Endothelin-mediated constriction of prenodal lymphatic vessels in the canine forelimb.

Endothelin is a 21 amino acid peptide which is produced by the vascular endothelium and is believed to be the mediator of endothelium-dependent vasoconstriction. In the current study we assessed the ability of synthetic human endothelin-1 to affect prenodal lymphatic vessel contractility in the canine forelimb. Intralymphatic infusion of endothelin at 1.09 x 10(-9), 1.09 x 10(-8) and 1.09 x 10(-7) M significantly constricted lymphatic vessels as evidenced by dose-dependent increases in lymphatic perfusion pressure. The increase in lymphatic perfusion pressure seen during intralymphatic infusion of endothelin at 1.09 x 10(-8) M during the intra-arterial infusion of phentolamine was not significantly different from that seen prior to phentolamine, indicating that endothelin-mediated lymphatic constriction is not alpha-receptor mediated. Intra-arterial infusion of endothelin at three infusion rates significantly increased forelimb arterial, systemic and lymphatic perfusion pressures. The constriction seen when endothelin (1.09 x 10(-8) M) was infused intralymphatically in the intact lymphatic system was not significantly different from that observed when only the prenodal lymph vessel was perfused. This indicated that the lymph nodes and efferent lymph vessels do not contribute significantly to the lymphatic constriction produced by endothelin. These data are consistent with the hypothesis that endothelin may modulate lymphatic function under either normal or pathophysiological conditions.

Relationship between substance P, intestinal wall compliance and vascular resistance in the canine ileum.

Substance P (SP) is one of many vasoactive peptides found within the gastrointestinal tract with actions on intestinal smooth muscle. Thus, its vasodilatory action could be attenuated through its stimulatory effect on intestinal smooth muscle producing subsequent elevations in extravascular pressure and thus, passively opposing the vasodilation. The aim of this study was to examine for such a possibility for SP by simultaneously assessing ileal perfusion pressure and intestinal wall compliance in the canine ileum during intra-arterial infusion of SP. Infusion of SP at either 0.74 or 7.4 pmol/min significantly decreased ileal perfusion pressure by 8 and 30%, respectively, without affecting wall compliance. During SP infusion at 74 pmol/min, perfusion pressure fell by 49% while wall compliance decreased by 43%, reflecting a significant increase in ileal wall tension. During SP infusion at 7.4 and 74 pmol/min, a 'two-phase' reduction in perfusion pressure was observed. These data suggest that although SP markedly increases ileal wall tension, the elevation in extravascular pressure produced is not strong enough to overcome its potent vasodilatory action in the canine ileum. The potential of a physiologic role for blood-borne SP is discussed.

Digital vascular responses and permeability in equine alimentary laminitis.

Digital vascular pressures, blood flow, and vascular resistances were measured in 11 control ponies and in 8 animals (7 ponies and 1 horse) affected with laminitis created by feeding a high starch ration. Animals with laminitis had increased digital blood flow, increased arterial, small vein, and large vein pressures, and decreased vascular resistances. Comparison of digital lymph flow rates and protein concentrations in animals with laminitis and control animals revealed no differences. Digital vascular responses of the 2 groups to acetylcholine, epinephrine, histamine, or serotonin also did not differ. Thus, the increased digital blood flow observed in animals with laminitis could not be attributed to altered responsivenss to the previously mentioned vasoactive agents. The studies also provided no evidence for increased capillary permeability in digits of animals affected with laminitis.

Vascular responses in the equine digit.

The digital circulation was isolated in 12 ponies under pentobarbital anesthesia. Blood flow was either controlled by a pump or measured under natural perfusion. The responses to rapid changes and stoppages of blood flow indicated no evidence of autoregulation or reactive hyperemia. Local administration of acetylcholine, histamine, and prostaglandins E1 and E2 decreased prevenous resistance, whereas epinephrine and serotonin caused prevenous constriction. Large doses of epinephrine and serotonin decreased venous caliber. The effects of prostaglandins A1 and F2alpha were variable. The equine digital vasculature responds to changes in flow and to vasoactive agents like the canine forelimb skin vasculature.

Effects of bolus injections of leukotrienes and norepinephrine on forelimb vascular and lymphatic pressures.

