Interaction between canine platelets and adult heartworms: platelet recognition of heartworm surfaces

An interaction between blood platelets and adult heartworms was examined in vitro. Surfaces of glutaraldehyde-fixed heartworms, which were taken from infected dogs washed, and incubated in platelet-rich plasma (PRP), were evaluated by scanning electron microscopy. Adherence of platelets to heartworms occurred only with PRP from infected dogs. Aggregation to epinephrine and adenosine diphosphate of PRP incubated with heartworms was monitored. Seemingly, platelet activation to heartworm membranes occurs in dogs with heartworm disease. The increased platelet reactivity was also observed in dogs with occult heartworm disease, indicating that the presence of circulating microfilaria was not important for this process. The ability to transfer the reactivity to heartworm-negative platelets by suspending them in heartworm-positive plasma indicated that this reactivity resided in the plasma. The processes leading to platelet activation may be responsible for the platelet-associated vascular disorders of canine heartworm disease.

Chlorpromazine inhibits arterial ACAT and reduces arterial cholesterol and cholesteryl ester accumulation in cholesterol-fed rabbits

Rabbits were fed an atherogenic diet concurrently with chlorpromazine for 2 weeks (10 mg/kg/day in the diet) or 12 weeks (20 mg/kg/day given orally in a single capsule). After 2 weeks, arterial acyl CoA: cholesterol acyltransferase (ACAT) activity tended to be reduced by chlorpromazine treatment with no affect on net arterial cholesterol. After 12 weeks of treatment, arterial ACAT activity was significantly reduced and was paralleled by a reduction in net arterial cholesterol, a reduction in the esterified cholesterol/unesterified cholesterol ratio, and a reduction in lipid staining intensity as determined histologically with oil red O staining of aortic cross sections. Paradoxically, there was no histological evidence for a reduction in the size of atheromatous lesions with chlorpromazine treatment as determined by morphometric analysis of tissue cross sections. The results support the hypothesis that the increased esterification of cholesterol that characteristically accompanies the atherogenic process serves as a biochemical trapping mechanism for cholesterol entering the vessel wall and suggest that regulation of the enzyme in vivo can reduce the net accumulation of arterial cholesterol.

Effect of acetylsalicylic acid on pulmonary arteriosclerosis induced by a one-year Dirofilaria immitis infection

The ability of aspirin to block arteriosclerosis that developed in response to chronic, low-level injury to pulmonary arteries was evaluated in 21 dogs during their 1-year infection with Dirofilaria immitis. Three groups, with seven dogs in each group, were studied before and after sustained injury produced by the transplantation of 28 adult Dirofilaria immitis into each dog. Group A received no treatment and served as controls; Group B received no treatment for 6 months and then received 7 mg/kg of aspirin daily) for 6 months; Group C received 7 mg/kg of aspirin daily for the entire year. The pulmonary arterial response was evaluated by hemodynamic and arteriographic studies at 6 and 12 months and by scanning electron microscopy at the end of the 12-month study. All groups developed a similar, mild pulmonary hypertension. The arteriographic changes of dilation and flow obstruction were worse in Groups A and B than in Group C at 6 months, and at 12 months both Groups B and C were less obstructed than Group A. Scanning electron microscopy revealed large, complex myointimal proliferations in Group A, whereas the two aspirin-treated groups had smaller, less complex lesions that covered a much smaller surface area. We concluded that: 1) aspirin markedly reduced the microscopic and macroscopic arteriosclerosis in Groups B and C; 2) aspirin in Group B not only arrested further development but also permitted resolution of arteriosclerosis while the arteries were still being injured.

Smooth muscle proliferation in chronically injured canine pulmonary arteries is reduced by a potent platelet aggregation inhibitor U-53,059

Dirofilaria immitis (DI) infection chronically injures canine pulmonary arteries. This injury produces endothelial cell loss, platelet/leukocyte adhesion, and smooth muscle proliferation. In the present study we assessed the effect of the cyclooxygenase inhibitor, U-53,059, on platelet function, platelet kinetics, coagulation, and smooth muscle proliferation in DI infected dogs. Platelet aggregation to the combination of arachidonic acid/ADP was significantly inhibited by U-53,059. Coagulation and hematologic parameters were not effected by either DI infection or U-53,059 treatment. Platelet survival and the number of platelet dense granules were reduced in DI infection. Quantification of the lesions demonstrated that U-53,059 reduced both severity and density compared to non-treated dogs. U-53,059 is a potent and effective inhibitor of platelet aggregation which modifies smooth muscle proliferation produced by chronic vascular injury.

