, nifedipine and magnesium sulphate were compared in vessels precontracted with serotonin (1 micromol/l).

RESULTS

The maximal relative relaxation responses (E(max), expressed as percentage of serotonin-induced precontraction) to magnesium sulphate, nifedipine and levcromakalim in umbilical arteries were identical in the healthy (85.06+/-3.31, 84.80+/-3.01 and 80.37+/-5.32%, respectively) and preeclamptic (77.20+/-5.30, 83.36+/-2.37 and 79.13+/-4.30%, respectively) groups.

CONCLUSION

Levcromakalim has a vasodilatory effect on the umbilical artery like magnesium sulphate and nifedipine, and serves as an antihypertensive potential that might be used in the treatment of preeclampsia. However, further experimental and clinical studies are needed to propose that ATP-sensitive potassium channel openers are beneficial drugs in cases of clinical preeclampsia.

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

Sodium pump activity and contractile effect of ouabain in human placental veins

The aim of the present study was to determine the number and affinity of [3H]ouabain binding sites, the sodium pump activity and the mechanisms involved in the contractile effects of ouabain in human placental veins. Scatchard analysis suggested the existence of a single population of binding sites with a KD of 196.7 nM and a Bmax of 1606 fmol/mg protein. The sodium pump activity was determined from the 86Rb+ uptake, which was reduced concentration dependently by ouabain (10(-8)-10(-4) M), and from the K+ (7.5 mM)-induced relaxation in veins preincubated in a K(+)-free medium and precontracted with PGF2 alpha (10(-6) M), which was also blocked by the glycoside (10(-6) M). Ouabain (10(-7)-10(-4) M) induced concentration-dependent contractions, which were not modified by either nifedipine or Bay K 8644 (10(-7) and 10(-6) M). Ca2+ omission from the medium or amiloride (10(-4) M) inhibited these contractions, whereas monensin (10(-6) M) potentiated them. These data indicate that human placental veins possess sodium pump activity and that its inhibition by ouabain induces potent contractions mainly mediated by Ca2+ entry through the Na(+)-Ca2+ exchange system.

Early and late contraction induced by ouabain in human umbilical arteries

1. Ouabain (3 x 10(-7)-10(-4) M) evoked a biphasic contraction in human umbilical artery that consisted of an early and a late contraction. The Ca(2+)-antagonists, verapamil (10(-7)-10(-5)) M), diltiazem (10(-7)-(10(-5)) M) and nifedipine (10(-9)-10(-7)) M) inhibited the early but not the late contraction. Caffeine (5 mM) changed neither the magnitude of the peak of the biphasic contraction nor the time needed to reach it. 2. Sodium concentration reduction (140 to 0 mM, replaced by N-methyl-D-glucamine, NMG) produced dose-dependent contraction of the arterial strip in 2.5 mM Ca2+ solution after the first treatment with verapamil (10(-5) M) and caffeine (5 mM), but not in Ca(2+)-free solution. 3. After prior treatment with verapamil and caffeine, the amplitude of the ouabain (10(-4) M)-induced late contraction varied, depending on the concentration of Ca2+ (0-2.5 mM) in the medium. 4. Amiloride (5 x 10(-5) M-5 x 10(-4) M), an inhibitor of the Na(+)-Ca2+ exchange system, produced dose-dependent inhibition of the late contraction induced by ouabain (10(-5) M) after prior treatment with verapamil and caffeine. 5. The time needed to reach the peak tension induced by monensin (5 x 10(-7) M) together with ouabain (10(-6) M) was less than that with ouabain alone, but the magnitude of the peak tension was not changed. 6. These results suggest that the early and late contractions caused by ouabain respectively produce a Ca2+ influx through voltage-sensitive Ca2+ channels and Ca2+ entry through sodium-calcium (Na(+)-Ca2+) exchange.

