2-Adrenoceptor is involved in connective tissue remodeling in regenerating muscles by decreasing the activity of MMP-9

We investigated the role of [beta]2-adrenoceptors in the connective tissue remodeling of regenerating muscles from [beta]2-adrenoceptor knockout ([beta].sub.2KO) mice. Tibialis anterior muscles from [beta].sub.2KO mice were cryolesioned and analyzed after 3, 10, and 21 days. Regenerating muscles from [beta].sub.2KO mice showed a significant increase in the area density of the connective tissue and in the amount of collagen at 10 days compared with wild-type (WT) mice. A greater increase occurred in the expression levels of collagen I, III, and IV in regenerating muscles from [beta].sub.2KO mice evaluated at 10 days compared with WT mice; this increase continued at 21 days, except for collagen III. Matrix metalloproteinase (MMP-2) activity increased to a similar extent in regenerating muscles from both [beta].sub.2KO and WT mice at 3 and 10 days. This was also the case for MMP-9 activity in regenerating muscles from both [beta].sub.2KO and WT mice at 3 days; however, at 10 days post-cryolesion, this activity returned to baseline levels only in WT mice. MMP-3 activity was unaltered in regenerating muscles at 10 days. mRNA levels of tumor necrosis factor-[alpha] increased in regenerating muscles from WT and [beta].sub.2KO mice at 3 days and, at 10 days post-cryolesion, returned to baseline only in WT mice. mRNA levels of interleukin-6 increased in muscles from WT mice at 3 days post-cryolesion and returned to baseline at 10 days post-cryolesion but were unchanged in [beta].sub.2KO mice. Our results suggest that the [beta]2-adrenoceptor contributes to collagen remodeling during muscle regeneration by decreasing MMP-9 activity.