Acetyl glyceryl ether phosphorylcholine (PAF-acether) and leukotriene B4-mediated neutrophil chemotaxis through an intact endothelial cell monolayer

Human endothelial cell monolayers were grown on nucleopore filters, and used to partition the two halves of a modified Boyden chamber. Human neutrophil chemotaxis through the monolayer was studied in response to leukotriene B4 and acetyl glyceryl ether phosphorylcholine (PAF-acether). Both leukotriene B4 and PAF-acether concentration-dependently stimulated neutrophil chemotaxis through intact monolayer. The biologically inactive lyso-PAF, and leukotriene C4 and D4 were inactive as chemotactic agents. Leukotriene A4 was weakly chemotactic. In the absence of chemotaxin, little penetration of the monolayer by neutrophils was observed. Agents that elevate neutrophil cyclic AMP levels inhibit both leukotriene B4 and PAF-acether-stimulated chemotaxis through the endothelial cell monolayer. The specific 5-lipoxygenase inhibitor, 6,8-de-epoxy-6,9-(phenylimino) delta 6,8-prostaglandin I1 (U-60257), inhibits PAF-acether, but not leukotriene B4-mediated chemotaxis. These data suggest that an intact 5-lipoxygenase may be required for normal PAF-acether-mediated chemotaxis, but leukotriene B4-mediated chemotaxis is independent of 5-lipoxygenase activity. This system may prove to be a useful model for the study of neutrophil-endothelial cell interactions.

Early events in the formation of a venous thrombus following local trauma and stasis

A jugular vein was exposed in 20 cats divided into four groups of five cats each. In group 1 the vein was removed immediately after exposure. In group 2 the vein was removed after three 5-minute periods of stasis and reflow. Groups 3 and 4 had the jugular vein occluded for 24 and 72 hours, respectively. In all groups, veins were perfused under physiologic pressure by heparinized saline to remove blood and immersed in 2.5% glutaraldehyde for fixation. All vessels were prepared for scanning and transmission electron microscopy. Group 1 cats had a normal-appearing luminal surface. Group 2 cats had deposition of leukocytes with few erythrocytes or platelets. Groups 3 and 4 had deposition of leukocytes, platelets, and erythrocytes. Leukocytes were found in all areas and associated with all cell types. Platelets and erythrocytes were seldom found in the absence of leukocytes. Thrombi were found on normal-appearing and damaged endothelium. The majority of thrombi were found at side branches and valve pockets. Our results suggest that leukocytes play a primary role in the initiation of deep vein thrombosis. Platelets may have only a secondary role.

Histologic examination of selected areas of canine pulmonary arteries

Selected areas of pulmonary arteries from 18 healthy mixed-breed dogs were examined using histologic staining techniques. Smooth muscle cell, collagen, and elastin content of the tunica intima and tunica media were assessed. Fifteen dogs had abnormalities of tunica intima or tunica media in at least one arterial section examined. Of all arterial sections examined, 40% had histologic changes of the tunica intima or tunica media, and 42% of these vascular lesions were in the main pulmonary artery. The most commonly occurring pathologic change was loss of smooth muscle cells and elastin of the tunica media and replacement by collagen. This lesion is similar to cystic medionecrosis of the aorta. Seemingly, a high frequency of spontaneous vascular lesions exist in pulmonary arteries of young dogs.