Biphasic contraction induced by ouabain in human umbilical arteries

The effects of nifedipine on the contraction induced by ouabain were investigated in helically cut human umbilical arterial strips. Ouabain (10(-7)-10(-4) M) induced a rapid transient (early) contraction and a persistent slow (late) contraction. Both effects were dose-dependent. The early contractions were inhibited by nifedipine (5 x 10(-9) and 10(-7) M) and by Ca2+-free solution containing 0.2 mM EGTA but the late contractions were not altered by nifedipine. The results suggest that ouabain could facilitate the transmembrane Ca2+ entry through voltage-sensitive Ca2+ channels.

Hyperemic response of the coronary circulation to brief diastolic occlusion in the conscious dog

This study was undertaken to determine whether coronary blood flow can be regulated in response to coronary arterial occlusions briefer than a single diastole. The possible involvement of metabolic vs. myogenic mechanisms in such regulation was investigated. Eleven conscious dogs with experimentally produced complete heart block, chronically implanted electromagnetic flow probes, and pneumatic occluders on the left circumflex coronary artery were studied. Diastolic coronary occlusions lasting 100 to 400 msec were performed at paced heart rates of 40, 60, and 120 beats/min. At a heart rate of 60 beats/min, a 200-msec occlusion was sufficiently long to produce a significant reactive hyperemic response; 400-mec occlusions resulted in larger responses, while 100-msec occlusions did not generate a discernible response. The onset of reactive hyperemia was delayed from the end of the occlusion until the first post-occlusion systole. The length of this delay could be altered by changing the heart rate or occlusion duration, but no significant response was detected before the first post-occlusion systole. This characteristic of the data is more consistent with a metabolic than with a myogenic mechanism. If the response is metabolic, the data demonstrate that autoregulation of coronary flow by such a mechanism is very rapid, occurring during the first systole in which a flow deficit is detected by the myocardium.

Thermodilution measurement of human umbilical blood flow at delivery

A local thermal dilution technique was used to estimate blood flow in the human umbilical vein during the first minute after delivery of the neonate. Estimates of flow were attempted at 101 deliveries and thermal dilution curves obtained at 52. The mean estimation of all flows recorded up to 20 seconds after delivery was 171 ml. per minute reducing to 58 ml. per minute between 40 and 60 seconds.

Static and dynamic components in the vascular myogenic response to passive changes in length as revealed by electrical and mechanical recordings from the rat portal vein

The effects of static and dynamic passive stretch and shortening on electrical activity and active force were analyzed in the isolated rat portal vein. Static stretch by 40% of muscle length evoked moderate excitatory effects with enhanced mechanical activity and an average increase in spike discharge of 12% above the control value of 55 plus or minus 2.6 spikes/min. The dynamic responses studied at various rates of length change (dL/dt) over the range between minus 12 and plus 12 mm/min, i.e., minus 3 and plus 3% muscle length/sec, were much more pronounced. Active force and spike activity showed graded increases with increasing rates of stretch. The electrical activity reached a value of 180 spikes/min (approximately equal to 325% of control) at 5 mm/min; this frequency was then maintained for stretch rates up to 12 mm/min. Mechanical activity during stretch was further reinforced by the shift along the length-tension diagram. Passive shortening at rates from minus 1 to minus 12 mm/min caused graded decreases in mechanical and electrical activity below the control levels, complete inhibition being observed at the latter dL/dt. Blockade of alpha and beta receptors indicated that the responses were myogenic in nature. The findings seem to provide direct support for the myogenic hypothesis of vascular tone and responses to stretch of the vascular wall, but they indicate that emphasis should be placed on the dynamic characteristics of the stimulus rather than its static nature. This emphasis constitutes a new concept in the myogenic control of the peripheral circulation.