Leukotrienes, lypoxygenase metabolites of arachadonic acid, have been reported to be potent vasoconstrictors in some organs. This study was undertaken to delineate the actions of leukotrienes on both vascular and lymphatic vessels in the canine forelimb. Bolus intra-arterial injections of 1 microgram and 10 micrograms of leukotriene B4, C4, and D4 and 1 microgram of norepinephrine were made into forelimbs perfused at constant flow. Norepinephrine significantly increased systemic, forelimb perfusion and small artery pressures. Lymphatic pressure was significantly increased from a control of 6.6 mmHg to a peak of 14.4 mmHg. Leukotriene B4 in either dosage, did not significantly affect vascular or lymphatic pressures. Leukotriene C4 (1 microgram or 10 micrograms) significantly increased systemic and forelimb arterial pressures but did not alter lymphatic pressure. Leukotriene D4 (1 microgram) significantly increased small artery pressure. Leukotriene D4 (10 micrograms) increased systemic and forelimb arterial pressures. Neither dosage of leukotriene D4 significantly affected lymphatic pressure. Repeat injection of norepinephrine after completion of all leukotriene injections again markedly increased systemic, forelimb arterial and lymphatic pressures. These data indicate that leukotrienes exhibit only mild constrictor effects on forelimb blood vessels and do not significantly affect forelimb prenodal lymphatic vessels.

Bradykinin-mediated edema formation is blocked by levorotatory but not dextrorotatory terbutaline.

The ability of the purified stereoisomers of the beta 2-receptor agonist terbutaline to block bradykinin-mediated increases in lymph flow and protein concentration was assessed in the canine forelimb perfused at constant arterial flow. Intra-arterial infusion of bradykinin (2 micrograms/min, n = 8) decreased forelimb arterial pressures but did not affect skin small vein pressure or systemic pressure. Lymph flow, protein concentration and protein transport were significantly increased. Intra-arterial infusion of 1-terbutaline (1 microgram/min, n = 9) decreased forelimb arterial pressures and systemic pressure but did not affect lymph parameters. Subsequent infusion of bradykinin during the continued infusion of 1-terbutaline failed to alter forelimb lymph parameters. Intra-arterial infusion of d-terbutaline (1 microgram/min, n = 11) did not alter vascular pressures or lymph parameters. Subsequent infusion of bradykinin during the continued infusion of d-terbutaline decreased forelimb arterial pressures and significantly increased lymph flow, protein concentration and protein transport. Intra-arterial infusion of a high dose (100 micrograms/min, n = 9) of d-terbutaline significantly decreased forelimb arterial pressure but was likewise ineffective in blocking the increases in lymph parameters produced by subsequent bradykinin infusion. These data indicate that the beta 2-receptor agonistic and anti-permeability actions of terbutaline are found solely in the levorotatory enantiomer.

Constriction of lymphatics by catecholamines, carotid occlusion, or hemorrhage.

Regulation of lymphatics by sympathetic nerves or hormones seems probable. To elucidate this, we perfused a lymphatic vessel in the paw of the anesthetized dog while measuring lymphatic perfusion pressure. We studied the effects of norepinephrine, epinephrine, hemorrhage, and carotid occlusion on lymphatic pressure. Blood was pumped to the forelimb via the brachial artery. Cannulas were placed to measure systemic, central venous, and forelimb vascular pressures. Catecholamines, whether added to the lymphatic perfusate or infused into the forelimb arterial blood, and bilateral carotid occlusion significantly increased lymphatic perfusion pressure. Perfusion of prenodal lymphatics disconnected from downstream vessels and nodes indicated that this increase occurred primarily in prenodal lymph vessels. Hemorrhagic hypotension to 55 mmHg did not affect lymphatic pressure but reduction to 35 mmHg did. The increase in lymphatic pressure produced by epinephrine and norepinephrine was blocked by phentolamine. Increased lymphatic perfusion pressure subsequent to exogenous catecholamines, severe hemorrhagic hypotension, or bilateral carotid occlusion supports the possibility that lymphatic function is modulated by adrenergic mechanisms in physiological and/or pathophysiological states.