Medial smooth muscle cell proliferation in the balloon injured rabbit aorta: effect of a thiazole compound with platelet inhibitory activity

Twenty-seven adult male New Zealand rabbits (3-4 kgs) were used in this study. Six rabbits received vehicle, 3 groups of 6 each received doses of 4,5-bis(p-methoxyphenyl)-2-(trifluoromethyl)-thiazole, (U-53,059), at 0.3 mg/kg, 3.0 mg/kg and 30.0 mg/kg/day respectively. Drug and vehicle doses were given orally each day starting 3 days before balloon injury and continuing for the entire 2 week time period. Three rabbits were used as nontreated sham controls. In the vehicle and U-53,059 treated groups aortae were denuded of endothelial cells by balloon catheter injury. Two weeks after injury platelet aggregation to collagen was measured and the aortae removed for analysis of surface characteristics by scanning electron microscopy and lesion size by morphometry. All doses of U-53,059 inhibited platelet aggregation. The 3.0 and 30.0 mg/kg groups had the greatest inhibitory effect. All balloon injured aortae had the same morphologic characteristics. All vessels had similar extent and intensity of Evan's blue staining, similar areas of leukocyte/platelet adhesion, and a myointimal cell cover of transformed smooth muscle cells. The myointimal proliferative response was not inhibited at any of the drug doses studied.

Suppression of experimental atherosclerosis in rabbits by interferon-inducing agents

The effects of two chemically different interferon inducers on the suppression of atherosclerosis were studied in rabbits fed an atherogenic chow diet. One group (10 rabbits per group) was fed normal rabbit chow, and three groups were fed an atherogenic chow. One of the latter groups received the atherogenic feeding alone; the other two were treated with either polyinosinic-polycytidylic acid (poly I:C) or 2-amino-5-bromo-6-phenyl-4-pyrimidinone (ABPP). Neither of the drugs reduced significantly the hypercholesterolemia induced by the feeding. However, both poly I:C and ABPP treatment significantly reduced the percent area of the aortic intimal surface lesions, stained for lipid with Sudan IV, compared with that in untreated rabbits fed atherogenic chow. Microscopic sections of typical aortic plaques showed that both drug treatments significantly reduced the size and number of intimal lipid deposits compared with those observed in the aortas of untreated animals. Chemical analysis for cholesterol and collagen content revealed that interferon-inducing agents significantly reduced cholesterol deposits in the aorta, with little effect on fibrous protein deposition. The results indicate that two unrelated interferon-inducing drugs suppressed atherogenesis without reducing serum cholesterol and low density lipoprotein levels. Whether the protection against atherosclerosis is exerted by endogenous interferon production remains to be determined.

Pulmonary thromboembolism during therapy of dirofilariasis with thiacetarsamide: modification with aspirin or prednisolone

The effects that 4 weeks of treatment of dirofilaria-infected dogs with either aspirin or prednisolone had on the pulmonary thromboembolism which occurs after they are given thiacetarsamide were determined, using light and electron microscopies. Pulmonary lesions in control dogs at 4 weeks after thiacetarsamide was given were compared with lesions in dogs which were treated with either aspirin (22 mg/kg daily) or prednisolone (1 mg/kg daily) during the 4-week period after adulticide was given. Pulmonary vascular and perivascular lesions were most severe in the prednisolone-treated dogs and least severe in the aspirin-treated dogs. The aspirin-treated dogs had greater resolution of pulmonary arterial proliferative disease, and prednisolone-treated dogs had the lesser resolution.

Treatment of canine dirofilariasis: pulmonary thromboembolism caused by thiacetarsamide--microscopic changes

Light and scanning electron microscopies were used to study the pulmonary embolism occurring in Dirofilaria immitis-infected dogs after treatment with thiacetarsamide. Lesions in control dogs (nontreated) which had been infected for 4 weeks with 9 Dirofilaria immitis adults were compared with lesions occurring in infected dogs at 2 weeks and at 4 weeks after they were treated with the adulticide. Extensive thromboembolism occurred in the caudal lobar pulmonary arteries of dogs at posttreatment weeks 2 and 4. Complicated villous proliferations were present at posttreatment week 2. The characteristic myointimal proliferation of dirofilariasis showed resolution in the large pulmonary arteries of the dogs at week 4. However, the caudal lobar pulmonary arterial and lung lesions were more severe in the later group. The pathophysiology of adulticide-induced thromboembolism and associated lung parenchymal changes were discussed.

