In chronic models of hypertension such as the spontaneously hypertensive rat (SHR), thickening of the media of large arteries occurs mainly through smooth muscle cell (SMC) hypertrophy accompanied by DNA replication resulting in large polyploid cells. In resistance vessels of SHR, medial hypertrophy occurs through a hyperplastic response. It has been suggested that this hyperplasia is due to mitogens such as platelet-derived growth factor (PDGF), while the hypertrophied polyploid cells occur from stimulation by angiotensin II from within the vessel wall. Angiotensin II activates many of the same cellular pathways as PDGF, including stimulation of phospholipase C, mobilization of intracellular calcium and activation of Na+/H+ exchange. Both induce transient increases in the proto-oncogenes c-fos and c-myc. However, a possible explanation for the difference in SMC response may be involvement of an intracellular pathway stimulated by PDGF (but not by angiotensin II), such as stimulation of JE (a cytokine-like molecule), which may activate transcriptional events necessary for mitogenesis. In atherosclerosis vascular hypertrophy occurs in the form of focal intimal thickening and results from hyperplasia of diploid SMC and their greatly increased production of extracellular matrix, (particularly collagen) and the accumulation of intra- and extracellular lipid. The SMC involved in atherogenesis are phenotypically modified compared with the SMC of undiseased regions, and amongst other features have a lower volume fraction of myofilaments (V(v)myo). Associated with modulation to a low V(v)myo are increases in SMC expression of mRNA for collagens type I (alpha-1 and alpha-2) and type III (alpha-1), elastin, fibronectin, as well as massive increases in collagen protein (26- to 45-fold), glycosaminoglycans (5-fold), and lipid accumulation (7-fold). SMC with a high V(v)myo do not proliferate in response to mitogens such as PDGF, but will undergo logarithmic growth upon modulation to a low V(v)myo. Thus, there are several forms of vascular hypertrophy. How these different growth responses of vascular SMC occur, their initiating factors, and why different responses occur in different vessels of the same animal and within different layers of the same vessels, are only just beginning to be unravelled.
