Influence of lipoprotein(a) plasma concentration on neointimal growth in a monkey model of vascular injury.

Lipoprotein(a) [Lp(a)] has been proposed as a risk factor for both restenosis and coronary heart disease. Recently, we identified Lp(a) in the arterial wall during the initial rapid neointimal growth phase that occurs after balloon injury in cynomolgus monkeys. The purpose of this study was to determine the relationship between circulating Lp(a) levels and the extent of early neointimal formation. Initially, 348 cynomolgus monkeys were screened to identify 15 monkeys that had either high or low circulating Lp(a) levels. In the 15 monkeys, circulating Lp(a) levels were confirmed by two separate measurements over 6 weeks using an immunoturbidimetric assay. Cohorts were identified with plasma Lp(a) levels that differed by four fold. Lp(a) levels expressed as total mass averaged 32 +/- 4 (N = 8) and 136 +/- 12 (N = 7) mg/dl in the low and high groups, respectively. Between the two assays absolute Lp(a) levels differed by less than 6%. Iliac arteries were harvested 14 days after injury induced by expansion of the internal vessel diameter 1.4 times its initial size with an angioplasty balloon. The neointimal area in the high Lp(a) monkeys was 16% greater (0.49 +/- 0.12 mm2, N = 8 versus 0.57 +/- 0.10 mm2, N = 7) than in the low animals; however, this difference was not statistically significant (P = 0.63). Medial areas averaged 1.27 +/- 0.11 and 1.44 +/- 0.20 mm2 (P = 0.48) in these groups, respectively. Tissue Lp(a) quantification, using a mouse monoclonal anti-Lp(a) antibody, indicated that the percent total area staining positive for Lp(a) was 1.7-fold higher in the high versus the low Lp(a) group (2.7 +/- 0.4% versus 1.6 +/- 0.4%, N = 6-8); this difference was not statistical significant (P = 0.28). In summary, a four-fold increase in circulating plasma Lp(a) levels did not result in a statistically significant enhanced neointimal formation at 14 days after balloon injury. This finding suggests that studies of longer duration may be needed to amplify the trend toward increased neointimal growth observed in this study.

Vascular remodeling in balloon injured rabbit iliac arteries.

To characterize long-term vascular remodeling associated with neointimal formation in vivo, we established a model of balloon injury in normal chow fed rabbits. The iliac artery was injured by denudation using a 2F embolectomy catheter. Injured vessels were removed after perfusion fixation (90 mm Hg) in situ at 2, 4, 6, and 12 weeks post-injury; control vessels were obtained from 2- and 12-week age-matched, uninjured animals. Intimal growth was observed in all animals post-injury. Intimal area averaged 0.13 +/- 0.02 mm2 2 weeks post-injury and continued to increase at 4 and 6 weeks post-injury; +38% and +77% relative to the 2-week time point, respectively. Medical areas were similar among the 2-, 4-, and 6-week injury groups and the 2- and 12-week control groups. From 6 to 12 weeks post-injury, both intimal and medial areas decreased significantly (30% and 34%, respectively); while lumen area increased 53% from 4 to 12 weeks and overall vessel size (area enclosed by the external elastic lamina) remained the same. These data demonstrate that intimal and medial thinning contribute to long term maintenance of lumen area in response to neointimal formation.

Vascular remodeling in balloon injured rabbit iliac arteries.

To characterize long-term vascular remodeling associated with neointimal formation in vivo, we established a model of balloon injury in normal chow fed rabbits. The iliac artery was injured by denudation using a 2F embolectomy catheter. Injured vessels were removed after perfusion fixation (90 mm Hg) in situ at 2, 4, 6, and 12 weeks post-injury; control vessels were obtained from 2- and 12-week age-matched, uninjured animals. Intimal growth was observed in all animals post-injury. Intimal area averaged 0.13 +/- 0.02 mm2 2 weeks post-injury and continued to increase at 4 and 6 weeks post-injury; +38% and +77% relative to the 2-week time point, respectively. Medical areas were similar among the 2-, 4-, and 6-week injury groups and the 2- and 12-week control groups. From 6 to 12 weeks post-injury, both intimal and medial areas decreased significantly (30% and 34%, respectively); while lumen area increased 53% from 4 to 12 weeks and overall vessel size (area enclosed by the external elastic lamina) remained the same. These data demonstrate that intimal and medial thinning contribute to long term maintenance of lumen area in response to neointimal formation.

Localization of lipoprotein(a) in a monkey model of rapid neointimal growth.

Lipoprotein(a) [Lp(a)] has been proposed as a restenosis risk factor, but it is not known if Lp(a) is present in the injured arterial wall during the initial neointimal growth. The purpose of this study was to determine if Lp(a) is incorporated into the vessel wall during rapid neointimal formation after arterial injury in primates. In this model, distention of the iliac artery with an angioplasty catheter caused focal breaks in the internal elastic lamina (IEL) in 80% of the vessels and extensive IEL fragmentation with medial disruption in 20% of the vessels. Neointimal growth was noted in all injured arteries; thrombus formation was noted in 40% of the vessels. Based on morphometric measurements, injured arteries had neointimal areas of 0.41 +/- 0.05 (n = 4) and 0.83 +/- 0.23 (n = 6) mm2 at 14 and 28 days after injury, respectively. Control arteries had an intact IEL and a monolayer of intimal cells. Lp(a) localization was examined histologically by using a mouse monoclonal anti-Lp(a) antibody. Lp(a), found in all injured arteries, was localized primarily in the neointima in 50% of the vessels. In the subset of vessels with evidence of thrombus formation, intense Lp(a) immunostaining was associated with the thrombus. Lp(a) was specific to injured arteries as uninjured vessels did not stain. In addition, staining was not seen with a negative control, a nonspecific mouse IgG1 antibody. The presence of Lp(a) at the site of rapid neointimal growth supports a role for this lipoprotein in the response to vascular injury after balloon angioplasty.