Evidence for constriction of canine prenodal lymphatic vessels by vasoactive agents and carotid occlusion.

We measured pressure in a prenodal lymphatic in the canine forelimb during constant flow pump-perfusion of the brachial artery. We made bolus i.a. injections of 1.0 micrograms angiotensin II, norepinephrine, bombesin, or bradykinin, 20 micrograms 5-hydroxytryptamine (5HT), or occluded the carotid arteries. Norepinephrine, 5HT, or carotid occlusion produced regular rises in forelimb perfusion pressure and in lymphatic pressure. Angiotensin II increased forelimb arterial pressures but increased lymphatic pressure in only four experiments. Bombesin increased artery pressures but did not affect lymphatic pressure. Small vein pressure was increased by carotid occlusion, 5HT and norepinephrine. Increases in lymphatic pressure were coincident with increases in vein pressure but no related in magnitude. Bradykinin decreased forelimb arterial and venous pressures but did not affect lymphatic pressure. Either active constriction of lymphatic vessels or passive compression by movements of adjacent blood vessels could increase lymphatic pressure. These data do not preclude a passive component of pressure rise in the lymphatics nor do they support the concept. We conclude that active constriction of prenodal lymphatic vessels in the dog forelimb can occur in response to circulating vasoactive agents and bilateral carotid occlusion.

Constriction of canine prenodal lymphatic vessels following the intra-arterial injection of vasoactive agents and hemorrhage.

In the forelimbs of anesthetized dogs, perfused at constant arterial inflow, we measured the pressure in a prenodal lymphatic vessel before and following arterial hemorrhage to a mean systemic arterial pressure of approximately 55 mmHg. We also made bolus intra-arterial injections of 1 microgram epinephrine and arginine vasopressin or 20 micrograms dopamine, prostaglandin F2 alpha and tyramine. Hemorrhage and all vasoactive substances significantly increased forelimb perfusion pressure and skin small artery pressure. Skin small vein pressure was significantly decreased by hemorrhage or injection of epinephrine, dopamine or tyramine, but was not significantly altered by arginine vasopressin or prostaglandin F2 alpha. Mean systemic arterial pressure was decreased by hemorrhage, increased by arginine vasopressin, tyramine and prostaglandin F2 alpha but remained unchanged following the injection of either epinephrine or dopamine. Lymphatic pressure was significantly increased following hemorrhage or the injection of all vasoactive agents. The increase seen with tyramine was small but consistent and thus statistically significant. These data indicate that the prenodal lymphatic vessels of the canine forelimb actively constrict in response to the neural and/or hormonal consequences of arterial hemorrhage or the introduction of exogenous vasoactive substances into the arterial blood supply to the forelimb. The results of the current study support the possibility that lymphatic function, through activation of lymphatic smooth muscle, is subject to neural and/or hormonal regulation in certain physiological and/or pathophysiological states.

Stereospecificity of the anti-inflammatory actions of terbutaline.

Intra-arterial infusion of a racemic mixture of the beta 2-agonist terbutaline blocks histamine-mediated increases in lymph flow and protein concentration in the canine forelimb. In the current study we have assessed the relative anti-inflammatory potencies of the purified stereoisomers of terbutaline. Infusion of histamine (4 micrograms base/min) increased lymph flow, protein concentration and protein transport. The intra-arterial infusion of 1-terbutaline (1 microgram/min) significantly decreased forelimb arterial pressures and prevented any changes in lymph parameters due to subsequent histamine infusion. Intra-arterial infusion of d-terbutaline (1 microgram/min) did not significantly affect forelimb vascular pressures but subsequent to histamine administration, lymph parameters increased similar to that seen with histamine alone. Infusion of a high dose of d-terbutaline (100 micrograms/min) slightly decreased forelimb arterial pressures but failed to inhibit histamine-mediated increases in lymph parameters. Infusion of 1-terbutaline alone (1 microgram/min) significantly decreased forelimb arterial pressures, lymph flow and protein transport and slightly but significantly increased lymph protein concentration. These data indicate that the beta 2-agonistic and anti-inflammatory properties of terbutaline are confined solely to the levorotatory enantiomer.

Neurokinin A and B: potent vasodilators in the canine forelimb.