Inactivation of bradykinin in the pulmonary vascular bed of newborn and fetal lambs

Bradykinin is largely inactivated in one passage through the pulmonary vascular bed of adult animals. We studied pulmonary inactivation of bradykinin in newborn lambs, fetal lambs at term, fetal lambs preterm, and adult ewes by comparing the systemic pressure response to infusions proximal to the pulmonary vascular bed with the response to infusions distal to the pulmonary vascular bed. A high degree of inactivation was confirmed in mature ewes (93 ± 1.2%); newborn lambs showed significantly less pulmonary inactivation (68 ± 5.2%) and fetal lambs at term showed only 46 ± 4.8% inactivation. In the preterm fetuses (gestational age 110-128 days), no inactivation could be demonstrated in the pulmonary vascular bed. The return to base-line diastolic pressure after the infusion was discontinued was much slower in the fetus than it was in the newborn and the adult animal, indicating a prolonged effect of bradykinin. The results suggest that inactivation of bradykinin is a pulmonary function that develops late in fetal life and is still less than adult levels at birth. This finding could be of particular interest if bradykinin participates in mediating the circulatory changes occurring at birth.

Effects of acetylstrophanthidin on isolated veins of the dog

Acetylstrophanthidin, 1.0 to 10.0 µg/ml, caused contraction of strips of the lateral saphenous vein of the dog in an organ bath at 37°C. The superior mesenteric vein, which exhibited spontaneous activity in the organ bath, responded to acetylstrophanthidin with an almost immediate increase in tension and an increased frequency of contractions; this was followed by a sustained contraction which had the same time lag as the contraction of a saphenous vein strip from the same dog. In strips from both veins, the contraction was depressed by cooling and augmented by warming, an effect opposite to that seen with norepinephrine-induced contractions. A latent period of 10 to 60 minutes preceded the contraction of the saphenous vein; during this time, the vein showed increased reactivity to added norepinephrine, 5-hydroxytryptamine, acetylcholine, barium chloride, increased bath concentration of K + , and decreased bath concentration of Na + . The increased reactivity and contraction still occurred after catecholamine depletion of the vein by chronic lumbar sympathectomy and in umbilical veins, which are devoid of adrenergic nerves. In addition to its direct constrictor action on cutaneous and mesenteric veins, acetylstrophanthidin sensitized cutaneous veins to vasoactive agents and to changes in ionic environment. These actions did not depend on the presence of norepinephrine in the veins.

Bradykinin production associated with oxygenation of the fetal lamb

Previous studies showed that synthetic bradykinin produced in-vitro constriction of the ductus arteriosus of the fetal lamb and guinea pig, constriction of isolated human and lamb umbilical vessels, and pulmonary vasodilatation in the fetal lamb.

The present study showed that bradykinin precursor, kininogen, is present in arterial blood of the fetal lamb by 61 days of gestation and that its concentration increases toward term. We studied the bradykinin-generating system in 6 exteriorized fetal lambs. Kininogen concentrations in left atrial blood decreased within 1 to 2 minutes after beginning ventilation with O 2 , and free bradykinin was detected in left atrial blood. Pulmonary arterial blood kininogen concentration was not significantly altered. Kininogen concentration in left atrial blood did not fall in 4 other lambs ventilated with N 2 but did fall after subsequent ventilation with O 2 . To study the effects of increased oxygenation in utero without lung expansion, 6 pregnant ewes were exposed to hyperbaric O 2 at 3.63 atmospheres absolute. Associated with a mean increase in brachial arterial blood Po 2 to 44 mm Hg, a value equivalent to that in the ventilated exteriorized fetus, kininogen concentration fell, and free bradykinin was detected in the brachial arterial blood of 3 of the 6 fetuses. In 4 other exteriorized lambs we measured kininogen concentration in brachial arterial and right ventricular blood at frequent intervals up to 30 minutes after ventilation with O 2 . The concentration in brachial arterial blood fell before that in right ventricular blood.

The hypothesis is presented diat bradykinin is produced from kininogen in the lungs of the fetal lamb when oxygenated, and that the maximal rate of production occurs during the first few minutes after expansion of the lungs or exposure of the ewe and fetus to hyperbaric O 2 .