Effect of acetylsalicylic acid on vascular damage and myointimal proliferation in canine pulmonary arteries subjected to chronic injury by Dirofilaria immitis

The effect of aspirin (ASA) on pulmonary artery response to chronic injury induced by Dirofilaria immitis was assessed in the dog. Eight dogs were studied for 30 days after adult heartworms were transplanted into the pulmonary arteries. Four dogs were treated with ASA (325 mg/day given orally) starting 3 days before the transplantation was done. Evan's blue dye was used to locate areas of vascular damage. Vascular morphology was assessed by scanning electron microscopy and light microscopy. According to results of the morphologic studies, nontreated dogs had extensive endothelial damage and significant adhesion of leukocytes and blood platelets. Blood vessels of ASA-treated dogs had less endothelial damage. Platelet adhesion on areas of vascular damage was less than that observed in nontreated dogs. Myointimal proliferative lesions were observed in both nontreated and ASA-treated dogs. However, the lesions in nontreated dogs were larger (means 68 +/- 7 mm2) and more complex than those in ASA-treated dogs (means 20 +/- 1 mm2). The results indicated that aspirin may protect against development of myointimal proliferation resulting from chronic vascular injury. The mechanism of the ASA protection may result from a combination of antiplatelet and anti-inflammatory effects, as well as a direct protective effect on vascular endothelium.

Early arterial injury-induced myointimal proliferation in canine pulmonary arteries

Transmission electron microscopy was used to study the early vascular response induced by arterial damage with heartworm infection. The pulmonary arteries were examined in dogs 4 days after experimental transplantation of 6 to 8 Dirofilaria immitis adults. Evan's blue dye was given IV and followed in 60 minutes by perfusion fixation with 1% glutaraldehyde. Endothelial cell junctions were disrupted and rounded endothelial cells were observed. Macrophages and neutrophils adhered to abnormal endothelial cells. Focal areas had platelet aggregates adhered to exposed subendothelial structures. Platelets were activated and degranulated. Areas of the internal elastic lamina appeared to be disrupted, and smooth muscle cell processes from the tunica media extended through these regions of disruption. Smooth muscle cells were oriented toward the arterial lumen and some had migrated to the surface. These findings are compatible with the response to injury theory of myointimal proliferation.

Effect of aspirin on collateral blood flow after experimental thrombosis of the feline aorta

In 11 cats, the caudal aorta was occluded with a thrombus to inhibit opening of collateral vessels to the hindlimbs. Five cats were not treated, and 6 cats were given aspirin (650 mg, orally) 1 hour before aortic occlusion. Before the aortas were occluded, platelet counts were measured in the 11 cats, and platelet aggregation by adenosine diphosphate was assessed in 4 cats scheduled for aspirin treatment. Cats were maintained for 3 hours after surgery, and the tests were repeated. In addition, serum salicylate concentrations were determined for all aspirin-treated cats. Collateral circulation was assessed as the time necessary for contrast media to appear in the iliac arteries caudal to the thrombus during aortography. Platelet counts were significantly (P less than or equal to 0.05) reduced in nontreated cats, but not in the aspirin-treated cats. Serum salicylate concentrations indicated that aspirin had reached therapeutic blood concentrations. Inhibition of adenosine diphosphate-induced platelet aggregation was observed in all postthrombosis aspirin-treated cats. Contrast media did not appear in the iliac arteries of nontreated cats until 7.5 +/- 0.5 s after injection. In all aspirin-treated cats, contrast media appeared in the iliac arteries within 2.3 +/- 0.3 s after injection. Platelet thromboxane A2 may be the factor promoting collateral inhibition after aortic thrombosis in cats.

Effect of long-term aspirin treatment on platelet adhesion to chronically damaged canine pulmonary arteries

The effect of aspirin on platelet adhesion to chronically damaged pulmonary arteries was studied in 18 dogs. Chronic injury was produced in all dogs by infection with the canine heartworm Dirofilaria immitis (DI). Ten dogs were subjected to 4 days of chronic injury. Eight dogs were subjected to 30 days of chronic injury. Five of the 4 day injury and 4 of the 30 day injury dogs received aspirin daily; (325 mg/day orally). Aspirin was started 3 days prior to infection with DI. Pulmonary arteries were perfusion fixed in situ at physiologic pressure. The damaged pulmonary arteries were located by Evan's blue staining (2 ml/kg of 1% dye given one hr prior to perfusion) and prepared for scanning electron microscopy. Both groups of dogs with 4 day DI infection had monolayers of platelets adhered to exposed subendothelium. Aspirin treated dogs had enhanced platelet adhesion to damaged arteries. Aspirin treatment for 33 days reduced platelet adhesion. The damaged arteries of treated dogs infected with DI for 30 days had very few platelets adhering to the damaged surface. However, non-treated dogs subjected to 30 days of infection had platelet adhesion equivalent to the 4 day non-treated infection group. These results suggest that although aspirin is ineffective in preventing platelet adhesion in short term therapy it is effective when given for longer time periods. This inhibitory effect may occur due to platelet membrane changes rather than because of aspirin inhibition of cyclooxygenase.

