Age-related interrelationships of blood pressure and arterial sterol accumulation in spontaneously atherosclerosis-susceptible and atherosclerosis-resistant pigeons

Pulse wave velocity and age- and gender-dependent aortic wall hardening in fowl

Before sexual maturation, chickens (Gallus gallus) show high blood pressure (BP) and neointimal plaques in the lower abdominal aortae (AbA). We investigated age/sex-related changes in pulse wave velocity (PWV), elastin, collagen, and protein levels in AbA, and cardiac morphology to determine whether PWV increases during incremental increases in BP of maturing fowl, while arterial stiffness becomes dominant with aging. PWV (m/s) was significantly greater in male chicks (6-7 weeks, 9.3+/-0.8; females, 6.1+/-0.5) and remained high in cockerels (13 weeks), young (27-28 weeks), and adults (44-66 weeks). PWV increased in prepubertal pullets (10.0+/-0.9), dropped significantly in young hens, and remained low in adults. In contrast, medial thickness, protein levels, and collagen levels increased, while elastin/collagen ratios decreased, with maturation/aging. Males had heavier ventricular mass and thicker ventricular walls than females at all ages; left ventricular thickness decreased with maturation/aging. Thus, sustained high BP may have caused progressive medial hypertrophy, increased aortic rigidity, and enlarged hearts with left ventricular dilation. PWV of AbA was already greater in male chicks at an age when both sexes have similar collagen levels and low protein levels, suggesting that a factor other than structural stiffness may be an important determinant of PWV.

Age- and sex-dependent changes in pulse pressure in fowl aorta

Chickens (males more than females) have higher blood pressure (BP) than most mammals and spontaneously develop vascular neointimal plaques (NP) and diffuse subendothelial thickening in the lower segment of the abdominal aorta (AbA, referred to as 'NP-prone area') that partly resemble atherosclerotic lesions in mammals. NP areas, which are larger in males, have a causal relationship with incremental increases in BP during maturation. We hypothesize that decreased wall distensibility and altered hemodynamic forces at the NP-prone area may contribute to the NP formation. We measured pressure pulse wave (PW) and systolic and diastolic BP along the descending aorta in anesthetized chickens at different ages using an intravascular microtip transducer and calculated pulse pressure (PP) as an indicator for artery distensibility. At all ages examined and in both sexes, the PW showed a sharper peak at the more peripheral locations and the amplitude of the PW increased as it descended the aorta. PP, expressed as relative increases from the PP in the aortic arch (%), was 40.4+/-12.6 and 71.4+/-18.6 at the AbA and ischiadic artery, respectively, in young males (24-27 weeks); 23.5+/-8.6 and 43.8+/-16.2 in adults (72-75 weeks); and 5.4+/-3.4 and 9.1+/-4.9 in chicks (5-7 weeks). Location-dependent increases in PP were significantly higher in young males (P<0.05). The PP increases in females were not different among the three age groups. The contour of the PW in the proximal aorta changes in older birds, exhibiting steeper increases in the ascending and descending limbs, suggesting that faster wave reflection from the periphery augments peak systolic pressure. NP was most frequently seen in the lower segment of the abdominal aorta in older males. These results suggest that: (1) site-dependent increases in PP amplitude are marked in young males, possibly reflecting a reduction in arterial wall elasticity enhanced by incremental rises in BP, and (2) NP formation may contribute to the stiffness of aortic walls in the NP-prone area.

Maturation-dependent neointima formation in fowl aorta

Fowl show spontaneous elevation of blood pressure (BP) and neointimal plaque formation in the abdominal aorta at young ages. Maturation/age-dependent modulation of vascular lesions and a causal relationship between elevated BP and neointima formation, however, have not been clarified. We therefore intended to characterize, first, maturation/age-dependent neointimal plaque formation and vascular lesions and, second, their relationship to BP elevation. The BP measured in conscious domestic fowl, Gallus gallus, White Leghorn breed, DeKalb strain, via an indwelling catheter inserted into the ischiadic artery, increased with maturation in males; and at plateau level, BP (mmHg) was significantly (P<0.01) higher in males (194.0+/-4.6, n=11) than in females (169.3+/-3.1, n=10). Neointimal plaques consisting of neointimal cells and abundant extracellular matrix appeared initially in the distal segment of the abdominal aorta (lesion-prone area) of chicks as early as 6 weeks old. The area (size) of neointimal plaques right above the ischiadic bifurcation increased with maturation, whereas the plaque area became smaller with some degenerative changes in adult birds. In some birds, diffuse subendothelial hyperplasia and more extensive plaque formation at the branching points of the aorta were observed. The plaque area appears to be larger in birds, particularly in males that have higher BP (r=0.68). The width of aortic smooth muscle (SM) layers, measured in regions with no plaque, increased with age, whereas the number of cells per unit of area decreased, suggesting that hypertrophy of vascular SM occurs in response to exposure of the vascular wall to high BP. The number of cells was significantly (P<0.01) higher in the plaque than in underlying aortic SM layers or in layers with no plaque formation. Both neointimal plaques and underlying SM layers are immunohistochemically positive for alpha SM actin, suggesting that neointimal cells are modulated SM cells, whereas the staining with SM myosin heavy chain antibody is low in neointimal plaques. Furthermore, plasma arginine levels dropped in accordance with the time of neointimal plaque formation, whereas plasma cholesterol levels showed an age-dependent increase. The results suggest that spontaneous development of neointimal plaques may be a consequence of exposure to high BP and associated local hemodynamic changes.