Neurokinin A and neurokinin B may play a role in control of the peripheral circulation in either physiological or pathophysiological conditions. We have infused these peptides intra-arterially at three infusion rates each to assess their actions on vascular pressures, blood flows and total and segmental resistances in skin and skeletal muscle in the canine forelimb. Neurokinin A infusions (.01, .1, and 1 micrograms/min) decreased total forelimb resistance; transiently, 26% and 57%, respectively. The decrease in resistance was equally distributed between the skin and skeletal muscle circulations and was manifest in both large artery and small vessel resistances. Systemic and forelimb arterial pressures were decreased in a dose-dependent manner. Neurokinin B infusions (.5, 1 and 5 micrograms/min) decreased total forelimb resistance 29%, 31%, and 52%, respectively. The decrease in resistance was equally distributed between the skin and skeletal muscle circulations and was the result of decreases in both large artery and small vessel resistances. Systemic and forelimb arterial pressures were decreased in a dose-dependent manner. The potent effect of neurokinins on vascular resistance and their concentration in perivascular nerves innervating the resistance vessels of the circulation suggests a potential role for these neuropeptides in circulatory control.

Constriction of perfused lymphatics by acetylcholine, bradykinin and histamine.

We have previously reported that perfused lymphatic vessels in the canine forelimb constrict in response to increased sympathetic nerve activity or local infusions of endogenous vasoconstrictor substances. In the present study we have assessed the effects of three endogenous vasodilators; acetylcholine, bradykinin and histamine on lymphatic vessel contractility. Each one of these agents, when infused intralymphatically, produced lymphatic constriction as evidenced by significant increases in lymphatic perfusion pressure. The threshold concentrations which produced lymphatic constriction were between 10(-6) and 10(-5) molar for acetylcholine and bradykinin and between 10(-5) and 10(-4) molar for histamine. Surgical exclusion of the lymph nodes and efferent lymph vessels from the perfused tissue did not significantly affect the observed response, indicating that the response occurs predominately in the prenodal segments of the lymphatic system. Infusion of acetylcholine and bradykinin into the arterial supply to the forelimb did not significantly alter lymphatic perfusion pressure, unlike the response seen when catecholamines are infused intra-arterially. Histamine displayed an unusual property in that it constricts lymph vessels upon initial administration but was thereafter completely ineffective. Constriction of lymphatic vessels by substances which are potent vasodilators clearly indicates that significant functional differences exist in endothelial cell/smooth muscle relationships between blood vessels and lymph vessels.

The inflammatory actions of platelet activating factor are blocked by levorotatory terbutaline.

Platelet activating factor (PAF), a potent vasoactive lipid, may play an important role in the inflammatory process. In this study, we infused PAF intra-arterially to characterize its edematogenic potency in the canine forelimb. We have also assessed the ability of the beta 2-receptor agonist l-terbutaline to block PAF-mediated edema formation. The infusion of PAF at .25 micrograms/min, .5 micrograms/min and 1 micrograms/min increased forelimb arterial pressures and, at the two higher dosages, significantly decreased systemic arterial pressure. PAF infusions increased transvascular fluid and macromolecular flux as indicated by significant increases in skin lymph flow, protein concentration and protein transport. The intra-arterial infusion of l-terbutaline at 1 micrograms/min significantly decreased forelimb arterial pressures but did not affect small vein pressure, systemic pressure or forelimb lymph parameters. The subsequent infusion of PAF at .5 micrograms/min, during the continued infusion of l-terbutaline, failed to significantly affect forelimb lymph parameters. These data indicate that PAF is significantly more potent as an edematogenic agent in the forelimb than histamine or bradykinin. Furthermore, the blockade of PAF-mediated edema formation by l-terbutaline suggests that beta 2-receptor agonists may be capable of antagonizing the inflammatory actions of a wide variety of putative inflammatory mediators.

Perfused prenodal lymphatics are constricted by prostaglandins.

