Phenotypic Characteristics of SMC in the Distal Pulmonary Circulation In Vivo and In Vitro Analysis

Due to its primary functions of vasoconstriction and regulation of blood flow, it seems likely that SMC composition of the distal pulmonary arterial bed is significantly different from that of the vessels in the more proximal pulmonary circulation. Unfortunately, there is a paucity of published data describing the phenotype of cells comprising the media in distal pulmonary arterial segments, particularly resistance size vessels which are, the locus of pulmonary circulatory control. Further, it remains unclear whether the complex structure seen in the main pulmonary artery media extends to the distal arteries, and whether the multiple distinct SMC phenotypes observed in the proximal pulmonary arteries are also present in small resistance vessels.

We thus evaluated SMC phenotypes at various points along the longitudinal axis of the mature pulmonary artery bed. Interestingly, we found that the complex mosaic of cells observed in large proximal vessels is lost as distance from the heart increases (Fig. 5) (32). Unlike large conduit pulmonary arteries, the media of small resistance vessels, based on immunohistochemical studies, appears to be composed of a uniform population of differentiated SMC that express all the SM markers including metavinculin and SM-P-MHC isoform. These cells exhibit a phenotype similar to that of one population of SMC in the outer media of the main pulmonary artery (well-differentiated "L3-S" SMC).

Recent studies on porcine coronary arteries have demonstrated that even tunica media composed of an apparently uniform cell phenotype in vivo, can give rise to phenotypically and functionally distinct cell populations in culture (14). Therefore, to further examine the cellular composition of the distal pulmonary artery, media cells from arteries with a diameter ranging from 600 to 1300 (im were isolated and expanded in culture. The isolated cells were of a uniform spindle-shaped morphology, and expressed a-SM-actin, SM-MHC, and metavinculin thus supporting the in vivo observations of their "well-differentiated" SM phenotype (32).

Figure 5. Primary culture of cells isolated from distal (<1000nm, diameter) pulmonary arteries of hypoxic hypertensive neonatal calves. Two morphologically distinct cell populations are observed. Cells in one population (SMC, smooth muscle cells) are large, elongated and form "hill-and-valley" pattern at confluence, whereas cells in another population (NMC, nonmuscle cells) are markedly small and rhomboidal in shape, and pile on top of each other at confluence.

Figure 5. Primary culture of cells isolated from distal (<1000nm, diameter) pulmonary arteries of hypoxic hypertensive neonatal calves. Two morphologically distinct cell populations are observed. Cells in one population (SMC, smooth muscle cells) are large, elongated and form "hill-and-valley" pattern at confluence, whereas cells in another population (NMC, nonmuscle cells) are markedly small and rhomboidal in shape, and pile on top of each other at confluence.

We evaluated the proliferative capabilities of these distal SMC and compared them to cell populations isolated from the proximal pulmonary circulation. The distal SMC were slow growing, with a growth rate similar to that of "well-differentiated" "L3-S" SMC from the outer media of the main pulmonary artery (MPA), and much slower than all other MPA cell populations (Table 2) (32). The proliferative responses of distal SMC to purified peptide mitogens (PDGF, IGF-1 and bFGF) were much less than those of the non-muscle cell populations from the MPA and also significantly less than those of L2-SMC from the middle MPA media. The responses were very similar to those observed in the "well-differentiated" L3-S-SMC from the outer MPA media. The rank order of DNA synthesis in response to mitogen stimulation was as follows, MPA non-muscle cells (LI, "L3-R") > MPA middle media L2-SMC > distal SMC > MPA outer media "L3-S" SMC (Table 2) (32).

Because hypertrophy of cells has been suggested as a response of small vessels to hypertensive stress, the ability of distal SMC to increase 35S-incorporation (as a measure of protein synthesis) in response to purified peptide mitogens was also evaluated. We found that, in distal PA SMC and in MPA-"L3-S", PDGF, angiotensin II and TGFßl stimulated significant increases in incorporation (protein synthesis) while causing only minimal increases in 3H-thymidine incorporation (cell proliferation). Thus, these responses were significantly different from those observed in the middle media SMC where PDGF stimulated proliferation rather than hypertrophy.

The responses of distal SMC to hypoxia were also evaluated. Hypoxia consistently inhibited the growth of distal pulmonary artery SMC under all conditions tested, (i.e. both in the presence and/or absence of 10% calf serum, Table 2). Thus, it appears as though medial SMC in the distal pulmonary circulation, at least in the bovine species, are of a uniform phenotype, "well-differentiated", and relatively growth-resistant.

There are at least three recent in vitro studies, which have described SMCs isolated from distal segments of the human pulmonary arterial tree. Yang and coworkers (37) described two SMC phenotypes isolated from distal segments of the human pulmonary arterial bed that differed in their proliferative response to hypoxia. Hypoxia inhibited proliferation in one cell phenotype, while stimulated in another. Using SMC derived from the pulmonary artery of both normal and hypertensive patients, Wharton and co-workers described that distal SMC exhibited marked responsiveness to serum-stimulated proliferation and adenylyl cyclase-induced inhibition of growth, compared to SMC from more proximal regions (34). Additional work from this group demonstrated that SMCs from distal segments express both endothelin A and endothelin B receptors and that both receptor subtypes mediate cell proliferation (6). These studies, when taken at face value, do confirm findings in many animal species of significant differences in SMC phenotype between proximal and distal vessels. In some ways, they suggest the existence of SMC heterogeneity in small distal pulmonary arteries in humans; however, because the cells utilized for study were often obtained from diseased as well as normal vessels, and because they were often only minimally characterized as SMC, the studies do not definitely exclude the possibility that some cells described were of non-medial origin.

Was this article helpful?

0 0
Reducing Blood Pressure Naturally

Reducing Blood Pressure Naturally

Do You Suffer From High Blood Pressure? Do You Feel Like This Silent Killer Might Be Stalking You? Have you been diagnosed or pre-hypertension and hypertension? Then JOIN THE CROWD Nearly 1 in 3 adults in the United States suffer from High Blood Pressure and only 1 in 3 adults are actually aware that they have it.

Get My Free Ebook


Post a comment