Osteopontin expression in spontaneously developed neointima in fowl (Gallus gallus)

Fowl show spontaneous elevation of blood pressure and neointimal plaque formation in the abdominal aorta at young ages. A similar neointima can be induced by a balloon-catheter-induced endothelium injury to the fowl aorta. Both spontaneously developed and injury-induced vascular lesions exhibit subendothelial hyperplasia consisting of neointimal cells with a synthetic phenotype and abundant extracellular matrix. The role of the extracellular matrix in the formation of neointima is not known. In this study, we investigated whether osteopontin, an adhesive glycoprotein present in the extracellular matrix, is expressed in aortic smooth muscle tissue of the fowl abdominal aorta, in spontaneously developed neointimal plaques and in the aortic smooth muscle underlying neointimal plaques. Crude protein extracted from isolated aortic smooth muscle tissues and neointimal plaques was fractionated by SDS-polyacrylamide gel electrophoresis and analyzed by immunoblotting with rabbit anti-fowl osteopontin (provided by Dr L. C. Gerstenfeld, Boston University) or anti-α smooth muscle actin antibodies. The anti-fowl osteopontin antibody predominantly recognized a 66–70 kDa protein band in neointimal plaques that co-migrated with the osteopontin phosphoprotein from chick bone. In contrast, intact aortic smooth muscle and the smooth muscle underlying neointimal plaques equally expressed three proteins (66–70 kDa, approximately 50 kDa and approximately 43 kDa) recognized by the anti-osteopontin antibody. Anti-α smooth muscle actin antibody recognized a 43 kDa protein band, and the expression of α smooth muscle actin was higher in aortic smooth muscle than in neointimal plaques. Osteopontin mRNA expression was examined using reverse transcription-polymerase chain reaction (RT-PCR) of total RNA from vascular tissues with specific primers constructed on the basis of the reported fowl osteopontin nucleotide sequence. The PCR products from intact aortic smooth muscle and neointimal plaques correspond to the product from recombinant plasmid cDNA (a gift from Dr L. C. Gerstenfeld) transcribed in vitro. These results suggest that osteopontin is synthesized in intact aortic smooth muscle and neointimal plaques in fowl and that unmetabolized approximately 66 kDa osteopontin protein is a predominant form in the neointima, indicating that osteopontin protein may be actively synthesized in the neointima.

Control of vascular smooth muscle cell growth in fowl

In adult domestic fowl, angiotensin (ANG) receptors are present in the vascular smooth muscles (VSM) and in the endothelium, mediating vasorelaxation via endothelium-derived relaxing factor/cGMP. ANG II-induced relaxation is minor in chicks and becomes more marked as they mature but diminishes in adult birds, whereas ANG II neither relaxes nor contracts endothelium-denuded aortae from mature chickens. The present study examines in cultured fowl aortic SM cells whether (1) ANG II stimulates or inhibits VSM cell growth and, if so, whether this growth-stimulatory or -inhibitory effect changes with maturation/aging, and (2) S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide donor, and cGMP attenuate the basal or stimulated VSM cell growth. [Asp1, Val5]ANG II (native fowl ANG II, 10(-6) M) markedly increased (increase from vehicle control, 226.5%; P < 0.01) [3H]thymidine (Thd) incorporation into DNA of quiescent VSM cells (first subculture) from 6-week-old chicks. This growth-stimulating effect was reduced with age (41.4, 29.6, and 3.2% at 9, 19, and 43 weeks of age, respectively). In contrast, platelet-derived growth factor (PDGF, 20 ng/ml) increased [3H]Thd incorporation similarly in chicks, pullets, and hens. Furthermore, ANG II significantly (45.9%, P < 0.01) attenuated the growth-promoting effect of fetal calf serum in cultured VSM cells from 6-week-old chicks. This inhibitory effect also decreased in older birds. ANG II showed neither a growth-stimulatory nor -inhibitory effect in cultured neointimal cells. SNAP attenuated dose dependently (20-60 microM) the basal and PDGF-induced VSM cell growth, whereas cGMP inhibited basal growth only at a high dose (100 microM). These results indicate that in fowl VSM cells, ANG II is mitogenic and antimitogenic in chicks but not in mature birds, suggesting that phenotypic modulation occurs in the ANG receptors/signaling mechanism with maturation/age or in neointimal cells, whereas the mitogenic mechanism via PDGF remains in both young and mature birds.

