Matrix metalloproteinase activity is required for activity-induced angiogenesis in rat skeletal muscle

Proteolysis of the capillary basement membrane is a hallmark of inflammatio n-mediated angiogenesis, but it is undetermined whether proteolysis plays a critical role in the process of activity-induced angiogenesis. Matrix meta lloproteinases (MMPs) constitute the major class of proteases responsible f or degradation of basement membrane proteins. We observed significant eleva tions of mRNA and protein levels of both MMP-2 and membrane type 1 (MT1)-MM P (2.9 +/- 0.7- and 1.5 +/- 0.1-fold above control, respectively) after 3 d ays of chronic electrical stimulation of rat skeletal muscle. Inhibition of MMP activity via the inhibitor GM-6001 prevented the growth of new capilla ries as assessed by the capillary-to-fiber ratio (1.34 +/- 0.08 in GM-6001- treated muscles compared with 1.69 +/- 0.03 in control 7-day-stimulated mus cles). This inhibition correlated with a significant reduction in the numbe r of capillaries with observable breaks in the basement membrane, as assess ed by electron microscopy (0.27 +/- 0.27% in GM-6001-treated muscles compar ed with 3.72 +/- 0.65% in control stimulated muscles). Proliferation of cap illary-associated cells was significantly elevated by 2 days and remained e levated throughout 14 days of stimulation. Capillary-associated cell prolif eration during muscle stimulation was not affected by MMP inhibition (80.3 +/- 9.3 nuclei in control and 63.5 +/- 8.5 nuclei in GM-6001-treated animal s). We conclude that MMP proteolysis of capillary basement membrane protein s is a critical component of physiological angiogenesis, and we postulate t hat capillary-associated proliferation precedes and occurs independently of endothelial cell sprout formation.