Platelet adhesion and myointimal proliferation in canine pulmonary arteries

Pulmonary arteries were studied by scanning and transmission electron microscopy in 15 preconditioned dogs. Five dogs were control animals, while 10 dogs were studied 4 and 30 days following transplantation of adult heartworms into the pulmonary arteries. Evan's blue dye was used to locate areas of vascular damage. Pulmonary arteries from control dogs exhibited no Evan's blue staining. The surface and ultrastructural characteristics of these blood vessels were comparable to normal peripheral blood vessels. Pulmonary arteries removed from dogs after 4 days of heartworm infection exhibited extensive staining with Evan's blue. These stained areas had disrupted endothelium with many platelets adhered to the exposed subendothelium. In addition, leukocytes were attached to adjacent areas of damaged endothelium. Pulmonary arteries of dogs infected with heartworms for 30 days also exhibited extensive staining with Evan's blue. The blue-stained areas in this group had two typical responses. On some portions the lesions were similar to those seen at 4 days (ie, loss of endothelium with platelet and leukocyte adhesion), while other stained areas had complex lesions that projected from the surface into the lumen of the blood vessel. These lesions were endothelialized, and transmission electron microscopy revealed that they consisted of large numbers of smooth muscle cells that had migrated through the internal elastic lamina. The findings in the 30-day infection group suggest that the proliferative lesion formation was a result of an ongoing active process of endothelial loss and plateletleukocyte adhesion. The characteristic response of canine pulmonary arteries to the presence of heartworms (endothelial loss, platelet-leukocyte adhesion, and development of myoproliferative intimal lesions) suggests that this condition is a potential model for study of the early vascular changes that produce myointimal proliferation.

Effects of stress on blood pressure and cardiac pathology in rats with borderline hypertension

Repeated attempts to produce hypertension (HT) through psychological stress have failed to elevate blood pressure (BP) to levels seen in chronic, untreated essential HT in humans. In general, these studies have two characteristics in common: they utilize the normotensive animal, with no genetic history of HT, and they involve stressors to which animals readily adapt. The present study utilized offspring with one HT parent. The male F1, offspring of SHR x WKY had borderline HT (-/x +/- SEM = 152.4 +/- 1.34 mm Hg). With a conflict paradigm used as the stressor, experimental animals eventually developed severe HT (188.3 +/- 2.70 mm Hg) compared to two non-stressed control groups (158.4 +/- 2.31 mm Hg and 151.9 +/- 2.25 mm Hg). After 15 weeks of stress for 2 hours daily, termination of conflict for 10 weeks did not reduce the HT in experimental animals. Subsequent analyses revealed that stressed animals, when compared to nonstressed controls, exhibited elevated heart-weight-to-body-weight ratios and significant cardiac pathology in the form of myofibrillar degeneration, accumulation of inflammatory cells, and fibrosis. The implications of using this model for the analysis of cardiovascular concomitants of stress-induced HT are discussed.

Pathophysiological changes associated with stress-induced hypertension in the borderline hypertensive rat

1. This study sought to develop an animal model which would be prone to stress-induced hypertension without spontaneously developing it. 2. Eighteen male rats, F1 offspring of spontaneously hypertensive x Wistar-Kyoto rats, had tail-cuff systolic blood pressures of 152 mmHg at 12 weeks of age. Animals were randomly assigned to three groups of six each: experimental (subjected to 'conflict' 2 h daily for 12 weeks), restraint control (placed in conditioning cages but not subjected to conflict) and maturation control (neither restrained nor stressed). 3. Systolic blood pressure rose significantly in experimental animals to 186 mmHg. Experimental animals studied for a 10 week follow-up period without conflict maintained the elevated pressure. In addition, these animals showed myofibrillar degeneration and an elevation in heart weight/body weight ratios. Restraint animals showed a modest elevation in pressure during the study, but not during the 10 week follow-up. Maturation animals showed no changes in blood pressure. 4. The development of a model of hypertension combining genetics with psychological stress may serve as a means for determining the factors involved in triggering and sustaining stress-induced hypertension which may prove relevant to essential hypertension in man.

