Plasminogen activators and plasminogen activator inhibitors in connective tissues and connective tissue cells: influence of the neuropeptide substance P on expression.

Tissue segments isolated from ligament, epiligament, and synovial tissues from mature female New Zealand White Rabbits were demonstrated to constitutively secrete a plasminogen activator. Several tissues were also observed to constitutively secrete a plasminogen activator inhibitor which was detected in the form of a PA-PAI complex. Heterogeneity was observed in PA and PAI activity between the different connective tissues. Heterogeneity also existed between and within the medial collateral (MCL), lateral collateral (LCL), and the anterior cruciate (ACL) ligaments. In addition to the differences in constitutive expression of PA and PAI activity, differences in the responsiveness to the neuropeptide substance P (10(-5)-10(-9) M) were also detected. This responsiveness to substance P was displayed by an increase in PA and PAI activity in the conditioned medium. The pattern of responsiveness reflected the degree of innervation of these tissues. That is, synovium and epiligament tissue were the most responsive tissues to substance P while the MCL, LCL and ACL were less responsive to the neuropeptide. Parallel results were obtained using cell culture with fibroblasts isolated from the above mentioned tissues. That is, the pattern of responsiveness was similar between cells and tissue segments. More specifically, cells isolated from both synovium and epiligament increased their both their PA (slightly) and PAI activity following exposure to substance P. This was demonstrated at both the protein and RNA level. Thus, cells within a tissue maintain their phenotype when removed from their three-dimensional matrix. These results are unique in demonstrating that normal ligament and synovial cells and tissue respond to substance P by altering the expression of PA and PAI activity. This investigation further supports the concept that innervation may be important in normal connective tissue function.