Catecholamines in arteries and veins of the foreleg of the dog

The catecholamine content of various blood vessels of the foreleg of the dog was measured by a fluorimetric method (48 vascular segments from 11 dogs) and also by bioassay (33 segments from 8 dogs). The results obtained by the fluorimetric method indicate that the average concentration of catecholamines in the brachial artery (0.14 µg/g of wet tissue) was significantly lower than that in the ulnar (0.70 µg/g) and metacarpal arteries (0.66 µg/g); the average concentration in the brachial vein (0.17 µg/g) was significantly lower than that in the cephalic (1.30 µg/g) and metacarpal veins (0.47 µg/g). The values obtained by bioassay were generally higher than those obtained by the fluorimetric method, but the differences between the arterial segments and between the venous segments were similar by both methods. There was a close correlation between the concentration of catecholamines in the different vascular segments and the relative responsiveness of these segments to nerve stimulation as reported previously (Circulation Res. 18: 263, 1966). Pretreatment with reserpine decreased markedly the catecholamines in segments of ulnar artery and cephalic vein. Intravenous infusion of norepinephrine in amounts sufficient to raise blood levels of catecholamines five- to tenfold did not alter the content of segments of ulnar artery.

Constriction of human umbilical arteries. Interaction between oxygen and bradykinin

Experiments on perfused segments or isolated rings of human umbilical artery showed that the magnitude of their contraction in response to bradykinin, epinephrine, or serotonin depended on the Po 2 of the perfusing or bathing solution. Constriction to bradykinin varied directly with the O 2 tension and was greatest when the Po 2 was approximately that of umbilical arterial blood during the transition from the fetal to the neonatal period. Catecholamine beta-receptors were not demonstrated (not present or not functional) in the umbilical artery, since there was no vascular response to either isoproterenol or to propranolol. Alpha-receptors were not required for O 2 or bradykinin constriction since these agents constricted the umbilical artery after enough phentolamine had been given to block constriction by norepinephrine. Because of the interaction between O 2 and vasoconstrictor agents the O 2 environment of umbilical vessels must be rigidly controlled in experiments measuring vascular reactivity. Sensitization by O 2 to the effects of chemical substances endogenous to the fetus or mother may play an important role in circulatory adjustments essential for neonatal life.

A method for studying isolated resistance vessels from rabbit mesentery and brain and their responses to drugs

A technique has been developed for studying the reactivity of single, isolated resistance vessels, 50 to 250 µ o.d., perfused at constant flow rate. The validity of the method is established because responses to a given stimulating agent are reproducible and stable over a reasonable period of time. The magnitude of the response is dependent on the perfusion pressure, being maximal at a physiological pressure level. Resistance vessels from mesentery and brain of normal rabbits were compared with respect to their threshold for response to several vasoconstrictors. The results reaffirmed the individuality of smooth muscle from different vascular beds. Cerebral and mesenteric vessels are alike in the dose required for threshold constrictor response to KCl, angiotensin and plasma, but differ in that cerebral vessels have a higher threshold for response to epinephrine, norepinephrine and serotonin, and a lower threshold for response to vasopressin, than mesenteric vessels. Since the differences in threshold between vessels from the two sources are not the same for all stimulating agents, it seems probable that smooth muscle of the cerebral vessels is not generally less sensitive to all stimuli than that of mesenteric vessels, but that vessels from the two sources differ in the "number of receptors" for the several vasoactive agents.

Angiotensin tachyphylaxis and its reversal

Tachyphylaxis to angiotensin and some analogues has been demonstrated on spirally cut arterial strips from cat, dog, sheep and rat, and on venous strips from rabbit and cat. Rabbit and guinea pig arteries do not appear to become tachyphylactic. A free C-terminal carboxyl group of angiotensin is necessary for binding with receptor sites and development of tachyphylaxis. Tachyphylaxis seems to represent saturation of receptor sites. It can be reversed by plasma fractions rich in angiotensinase A, possibly by metabolizing the N-terminal part of angiotensin directly from the bound state. Dowex 50 can also reverse it, probably by physical adsorption and stronger binding of angiotensin. Angiotensinase A does not metabolize βaspartyl 1 -angiotensin and does not reverse tachyphylaxis to this peptide. A possible scheme of interaction between peptide and receptor site is presented.