Angiotensin II receptors in the fowl aorta

In the domestic fowl, angiotensin II (ANG II) causes an in vivo depressor response and in vitro relaxation of aortic rings which appear to be a direct action of ANG II on the blood vessels. Thus, we determined whether binding sites specific to ANG II exist in the membrane fraction of the fowl aorta. The particulate fraction of aortas from adult female fowl, Gallus gallus, exhibits high specific binding to ANG II ligand. 125I-[Ile5]ANG II (0.5 nM) binding to the above fraction (30 micrograms protein) in 50 mM Tris (pH 7.2), 10 mM MgCl2, and 0.2% bovine serum albumin at 12 degrees (1) is rapid, saturable, and reversible; (2) increases as a function of ligand or membrane concentration, time, and temperature; and (3) optimally fits to a two-site (high and low affinity) model. The equilibrium dissociation constant (0.15 +/- 0.03 nM) and binding site concentration (28.7 +/- 8.1 fmol/mg protein) of the high affinity site as well as association (0.055 nM-1.min-1) and dissociation (0.0122 min-1) rate constants are similar to those of mammalian vascular ANG II receptors. Both 125I-[Ile5]ANG II and 125I-[Val5]ANG II are competitively displaced by unlabeled ANG II. These results suggest that specific ANG II receptors exist in the fowl aorta.

Regional aortic differences in atherosclerosis-susceptibility: changes in prostaglandin biosynthesis and cholesterol accumulation in response to desoxycorticosterone (DOCA)-salt induced hypertension

In spontaneously atherosclerosis-susceptible White Carneau pigeons, intimal cushions that appear at birth near the coeliac branch of aorta do not progress into atherosclerotic lesions. However, the area across from the intimal cushion (so called 'lesion area') a) accumulates cholesteryl esters b) synthesizes more PGE2 and c) eventually develops into complicated atherosclerotic plaques. When DOCA-salt hypertension is induced in the pigeons, the 'initimal cushion' area displays a) accumulation of increasing amounts of cholesteryl esters and b) increase in the synthesis of all prostaglandins (particularly PGE2) from C14-arachidonic acid and c) approaches similarity to the 'lesion area' in the magnitude of these changes. These results suggest that under the influence of a risk factor, the 'intimal cushion' can acquire biochemical properties of the atherogenic areas of the aorta.

Renin-angiotensin and adrenergic mechanisms in control of blood pressure in fowl

Many avian species demonstrate atherosclerosis and high blood pressure (BP) that are influenced by age, sex, diet, and environment, but show no arteriosclerosis in small vessels. Thus, we aimed to define neural and humoral control of BP in conscious, 32-wk-old female chickens, Gallus gallus. Mean aortic pressure (determined by chronically implanted catheter) was 137.6 +/- 2.0 mm Hg; heart rate was 295 +/- 4 beats/min. Plasma renin activity (PRA), measured by radioimmunoassay of fowl angiotensin I ([Asp1, Val5, Ser9]AI), and plasma angiotensinogen levels were 3.55 +/- 0.31 ng/ml/hr and 1229 +/- 66 ng/ml respectively. Repeated injection of propranolol (4 to 8 mg/kg/day, i.m.) decreased (p less than 0.01) the BP 19.1 +/- 3.0 mm Hg and heart rate 76 +/- 6 beats/min. Acute infusion of propranolol also markedly reduced BP and heart rate, and increased plasma levels of norepinephrine and epinephrine. SQ 14,225 (20 mg/kg/day) reduced BP (p less than 0.01), but BP returned towards original levels unless a higher dose was given. PRA increased 2- to 6-fold. BP also decreased 31.0 +/- 2.1 mm Hg after reserpine treatment, but not after [Sar1, Ile8]AII. These results suggest that in maintaining BP in fowl the beta-adrenergic function is important, whereas the renin-angiotensin system may not have a primary role.

Regional differences in cholesterol content of aorta in response to experimental coarctation in spontaneously atherosclerosis-susceptible pigeons

The effect of experimental coarctation of the aorta at a site above the coeliac bifurcation (site of occurance of spontaneous atherosclerotic lesions) in White Carneau pigeons was examined. Cholesterol content of the aorta in the site where spontaneous atherosclerosis usually occurs (coeliac bifurcation = lesion site) was decreased in pigeons with aortic coarctation. On the other hand, in the region proximal to the site of coarctation which is usually free of atherosclerotic events cholesterol accumulation was increased. A decrease in percent composition of oleic acid (the major fatty acid which increases during atherosclerosis) was noted in the lesion site. These studies have shown regional differences in response of aorta to experimental coarctation in spontaneously atherosclerotic-susceptible pigeons and suggest that lowering of blood pressure at lesion site might decrease cholesterol accumulation and perhaps might retard subsequent atherogenic process.

Endothelial damage and thrombocyte adhesion in pigeon atherosclerosis

Scanning electron microscopy studies of spontaneously occurring atherosclerosis in pigeons reveal dramatic alterations in endothelial integrity. An irregular endothelium at the intimal cushion region of 5-week-old birds gives rise to extensive areas of pitted endothelium and subendothelial exposure. Thrombocytes, thrombocyte aggregates, and leukocytes are associated with the developing lesion.