Scanning electron microscopy of canine pulmonary arteries and veins

In this communication we describe (1) a method for preparing pulmonary arteries and veins for examination by scanning electron microscopy and (2) the topographic characteristics of the luminal surface of these vessels. Seven healthy adult dogs were used in this study. The pulmonary vasculature was washed free of blood by perfusion with Tyrode's solution. Blood vessels were partially fixed in situ under physiologic pressure in 3 dogs (15 mm of Hg) or 2 times physiologic pressure in 4 dogs (30 mm of Hg) by perfusion with 1% glutaraldehyde. The pulmonary artery and aorta were tied off and the lungs were removed and immersed in 1% glutaraldehyde for 4 hours for additional fixation under pressure. Blood vessels were dissected free of surrounding tissue, further fixed in 2.5% glutaraldehyde, postfixed in osmium tetroxide, dehydrated, and critical point dried. The luminal surfaces were sufficiently flat to allow visualization of most of the topographic features of the luminal surface. Even 1 missing endothelial cell wound have been obvious. Contrary to previous reports, we did not find a dense population of finger-like projections on the surface of arterial endothelium. Rather, we found a modest number of small rounded blebs. The venous endothelial sheet was similar in appearance. These observations demonstrate that scanning electron microscopy may be used to study vascular lesions in pulmonary vaessels, as well as in more accessible peripheral vessels.

Products of tissue injury. Their induction of venous endothelial damage and blood cell adhesion in the dog

To investigate the effects of the continuous entry into the circulation of selected products of tissue injury, histamine, serotonin, and bradykinin were infused intravenously for four hours into anesthetized dogs. Infusion of Tyrode's solution served as the control. Arterial blood pressure and ECG were unchanged at the levels of agents used. In jugular veins, test agents but not Tyrode's solution caused subtle changes over the endothelial sheet and multiple parallel tears of endothelium near valves and side branches. Numerous leukocytes and erythrocytes, and occasional platelets, adhered singly or in small patches to intact but altered endothelium. Masses of leukocytes, erythrocytes, and some platelets accumulated on areas of disrupted endothelium. Arterial endothelium from treated dogs was the same as from controls and was free of blood cells. Jugular veins and carotid arteries were free of noncellular material except for two of six veins from dogs receiving histamine. Thus, the continuous entry of low levels of histamine, bradykinin, and serotonin caused venous damage and blood cell accumulation, both of which have been implicated in initiation and propagation of venous thrombosis.

Pulmonary vascular response during phases of canine heartworm disease: scanning electron microscopic study

Pulmonary arteries and veins of 14 dogs in phases of heartworm disease (Dirofilaria immitis infection) were examined by scanning electron microscopy. Two dogs were infected with D immitis microfilaria only, whereas 12 dogs were infected with adult D immitis. Seven of the dogs infected with adult worms were untreated. Two of these 7 dogs had natural infections of unknown duration introduced by mosquito bite, whereas 5 were experimentally infected for 30 days. The remaining five dogs were experimentally infected for 1 year and had worms removed by drug therapy. These five dogs were maintained 12 months after treatment. Arteries and veins from dogs infected with microfilaria had a continuous sheet of endothelial cells. Arterial endothelium from the seven nontreated dogs infected with adult heartworms exhibited swirling patterns, pore formation, and separation of intercellular junctions. Arteries from all dogs had numerous endothelialized villus protrusions; veins had similar, less extensive changes. Arteries and veins from experimentally infected dogs were similar to naturally infected dogs, indicating the infection procedure produced lesions similar to that normally seen in heartworm disease. The extent of vascular lesions was reduced in four of the five treated dogs that had been infected with adult worms. Adult worms, not microfilaria, may produce the vascular lesions seen in heartworm disease. Lesions will regress if worms are removed from the circulation. Lesions may be caused by generation of humoral factors initiated by the presence of adult worms.