Prostaglandins may contribute to the control of lymph flow by affecting lymphatic vessel contractility. We measured the pressure in perfused prenodal lymphatic vessel in the paw of the anesthetized dog as affected by administration of prostaglandins E1, E2, F2 alpha or arachidonic acid. The forelimb was perfused at constant flow with blood obtained from a femoral artery. Systemic arterial, central venous, and forelimb vascular pressures were measured. When added to the lymphatic perfusate, all of the prostaglandins and arachidonic acid caused constriction of lymphatic vessels. Perfusion of prenodal lymphatics separated from downstream nodes and vessels showed that this constriction occurred primarily in prenodal vessels. However, only prostaglandin F2 alpha caused lymphatic constriction when infused into the blood to the forelimb. Because prostaglandins are a common component of the lymph leaving an area of tissue damage, these results are compatible with the possibility that prostaglandins, by directly affecting lymphatics, help modulate lymph flow following local injury.

Comparison of vascular effects of gastrointestinal hormones on various organs.

Vascular effects of raising local arterial concentration of pentagastrin (2-1,500ng/ml), secretin (0.2-150mU/ml), and cholecystokinin (0.2-150mU/ml) in the duodenum, jejunum, heart, kidney, forelimb, spleen, and the skin and muscle of the forelimb were studied in 54 anesthetized dogs. Secretin produced similar vasodilation in all organs. The minimal increment in local blood secretin concentration for vasodilation ("concentration requirement") was between 7 and 32 mU/ml. Pentagastrin produced vasodilation only in the duodenum and jejunum and the concentration requirement was between 25 and 50 ng/ml. Cholecystokinin did not affect vascular resistance of the forelimb, skin, or muscle. In the heart, kidney, and spleen, cholecystokinin produced vasodilation but the concentration requirement was above 21-33 mU/ml. In contrast, vasodilation in the duodenum and jejunum appeared when cholecystokinin concentration was increased by only 2.5 mU/ml. Furthermore, almost all its vasodilating effect occurred below an increment of 10 mU/ml. Comparison of our data with the reported cardiovascular adjustments and blood concentration of gastrointestinal hormones following a meal suggests that cholecystokinin may contribute to postprandial intestinal hyperemia.

O-(beta-hydroxyethyl)-rutoside (Venoruton) fails to block histamine or bradykinin-induced edema formation in the canine forelimb perfused at constant arterial inflow.

O-(beta-hydroxyethyl)-rutoside (Venoruton) has been reported to alleviate edema formation in chronic venous insufficiency. In an attempt to elucidate Venoruton's potential as an antiinflammatory agent, we infused Venoruton (20 mg/minute) intraarterially into the canine forelimb perfused at constant flow during the simultaneous intraarterial infusion of histamine (4 micrograms base/minute) or bradykinin (2 micrograms/minute). The infusion of Venoruton alone for forty minutes resulted in a small but significant increase in forelimb arterial pressures but no change in systemic pressure or forelimb skin lymph flow, protein concentration or protein transport. Subsequent infusion of either histamine or bradykinin resulted in a significant decrease in forelimb arterial pressures and a marked increase in skin lymph flow, lymph total protein concentration and lymph total protein transport. The changes in forelimb vascular pressures and skin lymph parameters were similar to those seen during the infusion of either histamine or bradykinin alone. These data indicate that the intraarterial infusion of Venoruton at this dosage does not inhibit the ability of simultaneously infused histamine or bradykinin to increase transvascular fluid and macromolecular efflux in the canine forelimb perfused at constant arterial inflow.

Effects of histamine and acetylcholine on equine digital lymph flow and composition.

We measured the flow rate and protein concentration of lymph collected from a digital lymphatic in eight anesthetized ponies. Additionally, we recorded systemic arterial pressure (Part), and small vein pressure (Psv). Control lymph flow averaged 0.068 ml/min, and contained 3.11 g/100 ml of protein with albumin/globulin ratio of 0.75. Twenty-minute local intra-arterial infusion of acetylcholine (10 mug/min.) elevated Psv but did not increase lymph flow rate or protein concentration. A 60-min local intra-arterial infusion of histamine (10 mug/min) produced a marked sustained increase in Psv and both lymph flow and protein concentration. Edema developed in the digit receiving histamine. These data support the conclusion that in the horse, as in other species, histamine edema is due primarily to a decreased transcapillary colloid osmotic pressure gradient rather than an increased transcapillary hydrostatic pressure gradient.